Activity Report
2006
Preface1

Research Highlights

2 - 35Highlights of Joint Research

36 - 51

International Conferences
and Workshops

52 - 53

ISSP Workshops

54 - 56

Subjects of Joint Research

57 - 101

Publications

102 - 149

ISSP Activity Report 2006

Preface

The ISSP will celebrate its 50th anniversary this fall. It wasestablished in 1957 by the recommendation of the National
Council of Japan to set up a new institute for condensed
matter physics. The idea was that to catch up the world levelof research in the field at that time it was necessary toestablish a top class research institute for experimental andtheoretical studies of condensed matter and to promote
cooperative research throughout the country.
About twenty years after the inauguration the ISSP wasrecognized as one of the major research institutes in thecommunity of condensed matter physics and decided to reorganize the institute to promote theproject of the "Physics in extreme conditions", which includes ultra-high magnetic fields, laser
physics, surface physics, ultra-low temperature physics and very high pressures. After anothertwenty years, the ISSP completed its relocation in 2000 to the new Kashiwa campus of the
University of Tokyo to make further progress and to extend its activities into new fields.
This copy is the Activity Report for the fiscal year 2006 of the research done by the staff of the ISSP
and also of the joint research conducted by the ISSP. We are happy to receive any comments on theReport for the possible improvement of the research activities of the ISSP.
At present we are making plans for the events commemorating semicentennial of the ISSP. One is a
symposium on the physics of strongly correlated systems, which is one of the major activities of the
ISSP. In another symposium the past and present of the ISSP will be reviewed and future directionsof the ISSP will be discussed. The fifty year anniversary of the ISSP will be also a good occasion to
attract a general audience and to provide them an opportunity to think about the role of materials
science in the modern world. The third symposium will be devoted to the general audience including
young students. Any suggestions for the events will be most welcome.

June 30, 2007

Kazuo Ueda
Director
Institute for Solid State Physics
The University of Tokyo

ISSP Activity Report 2006

1

Research Highlights

Lattice Preferred Orientation in
Post-Perovskite type MgGeO3
Yagi Group
Since the discovery of post-perovskite type MgSiO3 in2004 [1, 2], numerous studies have been made to clarify its
properties such as stability field, equations of state, elastic
property, and plastic property. Because this new phase may
explain various strange properties of the layer at the bottomof the lower mantle, a layer called D" layer, which weredifficult to explain by any known materials. If the postperovskite phase forms lattice preferred orientation (LPO) inthis layer, strong elastic anisotropy observed in D" layer
could be explained and various studies have been made toclarify this possibility. Unfortunately the stability field of the
post-perovskite type MgSiO3 is too high to conduct this kindof study and the experiments were made using various
analog materials such as MgGeO3 [3] and CaIrO3 [4].
Although these two model materials have the same crystal
structure, the resulting LPO are quite different and it is difficult at this moment to estimate the behavior of MgSiO3 in
the Earth.
In the present study we have studied the behavior of postperovskite type MgGeO3 in detail and concluded that thereported LPO in MgGeO3 [3] is not the deformation texture
and cannot be used for the discussion of the D" layer.
Experiments were made by squeezing the powdered sample
between two diamond anvils and powder X-ray diffraction
experiments were made adopting radial diffraction geometry
using X-ray transparent gasket [5], as shown in Fig. 1.

Fig. 2. Unrolled X-ray diffraction profile of the radial diffraction of
post-perovskite type MgGeO3 at 77 GPa. Strong lattice preferred orientation different from the one reported before [ref. 2] is observed.

Orthopyroxene (Opx) type MgGeO3 was prepared at atmospheric pressure. The powdered starting material was
squeezed to 37 GPa at room temperature and converted into
perovskite (Pv) structure by heating it using YAG laser.
Then further compression was made to 77 GPa at room
temperature and then the post-perovskite phase was formedby heating in a uniaxial stress field. Radial diffraction experiments were made at BL13A of the Photon Factory andobserved 2D diffraction patterns were recorded on IP
detector. The unrolled X-ray diffraction pattern is shown in
Fig.2. It is clear from this figure that strong LPO is formed in
the post-perovskite phase formed under such condition.
However, the observed LPO is quite different from thatreported in the previous study [3], which was formed directlyfrom orthopyroxene phase. This result strongly suggests that
the LPO in MgGeO3 so far reported is not the texture representing the deformation fabric but the texture formed by the
topotactic relation from the pre phase before the transformation. In order to confirm it, experiments were made tocompress post-perovskite phases formed from Opx and from
Pv to much higher pressures. The result indicate that the each
LPO formed just after the formation remained unchanged
even when the pressure is increased to more than 45 GPa
after the formation. This is because the sample chamber is
too thin to deform during compression. By using diamondgasket technique, we could increase the sample thickness
considerably and could see some change in LPO during
compression. Further careful studies are needed to clarify the
LPO which can be applicable to the discussion of the Earth's
deep interior.
References[1] M. Murakami, K. Hirose, K. Kawamura, N. Sata, and Y. Ohishi,
Science 304, 855 (2004).
[2] A. R. Oganov and S. Ono, Nature 430, 445 (2004).
[3] S. Merkel, A. Kubo, L. Miyagi, S. Speziale, T. S. Duffy, H. K.
Mao, and H. R. Wenk, Science 311, 644 (2006).
[4] D. Yamazaki, T. Yoshino, H. Ohfuji, J. Ando, and A. Yoneda,
Earth Planet. Sci. Lett. 252, 372 (2006).
[5] S. Merkel and T. Yagi, Rev. Sci. Instrum. 76, 046109 (2005).

Fig. 1. Diamond anvil with large side openings developed for the radialdiffraction study. Incident X-ray passes through the transparent gasket
made of boron and Kapton and irradiates the sample perpendicular tothe compression axis of the diamond anvil.

2

ISSP Activity Report 2006

Authors
T. Okada, T. Yagi, and K. Niwa

Site-Selective NMR in the Quasi One
Dimensional Conductor β-Sr0.33V2O5
Takigawa and Y. Ueda Groups
The family of quasi-one-dimensional mixed valent
vanadium oxides β-A0.33V2O5 (A=Li, Na, Ag, Ca, Sr, andPb) exhibits a variety of exotic phenomena such as metalinsulator transition, charge ordering, and pressure-induced
superconductivity [1], providing fascinating opportunities tostudy competing electronic orders in one series ofcompounds. There are three types of one-dimensional V-O
network extending along the most conducting b-axis (Fig. 1):
two types of zigzag chains formed by the V1 and the V3 sites
and a two-leg ladder formed by the V2 sites. The ordering ofA ions doubles the unit cell along the b-axis and generates
two inequivalent V sites in each of the three 1D structuralunits denoted as Vna and Vnb (n=1 - 3) in the monoclinic
space group P21/a.
We have performed nuclear magnetic resonance (NMR)
experiments on 51V nuclei in β-Sr0.33V2O5, which shows ametal-insulator transition at T=170 K (see the article in thisreport by Y. Ueda’s group). We have resolved NMR linefrom all six V sites in the high temperature metallic phase(Fig. 2) and made site-selective measurements of the Knight

Fig. 3. The Knight shift (K) is plotted against the field direction for all
six sites (labeled as A - F) at T=190 K. The angles for the maximum of|K| match precisely with the directions of the shortest V-O bonding in
the ac-plane, leading to the assignment of the pairs of lines (A, B),(C, D) and (E, F) to (V1a, V1b), (V2a, V2b) and (Va3, V3b) sites,
respectively.

Fig. 4. (a): Temperature (T) dependence of K at various sites normalized by the value at T=300 K. (b) - (d) : T- dependences of 1/(T1T). (e):T- dependences of 1/(T1T) and 1/(T1TK2) at the D sites (one of the V2sites). All measurements were done for the field oriented along one ofthe "magic angles" shown in the figures, where the quadrupole splitting
vanishes.

Fig. 1. The crystal structure of β-Sr0.33V2O5 with the space group P21/aviewed along the (a) b-, (b) a*-, and (c) c-directions.

shift (K) (Figs. 3 and 4a) and the nuclear spin-lattice relaxation rate 1/(T1T) (Figs. 4b - 4e) [2].
We found that different sites show contrasting temperature dependences of K and 1/(T1T) (Fig. 4). Thus themagnetic properties are remarkably heterogeneous not only
among the three 1D structural units but also among the two
V sites in the same unit. In particular, the V2 ladder unit
shows pronounced charge disproportionation of d-electrons
among the two (V2a and V2b) sites as evidenced by the large
difference in angular variation of K for the C and D sites(Fig. 3). Only one of them (D site) shows strong increase of|K| and 1/(T1T) with decreasing temperature (Fig. 4),
suggesting development of ferromagnetic correlation. The
strong heterogeneity even in the metallic phase should be a
key feature to understand the metal-insulator transition inthis family of compounds.
References[1] T. Yamauchi, M. Isobe, and Y. Ueda, Solid State Science 7, 874(2005).
[2] T. Waki, M. Takigawa, T. Yamauchi, J. Yamaura, H. Ueda, andY. Ueda, J. Phys. Chem. Solid, to be published.

Fig. 2. 51V NMR spectra at T=190 K obtained from a needle-shape
single crystal (0.5×1.8×0.2 mm3). The magnetic field of 10.5 T was
rotated in the ac-plane (parallel to the a-axis at 0 degree). A single V siteyields seven resonance lines split by the electric quadupole interaction.

Authors
T. Waki, M. Takigawa, T. Yamauchi, J. Yamaura, H. Ueda, andY. Ueda.

ISSP Activity Report 2006

3

Angle-Resolved Magnetization
Measurements of PrFe4P12:
Evidence of a Scalar Order Parameter
Sakakibara Group
Cubic filled skutterudite compounds with the generalformula RT4X12 (R: rare earth; T: Fe, Ru, and Os; X: P, As,
and Sb) have been extensively studied recently because they
exhibit a number of unusual properties. Among them, one of
the fascinating Pr-based skutterudite compounds is PrFe4P12
which shows an intriguing phase transition at TA=6.5 K. The
order parameter (OP) of the phase below TA (A-phase) is
considered to be non-magnetic, because no magnetic superlattice reflection is observed in powder neutron diffraction
experiment. In the early stage of the study, the A-phasetransition has been thought to be an antiferro-quadrupolar(AFQ) order (alternating alignment of the f orbitals) [1].
Some important features of the A-phase, however, haveremained unresolved within the AFQ ordering scenario.
First, no significant magnetic anisotropy that could be
expected in the AFQ phase develops in the A-phase. Second,no field-induced antiferromagnetic moment perpendicular to
applied field has been observed in the NMR experiment [2].
Very recently, a new type of OP that preserves the local
cubic symmetry has been proposed to explain the A-phasetransition [3, 4]. This order parameter is called the scalar
order since the symmetry belongs to the identity representation of the cubic (Th) point group. A number of unusual
properties of the A-phase have been accounted for byassuming a scalar OP. In order to obtain further evidence forthe scalar OP, we have studied the field-orientation dependence of the magnetization and the transition temperature indetail [5].
Figure 1 shows the field-orientation dependence of themagnetization M of PrFe4P12 in the A-phase at T=0.3 K withmagnetic fields H rotated in the (1-10) plane. A smooth
angular variation of M with weak anisotropy has been
observed. The anisotropy is found to diminish as H→0. In
general, the magnetization can be expanded in odd powers of
H as M= χ1H+ χ3(h)H3+..., where h=(hx, hy, hz) is the unit
vector parallel to H. The linear susceptibility χ1 is isotropic
in cubic systems but becomes anisotropic for lower symmetries. The observed weak anisotropy at low fields (see theinset) suggests that the cubic symmetry is preserved in theordered state. Further evidence of the cubic symmetry is

Fig. 2. Field-angle dependence of the transition temperature of
PrFe4P12 measured at 2.8 T. The solid line is a fit by the fourth-order
cubic invariant term.

provided by the anisotropic part of M. Within the cubic
symmetry, the anisotropic component of the non-linear
susceptibility χ3(h) obeys the fourth-order cubic invariant
h4=hx4+ hy4+ hz4-3/5 [3]. The solid line in Fig.1 is a fit by theh4 term, which reproduces the data fairly well.
Figure 2 shows the angular dependence of the transition
temperature TA (H) with H rotated in the (1-10) plane. A
smooth variation is observed over the entire range of the
field angle. Here, the solid line again represents the fit by the
fourth-order cubic invariant h4, in good agreement with theexperimental data. These observations strongly suggest thatthe cubic symmetry is preserved in the A-phase, i.e., the OP
is of scalar type.
References[1] Y. Aoki, T. Namiki, T. D. Matsuda, K. Abe, H. Sugawara, and
H. Sato, Phys. Rev. B 65, 064446 (2002).
[2] J. Kikuchi, M. Takigawa, H. Sugawara, and H. Sato, J. Phys. Soc.
Jpn. 76, 043705 (2007).
[3] A. Kiss and Y. Kuramoto, J. Phs. Soc. Jpn. 75, 103704 (2006).
[4] O. Sakai, J. Kikuchi, R. Shiina, H. Sato, H. Sugawara,
M. Takigawa, and H. Shiba, J. Phys. Soc. Jpn. 76, 024710 (2007).
[5] H. Sato, T. Sakakibara, T. Tayama, T. Onimaru, H. Sugawara, and
H. Sato, J. Phys. Soc. Jpn. 76, 064701 (2007).
Authors
H. Sato, T. Sakakibara, T. Tayama, H. Sugawaraa, and H. SatobaFaculty of Integrated Arts and Science, Tokushima University
bDepartment of Physics, Tokyo Metropolitan University

Magnetophotocurrent Effect at Low
Temperature in Polymer Solar Cell
Tajima Group

Fig. 1. Field-angle dependence of the magnetization of PrFe4P12
measured at 0.3 K. The solid line is a fit by the fourth-order cubic
invariant term. Inset shows the magnetization curves at low fields.

4

ISSP Activity Report 2006

Polymer solar cells have been intensively studied forseveral years because of their features such as low cost, lowweight, and mechanical flexibility. Today's mainstream
device structure is one using a bulk hetero junction, andinterpenetrating donor and acceptor materials. Soluble conjugated polymers, such as poly(3-hexylthiophene) (=P3HT)
and [6,6]-phenyl C61 butyric acid methyl ester (PCBM),
which is a soluble derivative of fullerene, are often used as
donor and acceptor substances, respectively [1].
From the view point of the organic solid-state science,
the bulk-hetero-junction solar cell is an interesting system.
This system is an artificial molecular conductor in whichcarrier density can be controlled by light illumination. No
studies have been reported for such systems at low temperatures and high magnetic fields. In this paper, we report on a
photocurrent decrease at low temperatures under appliedmagnetic field. This is a new phenomenon of organic photovoltaic devices [2].
We fabricated several devices having a structure of
ITO/PEDT:PSS/P3HT:PCBM/Al (see Fig. 1(a)). The thicknesses of the active layer were d = 80 nm and d = 30 nm.
The light source was green LED (peak wavelength = 525nm) chopped at various frequencies.
Figure 1(b) shows the temperature dependences of lightchopped photocurrents at various frequencies for the thin (d =30 nm) sample. All the curves were measured in one thermal
cycle. Although photocurrent decreased on reducing thetemperature, the values at 4.2 K were still 1/10-1/5 of thoseat room temperature. Interestingly, photocurrent exhibits
different temperature dependence by changing the chopping
frequency. This phenomenon is marked above 100 K.
Figure 2 shows the effect of magnetic field on photocurrent for a thin sample. Thick samples also exhibited a fielddependent photocurrent. By increasing the chopping frequency,
and on lowering the temperature, the decrease in photocurrent becomes more prominent. We ascertained that themagnetoresistance in Al and ITO films is negligibly small.
There are several papers reporting on the magnetoresistance and magneto-photocurrent of organic thin films.
Frankevich et al. reported the effect of magnetic field on thephotocurrent of polymer thin films [3]. In their study, ananomalous increase in photocurrent was observed at amagnetic field below 0.1 T in the temperature range between
130 and 350 K. There are several reports on the anomalous
negative magnetoresistance of a particular metal/organic
semiconductors/metal junction containing organic molecules[4]. However, we consider that the photocurrent decrease

Fig. 2 (a) The effect of magnetic field on photocurrent at 2.0 K. Notethat the field dependence differs at two chopping frequencies (20 Hz
and 20 kHz). (b) Temperature dependence of the photocurrent below 10
K at B = 0 T and B = 8 T. Chopping frequency is 20 kHz, and the lightintensity is 9.25 µW.

observed in our studies originates from a different mechanism on the basis of the following reasons. First, weobserved the photocurrent decrease only at low temperatures,
whereas the previously reported phenomena occur at
relatively high temperatures. Secondly, we observed acurrent decrease in the presence of magnetic field, whereas acurrent increase in the presence of magnetic field wasobserved in the previous report.
At the present stage, we are considering that thephotocurrent decrease found in this study originates frompositive magnetoresistance in grains forming bulk-hetero
junction. According to literatures, the sizes of P3HT grainsin the polymer solar cell increase up to around 8 nm after
annealing. The sizes of the PCBM grains are micrometerorder. Thus, the grain sizes are comparable to or larger thanthe magnetic length at 1 T (~26 nm).
References[1]. For example, see "Organic Photovoltaics: Concepts andRealization", ed. C. J. Brabec, V. Dyakonov, J. Parisi, and N. S.
Sariciftci (Springer, Berlin, 2003).
[2] N. Suzuki et al., J. Phys. Soc. Jpn. 76, 023709 (2007).
[3] E. L. Frankevich, et al., Phys. Rev. B. 46, 9320 (1992).
[4] For example, Ö. Mermer et al., Phys. Rev. B. 72, 205202 (2005).
Authors
H. Tajima, N. Suzuki, M. Miyakawa, and M. Matsuda

Fig. 1 (a) A schematic diagram of polymer solar cell. (b) Temperaturedependence of photocurrent at various chopping frequencies.

ISSP Activity Report 2006

5

Intrinsic Carrier Doping in
Antiferromagnetic Supramolecular
Copper Complexes: [CuCl2(Pyra-TTF)]
Mori GroupThe synergy or interplay between electrical conductivity,
magnetism, permittivity, and optical properties in use of thespin, charge, lattice and molecules degrees of freedom has
attracted great interest from the fundamental science topotential applications. Among these applications, the use oforganic-inorganic supramolecules has been tried with a view
toward the creation of new molecular-based magnetic
conductors. Recently, we were the first to synthesize a 3D
supramolecular copper(II) complex, [CuCl2(BP-TTF)] (BPTTF = bis(pyrazino)tetrathiafulvalene), with a antiferromagnetic interatction between CuII through organic π-donors,
namely π-d interaction [1]. Moreover, we successfully foundmore conductive supramolecular copper(II) complex[CuCl2(Pyra-TTF)] (1) by the intrinsic carrier doping fromCuII to donor molecule, Pyra-TTF.
Figure 1 revealed the 3D supramolecular network by the
X-ray analysis. The linear chain [-CuIICl2-(Pyra-TTF)-]nextends along the b axis, where the CuII atom in a planar
geometry is coordinated by two Cl– and two N atoms of
Pyra-TTF as ligands [Cu-Cl = 2.23 Å, Cu-N = 2.04 Å]. The
Pyra-TTF donors are stacked in a head-to-tail and ring-overbond configuration, where the pyrazino-rings stick out of thedonor stacking column. In the ab plane, the donors do notform uniform stacking columns, but that two donor units
form a honeycomb network. The intermolecular overlap
integrals of Pyra-TTF including N atoms are calculated
based on the molecular orbitals obtained by WinMOPAC(ver 3.9.0, Fujitsu Limited) to be r = 1.78, p = 0.538 and q =4.64 (×10 –3). The largest interaction, q, is observed in the
diagonal direction, and the second largest one, r, is observed
within a unit, so that 2D interactions spread in the ab plane.
In addition, there exist contacts between donor sheets,
C1...Cl–1 (3.35 Å), Cl–1...H1 (2.70 Å) and Cl–1'...H2 (2.65
Å). Therefore, this material is the 3D supramolecular copper(II) complex. The temperature dependences of the resistivities of the single crystals of 1 was measured by the ac fourterminal method at 27 Hz with carbon paint contacts. Due tothe intrinsic slight carrier doping from CuII to the coordinated Pyra-TTF, this complex is semiconducting with
σRT = 1.0 × 10 –4 Scm–1 and Ea = 0.33 eV.
To confirm the intrinsic carrier doping of 1, the temperature dependences of the real (ε’) and imaginary (ε”) parts ofthe dielectric constants of the single crystal of 1 were
measured by impedance analyzer (Agilent 4294A) as shownin Fig. 2. The room temperatures ε’ and ε” at 1 kHz are 45
and 2240, respectively. The obtained ac conductivity σ =Fig. 2. The temperature dependences of real (ε’) and imaginary (ε”)
parts of complex dielectric constants of [CuCl2(Pyra-TTF)] (1) (// c*).
The obtained ac conductivity σ = 2πfε0ε” = 1.25 × 10 –6 Scm–1 (f:frequency; ε0: vacuum permittivity) and Ea = 0.39 eV and thefrequency dependence suggests that the carriers of 1 are really doped.

Fig. 3. The temperature dependences of magnetic susceptibility, which
follows 2D Heisenberg model from 300 to 4 K with S = 1/2 and 2J =–20 K. The larger antiferromagnetic interaction between CuII (S =1/2)
spins might be caused not only by 3D interaction but also by theslightly doped carriers in Pyra-TTF donors as ligands.

2πfε0ε” = 1.25 × 10 –6 Scm–1 (f: frequency; ε0: vacuum
permittivity) and Ea = 0.39 eV and the frequency dependence suggests that the carriers of 1 are really doped. On theother hand, no dielectric response is observed for the
insulating [CuCl2(BP-TTF)]. The temperature dependencesof magnetic susceptibilities for 1 was investigated by a
Quantum Design MPMS-XL SQUID magnetometer under
10,000 Oe, as shown in Fig. 3. The magnetic susceptibility
χ1 at 300 K after the subtraction of the Pascal diamagnetic
contribution is 1.41×10 –3 emu mol–1 and follows 2D
Heisenberg model from 300 to 4 K with S = 1/2 and 2J =–20 K. The larger antiferromagnetic interaction between CuII(S =1/2) spins, compared with that of [CuCl2(BP-TTF)],
2J = –7 K, might be caused not only by 3D interaction but
also by the slightly doped carriers in Pyra-TTF donors as
lingands. Therefore, our complexes have posed the newmaterial design for molecular magnetic conductors,
composed of the conducting donors and the coordinated CuIIas a spin source.
References[1] S. Ichikawa, S. Kimura, H. Mori, G. Yoshida, and H. Tajima, Inorg.
Chem. 45, 7575 (2006).

Fig. 1. Crystal structure of the supramolecular CuII complex[CuCl2(Pyra-TTF)] with the 3D framework consisted of the 1D linear
coordinated chain [CuCl2(Pyra-TTF)]n, 2D donor sheet, and C...Cl–,
and two kinds of H...Cl– contacts between donor sheets.

6

ISSP Activity Report 2006

Authors
S. Ichikawa, S. Kimura, K. Takahashi, H. Mori, G. Yoshida, Y.
Manabe, M. Matsuda, H. Tajima, and J. Yamaura

Unconventional Anomalous Hall EffectEnhanced by a Noncoplanar Spin
Texture in the Geometrically Frustrated
Kondo Lattice Pr2Ir2O7
Nakatsuji and Sakakibara Groups
Anomalous Hall effect of magnetic conductors has
attracted great interest both in the field of fundamental
science and applications. Empirically, the Hall resistivity ρxy
is written as ρxy=R0B+4πRsM (1), where B is the magnetic
field and M is the magnetization. The first term is theordinary Hall resistivity. The second term describes theanomalous Hall effect (AHE) whose origin is theoreticallyattributed to the magnetization and spin-orbit coupling.
Recently, a new mechanism of the AHE, related to thespin chirality under non-coplanar spin configurations has
been proposed. A finite spin chirality, which is proportional
to a solid angle subtended by spins, induces a finite Berryphase of conduction electrons. This Berry phase acts as a
fictitious magnetic field on the conduction electrons andleads to the AHE. Up to now, however, the anomalous Hall
effect driven by the spin chirality has been experimentally
examined mainly in ferromagnets such as manganites andpyrochlore molybdates. In contrast, a paramagnetic metal witha non-coplanar spin texture is ideal to search for a large Hall
effect and to test the validity of spin chirality mechanism.
Thus the recently discovered geometrically frustrated Kondo
lattice Pr2Ir2O7 stands uniquely for such possibility [1].
Here, we report the observation of the divergingly
enhanced low T Hall resisitvity in the spin-liquidlike regime
in Pr2Ir2O7 [2]. In this system, the localized Pr 4f momentswith <111> Ising anisotropy form a pyrochlore lattice, andprovide the non-coplanar spin texture. Ir 5d-conduction
electrons are only weakly correlated and Pauli paramagnetic.
Significantly, because of strong geometrical frustration,
Pr2Ir2O7 has no magnetic long-range order down to 120 mK,
despite an antiferromagnetic RKKY interaction of T*=20 K
between the Pr spins. Instead, the Kondo effect emerges andleads to partial screening of the 4f moments below the Kondo
temperature of 20 K. On further cooling, the susceptibility
exhibits a logarithmically diverging behavior, suggesting thata ferromagnetic coupling eventually develops below 2 K,
and the Pr spins form a correlated liquidlike state.
In the spin-liquidlike regime, ρxy under a field of 0.3 T
increases logarithmically on cooling, and reaches a large

Fig. 1. Temperature dependence of the Hall resistivity ρxy (T) under afield of B=0.3 T along the [111] direction. Inset Left: the Hall coefficient RH vs the susceptibility 4πM/B under a field of B=0.3 T along the[111] direction. Inset Right: Pyrochlore lattice.

value up to ~ 3 µΩcm, which is comparable with or larger
than typical values for ordinary ferromagnets (see Fig. 1).
Furthermore, in the same temperature region, strong
anisotropy and non-monotonic dependence on M have been
observed in ρxy, which sharply contrasts with the empirical
relation given by Eq.(1) (see inset of Fig. 1). Moreover, thefield dependence of the Hall resistivity in the low T regime
does not obey the empirical rule of Eq. (1), but shows strong
anisotropy and nonmonotonic M dependence.
In terms of the conventional mechanism due to the spinorbit coupling, it is highly difficult to understand the above
nontrivial temperature and field dependences of the large
Hall effect observed below 2 K. Instead, it is natural toexpect the spin-chirality contribution to the Hall effectbecause the Pr <111> Ising-like spins under fields may wellhave a sizeable spin chirality due to their noncoplanar spin
texture and produce a fictitious magnetic field on the Ir sites
through the Kondo coupling. Especially for high fields, wefound that the anisotropy of the Hall resistivity can be mainly
ascribed to the anisotropy of the fictitious field at the Ir site
due to the spin chirality of Pr moments.
References[1] S. Nakatsuji, Y. Machida, Y. Maeno, T. Tayama, T. Sakakibara,
J. van Duijn, L. Balicas, J. N. Millican, R. T. Macaluso, and Julia
Y. Chan, Phys. Rev. Lett. 96, 087204 (2006).
[2] Y. Machida, S. Nakatsuji, Y. Maeno, T. Tayama, T. Sakakibara,
and S. Onoda, Phys. Rev. Lett. 98, 057203 (2007).
Authors
S. Nakatsuji, Y. Machidaa, Y. Maenoa, T. Tayama, T. Sakakibara, and
S. Onodab
aKyoto University
bCREST, Department of Applied Physics, University of Tokyo

Nonequilibrium Kubo Formula ofFinite Conductor Connected toReservoirs based on Keldysh Formalism
K. Ueda GroupThe correlation effects in mesoscopic devices havebecome a subject of intensive studies. Especially thesesystems have provided good platform to study the nonequilibrium nature induced by the finite bias voltage. It is wellknown that in the linear response regime, there is the generalrelation between fluctuation and dissipation, which is theessence of the Kubo formula. This gives rise to the NyquistJohnson relation between conductance and noise power.
However concerning the extension of the Kubo formula andthe relation between physical quantities in the nonequilibrium situation, there have been few attempts to explore it inmesoscopic systems with interactions.
On the basis of Keldysh formalism we find that thedensity matrix of mesoscopic systems has a MacLennanZubarev form. Using the MacLennan-Zubarev form, it ispossible to derive the general formula of differential conductance, which is given by the sum of the well-known currentcurrent correlation function and the non-trivial correlation
function between current and the difference of the twoparticle numbers of reservoirs. In the linear response regime,
the differential conductance is given only by the currentcurrent correlation function. Our results show that a naive
extension of the fluctuation dissipation theorem into thenonequilibrium case is not possible and requires the
additional contribution. We call the generalization the

ISSP Activity Report 2006

7

nonequilibrium Kubo formula of a finite conductor attached
to reservoirs [1].
Here we stress that our result provides new insight toobservable quantities in mesoscopic systems. Let us discuss
the nonequilibrium Kubo formula in view of physical quantities. In mesoscopic systems the current-current correlation
function expresses noise power at the zero frequency. We
would like to propose that the non-trivial correlation function
generally gives the formula for shot noise. This fact ischecked based on the noninteracting Anderson model,
however it is also valid for interacting systems.
Theoretically for noninteracting systems, noise power
may be separated into equilibrium noise and shot noise. In
this case shot noise can be defined from only noise power.
With interaction such a separation cannot be expected ingeneral. In many cases with interaction, it has been considered that shot noise is included in noise power for eV>kBT.
Thus the definition of shot noise has been ambiguous ininteracting systems. On the basis of the nonequilibrium Kubo
formula, it has become possible to define shot noise and givean explicit formula for it. Finally a generalization of the
Nyquist-Johnson relation is naturally derived, which we callthe nonequilibrium identity between differential conductance, noise power and shot noise. Using the nonequilibrium
identity, we may define shot noise experimentally by thedifference between observable quantities: noise power anddifferential conductance.
References[1] T. Fujii, J. Phys. Soc. Jpn. 76, 044709 (2007).
Authors
T. Fujii

Polarons in Jahn-Teller Crystals: Intrinsic
Difference between eg and t2g Electrons
Takada Group
About a decade ago, Tokura's group made an interesting
comparative study of the effective mass m* between eg andt2g electrons by measuring the low-temperature electronic
specific heat Ce(T) of the manganese oxides La1-xSrxMnO3
as well as the titanium ones La1-xSrxTiO3 [1]. They havefound that the t2g (titanium) system has a larger T-linear
coefficient in Ce(T) than the eg (manganese) one by morethan a factor of two, irrespective of x, implying that t2g
electrons may be intrinsically much heavier than eg ones.
Since Mn3+ and Ti3+ ions are, respectively, considered as
typical E⊗e and T⊗t Jahn-Teller (JT) centers, we may anticipate that the above experimental result directly reflects aninherent difference in the mathematical structure of JT
centers, namely, either E⊗e or T⊗t. Motivated by this anticipation and as a work subsequent to a previous one of ourgroup for the E⊗e polarons [2], we have made a comparative
study of the polaronic effective mass m* between the E⊗eand the T⊗t JT polarons. A comparison with the conventional Holstein polarons is also made.
If the electron-phonon (el-ph) coupling is weak, it isstraightforward to determine the ratio of m* to the bare band
mass m by resorting to second-order perturbation calculation,from which we obtain m*/m as 1+2α for both JT polarons
with α the nondimensional el-ph coupling constant in its
conventional definition. Exactly the same mass ratio can be

8

ISSP Activity Report 2006

Fig. 1. Inverse of the mass enhancement factor, m/m*, as a function of
α for the T⊗t (solid curve) and the E⊗e (dashed curve) JT polarons incomparison with the Holstein one (dotted-dashed curve). All the results
are obtained in the strong-coupling and/or antiadiabatic region.

reproduced in the Holstein model, if we redefine α in themodel in an appropriate way. Thus we see that there is no
essential difference in the enhancement of m* among those
three polarons in the small-α limit.
In the limit of α → ∞, a polaron will be completely localized at a single site, implying m*/m = ∞. For a finite but verylarge α, the localized polaron will begin to hop betweensites, but the hopping in this case is a very rare event. Thus
physics connected with such a hopping can be well captured
by just considering a two-site system. In fact, by implementing the two-site calculation by exact diagonalization, we
can obtain a generic feature of m*/m in the strong-coupling(α → ∞) and/or antiadiabatic (the electron hopping integral t
much smaller than the local optic phonon energy ω0) region.
The results of the two-site calculation are shown in
Fig. 1 [3]. We note that the Holstein polaron (the dotteddashed curve) is characterized analytically by the relation of
m/m*=e-2α, while the E⊗e polaron (the dashed curve) by
m/m*=(πα/2)1/2e-α [2], indicating the much lighter effective
mass of the latter system.
Physically the mass enhancement is brought about by thevirtual excitation of local phonons in those model
Hamiltonians describing the coupling of an electron withlocal optic phonons. In the Holstein model no restriction is
imposed on exciting multiple phonons, but in the E⊗e JT
model there is a severe restriction due to the existence of aconserved quantity intimately related to the SO(2) rotational
symmetry in the pseudospin space representing the twofold
degenerate orbitals.
As for the T⊗t model, no corresponding symmetry exists
in the pseudospin space, explaining its difference in m/m*from the E⊗e model in Fig. 1. In order to understand its
difference from the Holstein model, however, a moredetailed analysis is needed. It is well known that the
adiabatic potential energy surface for the T⊗t model contains
four equivalent wells for sufficiently large α, but the wells
are not isotropic; the vibrational t-mode splits into an a1mode and two e-modes (i.e., t → a1⊕e). This fact indicatesthat the T⊗t model is a combination of the Holstein and theE⊗e models, explaining the reason why the result of m/m* inthis model (the solid curve in Fig. 1) comes between those of
the Holstein and the E⊗e models.
Our model is too primitive to make a quantitative
comparison with experiment, but we find that, at least in thestrong-coupling and/or antiadiabatic region, the difference inthe behavior of m* as a function of α can be ascribed to thepresence/absence of rotational symmetry in the pseudospin
space representing the internal mathematical structure ofeach JT center.
References[1] T. Okuda, A. Asamitsu, Y. Tomioka, T. Kimura, Y. Taguchi, andY. Tokura, Phys. Rev. Lett. 81, 3203 (1998).
[2] Y. Takada, Phys. Rev. B 61, 8631 (2000).
[3] Y. Takada and M. Masaki, J. Molecular Structure 838, 207 (2007).
Authors
Y. Takada and M. Masaki

Absence of Aharonov-Bohm Effect in
One-Dimensional Bose Liquid
Oshikawa GroupAharonov-Bohm effect is one of the most striking
phenomena in quantum mechanics. It occurs when aquantum mechanical particle is transmitted through a ringinterferometer which encloses a magnetic flux, as shown in
Fig. 1. In classical mechanics, the path of the particle musttake one of the branches. In contrast, according to quantum
mechanics, the particle travels both of the branches “at thesame time”. Namely, the probability amplitude of the particle
propagates like a wave, and the contributions through thetwo branches interfere. This is a famous manifestation of the
wave nature of a quantum mechanical particle. The
Aharonov-Bohm effect refers to the effect of the enclosed
magnetic flux on this interference. In classical mechanics,
there should be no effect of the enclosed magnetic flux, as
the particle never touches the flux. However, in quantum
mechanics, the vector potential rather than the magnetic fieldis the fundamental quantity, although it is gauge dependent.
Even though the enclosed magnetic flux is not touched by
the particle, it induces a vector potential which gives rise to aphase difference for the probability amplitude through thetwo paths. This phase difference is gauge invariant andaffects the interference. While being counter-intuitive, theAharonov-Bohm effect is ubiquitous in quantum mechanics
and is related to many important phenomena.
Here we report our recent theoretical work on BoseEinstein condensate (Bose liquid) confined in one-dimen-

sional potentials. Nowadays Bose-Einstein condensates canbe experimentally realized by cooling of atoms to ultracold
temperatures, and are studied vigorously. When the ringinterferometer discussed above is filled with the Bose liquid,
we found the surprising result that the Aharonov-Bohm
effect is absent in the low-energy limit. Namely, the transport through the interferometer becomes completely insensitive to the enclosed (effective) flux [1].
In one dimension, quantum fluctuation is so strong that it
destroys the long-range phase coherence. Thus the onedimensional Bose liquid is actually not a true Bose-Einsteincondensate. The fluctuation of the local phase can be
described by the Tomonaga-Luttinger liquid theory, which isan asymptotically exact effective theory in the low-energy
limit. The Tomonaga-Luttinger liquid theory is a universal
theory in one dimension, describing also the low-energy
physics of one-dimensional electron systems and spin
systems. The ring interferometer consists of two Y-shaped
junctions of one-dimensional branches. We first studied the
Y-junction of three one-dimensional Bose liquids, comparing
with our previous analysis on the corresponding electron
problem. We found that the strongly coupled junction isstable for a wide range of realistic parameters for Bose
liquids, in contrast to the electron systems where an attractive interaction is necessary to stabilize the strongly coupledjunction.
We found that a negative density reflection analogous to
the Andreev reflection occurs even in the Y-junction of Bose
liquids, although the standard Andreev reflection is attributed
to the Cooper pair formation of fermions. In the ring interferometer, the Tomonaga-Luttinger liquid can be decomposed
into symmetric and antisymmetric combinations of the two
branches. The enclosed magnetic flux affects only theantisymmetric mode, which corresponds to the induced
persistent current. The transmission between the two leads isonly coupled to the symmetric mode, which does not feel theenclosed flux at all. As a consequence, the Aharonov-Bohm
effect is absent in the transmission through the Bose liquid in
the ring interferometer.
References[1] A. Tokuno, M. Oshikawa, and E. Demler, preprint arXiv:condmat/0703610.
Authors
A. Tokuno, M. Oshikawa, and E. Demlera
aHarvard University

New Universal Distribution of SpectralFlow Gaps in the Rashba Spin-Orbit
Model
Kohmoto Group

Fig. 1. The ring interferometer is a typical setting for the AharonovBohm effect. The quantum mechanical motion of the particle transmitted through the system exhibits the interference between the probability amplitudes attributed to the two paths. The Aharonov-Bohm
effect is the dependence of the interference on the enclosed flux. Whenthe interferometer is filled with Bose liquid, we find that the transport
between the two leads is independent of the enclosed (effective) flux,
namely the Aharonov-Bohm effect is absent.

In recent years, there is a surge of interests in spintronics,
i.e. to manipulate and to control electron spins in solid state
devices by an external electric filed, instead of a magnetic
field. Potential advantages include efficient generation ofspin polarization inside devices, handy manipulation ofindividual spins at nano-scales, and desired reduction of heatdissipation during information processing. Since there is nofundamental direct coupling between spin and an electricfield, the spin-orbit coupling plays a central role in inducing
spin transport. The study of spin transport and, in particular,

ISSP Activity Report 2006

9

Fig. 1. Distributions for level-avoiding gaps.

the spin Hall effect in systems with the spin-orbit coupling
has recently attracted much attention. A mechanism for spintransport due to the spin-orbit coupling is called extrinsic or
intrinsic, depending on whether disorder (such as impurities
or imperfections) is involved or not. In a real sample, there isalways disorder. To study possible interplays between
intrinsic and extrinsic mechanisms, it is desirable to study
them simultaneously in one unified framework, rather than to
study them separately and add their contributions. We reportsuch a study of the systems with the spin-orbit coupling
using the methods of random matrix ensembles.
A prototype of the intrinsic spin-orbit coupling is the
Rashba model in a two-dimensional electron gas. We put the
Rashba model with on-site randomness on a cylinder, andinclude an infinitesimal electric field by turning on slowly amagnetic flux threading the cylinder. The latter is equivalent
to imposing a twisted phase in the boundary condition. Witha threading flux turned on, the single particle levels will
generally avoid, rather than cross. As is shown in Fig.1, ournumerical study of level-avoiding gaps in the disordered
Rashba model demonstrates that the normalized gap distribution PG(ξ) is of a universal form, independent of the random
strength W and the system size L. For small gaps it exhibits alinear behavior, while for large gaps it decays exponentially.
In Ref. [1], we suggested a matrix model for the underlying random ensemble, which is verified to exhibit thedesired small-gap behavior. Usually one uses the levelspacing statistics as a characteristic of universality in arandom system or ensemble. Our results suggest a different
type of characterization of random ensembles using gap
statistics in spectral flow due to flux threading.

of material. Simulations of the dynamics – transient motion ofelectrons at femtosecond, adiabatic/nonadiabatic motion of
nuclei at picosecond, and fast/slow structural change – havebeen given more firm theoretical foundations and made morefeasible by the development of supercomputers. Therefore
computational algorithms are now actively developed toadvance the simulations. In this context, we developed
algorithms and performed intensive simulations using thesupercomputers of ISSP and other centers. Here we reportour recent activity.
Near the conical intersection, where two adiabatic potential surfaces cross, the Born-Oppenheimer approximation
breaks down even though electron mass is significantly
smaller than nuclear mass. The nonadiabatic effect, which
thereby gives essentially important contribution to the
dynamics, has thus far been solely studied by the accurate
configuration interaction (CI) methods. But they become
extremely demanding as the size of the system increases. On
the other hand, we developed a scheme to compute thenonadiabatic coupling constant using the time-dependent(TD) version of the density functional theory (DFT), which
is based on the electron density and thus is potentially applicable to larger systems. Our scheme uses a local approximation within DFT but benchmark calculation with H3 indicatesa CI-level accuracy. The coupled electron-nucleus dynamics
has thus made simpler, and intensive applications are now inprogress.
At the electrode surfaces, electrons or holes transfer(often rather adiabatically) to nearby molecules triggering anelectrochemical reaction. The role of the bias potential
applied to the electrode has not been given comprehensive
theoreticalexplanation,inparticularfortheelectrode/solution interface where the water molecules makethe reaction extremely easy. Here we used recently developed algorithm called Effective Screening Medium (ESM)[1] to clarify the most standard electrode dynamics, that is,
the evolution of hydrogen molecule at the platinum surface.
Through intensive simulations, we have found that under anegative bias condition, water molecules form a contact

References[1] D. Tobe, M. Kohmoto, M. Sato, and Y. -S. Wu, cond-mat/0611696;
Phys. Rev. B. in press.
Authors
M. Kohmoto and M. Sato

Advanced First-principles Simulations on
Nonadiabatic Dynamics, Electrochemical
Reactions, and Phase Transitions
Sugino Group
Elucidating dynamics of electrons and nuclei is an important topic of condensed matter physics and is regarded as atarget of recent and future first-principles simulation, although
it has thus far focused more attention on the static properties

10

ISSP Activity Report 2006

Fig. 1. A representative configuration of the biased Pt(111)/water interface (left: side view and right: top view). The hydronium ion (yellow
circle) is attracted to the negatively charged Pt surface and is weakly
bound at the contact water layer that covers the Pt surface with thehydrogen-bond network (right panel). A hydrogen atom is afterwards
adsorbed on the surface accepting an electron from the electrode. Thisis the microscopic aspect of the electrochemical reaction H3O++e→H(ad)+H2O (Volmer step) in the hydrogen evolution reaction.

water layer at the interface and the double layer away fromit. Formation of these structures makes easy the diffusion of
the hydronium ions and the electrochemical adsorption
process H3O++e-→H(ad)+H2O. A general computational
scheme for the electrode dynamics has thus been establishedand is now applied to various electrodes.
Dynamics occur through a very complicated path in somechemical reactions and in most structural phase transitions.
To understand them thermodynamically, change in the freeenergy along the reaction path is a very important clue.
Existing algorithms to compute the free-energy are notgeneral enough or applicable to extended systems. Here wedeveloped a general algorithm on the basis of the multicanonical ensemble method and benchmarked it using themelting of silicon [2]. The result shows that the applicabilityof the free-energy-based approach has been greatly extended.
References[1] M. Otani and O. Sugino, Phys. Rev. B 73, 115407 (2006).
[2] Y. Yoshimoto, J. Chem. Phys. 125, 184103 (2006).
Authors
O. Sugino, CP. Hu, M. Otani, Y. Yoshimoto, Y. Morikawaa, I.
Hamadaa, Y. Okamotob, T. Ikeshojic
aOsaka University
bNEC Corporation
cNational Institute of Advanced Industrial Science and Technology

Phase Transition ofSuperconducting Wire Network
under Fully-Frustrated Condition
Iye Group
The nature of phase transition in artificially structured
superconducting systems such as superconducting wire
network (SWN) has been the subject of intensive studies
both experimental and theoretical. The superconducting
phase is described by the complex order parameter
ψ=|ψ|exp(iθ). Corresponding to the two (amplitude andphase) degrees of freedom, the phase transitions in thesesystems can occur in two steps. Upon cooling, the amplitude|ψ| of the superconducting order parameter grows first

Fig. 1 (a) Scanning electron microscope image of the Al network
decorated with an array of Co dots. (b) Electrode configuration.
(c) Cross-sectional view of the segment A-A’. Stray magnetic fieldemanating from the Co dots creates a staggered flux pattern.

Fig. 2 top: The I-V curves for α=β=0. bottom: Temperature dependenceof the power law exponent a of the I-V characteristics at α=1/2 forseveral values of β.

around the mean field transition temperature TMF of thesuperconducting material. Subsequent ordering of the phaseθ of the order parameter over the entire system takes place atTc (< TMF). It is the latter global phase ordering process in
artificially structured superconducting systems that has
attracted much interest, as it can be mapped onto theordering problem of an XY spin system.
In zero magnetic field, the system undergoes theso-called Kosterlitz-Thouless-Berezinskii (KTB) transition,i.e. transition via vortex-antivortex pair unbinding. The
nature of transition at arbitrary values of frustration parameter α, (α being the average number of flux quanta perplaquette) is not well-understood. The simplest non-trivial
case of α=1/2 can be mapped onto the fully frustrated XY(FFXY) model. The vortices tend to arrange themselves in acheckerboard pattern. The system has a double degeneracy
associated with the placement of the vortices either in whiteor black squares of the checkerboard. This leads to occurrence of antiphase domains at finite temperatures, in addition
to the vortex pair excitation.
In this work, the nature of transition at α=1/2 is investigated by measureing the I-V characteristics, the power-law
exponent (a of V~I a) of which reflects the helicity modulus
Γ, or the system's rigidity against phase twist. The effect ofstaggered field that lifts the double degeneracy is investigated.
We fabricated a superconducting Al wire network
decorated by an array of ferromagnetic Co dots as shown in
Fig.1. The stray field emanating from the Co dots creates a
staggered field of checkerboard pattern whose amplitude β
can be controlled by rotating the azimuthal angle of the inplane field and thereby changing the direction of Co magnetization [1].
At zero magnetic field, the system undergoes a KTB
transition with the universal jump of a from 1 to 3. For

ISSP Activity Report 200611

α=1/2, Tc is suppressed and the change in a at Tc is steeper.
These features are consistent with the picture first proposedby Korshnov [2] for the fully frustrated case. According to
the model, as Tc is approached from below unbinding of kink
and antikink at antiphase domain boundaries effectively
destroys the phase correlation across the boundary leading torapid decrease of helicity modulus.
The effect of non-zero β on the superconducting transition at α=1/2 is to recover Tc and make a less rapidlychanging at Tc. These features are qualitatively understoodby considering the role of β as quenching of the occurrenceof antiphase domains At β=1/2 and α=1/2 the canonical KTB
transition is restored.
References[1] Y. Iye et al., Phys Rev. B 70, 144524 (2004).
[2] S. E. Korshnov, Phys. Rev. Lett. 88, 167007 (2002).
Authors
H. Sano, A. Endo, S. Katsumoto, and Y. Iye

Berry's Phase Arisen from
Spin-Orbit Coupling
Katsumoto Group
When a quantum trips around a loop adiabatically,
it acquires a quantum mechanical phase called Berry's
phase [1]. When the physical properties of the system(Hamiltonian) are fixed, the phase only depends on thegeometry of the loop and hence Berry's phase is also called
geometrical phase. The celebrated Aharonov-Bohm (AB)phase is, in a sense, a kind of Berry's phase. (Some of thephysicists do not include the AB phase into Berry's phase.)
Berry's phase appears in electric transport through quantum
mechanical interference effect. The AB oscillation of theresistance in ring-shaped mesoscopic conductors (AB rings)
is again a typical example.
When there is a spin-rotation mechanism, such as spinorbit interaction, the interference in the spin part of the
wavefunction also plays an important role. In two-dimensional
system with strong Rashba-type spin-orbit coupling [2],particles feel effective in-plane magnetic field perpendicular
to the momenta. When such a particle is transported along acircle (e.g. by ring shaped boundaries) and the spin-orbit
coupling is strong enough, the spin also rotates adiabatically.
As the particle goes back to the starting point, the spin has
rotated 2π, which is a half rotation for a spinor and thus
Berry's phase of π is given to the particle wavefunction. The
magnetic field perpendicular to the plane gives verticalcomponent to the spin diminishing the Berry's phase. In thecase of an AB ring, this phenomenon results in phase shift in
the AB oscillation, which is non-linear for the magnetic fieldand gives dramatic peak splittings in the Fourier transform
spectra [3].
We have observed essentially the same phenomenon inan anti-dot lattice (ADL) system made of two-dimensionalhole system (2DHS) at an AlGaAs/GaAs heterointerface [4].
We made circular hollows on the surface of a 2DHS at thesquare lattice points as shown in the inset of Fig.1. The
hollows form repulsion potentials and exclude holes underneath them. Such repulsive dots are called anti-dots. Whenthe lattice constant of an ADL is small enough, the ADL canbe viewed as a set of loop orbits encircling anti-dots and

12

ISSP Activity Report 2006

Fig. 1. Fourier spectrum of the oscillatory part in the magnetoresistanceof an anti-dot lattice made of a two-dimensional hole system at an
AlGaAs/GaAs heterointerface. Clear peak splitting into the structures
marked as B, A, A', B' is observed at the main peak. The inset is ascanning electron micrograph of the sample, which has an anti-dot
lattice structure.

tunneling connections between them. Then the interferencein the loops results in the resistance oscillation called ABtype oscillation.
Figure 1 shows the Fourier spectrum of the oscillatory
part of the magnetoresitance. In the main peak structurearound the magnetic field that corresponds to single flux
quantum per unit cell, clear splitting into three peaks (A, A',B') is observed. Peak A has a shoulder marked as B. Thesefour positions are symmetric to the center indicated as C,
where a deep dip is located and just corresponds to single
flux quantum per circle with the diameter of the lattice
constant. At the second harmonic position (winding numbern=2) also a dip indicated as C* appears. Though we do not
observe any peak at n=3, a small peak is observed at n=4.
All of these features can be explained by accounting theexistence of two different carriers in the 2DHS and the
Berry's phase. The results constitute evidence of Berry's
phase arising from the spin-orbit coupling and adiabatic
rotation of hole spins when they are transformed along
circular potentials.
References[1] M.V. Berry, Proc. R. Soc. Lond. A 392, 45 (1984).
[2] E. I. Rashba, Sov. Phys. Solid State 2, 1109 (1960).
[3] A. G. Aronov and Lyanda-Geller, Phys. Rev. Lett. 70, 343 (1993).
[4] N. Kang, E. Abe, Y. Hashimoto, Y. Iye, and S. Katsumoto, J. Phys.
Soc. Jpn. to be published.
Authors
N. Kang, E. Abe, Y. Hashimoto, Y. Iye, and S. Katsumoto

Reversible Transformation between
Spin and Charge Currents Observedin Lateral Ferromagnetic-Nonmagnetic
Nanostructures
Otani GroupThe basic science for electronic devices aiming at manipulating the spin degree of freedom is spintronics, which
provides a possible means to realize advantageous functionalities for spin based recording and information processing.
For such functions, the usage of spin current, a flow of spin
angular momentum, is indispensable. There is a novel
phenomenon where the spin-orbit interaction converts a spin

Fig. 1 (a) Scanning electron microscope (SEM) image of the fabricatedspin Hall device together with a schematic illustration of the fabricateddevice. (b) Schematic spin dependent electrochemical potential map
indicating spin accumulation in Cu and Pt induced by the spin injection
from the Py pad. Dashed line represents the equilibrium position. (c)Schematic illustration of the charge accumulation process in the Pt
wire, where IS and Ie denote injected pure spin current and induced
charge current, respectively. (d) Spin dependent electrochemical potential map for the charge to spin-current conversion and (e) corresponding
schematic illustration.

current into a charge current and vice versa. This is known as
the spin Hall effect (SHE), which has recently been demonstrated only by applying optical detection techniques tosemiconductor systems such as GaAs at 80 K and Cl doped
ZnSe at room temperature, but not yet by the electricalmeans. In diffusive normal metals, the electrical observationof the inverse SHE has been made by using a nonlocal spin
injection in a lateral ferromagnetic-nonmagnetic aluminum
structure. However, the observation is limited only to lowtemperatures because of a small spin-orbit interaction ofaluminum. Here we report clear observation [1] of areversible spin Hall effect (comprising a spin Hall effect andan inverse spin Hall effect), that is the reversible transformation between spin and charge currents both at 77 K and room
temperature induced in the platinum wire with a nanoscale
spin diffusion length of about 4 nm using a speciallydesigned electrical detection technique combined with thespin-current absorption.
Our device consists of lateral ferromagnetic/nonmagnetic
metallic junctions as shown in Fig. 1(a). The device consistsof a large permalloy (Py) pad 30 nm in thickness, a Cu cross
100 nm in width and 80 nm in thickness, and a Pt wire 80 nm
in width and 4 nm in thickness. The size of the junction
between the Py injector and Cu wire, is chosen to be 100 nm
× 100 nm to induce an effective spin accumulation in the Cu
wire. The distance from the center of the injector to thecenter of the Pt wire is 400 nm. When the charge current is
injected from a ferromagnetic Py pad into a nonmagnetic Cu
cross and is drained from one of the top and bottom arms
[see inset of Fig. 2(a)], the accumulated spins in the vicinityof the junction induce the diffusive flow of the spin-current

along the Cu wire as can be understood from Fig. 1(b). Whenthe distance between the Py/Cu and Cu/Pt junctions isshorter than the spin diffusion length of about 500 nm, thespin current should be absorbed into the Pt wire from the Cu
cross. The injected spin current flows almost perpendicular
to the junction plane as in Fig. 1(c). The charge current Ie isgenerated perpendicular to the spin current IS in the Pt wire
via the inverse SHE. One should notice that the inverse
transformation is also possible as illustrated in Figs. 1(d) and1(e).
Figures 2(a) and 2(b) show clear hysteresis both in the
charge ∆VC/I and spin ∆VS/I accumulation signals as afunction of the external magnetic field measured at 77 K.
Important is that the overall resistance change ∆RS is exactly
the same as ∆RSHE and the same tendency is also observed at
RT. This corresponds to the experimental demonstration of
the fundamental relations called Onsager reciprocal relations
between spin and charge currents. The value of SH conductivity of the Pt wire is evaluated to be 2.4 × 104 (S/m) at RT
104 times larger than the values reported for semiconductor
systems and 10 times larger than that for Al. Furthermore,
the ratio of the SH conductivity to the electrical conductivity
is 3.7 × 10 –3, which is also the largest value reported so far.
This result opens up a new possibility to use normal metals
with high spin-orbit coupling as spin current sources
operating at room temperature for future spintronic applications.
References[1] T. Kimura, Y. Otani, T. Sato, S. Takahashi, and S. Maekawa, Phys.
Rev. Lett. 98, 156601 (2007).
Authors
T. Kimura and Y. Otani

Fig. 2 (a) The change in Hall resistance ∆VC/I due to the inverse Spin
Hall effect (SHE) at 77 K. (b) Spin accumulation signal ∆VS/I generated by SHE at 77 K. The insets of the figures (a) and (b) show theprobe configurations for the measurements.

ISSP Activity Report 200613

Self-assembled MnN Superstructure
Komori Group
Self-assembled nanostructures at surfaces have attracted
tremendous interest in nanoscience. Among them, the ones
with regular spatial array, uniform and well-definedgeometric and structural characters, are especially desirable.
However, it is still a challenge to prepare such high-quality
nanostructures. Recently, we have fabricated self-assembled
manganese nitride nano-islands on the Cu(001) surface andproposed a new mechanism of the self-assembly. [1]As shown in Fig. 1, each island shows a square shape andhas a well-defined size of 3 nm × 3 nm. They are regularly
arranged with a periodicity of 3.5 nm and form a two-dimensional square superstructure. The depth of the trench between
the islands is the same as the step height of the surface or theheight of the island on the superstructure. The nano-islands
adopt a 1 × 1 structure which is oriented in the same way as
the fcc Cu(001) substrate. They are reproducibly prepared inthree steps in ultrahigh vacuum by Mn deposition, atomic
nitrogen exposure and annealing, and stable up to 820 K.
The stoichiometry of the manganese nitride islands ismanganese mononitride as determined by in-situ x-ray
photoemission spectroscopy in Fig. 2. The MnN film covers(a)Fig. 2 N-1s core-level spectra from a N-saturated Cu(001)c(2×2)-N
surface and a single-layer manganese nitride covered surface. The
coverage of nitrogen on the former surface is 0.5 mono atomic layer(ML). Thus the nitrogen coverage of the latter surface is 0.9 ML.

the whole surface, and is just mono-atomic-layer thick. The
bulk MnN crystal has a distorted NaCl structure with a lattice
constant 14 % lager than that of the Cu(001) surface [2].
Thus, the nanostructure formation is attributed to strain-relief
at the interface. However, different from the conventional
stress-domain dominated self-assembly, the shape, size, andperiodicity of the MnN islands do not change even when
they coexist with the clean Cu(001) surface. This indicatesthat the self-assembling is driven by a short-range mechanism. The presence of the MnN strips surrounding the MnNisland can minimize the strain energy. We propose that MnNisland is stabilized by such a simultaneous bilayer organization.
References[1] X. Liu, B. Lu, T. Iimori, K. Nakatsuji, and F. Komori, Phys. Rev.
Lett. 98, 066103 (2007).
[2] K. Suzuki, T. Keneko, H. Yoshida, Y. Obi, H. Fujimori, and H.
Morita, J. Alloys Comp. 306, 66 (2000).

(b)

Authors
X. Liu, B. Lu, T. Iimori, K. Nakatsuji, and F. Komori

Regioselective Cycloaddition Reactionof Asymmetric Alkene Moleculeswith the Asymmetric Dimer
×2)
on Si(100)c(4×Yoshinobu Group(c)Fig. 1 (a) Large-scale STM image of the self-assembled MnN nanoislands on a wide terrace. Square shaped islands of 3 nm × 3 nm form a
two-dimensional square superstructure. (b) High-resolution STM
image. Cross section profile along the blue line is presented in (c). Mn
atoms are imaged on the surface, and the island is single atomic-layer
high.

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ISSP Activity Report 2006

Potential applications enabled by the chemical modification of Si surfaces include coatings, sensors, molecule-based
microelectronics and so forth. The cycloaddition reaction of
unsaturated organic molecules with the Si(100) surface is
one of the most important families of chemical reactions forthe modification of Si surfaces. The asymmetric dimer on thereconstructed Si(100) surface plays an important role, inwhich a partial charge transfer occurs from a down dimer
atom to an up dimer atom. Thus, the chemical nature of Sd
and Su is electrophilic and nucleophilic, respectively. In
organic chemistry, the regioselective addition reaction of

(a)

(b)

(c)

small open circles represent the position of the Su and Sd,
respectively. The bright protrusions indicated by arrows
originate from adsorbed 2-methylpropene molecules. The
protrusions of adsorbed 2-methylpropene are obtuse triangleshaped, and the center of the triangle-shaped protrusion is
located at the Su position. One of the three corners of thetriangle-shaped protrusion points in the dimer bond direction.
Previous STM studies attribute the bright protrusions tomethyl groups in chemisorbed organic molecules on Si(100).
This interpretation is confirmed by the first-principles
simulation of STM image in Fig.1b, which is calculated fromlocal density of states of theoretically optimized adsorption
structures (Fig.1c). Since the observed protrusions are always
located on the Su side in the present systems (nearly 100%),
we conclude that the alkene carbon atom having two methylgroups is selectively bonded to the nucleophilic Su site while
the other carbon atom with two hydrogen atoms is bonded tothe electrophilic Sd site.
In order to elucidate the reaction mechanism, wesearched for an adsorption structure other than the final disigma state by first-principles calculations. After the structure optimization process starting from various initial configurations with different orientations at the Sd site on Si(100),
a stable precursor structure was obtained as shown in Fig. 2.
The end carbon atoms without methyl groups are always
closer to the Si atom at the Sd site compared with the centercarbon atoms with methyl groups in any case. The C=C
double bond is elongated to 1.43 Å, while the other C-C
bonds in the molecule are slightly shortened from 1.53 Å to1.51 Å. In addition, the end carbon atom without methylgroups in Fig. 2 shows sp3-like geometrical features; on theother hand, the carbon atoms with methyl groups have sp2like geometrical features. These calculated results indicatethat the carbocation-like precursor state plays an importantrole in these reactions, and Markovnikov's rule works in thepresent surface reaction.
References[1] K. Oguchi, M. Nagao, H. Umeyama, T. Katayama, Y. Yamashita,
K. Mukai, J. Yoshinobu, K. Akagi, and S. Tsuneyuki, J. Am. Chem.
Soc. 129, 1242 (2007).
Authors
K. Oguchi, M. Nagao, H. Umeyama, T. Katayama, Y. Yamashita,
K. Mukai, J. Yoshinobu, K. Akagia, and S. Tsuneyukia
aThe University of Tokyo, Department of Physics

Fig. 1. (a) An STM image of 2-methylpropene molecules on
Si(100)c(4×2) at 80K. Vs=2.0 V, It=0.085 nA. (b) A contour map of theintegrated LDOS on a plane parallel to the surface. (Vs=2.3V,
z=11.0Å). The topmost hydrogen atoms of the system locate at
z=10.0Å. (c) A structure model of adsorbed 2-methylpropene on
Si(100)c(4×2). The large and small open circles represent silicon andhydrogen, respectively. The filled circle represents carbon.

asymmetric alkene with an ionic molecule like hydrogen
halide is known as “Markovnikov’s rule”. This reaction
undergoes two steps including a carbocation intermediate,
and the regioselective product is controlled by the stability of
the carbocation intermediate species. In this study, we investigated the adsorption states of 2-methylpropene and propene
on Si(100)c(4×2) using low temperature scanning tunnelingmicroscopy [1].
Figure 1(a) shows an unoccupied-state STM image of the
2-methylpropene adsorbed Si(100)c(4×2) surface. The
protrusions on the Si(100)c(4×2) “bare” surface correspondto Sd sites in the unoccupied state STM image; the large andFig. 2. A calculated precursor state of 2-methylpropene on a Si(100)– –√8 ×√8 surface. The front view is shown in the circle. The CC and two
SiC “bond” lengths are shown in the Å unit.

ISSP Activity Report 200615

Element Specific Imaging by Scanning
Tunneling Microscope Combined with
Synchrotron Radiation Light
Hasegawa Group andSynchrotron Radiation Laboratory
Atomically resolved imaging with a capability ofelemental identification is one of the ultimate goals in adevelopment of microscopy. Using scanning tunnelingmicroscopy (STM), which provides us atomically resolved
surface images, many attempts have been performed forelementally contrasted images. Most of them, however,merely distinguish one element from others. Here, we reporton a new method to obtain element specific imaging using
STM. In the method, core electrons of an element are excitedwith synchrotron radiation (SR) light and emitted electrons
are detected with an STM tip. A spatial resolution of thechemical imaging is 14 nm at the present moment and will
be improved by using higher brilliance SR beam line [1].
Since STM probes electronic states near the Fermi
energy, which are strongly affected by chemical environments, it is difficult to obtain definite "fingerprint" ofelements. To obtain elemental information, core electrons,
which are characteristic to elements, have to be probed. We
have, thus, developed an STM combined with SR so that the
STM imaging can be performed under soft-x-ray irradiation,
which excites core electrons of specific elements. So far,using the STM, we have taken X-ray absorption spectra(XAS) bearing elemental information of samples by exciting
core electrons with the irradiation and detecting emitted
electrons with an STM probe tip. In the present study, we
successfully obtained element-sensitive images by measuring
the photo-induced current during the scanning under photo
irradiation whose energy is just above an adsorption edge ofthe element.
Experiments were carried out at BL-13C at Photon
Factory, KEK, Tsukuba. In order to improve detection sensitivity, the light intensity was chopped and the synchronous
photo-induced current was measured using a lock-in amplifier. A tungsten tip covered with a glass layer except its apex
was used as a probe so that an undesired background component in the photo-induced current is eliminated. A Si
substrate having a regular array of Ni dots on it was used as asample.
XAS spectra measured with an STM tip clearly show
peaks due to the Ni L absorption edges (852.7 eV and 869.4
eV). We then took an image of the photo-induced current
under the irradiation whose photon energy is just above theadsorption edge (855 eV). The image is presented in Fig. 1
together with simultaneously taken topographic STM image.
The site where a Ni dot is found in the topographic image iscontrasted bright in the photo-induced current image. As a

Fig. 1. A topographic STM image of Ni-dots fabricated on Si substrate,and simultaneously obtained photo-induced current image. These
images were taken under the irradiation of SR light whose photon
energy was 855 eV, slightly above the Ni L3-edge (852.7 eV).

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ISSP Activity Report 2006

reference, the same imaging was performed on the sameisland with a photon energy of 830 eV, far below the Ni
absorption edge. Although the Ni dot is clearly visible in the
STM image, the photo-induced current at 830 eV does notshow any contrast. These results demonstrate possibility ofthe element specific imaging from a detection of the photoinduced current under the photo irradiation.
Spatial resolution of the element specific imaging wasestimated from cross-sectional profiles of the image andfound around 14 nm. The observed spatial resolution is better
than that of photoemission electron microscopy (PEEM),
whose best spatial resolution is currently 22 nm using stateof-the-art instrument with advanced high-brilliance synchrotron radiation light. Our resolution will be also improved byusing higher brilliance SR beam line.
References[1] T. Eguchi et al., Appl. Phys. Lett. 89, 243119 (2006).
Authors
T. Eguchi, T. Okuda, T. Matsushima, A. Kataoka, A. Harasawa, K.
Akiyamaa, T. Kinoshitab, and Y. Hasegawa
aTohoku University
bJapan Synchrotron Radiation Research Institute (JASRI) / SPring-8

Insulator-metal transition in SrTiO3
Lippmaa Group
Transition metal oxides are well known for their richelectronic and magnetic phase diagrams with many
competing phases. In our work, we study carrier densitydriven phase transitions in thin oxide layers. By using afield-effect transistor structure (Fig. 1), we can modulate thecharge density in a surface layer of a SrTiO3 single crystal by
about 1013 cm-2. This carrier density is sufficient to induce
an insulator-metal transition in SrTiO3. The main advantage
of using a transistor to study phase transitions is the ability tocontinuously tune the carrier density without altering any of
the other crystal parameters, such as disorder, which isalways created by chemical doping.
Intrinsic SrTiO3 is an insulator with a band gap of 3.2 eV.
The Fermi level is located within about 100 meV of the
conduction band bottom, typically within in-gap defect states

Fig. 1. The band structure of SrTiO3 single crystal surface in thevicinity of the conduction band bottom. The Fermi level of the crystal
is located within the localized states in the band gap. (Inset) Structureof a SrTiO3 field-effect transistor, showing the insulator layers (yellow)
and the channel region where charge is accumulated (blue).

successfully induced in a field-effect transistor configuration.
The same technique can potentially be used to study a variety
of other materials besides SrTiO3, although achieving asufficient carrier density modulation remains challenging.
Authors
M. Lippmaa, K. Shibuya, T. Ohnishi, and K. Nishio

Magnetization Plateau in
Two Dimensional Anti-ferromagnetic
Solid 3He on Graphite
Ishimoto Group

Fig. 2. (a) The channel current of a SrTiO3 FET as a function of gatebias and temperature. At low gate bias, the channel is insulating (violet
plots). The channel becomes metallic above a gate bias of 1.3 V (redplots). (b) Scaled sheet resistance of the FET channel under bias. All
curves collapse onto a metallic branch and an insulating branch.

that are always present in a crystal, as illustrated in Fig. 1.
Close to room temperature, conductivity is possible dueto thermal excitation of carriers from the occupied defectstates over the mobility edge. In a transistor structure, apositive gate bias accumulates electrons in the surface layerof the SrTiO3 crystal, raising the chemical potential closer tothe band edge. This results in an drop of the channel sheet
resistance, as shown in Fig. 2(a) for low gate bias values.
In this particular device, the chemical potential reachedthe mobility edge at a gate bias of 1.3 V, which corresponds
to a sheet carrier density of 2 × 1012 cm–2. Above this bias,
the device is in a metallic state. When the gate bias ismaintained at the critical value, where the chemical potential
coincides with the mobility edge, the activation energy fortransport vanishes and the sheet resistance is not sensitive totemperature.
It is interesting to note that the transition behavior is, atleast qualitatively, similar to the appearance of a metallic
state in two-dimensional ultra-clean silicon systems. It is
possible to re-plot the sheet resistance data in Fig. 2(a) byassuming a single-parameter scaling relation, ρs(T, ns) =ρs(T/T0(ns)), where T0 is different for each carrier density
value ns.
When such scaling is performed, all the sheet resistancecurves collapse onto two mirror-symmetric branches, one forthe insulating side and one for the metallic side of the transition, as shown in Fig. 2(b).
Our work shows that electronic phase transitions can be

3He film adsorbed on graphite surface is one of the mostideal two-dimensional (2D) Fermi systems which can realize
not only a Fermi liquid but also a quantum solid just bychanging the adsorption density. The transition between
liquid and solid is recently considered to be a sort of Mott
transition [1]. The 2D solid forms a quantum spin magnet
with nuclear spin S = 1/2 on a triangular lattice. Because of
the hard-core potential between 3He atoms, the higher ordermultiple spin exchange (MSE) processes as well as twoparticle exchange play important roles. The exchange ofeven number of particles is anti-ferromagnetic (AFM), whilethat of odd ones is ferromagnetic (FM). The competition
between them, in addition to the geometrical frustration
inherent to a triangular lattice structure, makes the solid 3Hefilm strongly frustrated.
According to recent theoretical studies in the exact diagonalization method for this finite size system, none of longrange ordered AFM phases survives even at T = 0 K due toquantum fluctuations. The exchange parameters obtained athigh temperatures for the AFM phase ( so called a 4/7 phase)suggest a quantum spin liquid (QSL) ground state with afinite excitation gap [2], and a magnetization plateau ataround 5 to 6 T. Experimentally the heat capacity andsusceptibility measurements indicate a gapless spin liquid
ground state [3, 4]. Our magnetization measurement with a

Fig. 1. Temperature dependence of magnetization in the AFM 4/7
phase at various fields. Solid lines are the Brillouin functions where theAFM exchange interaction is taken into account as a molecular field.

ISSP Activity Report 2006

17

Fig. 2. Magnetization curve for the AFM 4/7 phase at 0.68 mK. The
vertical axis is normalized by the saturation magnetization.

double-gradient Faraday-type magnetometer shows theexistence of magnetization plateau in the AFM 4/7 phase on
both 3He and 4He mono layer pre-plated graphite. However
the exact polarization at the plateau was not yet clear due to alimited S/N ratio.
To clarify the magnitude, we tried to obtain the magnetization curve in the low field side by employing NMR
technique where the resonance frequencies are variable in thewide range from a few MHz to 100 MHz in a single experimental run. The paramagnetic first layer solid 3He exhibits a
Brillouin function at 3 T in the temperature region down to
0.5 mK, confirming the proper performance of refrigerationand measuring system. Figure 1 shows the magnetization of
AFM 4/7 phase on 4He mono layer pre-plated graphite as afunction of temperature at various magnetic fields. Here thevertical axis is the polarization estimated from the temperature dependence in the high temperature region. The magnetization curve at 0.7 mK is given in Fig. 2, representing aclear plateau at half of saturation magnetization. This is thefirst step to know the whole magnetization curve and further
investigation in the high field region is eagerly desired inspite of many difficulties.

Bose particles and/or Cooper pairs. The macroscopic
coherent system is expressed by a single wave function with
an amplitude and a phase. The gradient in the phase implies
flow of condensate with vs. Since vs is gradient of a scalar
field, curl vs= 0. Therefore in a singly-connected volume with
fixed impermeable boundaries vs= 0. In a multiply-connected
region there can be a finite circulation with quantization
condition. And quantized vortex lines penetrate through thesuperfluid from rather slow rotation speed Ωc1 [1] and a socalled vortex state is formed. Most general situation is vortex
lattice formation. Since the first observations of a nonclassical rotational inertia (NCRI) fraction in solid 4He [2-6],
a vast wave of interest in the supersolid state of matter (seereview [7] and references therein) started. An obvious
question is what kind of vortex state would be there if supersolid state is similar to conventional superfluids. There are a
few quantities necessary to estimate vortex state properties.
Namely vortex core size, for example, becomes important toestimate the 2nd critical angular velocity Ωc2, where vortex
cores start to overlap each other and finally the 3D superfluidity is destroyed beyond this speed [8].
Another essential, but unsolved experimental problem inthe supersolid study has been the determination of the onset
temperature To, where this phenomenon starts to appear. To
has been reported so far to be about 200 to 300 mK by
gradual appearance of NCRI. We have clarified this situation
by measuring AC excitation velocity dependence of torsional
oscillator, which contains solid He sample. Figure 1 showsthe energy dissipation in the solid He sample at 32 bar (uppercolumn) and NCRI fraction (lower column). The uppercolumn data were obtained without any artificial shift of theReferences[1] A. Casey, H. Patel, J. Ny’ki, B. P. Cowan, and J. Saunders, Phys.
Rev. Lett. 81, 115301 (2003).
[2] G. Misguich, B. Bernu, C. Lhuillier, and C. Waldtmann, Phys. Rev.
Lett. 81, 1098 (1998).
[3] K. Ishida, M. Morishita, K. Yawata, and H. Fukuyama, Phys. Rev.
Lett. 79, 3451 (1997).
[4] R. Masutomi, Y. Karaki, and H. Ishimoto, Phys. Rev. Lett. 92,
025301 (2004).
Authors
H. Nema, A. Yamaguchi, T. Hayakawa, and H. Ishimoto

Torsional Oscillator Experiments ofSupersolid State of He andDetermination of the Onset Temperature
and Characteristic Lengths
Kubota Group
Conventional Superfluidity has been regarded in large as
manifestation of unusual properties of Bose Einsteincondensation (BEC) with the macroscopic wave function of

18

ISSP Activity Report 2006

Fig. 1. AC velocity dependence of Torsional Oscillator Responses with
Solid 4He sample at 32 bar pressure. A unique temperature is foundnear 0.5 K, below which energy dissipation in the solid He sample
starts to depend on small changes of AC excitation velocity. We findthe nonclassical rotational inertia, or supersolid fraction also starts toappear and change depending on the AC velocity below the same
temperature. The inset shows energy dissipation data with a clear peak
for another solid 4He sample. It is remarkable to see the small AC
velocity influences very much of the supersolid property. On the other
hand we find that AC velocity dependence of the energy dissipation persupersolid fraction has a maximum near 200 µ m/s, which seems tohave different origin as apparent critical velocity which characterizes
only NCRI decreasing velocity. We can argue provisionally that what
we found is real bulk energy dissipation critical velocity whereas the
NCRI critical velocity is a characteristic velocity which indicatesfluctuation of NCRI component.

data at all and we can see clearly that all the data start tochange depending on small AV velocity below a certain
temperature near 0.5 K. We can associate the same temperature for the change of NCRI as well as seen in the lower
column. These observations give us opportunity to determine
the onset temperature To for sure, thereby confirming thatsupersolid transition is not associated with possible liquid
phase which might have remained in solid samples [7].
By utilizing above information of To we could analyze
the supersolid fraction temperature dependence and proceed
to evaluate Josophson's length, for the first time. Vortex core
size is expected to be the same order as Josephson's length[8] and we can use it to estimate vortex state parameters. We
obtained two sets of length scale, which may reflect theappearance of vortex fluid state and the original supersolidstate as recently discussed by P.W. Anderson [9].
References[1] W. F. Vinen, in The Vortex State, ed. N. Bontemps, et al., NATOASI Series, Kluwer Academic Publishers, (1994).
[2] E. Kim and M.H.W. Chan, Nature 427, 225 (2004); Science 305,
1941 (2004).
[3] A.S.C. Rittner and J.D. Reppy, Phys. Rev. Lett. 97, 165301 (2006).
[4] M. Kondo, S. Takada, Y. Shibayama, and K. Shirahama, condmat/0607032; J. Low Temp. Phys. in press.
[5] A. Penzev, Y. Yasuta, and M. Kubota, cond-mat/0702632; J. Low
Temp. Phys. in press.
[6] A.S.C. Rittner and J.D. Reppy, cond-mat/0702665.
[7] N.V. Prokof'ev, Advances in Physics, 56, 381 (2007).
[8] M. Kubota, M. Fukuda, T. Obata, Y. Ito, A. Penzyev, T.
Minoguchi, and E. Sonin, AIP Proc. 850, 283 (2006) and references
therein; N. P. Mikhin, V. E. Syvokon, T. Obata, and M. Kubota,
Physica B 329-333, 272 (2003); B.D. Josephson, Phys. Lett. 21, 608(1966); M. E. Fisher, M. N. Barber, and D. Jasnow, Phys. Rev. A 8,1111 (1973).
[9] P. W. Anderson, Nature Physics 3, 539 (2007); condmat/0705.1174.

contribute to the CDW formation. This fact strongly suggeststhat the observed AMRO originates not from the quantizedquasi-particles but from the cylindrical FS.
Next, to confirm if the observed AMRO could be
explained as an effect of the cylindrical Fermi surface, wehave evaluated interlayer conductivity using the quantum
tunneling picture, which considers only the lowest-ordercontribution of single interlayer tunneling between neighbouring two layers. We have successfully reproduced theobserved features, that is, the coexistence of Lebed
resonances and Kajita oscillations as shown in Fig.2. Thiseffect is the AMRO of the magnetic breakdown system with
the CDW potential.
In conclusion, the anomalous AMRO observed in the
CDW phase in α-(BEDT-TTF)2XHg(SCN)4 originates fromthe cylindrical FS. The present result is the negative one
against the UCDW model.
Authors
T. Osada, R. Yamaguchi, T. Konoike, and K. Uchida

Authors
A. Penzyev, Y. Yasuta, and M. Kubota

Angle-Dependent Magnetotransport
Study in an Organic Conductor: Test
for Unconventional CDW State
Osada GroupThe unconventional charge density wave (UCDW) state,
where quasi-particles have nodal dispersion, has been
recently discussed in organic conductors α-(BEDTTTF)2XHg(SCN)4 (X=K, Rb, Tl). In contrast to the case of
unconventional superconductivity, the zero-gap quasi-particles could contribute to the transport phenomena. It has been
discussed that the anomalous angle-dependent magnetoresistance oscillation (AMRO) appears resulting from the Landau
quantization of quasi-particle spectrum. The unexplained
AMRO similar to the Lebed resonance, which is observed inthose organic salts, has been the main experimental basis of
the UCDW model.
In order to check the possibility of UCDW, we haveexperimentally and theoretically investigated the Lebedresonance-like AMRO observed in the CDW phase. Detailed
stereographic measurements of interlayer magnetoresistancein X=K salts have revealed that the amplitude of AMRO is
modulated in the different way from the conventional Lebed
resonance (Fig.1). We have found that the modulation
pattern quantitatively coincides with Kajita oscillations of
the remaining cylindrical Fermi surface (FS), which does not

Fig. 1. Interlayer resistance of α-(BEDT-TTF)2KHg(SCN)4 as afunction of magnetic field orientations. The amplitude of Lebedresonance-like AMROs (dashed line) are modulated by Kajita oscillations (horizontal solid lines).

Fig. 2. Interlayer resistance of the Q2D electron system using thetunneling picture. Lebed resonances modulated by Kajita oscillations
are successfully reproduced.

ISSP Activity Report 2006

19

Magnetization Measurements by Using
Turnbuckle Type Diamond Anvil Cell
–Magnetic Behavior of TmTe–
Uwatoko GroupThe Tm monochalcogenide series TmX (X=S, Se, Te)
has the NaCl structure and is among the mysterious strongly
correlated electron systems. Many aspects of TmSe havebeen investigated including the valence fluctuation between
Tm3+ and Tm2+, the Kondo effect, antiferromagnetic
ordering (TN = 3.4 K [1]), and the metal-insulator transition.
It was shown that the behavior of the electrical resistance
was not that of a typical metal, but displayed a Kondo latticelike behavior. The Te analog, TmTe, exhibits anomalous
high pressure electrical resistance behavior above 2 GPa [2]similar to the Kondo-like behavior seen in TmSe. To provide
a more complete understanding of this behavior, we have
performed single crystal magnetization measurements to 6.1
GPa.
The magnetization was measured by a Quantum Design
SQUID magnetometer. Pressure was generated in a novel
turnbuckle diamond anvil cell (DAC) with a diameter andheight of 6.4 mm and 7.4 mm, respectively. Figure 1 shows across section of the CuBe alloy DAC. The sample (TmTesingle crystal) had dimensions of 45 µm × 180 µm × 180 µm
and was placed into the 400 micron hole of a NiCrAl alloygasket together with ruby chips and the pressure-transmitting
medium, glycerin. The pressure was calibrated by measuring
the shift of the ruby fluorescence line, R1, at 80 K. In
conventional high-pressure-magnetic experiments, the large
cell background makes it difficult to observe the sample
signal. To overcome this, a large sample was used and thecell background was subtracted. The NiCrAl alloy gasket
was not aged, since the aged material has a large magnetic
moment if not handled correctly.
The field cooled (1000 Oe) temperature dependent
magnetization traces are shown in Fig. 2. It was found that thetemperature dependent magnetic susceptibility for 0 GPa and2.56 GPa showed paramagnetic behavior with no discernible
distinction between the two traces for temperatures between
2 K and 80 K. Also, the magnetic field dependent magnetization curves have been performed at 2.0 K. The 1.0 GPa curveshows paramagnetic behavior. A small hysteresis and saturation in magnetism were observed at 4.7 GPa indicative of
ferromagnetism. From the saturation of the magnetic

Fig. 1. Diamond anvil cell, designed for magnetic measurement by a
SQUID magnetometer and pressures up to 10 GPa.

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ISSP Activity Report 2006

Fig. 2. 1000 Oe field cooled temperature dependences of the magnetization. On upper figure, with increasing pressure, the temperature of
the transition decreases. The magnetic susceptibility for 3.6 GPa showsa paramagnet-ferromagnet transition at TC = 12.2 K, which was ascertained by differentiating the magnetic susceptibility. TC decreases withincreasing pressure, disappearing above 6.1 GPa within the accessible
temperature range. We note that the moment increases with decreasing
temperature below 8.9 K at 3.6 GPa. This same phenomenon is seen in
the 5.4 GPa data. The origin is unknown.

moment, the effective magneton numbers at 4.7 GPa and 5.4
GPa are1.85 µB/f.u. and 1.62 µB/f.u., respectively. That isbigger than the value 1.2 µB/f.u. at 5.4 GPa found by neutron
diffraction experiments [3]. The moment is smaller than thatfor Tm2+ (4 µB/f.u.) and Tm3+ (7 µB/f.u.). Ferromagnetic
behavior was not observed at 6.1 GPa. Electrical resistivity
measurements have been performed under high pressure [2].
At 5.7 GPa the resistivity showed an abrupt decrease which
was believed to be due to a structural phase transition. We
observed that the ferromagnetic phase disappeared between
5.4 GPa and 5.85 GPa. From these results, below 2.56 GPa,
TmTe displays paramagnetic behavior. Above 3.6 GPa, aferromagnetic phase suddenly appears. With increasingpressure, the transition temperature decreases. By X-ray
absorption spectroscopy in the partial fluorescence yieldmode, the valence of Tm in TmTe is mostly divalent for 0-3
GPa and 2.45 for 3-4.5 GPa [4]. This may be the cause offerromagnetic phase appearing in this pressure range.
Magnetization measurements were performed under
extremely high pressure using a diamond anvil cell. The
ambient and 2.56 GPa data showed similar behavior for
temperatures between 2 K and 80 K. At 3.6 GPa, thetemperature dependence of magnetization shows the onset ofa paramagnetism-ferromagnetism transition at TC = 12.2 K.
On increasing the pressure, TC decreases. Above 6.1 GPa,the transition disappears. We propose that a change in thevalence value of Tm plays an important role in this behavior.
References[1] E. Bucher, K. Andres, F. J. di Salvo, J. P. Maita, A. C. Gossard,
A. S. Cooper, and G. W. Hull, Jr., Phys. Rev. B 11, 500 (1975).
[2] T. Matsumura, T. Kosaka, J. Tang, T. Matsumoto, H. Takahashi,
N. Môri, and T. Suzuki, Phys. Rev. Lett. 78, 1138 (1997).

[3] P. Link, I. N. Goncharenko, J. M. Mignot, T. Matsumura, andT. Suzuki, Phys. Rev. Lett. 80, 173 (1998).
[4] I. Jarrige, J.P. Rueff, S.R. Shieh, T. Matsumura, H. Ishii, N.
Hiraoka, and Y.Q. Cai, Physica B: Condensed 378-380, 1154 (2006).
Authors
L. Chen, N. Kurita, K. Nakazawa, I. Oguro, Y. Uwatoko, T.
Matsumoto, M. Hedoa, T. Matsumurab, M. Tokumotoc, and S.W.
TozerdaUniversity of Ryukyu
bDepartment of Physics, University of Tohoku
cNanotechnology Research Institute, National Institute of Advanced
Industrial Science and Technology
dNational High Magnetic Field Laboratory, USA

Lissajous-like Motion of the
Wave-packets in a Two-dimensional
Adiabatic Potential Well
Suemoto Group
Recently, we have been studying the nuclear wave packet
dynamics in solids by using femtosecond time-resolved
luminescence spectroscopy. The wave-packet movie has
been taken for the self-trapped excitons (STE) in brominebridged platinum complexes (Pt-Br) [1]. Selective sensitivityof this method to the excited state has been demonstrated for
F-centers in alkali-halides [2].
Here, we present an evidence for Lissajous-like motion of
the wave-packet on a two-dimensional potential surface
observed in cooled Pt-Br [3]. The red curves in Fig. 1 are thetime evolution of the luminescence intensity at 0.95 eV (top)and 0.85 eV (bottom). The process we are observing isshown in the inset of Fig. 2. The system is excited (upward
arrow) from the ground state (blue bowl) to the excited one(red bowl). The wave-packet starts from the upper part of the
red bowl and makes an oscillatory motion. We observe theluminescence at certain points on the red bowl, shown by adownward arrow. When the wave-packet passes on thisFig. 2. Potential energy surfaces of the self-trapped exciton for the
ground state (blue) and the excited state (red). The yellow disk is thewave-packet and the green curve is its trajectory. The inset shows the
optical transitions between the two-dimensional potential surfaces.

point, the luminescence flashes. The fine oscillatory featureswith a period of ca. 300 fs in the red curves in Fig. 1 correspond to this flashing. In addition to this main oscillation, we
can notice a complicated behavior, i.e., the intensity is
modulated by a slower oscillation component at 0.95 eV. On
the other hand, at 0.85 eV, the oscillation persists for about
2.5 ps without this modulation and stops suddenly.
These complicated waveforms are understood in terms ofa model assuming a two-dimensional damped oscillation on
the excited state potential well. The wave-packet (yellow disk)
moves on a trajectory shown by a green curve (see Fig. 2).
The tilted black line shows the position where the luminescence photon energy is 0.95 eV. Crossing or approaching of
the wave-packet to this black line, shown by orange ovals,
bring about increase of the luminescence intensity. The
simulated time evolutions are shown in Fig. 1 by blue curves.
The numbers (1-6) indicated correspond each other in Fig. 1
and Fig. 2. The important features are well reproduced in termsof this simple model. More elaborate calculation based onresonant secondary radiation theory coupled with an interaction mode description of the wave-packet was performed [3].
References[1] T. Matsuoka, J. Takeda, S. Kurita, and T. Suemoto, Phys. Rev. Lett.
91, 247402 (2003)[2] T. Koyama, Y. Takahashi, M. Nakajima, and T. Suemoto, Phys.
Rev. B 73, 161102 (2006)[3]K. Yasukawa, Y. Takahashi, S. Kurita, T. Suemoto, Solid State
Communications 140, 197 (2006); J. Phys. Soc. Jpn. in press.
Authors
K. Yasukawa, Y. Takahashi, M. Nakajima, and T. Suemoto

Highly Advanced Coherent Soft X-ray
Generation Down to 7.9 nm Due to High
Harmonic Based on Novel Scheme ofLaser-ablation Controlled Plasma
Kuroda Group
Fig. 1. Time evolutions of the luminescence intensity at 0.95 and 0.85
eV. Red and blue curves correspond to experiment and calculation,
respectively. The large peaks at t=0 in the experimental curves aretentatively assigned to the radiation before self-trapping. The numbers(1-6) correspond to those in Fig.2.

Since the time when first ruby laser was demonstrated,
future development of x-ray laser remains as one of a dreamof laser physicists and almost all researchers engaged in laser

ISSP Activity Report 2006

21

Fig. 1. HHG spectra from tin and vanadium laser ablation irradiated byfemetosecond laser pulse. The intensity of the 17th harmonics was 20
times higher than those of its neighbors. The conversion efficiency of
the 17th harmonic was measured to be about 1.1×10-4, and this output
energy of 1.1 µJ was obtained from the pump laser energy of 10 mJ.

applications involving biomedical applications and basic
medicine. High-order harmonic generation (HHG), which
occurs as a result of the interaction of strong laser fields withatoms, ions, or molecules, is one of most promising shortwavelength unique source of coherent soft x-rays with
distinct advantages of high brightness, good beam quality,and femtosecond, or even attosecond duration. The demonstration of the HHG applications such as the control ofelectron dynamic in molecules, nonlinear optical phenomenain the XUV region, the photoelectron spectroscopy and ultrafast dynamics of multiphoton-induced photoelectron
emission has recently been reported. Thus the HHG havebeen recognized as very attractive tools. For the applicationof the HHG emission, it is important to increase the conversion efficiency of this process. In addition, it is necessary toextend the cutoff energy of the harmonic. From these objectives, we developed the high intensity harmonic generation atshort wavelength region (high photon energy region).
For the high intensity HHG, we report first demonstrationof the single harmonic enhancement by using multiphoton
resonance with the strong oscillator strength ions at the
wavelength of 47 and 37 nm [1, 2]. Figure 1 shows the
spectra of HHG from the laser-ablated tin plume. A strong
17th order harmonic at a wavelength of 46.76 nm wasobserved using tin laser ablation plume, as shown in Fig. 1.
The conversion efficiency of the 17th harmonic wasmeasured to be about 1.1×10-4, and this output energy of
1.1 µJ was obtained from the pump laser energy of 10 mJ.
Figure 2 shows the HHG spectra from the laser-ablated tin

(a)
(b)

with three different laser wavelengths of 795 nm, 782nm,
and 775 nm. In Fig. 3(a), one sees that the intensity of the
17th harmonic using 795 nm wavelength pump dominates the
harmonic spectrum. The intensity of the 17th harmonic is 20
times higher than that of other harmonics. However, in Fig
3(b), the intensity of the 17th harmonic using 782 nm
wavelength pump is decreased, and is almost same as that ofother harmonics. In Fig. 3(c), the intensity of the 17th
harmonic with 775 nm wavelength pump is further
decreased. In the case, the 17th harmonic intensity is weaker
than that of the 13th and 11th harmonics. The above results
show that the intensity of the 17th harmonic gradually
decreased as the wavelength of the pump laser become
shorter. As mentions above, the origin of this enhancement is
attributed to resonance with strong radiative transition of the
Sn II ion, produced within the laser-ablated plume. For thelaser-ablation Sb, we successfully demonstrated the intensity
enhancement of a single high-order harmonic at awavelength of 37.67 nm. The conversion efficiency of thisharmonic was 2.5×10 –5 and the output energy was 0.3 µJ.
Such an approach can pave the way toward a considerable
enhancement of a single harmonic in the short-wavelength
range using the appropriate target materials.
On the other hand, for the extension of cutoff energy inhigh harmonic (HH), we demonstrated first observation of
HH by using the doubly charged ion in the laser-ablation
plume via produced by pre-plasma. By using the laserablation V plume, we observed the 71st harmonic at thephoton energy of 110 eV. Very recent result of the HH cutoff
has successfully extended up to 150 eV by using the laserablation Mn plume [4]. Thus the HH from the lowly charged
ion have obtained the quite novel results, especially in theiroutstanding high efficiencies and attractive physics in related
ion and atomic plasma laser physics.
References[1] M. Suzuki, M. Baba, R. A. Ganeev, H. Kuroda, and T. Ozaki, Opt.
Lett. 31, 3306 (2006).
[2] M. Suzuki, M. Baba, R. A. Ganeev, H. Kuroda, and T. Ozaki, Opt.
Express 15, 1161 (2007).
[3] M. Suzuki, L. B. Elouga Bom, T. Ozaki, R. A. Ganeev, M. Baba,
and H. Kuroda, Opt. Express 15, 4112 (2007).
[4] R. A. Ganeev, L. B. Elouga Bom, J.-C. Kieffer, M. Suzuki, H.
Kuroda, and T. Ozaki, Phy. Rev. A. to be published.
Authors
M. Suzuki, M. Baba, R. A. Ganeev, T. Q. Jia, H. Kuroda, andT. Ozakia
aInstitut National de la Recherche Scientifique, Énergie, Matériaux etTélécommunications

Laser Controlled Nanostructures
Formation on Wide Band-gap
Semiconductors Using Two-beamFemtosecond Laser Pulses
Kuroda Group(c)Fig. 2. HHG spectra from tin laser ablation for pump laser with central
wavelength of (a) 795 nm, (b) 782 nm, and (c) 778 nm. The ordinate ofspectra (b) and (c) are 6 times increased. The intensity of the 17th
harmonic gradually decreased as the central wavelength of laser
become shorter.

22

ISSP Activity Report 2006

Advanced laser nano-fabrication has great potential
applications in optoelectronic and information technology,
and it has been studied intensively in recent several years.
Many kinds of nanostructures, such as nanogratings,
nanowires and three-dimensional photonic crystals were
fabricated by femtosecond laser ablation method. Wideband-gap semiconductors have excellent optical, thermal andcontrolled plasma and realized highly advanced coherent soft
x-ray due to high harmonic generation. In the near future, wewill extend the formation of nanostructures and related
physics to a new stage. By utilizing the soft x-ray coherent
lights developed in our group, we will study the high resolution microfabrication using ultra-short x-ray pulses.

Fig. 1. The dependence of Λn on laser wavelengths λ. Here Λ is the
nanograting period, n is refractive index. The solid line represents λ/2.

mechanical properties, and they attract much attention in
optoelectronic industry. We developed a pump-probe
method, and studied the formation of nanostructures on wideband-gap semiconductors induced by femtosecond laser
pulses.
We report nanogratings with periods of 150 and 80 nm
formed on the surface of 6H-SiC crystals irradiated by the ppolarized 800 nm and the s-polarized 400 nm femtosecond
lasers, respectively. When both of the two collinear laser
beams focus simultaneously on the sample surface, nanoparticles formed on the whole ablation area, and they array inparallel lines. If we change the laser wavelength with OPA(optical parametric amplifier) system, nanogratings withperiods in the range of 80-250 nm were fabricated [1, 2].
We report a simple route of growing ZnSe nanowires on
ZnSe crystal surface [3, 4]. Uniform ZnSe nanowires areobserved on the ablation crater irradiated by femtosecond
lasers in air, while other part of the sample surface is notdamaged. The nanowire growth rate is about 5 µm/s, it ishigher than that fabricated by chemical vapor deposition
method by a factor of 104. The nanowire length and diametercan be controlled by varying laser pulse energy and pulse
number. If the crystal wafer is horizontally dipped in purewater, needle-like nanowires are also found in the ablationcraters. Our discovery opens a rapid and simple way to grownanowires on the designed micro-patterns.
Besides the formation of nanostructures, we also studied
the ultrafast dynamic in dielectrics and multilayers irradiatedby femtosecond laser [5, 6]. We found avalanche ionization
plays an important role in the processes of the generation of
conduction band electrons and sample damage.
Nanofabrication on dielectrics, semiconductors andmetals using soft x-ray laser is a hot research topic. In ourgroup, we have developed a novel scheme of laser-ablation

Fig. 2. SEM images of the ablation craters and nanowires. The laserparameters are: (a) and (b), 800 nm, 120 fs, linear polarization, 1 mW,
500 pulses; (c) and (d), 400 nm, 120 fs, linear polarization, 10 mW,
defocus for 3 mm, 500 pulses.

References[1] T. Q. Jia, F. L. Zhao, M. Huang, H. X. Chen, J. R. Qiu, R. X. Li,
Z. Z. Xu, and H. Kuroda, Appl. Phys. Lett. 88, 111117 (2006).
[2] X. J. Wu, T. Q. Jia, F. L. Zhao, M. Huang, N. S. Xu, H. Kuroda,
and Z. Z. Xu, Appl. Phys. A 86, 491 (2007).
[3] T. Q. Jia, H. X. Chen, M. Huang, X. J. Wu, F. L. Zhao, M. Baba,
M. Suzuki, H. Kuroda, J. R. Qiu, R. X. Li, and Z. Z. Xu, Appl. Phys.
Lett. 89, 101116, (2006).
[4] T. Q. Jia, M. Baba, M. Huang, F. Zhao, J. Qiu, X. Wu, M. Suzuki,
R. Li, Z. Xu, and H. Kuroda, Solid State Commun. 141, 635 (2007).
[5] T. Q. Jia, H. X. Chen, M. Huang, F. L. Zhao, X. X. Li, S. Z. Xu,
H. Y. Sun, D. H. Feng, C. B. Li, X. F. Wang, R. X. Li, Z. Z. Xu,
X. K. He, and H. Kuroda, Phys. Rev. B 73, 054105 (2006).
[6] T. Q. Jia, H. Y. Sun, X. X. Li, D. H. Feng, C. B. Li, S. Z. Xu,
R. X. Li, Z. Z. Xu, and H. Kuroda, J. Appl. Phys. 100, 023103 (2006).
Authors
T. Q. Jia, M. Baba, M. Suzuki, R. A. Ganeev, and H. Kuroda

Photoluminescence of Single
GaInAsP/InP Quantum Wires Formedby Etching-Regrowth Techniques
Akiyama Group
Recent advances in the top-down nano-technology
techniques of GaInAsP/InP quantum-film (Q-Film) epitaxial
growth, electron-beam lithography, dry etching, andregrowth, provide excellent controllability in fabricating
various nano-structure devices operating at 1.5 µm
wavelength, such as low-threshold quantum-wire (Q-Wire)lasers for telecommunication applications [1]. However, no
optical measurement has ever been reported on a single QWire fabricated by this method, due to material damages
introduced during the etching-regrowth processes and also to

Fig. 1. Schematic of a Q-Wire sample. Excitation energy was 1.26 eV,
where the surrounding InP was transparent at the energy.

ISSP Activity Report 200623

blueshifts from the PL peak energy of the Q-Film. Since P2existed in all Q-Wires and converges to the Q-Film peak, thiswas ascribed to a recombination between electrons and holes
in the ground subbands in a Q-Wire. The energy blueshift
represented lateral quantum confinement, which is as large
as 90 meV at the W = 6 nm spectrum. While P3 is ascribed to
one of the excited subbands in the Q-Wires, the origin of P1
is still unknown. All the peaks P1, P2, and P3 were experimentally ascribed to the Q-Wires, because scanningmeasurements across the Q-Wires have shown that they
emerged only when the spot overlapped with the Q-Wires. In
the scanning measurements along the Q-Wires, we observed
uniform spectra over 30 µm, which revealed highhomogeneity of the Q-Wire sizes along the Q-Wires.
References[1] H. Yagi et al., Appl. Phys. Lett. 87, 223120 (2005).
Authors
H. Itoh, M. Yoshita, H. Akiyama, D. Plumwongrota, T. Maruyamaa,
and S. Araia
aResearch Center for Quantum Effect Electronics, Tokyo Institute ofTechnology

Development of Copper-CoatedInsertion-Device Vacuum Chamber
Nakamura Group
Fig. 2. Normalized PL spectra for Q-Wires with various W (as denoted)
and a Q-Film at 4 K. Peak energy of the Q-Film is represented by avertical broken line. Three series of peaks denoted as P1, P2, and P3,
were clearly observed. Intense PL at 1.1 eV is due to OCLs.

low sensitivity of spectroscopic systems around 1.5 µm
wavelength. In this work, we measured and analyzed photoluminescence (PL) spectra at 4 K of etching-regrowth single
GaInAsP/InP Q-Wires with lateral widths down to 6 nm andlateral quantum confinement energies up to 90 meV.
Figure 1 shows schematic of an etching-regrowth single
GaInAsP/InP Q-Wire sample. The sample was grown byorganometallic vapor-phase-epitaxy (OMVPE) technique in
two steps. In the first growth, an undoped strain-compensated single 6 nm-thick GaInAsP Q-film structure
sandwiched by optical confinement layers (OCLs), or
cladding layers, was grown on a n+-InP substrate. After thegrowth, Ti metal patterns of 0.91 mm-long isolated wires
with various widths in the nm scale were formed withelectron-beam lithography and a lift-off process. Then,
CH4/H2 reactive ion etching and additional slight wet
chemical etching for cleaning were carried out to form
single Q-Wires. The Q-Wires were buried in intrinsic InP
regrown in the second growth step. The final Q-Wire widths
W = 6 - 40 nm were characterized by cross-sectionalscanning electron microscope measurements.
We performed micro-PL scanning measurements on
single Q-Wires with a spatial resolution of about 1.5 µm
limited by the excitation spot size. Figure 2 shows observed
normalized PL spectra at 4 K of the Q-Wires with various W.
The spectrum of the narrowest Q-Wire (W = 6 nm) is shownat the top, and spectra of wider Q-Wires are sequentiallyshown below. As a reference, a PL spectrum of the as-grown
Q-Film in the same original wafer was measured and shownat the bottom. As the width W of Q-wires decreases, PL
peaks denoted as P1, P2, and P3 showed systematic energy

24

ISSP Activity Report 2006

Insertion devices (IDs) can provide highly brilliant lightfor SR users and enable the users to do state-of-the-art experiments. On the other hand, ID vacuum chambers cangenerate large long-range wakefields by interaction with amulti-bunch electron beam and cause resistive-wall coupledbunch instabilities of the beam, because they have a verysmall aperture and are made of stainless steel in order toobtain high mechanical strength in spite of thin thickness andto reduce eddy current effects in the magnetic gap or phasechanges. For reduction of the resistive-wall wakefields, we
proposed that the inner surfaces of the ID stainless-steel
vacuum chambers should be coated with a highly conductive
metal such as copper [1]. However, effects of copper coatingon the ultra-high vacuum conditions had not been investigated. Therefore we studied and developed a copper-coatedchamber usable under the ultra-high vacuum conditions [2].
A developed copper-coated chamber was produced as atest chamber for measurement of the outgassing rate by

Fig. 1. Photograph of test copper-coated chamber.

References[1] N. Nakamura et al., Proc. of the 6th European Particle Accelerator
Conference, Stockholm, 984 (1998).
[2] H. Sakai et al., Proc. of the 2005 Particle Accelerator Conference,
Knoxville, 2633 (2005).
[3] J. R. Young, J. Vac. Sci. Technol. 6, 398 (1969).
Authors
N. Nakamura, H. Sakai, and K. Shinoe

Fig. 2. Setup for measurement of the outgassing rate of the coppercoated chamber.

plating all the inner surface of a stainless steel chamber(1.4 m in length and 152 mm in diameter) with copper. In
this plating procedure, the stainless steel chamber was put
into the copper sulfate solution and then dipped for about 3
hours. The thickness of copper coating was about 150 µm,
which is sufficiently thick for suppressing the wakefields
significantly. Figure 1 shows the photograph of the test Cucoated chamber. For comparison, a test stainless steel (SS)chamber usable under the ultra-high vacuum conditions was
also produced. It had the same length and diameter. The
orifice method was applied to measuring the outgassing rateof the test Cu-coated and SS chambers. Figure 2 shows the
set-up for measurement of the outgassing rate. This system
consists of the test Cu-coated chamber (or SS chamber), two
B-A gauges (BAG), an orifice (φ 5mm) between them, a Qmass analyser, a metal valve, two molecular pumps (TMP),
two gate valves in the upstream of the TMPs and two cold
cathode gauges (CCG). First we opened the metal valve andpumped the entire chamber during baking out and thenclosed the metal valve after the baking out. The gas from theinner surface of the chamber passes through the orifice and ispumped out by the TMP1 as shown by the arrows of Fig.2.
The outgassing rate q from the chamber under the equilibrium condition is described by q=C(P1-P2)/A, where P1 andP2 are the measured pressures by BAG1 and BAG2, C is the
conductance of orifice and A is the area of the inner surfaceof the chamber.
Table 1 shows the results of the outgassing rate measurement under the ultra-high vacuum. For each test chamber,the total baking time was 200 hours at 300ºC and then thetest chambers were cooled down for 48 hours at 24ºC. The
measured outgassing rate of the SS chamber was consistent
with the other experimental data [3] and the measured
outgassing rate of the Cu-coated chamber was smaller thanthat of the SS chamber. In addition, the measured mass
spectrum of the Cu-coated chamber was almost the same asthat of the test SS chamber. It was concluded from themeasurement results that the developed copper-coatedchamber was usable under the ultra-high vacuum conditions.
Recently the same copper coating was adopted for a new ID
vacuum chamber of KEK-PF BL-16 to keep the beam stable.

chamberCu-coated
SS

Hyper Gels with High Mechanical
Properties: Clay-Polymer
Nanocomposite Gels
Shibayama GroupIn general, gels are made of a small fraction of polymer
network component and a large portion of solvent. Hence,gels are very eco-friendly material and are served in various
applications. Though high toughness and mechanicalstrength in gels have been anticipated, conventional gels arevery brittle and fragile. Haraguchi et al. reported a novel
clay-polymer nanocomposite gels (NC gels) consisting ofsynthetic hectorite and poly(N-isopropyl acrylamide)(PNIPA) [1]. This “nanocomposite hydrogel” (after theirnotation) has excellent physical properties, such as toughmechanical properties, large deformability, large swelling
ratio, rapid shrinking capability, and high transparency [2].
For example, NC gels are much tougher than conventional
PNIPA hydrogels (organic gels; OR gels) by a factor of 20 –50 in deformability and of 300 – 1000 in toughness. The mostinteresting fact is that the improved properties are obtainedby simply replacing N,N’-methylenebis(acrylamide) (BIS)(cross-linker of usual chemical gels) with clay before
polymerization of PNIPA. Note that a simple mixture of
PNIPA and clay in water does not behave like NC gels. In
order to elucidate the origin of the superiority in the mechanical properties of NC gels, we carried out small-angle
neutron scattering (SANS) experiments and discussed thedeformation mechanism.
SANS experiments were carried out at SANS-U, ISSP
with 7.0 Å- neutron beam. Figure 1 shows the two-dimensional scattering intensity patterns of uniaxially stretched NC
gel [3, 4]. Here, q is the magnitude of the scattering vector.
The NIPA and clay concentrations were 0.04 M and 700
mM, respectively. The molecular weight of PNIPA is
expected to be more than 2.0 × 103 kg/mol. The SANS
patterns clearly show that the scattering profiles become
anisotropic by stretching. Even in the case of contrast-

outgassing rate
[Pa m3 s -1 m-2] (N2, 24ºC)(0.90±0.06)×10-10(1.36±0.06)×10-10

Table 1. Results of outgassing rate measurement

Fig. 1. Two-dimensional SANS patterns for NC4 (SDD = 8m); upper(in D2O solvent) and lower (in contrast matched solvent). The
stretching direction is horizontal and λ denotes the stretching ratio.

ISSP Activity Report 200625

[4] S. Miyazaki, T. Karino, H. Endo, K. Haraguchi, and M. Shibayama,
Macromolecules 39, 8112 (2006).
[5] M. Shibayama, T. Norisuye, and F. Ikkai, J. Phys. Soc. Jpn. Suppl.
A 70, 306 (2001).
[6] M. Shibayama, Y. Isaka, and Y. Shiwa, Macromolecules 32, 7086
(1999).
Authors
S. Miyazaki, T. Karino, H. Endo, K. Haraguchi, and M. Shibayama

Localized Mode in the β-Pyrochlore
Superconductors AOs2O6
Hirota and Hiroi Groups
Fig. 2. Deformation model of (a) NC gels and (b) OR gels. PNIPA
chains in NC gels are effectively bridging clay platelets and the molecular weight is rather high compared with those of OR gels.

matched sample, where the scattering contrast between waterand clay is matched by tuning hydrogen/deuterium ratio, thescattering patterns accompanying two lobes in the stretchingdirection are clearly observed. Since clay platelets aresupposed to be “invisible”, this two-lobe pattern must be
ascribed to an anisotropic scattering from deformed polymer
chains. At the high-q region, on the other hand, the SANS
pattern stretched in the perpendicular direction is interpreted
to be due to elongated polymer chains along the parallel
direction. Hence, by combining ordinary and contrastmatched SANS, we could differentiate the two contributions,
i.e., clay platelet orientation and polymer chain stretching,and conjectured the following mechanism.
Figure 2 shows the difference in the structure of NC gels
and OR gels [3]. First of all, the “spatial” inter-cross-link
distance of a NC gel, i.e., the inter-platelet distance is much
longer (of the order of 400 Å) than the inter-cross-link
distance of OR gels (of the order of 10 to 20 Å for the caseof 700 mM PNIPA gels with 5% cross-links) [5]. Secondly,
the “topological” chain lengths <R2>1/2, are also very
different, where R is the end-to-end distance of PNIPA
chains between cross-links and < > denotes ensemble
averaging. That is, the molecular weight of PNIPA chainsbridging neighboring clay platelets is much larger than thatof subchains between cross-links for OR gels. The former
could be of the order of 105 to 106 [6], while the latter is of
the order of 101 to 102 (based on the ratio of NIPA and BIS).
Thirdly, the number of “effective” cross-links, which arecapable of transmitting the external load, is much larger in
NC gels than in OR gels. This is because most of PNIPA
chains in NC gels are anchored to clay platelets and play arole as active chains, while most of PNIPA chains in OR gels
are separated by cross-linkers to short chains and merely
form clusters with a high density of cross-linking. These
facts directly result in the high elongation ratio and highstrength at break of NC gels. Upon stretching, clay platelets
become oriented with their surface normal parallel to thestretching direction. It is concluded that the extraordinarily
good mechanical properties are ascribed to “plane crosslinks” with long polymer chains between platelets and highdensity of “force sustainable” polymer chains.
References[1] K. Haraguchi and T. Takehisa, Adv. Mater. 14, 1120 (2002).
[2] K. Haraguchi, R. Farnworth, A. Ohbayashi, and T. Takehisa,
Macrmolecules 36, 5732 (2003).
[3] M. Shibayama, T. Karino, S. Miyazaki, T. Takehisa, and K.
Haraguchi, Macromolecules 38, 10772 (2005).

26

ISSP Activity Report 2006

Yonezawa et al. [1, 2, 3, 4] have recently discovered a newseries of superconductors AOs2O6 (A=K, Rb,Cs). The superconducting transition temperature falls with increasing theionic radius of the alkali metal ion: Tc = 9.6, 6.3, 3.3 K for A= K, Rb, Cs. While the origin of their superconductivity wasstudied intensively, other interesting features were also
revealed: a localized mode of the alkali ion in all the threecompounds and a mysterious first-order phase transition in
KOs2O6 below Tc. An alkali ion is located in an oversizedcage made of Os and O (See Fig. 1). Due to weak bonding
between the alkali ion and the cage, it is expected that thealkali ion vibrates independently of the cage. Hiroi et al. [5]pointed out the existence of this independent vibration (localized mode) macroscopically through specific heat measurements. The first-order phase transition was also discovered
by Hiroi et al., and a small structural change has been found.
Now we speculate that anharmonicity of the localized mode
plays a significant role in the phase transition.
In order to identify the localized mode microscopicallyand to evaluate its anharmonicity, we performed neutron
scattering experiments. Inelastic neutron scattering (INS)experiments and neutron powder diffraction (NPD) experiments were performed on AOs2O6 (A=K, Rb, Cs) powder
samples using the ISSP triple-axis spectrometer PONTA andthe IMR powder diffractometer HERMES installed at the
JRR-3 reactor in JAEA. In INS experiments, we successfully
observed a low-lying peak around 6 meV, which indicatesthat the localized mode exists in all the three compounds(See Fig. 1). The values of the energy of the mode are

Fig. 1. Inelastic neutron spectra from AOs2O6. (open circle: A = K,closed square: A = Rb, open triangle: A = Cs) Inset: crystal structure ofthe β-pyrochlore superconductors AOs2O6.

Fig. 1. Schematic structure of copper rubeanate including watermolecules in pores.
Fig. 2. Temperature dependence of the atomic displacement parametersof AOs2O6. The solid curves are the calculated values of harmonic
oscillators with the energy determined from INS experiments.

consistent with those determined in the previous specific heat
experiments in RbOs2O6 and CsOs2O6 while disagrees in
KOs2O6. This discrepancy suggests strong anharmonicity ofthe mode in KOs2O6. In NPD experiments, we found thatatomic displacements of Rb and Cs ions are harmonic butthat of the K ion is quite anharmonic (See Fig. 2). Thus werevealed the existence of the localized mode in all the threecompounds microscopically and the strong anharmonicity ofthe vibration of the K ion.
References[1] S. Yonezawa, Y. Muraoka, Y. Matsushita, and Z. Hiroi, J. Phys.:
Condens. Matter 13, L9 (2004).
[2] S. Yonezawa, Y. Muraoka, Y. Matsushita, and Z. Hiroi, J. Phys.
Soc. Jpn. 73, 819 (2004).
[3] S. Yonezawa, Y. Muraoka, Y. Matsushita, and Z. Hiroi, J. Phys.
Soc. Jpn. 73, 1655 (2004).
[4] Z. Hiroi, S. Yonezawa, J. Yamaura, T. Muramatsu, Y. Matsushita,
and Y. Muraoka, J. Phys. Soc. Jpn. 74, 3400 (2005).
[5] Z. Hiroi, S. Yonezawa, T. Muramatsu, J. Yamaura, andY. Muraoka, J. Phys. Soc. Jpn. 74, 1255 (2005).
Authors
K. Sasai, K. Hirota, S. Yonezawa, Y. Nagao, J. Yamaura, and Z. Hiroi

Proton Dynamics in Highly
Proton-conductive Copper Rubeanate

protons since the incoherent neutron scattering cross sectionof H atom (81.67) is much larger than those of other atoms(e.g., C: 0, N: 0.49, O: 0, S: 0.01, Cu: 0.52). We used a highresolution pulse cold neutron spectrometer AGNES [2],which is installed by ISSP at C3-1 guide of JRR-3 (Tokai,
JAEA).
Figure 2 shows the temperature dependence of elastic
scattering intensities, reflecting mean square displacement of
H atoms, of both dry and hydrated CR. The data of bulk H2O
are also plotted for comparison. The data of dry CR and icelie on each straight line as predicted by the harmonic oscillator model. The intensity of ice decreased drastically at thefusion temperature (Tfus = 273.15 K) and the slope of thedata became steeper above Tfus due to diffusive motions ofwater molecules. We have found that CR hydrate exhibits
excess decrease of elastic intensity (bending of the straightline) and corresponding quasielastic neutron scattering(broadening of the elastic peak) above 200 K. Each quasielastic spectrum was fitted well by the combination ofLorentz and a Gauss functions. The momentum transfer (Q)dependence of elastic incoherent structure factors (EISF)
revealed that water molecules are not moving around the
crystal but localized in the pores. The Q dependence of the
HWHM of Lorentzian was reproduced by the jump diffusion
model as well as in bulk water. In Fig. 3, the logarithmic
self-diffusion constants D of bulk H2O and CR hydrate areplotted against inverse temperatures. It is of interest that theactivation energy of CR hydrate is much smaller than that ofbulk H2O. This result suggests that protons of CR hydrate

Yamamuro Group
It is a great and urgent subject in current material science
to develop high-performance proton conductors for hydrogenstorage and fuel cell technology. Copper rubeanate(CuC2N2S2H2) is a porous coordination polymer exhibiting
high proton conductivity at room temperature. Nagao et al.
[1] have revealed that this compound absorbs watermolecules depending on relative humidity (RH) and its
proton conductivity is enhanced by four decades. The
conductivity at RH=100% is 0.01 S/cm which is as large asthat of the best proton conductor known before. Figure 1
shows the schematic crystal structure of copper rubeanate
including water molecules inside the pores.
We have measured inelastic and quasielastic neutron
scattering data from copper rubeanate (CR) and its hydrated
sample (CR-3.6H2O) to investigate the mechanism of proton
conduction, especially the role of water molecules. Neutron
scattering method is quite effective for studying dynamics ofFig. 2. Temperature dependence of logarithmic elastic scattering intensities of dry and hydrated copper rubeanates and bulk H2O. The inset
shows the quasielastic spectrum observed at 340 K and the result offitting by Lorentz (blue curve) and Gauss functions.

ISSP Activity Report 2006

27

Fig. 3. Arrhenius plots of self-diffusion constants of copper rubeanate
hydrate and bulk water. The activation energies were obtained from theslopes of the fitted straight lines.

are carried by the Grotthuss mechanism (combination of
H3O+ reorientation and H+ hopping along O-H...O hydrogenbonds). We are now measuring the neutron scattering data ofa related compound CuC2N2S2(C2H4OH)2 and its hydrate
sample. This compound has more hydrophobic pore walls
than CR and so will provide further information on the roleof water molecules in the proton conducting process.
References[1] Y. Nagao, R. Ikeda, K. Iijima, T. Kubo, K. Nakasuji, and H.
Kitagawa, Synthetic Metals 135-136, 283 (2003).
[2] O. Yamamuro and Y. Inamura, Hamon 17, 85 (2007).
Authors
O. Yamamuro, R. Yonamine, Y. Nagaoa, and H. KitagawabaTohoku University
bKyusyu University

matics. Several computer programs have been developed toobtain magnetic representations recently. However, they are
not seamlessly combined with the diffraction data analysis.
Hence, to make a full use of the magnetic representation
analysis, we have started developing the representation
analysis and profile fitting code. At the present moment, wehave successfully developed the representation analysis code
called, and profile fitting code
, whichis specific to the time-of-flight diffractometer LAM-80D
installed at the KENS spallation neutron source, KEK, Japan.
As an example of their applications, in what follows we will
present our recent spin-structure analysis on the ternary rareearth compound CeB2C2.
Space group of CeB2C2 is known to be tetragonal P4/mbm;
the Ce atoms occupy the 2(a) site with the site symmetry C4h [2].
This compound shows a magnetic order at TN = 7.3 K, where
an incommensurate spin structure was inferred [3]. The
details of the spin structure have not been elucidated to date.
We have performed the neutron powder diffraction experiment using LAM-80D, in a hope that this will confirm ourapproach to the diffraction analysis.
Shown in Fig. 1 are the resulting neutron diffraction
patterns in a temperature range spanning TN. As seen in the
figure, new Bragg peaks appear in the diffractogram below
TN. The magnetic reflection positions were successfully
indexed by using the magnetic modulation vector of q =(δ, δ, δ') where δ = 0.167(3) and δ'=0.114(3).
The magnetic representation vectors for the Ce sites wereobtained for the given q vector (and symmetrically equivalents); of total six representation vectors, we try to find thesimplest combination that reproduces the observed diffraction pattern. As a result, a good coincidence between theobserved and calculated profiles is found for a combinationof IR2:1 + IR2:2. The basis vectors for the IR2:1 are a2:1,1 =(1, 0, 0) for the Ce (0, 0, 0) site, and a2:1,2 = (0, e2πiδ, 0) for
the Ce(1/2, 1/2, 0) site, whereas those for the IR2:2 are a2:2,1= (0,1,0) and a2:2,2 = (e2πiδ, 0, 0). The resulting spin structure

Representation Analysis Approach for
Magnetic Structure Determination

350
C e11B2C2 Powder LAM-80D
300

Sato Group28

ISSP Activity Report 2006

7.4K

250

6.9K
6.3K

200
Intensity (a.u.)

A salient feature of neutrons is their magnetic-dipole
interaction with spins in magnetic materials; magnetic
scattering of neutrons is a direct consequence of the interaction. Hence, in principle spin structure may be determinedby analyzing neutron scattering data. A key for successful
spin-structure determination is selection of initial spin-structure models. However, to date this is largely based on
experiences using a so-called trial-and-error method, whichmakes spin-structure analysis non-trivial for non-specialists.
Therefore, a systematic method for the construction of theinitial models is highly desired.
Magnetic representation analysis, introduced by Izyumov
et al. [1], provides such a systematic way for the initial structure selection. In this method, an initial model may beobtained by a linear combination of a selected number ofirreducible magnetic representations of the paramagnetic
phase; coefficients in the linear combination automatically
become fitting parameters, instead of spin directions Sx, Sy,
and Sz with a priori constraints as assumed in usual spin
structure analysis.
Although the mathematical theory of the magnetic representation analysis was constructed more than 20 years ago, it
has not been widely used because of its cumbersome mathe-

8.0K

5.8K

150

5.2K
4.7K

100

Iobs - Icalc

50
1:1or1:2
2:1or2:2
1:1+1:2
1:1-1:2
2:1+2:2
2:1-2:2

0
-50
-100
30

32

34

36
38
40
T ime of flight (ms)

42

44

46

Fig. 1. Neutron diffraction patterns in Ce11B2C2 obtained using LAM80D. In the lower part of the figure, the peak positions and intensities
are schematically shown for several spin structures defined by linear
combinations of the irreducible representations IRn:m.

is sinusoidally modulated with the spin polarization along[110] direction. It should be emphasized that the validity ofthe spin structure is not quantitatively claimed but only
qualitative comparison can provide evidence for thepresently proposed structure; as seen in the lower part of the
figure, only IR2:1+IR2:2 structure can reproduce theobserved peak positions.
As demonstrated here, the representation analysis is avery powerful tool for the spin structure determination. We
are now working on the application of the presently developed codes for reactor based diffraction data.
References[1] Yu. A. Izyumov, V. E. Naish, and R. P. Ozerov, Neutron
Diffraction of Magnetic Materials (Consultants Bureau, New York,
1991).
[2] T. Onimaru et al., J. Phys. Soc. Jpn. 68, 2287 (1999).
[3] K. Ohoyama et al., J. Phys. Soc. Jpn. 72, 3303 (2003).
Authors
T. J. Sato

Multiple Ground State Competition
under Pressure in β-Sr0.33V2O5
Y. Ueda Groupβ-vanadium bronzes are a member of vanadium bronzes
with a chemical formula AxV2O5 in which the stoichiometriccomposition is x = 0.33 (1/3). There are six compounds of βA0.33V2O5 (A = Li+, Na+, Ag+, Ca2+, Sr2+, Pb2+). The
monoclinic crystal structure consists of a characteristic
V2O5-framework and A-cations. The A-cations are located in
the tunnels formed by the V2O5-framework and occupy only
one of the two nearest-neighboring A-sites within the a-cplane in a zigzag-type ordered manner along the b-axis. The
V2O5-framework is formed by sharing the corners and edgesof (V1)O6, (V2)O6 and (V3)O5 polyhedra, where V1, V2 andV3 are the three crystallographically independent vanadium
atoms. Each polyhedron forms three kinds of infinite chain
along the b-axis (the conductive direction in these quasi-onedimensional conductors). Among six compounds, allcompounds except Pb-compound exhibit metal-insulator(MI) transitions with the nature of charge order (CO) at
independent temperatures, accompanied by the formation ofsuperstructure with a common 6-fold periodicity (qb = 1/6)
along the b-axis. This 6-fold periodicity comes from 2-foldby A-cation order multiplied by 3-fold by CO. The groundstate of insulating phase is Néel ordered state in A+compounds and spin-gapped state in A2+-compounds except
Pauli-paramagnetic Pb-compound. We have revealed that the
CO transitions are suppressed by hydrostatic high-pressure
and that the superconductivity appears in A+-compounds,
while in A2+-compounds no superconductivity is observed.
Very recently, magnetic susceptibility (χ) and resistivity(ρ) of β-Sr0.33V2O5 were successfully measured under highpressure [1]. Under ambient pressure, β-Sr0.33V2O5 exhibitsa sudden increase of χ at CO transition temperature (TCO =150 K) and below the temperature it shows a low dimensional spin gap behavior with a broad maximum around 50
K. With increasing pressure, TCO is gradually lowered andsome hump in χ-T curve appears around 50 K and 0.5 GPa in
addition to the jump at TCO. This hump grows as pressure
further increases and then χ-T curves around 0.6 GPa clearlyshows three transitions around at TCO = 140 K, Td = 65 K

Fig. 1. Pressure-temperature (P-T) phase diagram of β-Sr0.33V2O5. The
filled and open symbols are derived from magnetic susceptibility andresistivity measurements, respectively. The colored stars represent thesuperperiodicity with qb = 1/2 (2b), 1/6 (6b) and 1/10 (10b) observed in
x-ray oscillation photographs.

and Ti = 40 K. These are regarded as the transitions between
the different electronic states with various lattice modulations such as qb = 1/6 and 1/10, which have been confirmedby x-ray diffraction under high-pressure. With increasingpressure, both TCO and Ti decrease, while Td increases andmerges with TCO around 1.1 GPa. In addition to these threeanomalies in the CO phase, a kink both in χ-T and ρ-Tcurves was observed at around Tm = 140 K in the charge
disordered (CDO) phase (the metallic phase). The transition
might be an antiferromagnetic (AF) ordering, although there
is no direct evidence. Very similar behaviors of χ-T curves
under pressure have also been observed in β-Ca0.33V2O5.
Furthermore the χ-T curve of β-Pb0.33V2O5 shows ananomaly around 60 K above 0.4 GPa, which suggests any
magnetic transition. Thus we can give a speculation for theabsence of superconductivity in A2+-compounds, namely AF
order is realized before the system falls into superconductivity. The obtained P-T phase diagram (Fig. 1) displays thecompetition among multiple ground states under pressure.
References[1] T. Yamauchi, H. Ueda, J. Yamaura, and Y. Ueda, Phys. Rev. B 75,
014437 (2007).
Authors
T. Yamauchi, H. Ueda, J. Yamaura, and Y. Ueda

Search for Missing Link
– Binary Vanadium Oxide V4O9 –Y. Ueda Group
Vanadium atom takes various oxidation states, which
gives us rich variety of binary vanadium oxides. In addition
to V2O3, VO2 and V2O5 containing vanadium atoms withinteger oxidation states, many mixed-valent compounds,
Magnéli phases VnO2n-1 (n = 3-9) between V2O3 and VO2,and VnO2n+1 (n = 3, 6) between VO2 and V2O5, are reported.
Most of them have been intensively studied in the viewpointof correlated electron systems, especially of their metal-

ISSP Activity Report 2006

29

Fig. 1. Temperature dependence of magnetic susceptibility (χ) for
V4O9. χ(T) is obtained by subtracting Curie term and temperature
independent term from the raw data (χraw). The inset shows a crystal
structural model of V4O9.

insulator transitions.
In VnO2n+1, V4O9 with a mixed-valence state of V4+/V5+= 1, has been missing. We have succeeded in synthesizing
single phase powder samples, solving its crystal structure,and elucidating physical properties of V4O9. Though V4O9 iseasily decomposed during its synthetic process, we found aproper condition to synthesize single phase V4O9 samples
from V2O5 by reduction controlled precisely. By combining
x-ray, neutron, and electron diffractions, a crystal structure
model was built and refined. Different from other binary
vanadium oxides, the crystal structure of V4O9 has VO4tetrahedra, indicating the existence of definite V5+ sites.
Remaining highly distorted VO6 octahedra containing V4+
form ladders, which are bridged by VO4 tetrahedra, as shownin Fig. 1.
Reflecting the crystal structure, V4O9 is an insulator.
Magnetic susceptibility behavior is of low dimensional
magnetism as shown in Fig. 1, suggesting the existence ofspin gap ∆/kB ~ 70 K. Magnetization curve measured up to
70 T also indicates a spin gap ground state. Magnetic excitation energy measured by inelastic neutron scattering using
powder samples is about 8 meV, which is consistent withthat from magnetic measurements. Momentum dependence
of inelastic neutron scattering data reveals a weakly interacting spin dimer system.
Compared with a famous spin gap system (VO)2P2O7,
V4O9 has a similar structure with VO4 tetrahedra instead ofPO4 tetrahedra in (VO)2P2O7. We can expect almost thesame physics behind these two systems.

Fig. 1), which is now called the post-perovskite structure,comprises IrO3 layers and eightfold-coordinated Ca2+ ions in
between. Within each plane, IrO6 octahedra are connectedwith each other by sharing edges and corners along the aand c-axes, respectively, giving rise to a rectangular lattice of
Ir4+ ions with the t2g5 electron configuration (S = 1/2). This
electronic state well resembles that of the parent compoundsof high-Tc cuprates. Turning our attention to this similarity,
we introduced mobile carriers into CaIrO3 by substituting
Na+ for Ca2+ [3].
Figure 1 represents the electronic phase diagram forCa1-xNaxIrO3 (0 ≤ x ≤ 0.37) within a temperature-Na content(T-x) plane. The parent compound (x = 0) is a Mott insulator,
which undergoes the antiferromagnetic transition at 115 K.
Upon carrier doping, the resistivity (ρ) gradually decreases
until a metallic state characterized by dρ/dT < 0 is eventuallyachieved at x ~ 0.30. At this composition, the magnetic
ground state simultaneously changes from the antiferromagnetic to paramagnetic one. The absence of an intervening
antiferromagnetic-metallic phase between the antiferromagnetic-insulating and paramagnetic-metallic phase markedly
contrasts with many metal-insulator transition (MIT) systems
in three dimensions; instead, this resembles high-Tc cuprates
with a quasi-two-dimensional lattice. However, superconductivity is indiscernible in the vicinity of the MIT; this is
likely related to the fact that a doped carrier forms a spin
triplet state in the present system (left inset of Fig. 1), in
contrast to the Zhang-Rice singlet in high-Tc cuprates.
The metallic state on the verge of the MIT shows some
intriguing properties that cannot be understood in the framework of the Fermi liquid theory. The ρ ∝ Tα (α = 1.2)
relation indicates presence of the antiferromagnetic fluctuation; this actually manifests itself in the strongly T-dependent
susceptibility. The ρ shows lnT divergence at low Ts
probably owing to the weak localization effect in two dimensions. The T dependence of the orbital magnetoresistance is
so strong that Kohler's rule does not hold in the present
system. It should be noted that these three properties are also
observed in high-Tc cuprates, and their origins are still under
debate. The observation of these phenomena in Ca1-xNaxIrO3
offers a solid foundation to unravel anomalous electronic
properties in the normal state of high-Tc cuprates.

Authors
S. Yamazaki, H. Ueda, M. Isobe, and Y. Ueda

Metal-Insulator Transition in Ca1-xNaxIrO3
with Post-Perovskite Structure
Y. Ueda and Yagi Groups
CaIrO3 with Cmcm orthorhombic symmetry captures
great geophysical interests, since the isostructural MgSiO3 isconsidered to be the main constituent of the D'' region in thelowermost mantle [1, 2]. The crystal structure (right inset of30

ISSP Activity Report 2006

Fig. 1. The electronic phase diagram for Ca1-xNaxIrO3 (0 ≤ x ≤ 0.37)
within a temperature-Na content (T-x) plane. Left inset: Schematic viewof the electron configuration among the t2g orbitals. One doped carrier
denoted by an open circle forms a triplet (high-spin) state with thepreexisting carrier. Right inset: Post-perovskite structure composed of
IrO3 layers, and Ca2+ and Na+ ions in between.

References[1] M. Murakami, K. Hirose, K. Kawamura, N. Sata, and Y. Ohishi,
Science 304, 855 (2004).
[2] N. Miyajima, K. Ohgushi, M. Ichihara, and T. Yagi, Geophys. Res.
Lett. 33, L12302 (2006).
[3] K. Ohgushi, H. Gotou, T. Yagi, Y. Kiuchi, F. Sakai, and Y. Ueda,
Phys. Rev. B 74, 241104 (2006).
Authors
K. Ohgushi, H. Gotou, T. Yagi, Y. Kiuchi, F. Sakai, and Y. Ueda

Computational Experiment on
Field-Cooled Magnetization Processes
in Ising Spin-Glass Model
Takayama GroupOne of most fundamental problems yet unsettled in spinglass (SG) studies concerns the stability of the equilibrium
SG phase under a finite magnetic field h. By experiments at
an early stage of the study, the field-cooled magnetization(FCM), the magnetization measured while a spin glass isslowly cooled under h, was observed to exhibit cusp-likebehavior at a certain temperature Tcusp. This FCM behavior
is roughly consistent with the prediction of the SG meanfield theory with Tcusp being interpreted as the criticaltemperature of the SG transition under h. On the other hand,the droplet theory, which phenomenologically describes
properties of spin glasses with short-range interactions,predicts that the SG order longer than a certain length scaleLh, called as the field-crossover length, is destroyed by h.
Since Lh is finite for any finite h, it implies the absence of the
SG phase under h.
We have been attacking this fundamental SG problem bymeans of numerical simulations, or computational experiments, on the Ising SG model. In our previous work on thefield-shift aging protocol, we obtaind strong support for thepresence of Lh and so the absence of the SG phase underh [1]. Recently we have carried out simulation on FCM
processes by means of a standard Monte Carlo (MC) method[2]. The results are shown in Fig.1. The cooling rate isindicated by rn, namely, the system is cooled in temperature
steps of 0.01J, while it is held at each temperature for a time,
tsty=n MC steps. The FCM with slow cooling (large n)
exhibits, with this enlarged scale of the ordinates, a peak at a
certain temperature T*. Numerically, we can measure theFig. 1. Field-cooled magnetization (FCM) and equilibrium magnetization (EQM) of the 3-dimensional Ising SG model with random nearest
neighbor interactions which obey the Gaussian distribution with mean
zero and variance J. Its SG transition temperature under h=0 in thethermodynamic limit is estimated as 0.95J. The field strength is represented by the corresponding Zeeman energy in unit of J.

Fig. 2. Halted FCM after slow and rapid cooling (sl-HFCM and rpHFCM), and EQM at Thlt below T*. The QUFM (QUPM) is the time
evolution of magnetization after instantaneous quench from completely
polarized (random) states. In QUFM we can see its initial decrease up
to a value smaller than EQM as well as its upturn to EQM.

equilibrium magnetization (EQM) by making use of
methods, such as the replica MC method, by which we artificially accelerate the relaxation to equilibrium. Interestingly,
FCM exhibiting a peak at T* is smaller than EQM in a whole
range of T simulated. Much more interesting phenomena areobserved when the cooling process is halted at a temperature
denoted as Thlt. For Thlt > T*, the halted-FCM (HFCM)
relaxes smoothly to EQM as expected. For Thlt < T*, on theother hand, HFCM after slow cooling changes in the
opposite direction to EQM as shown in Fig. 2 .
The peak structure in FCM and the peculiar relaxation
process of HFCMs are observed also by real experiments on
Ising spin glass Fe0.55Mn0.45TiO3 [3] and other spin glasses,
though EQM is not measured since the equilibration cannot
be completed within the laboratory time window. A key idea
to interpret these intriguing FCM phenomena is that, in states
far from equilibrium, individual spins are thermally fluctuating under effective fields from their nearest-neighboring
spins (plus h) without knowing their preferred direction in
the destination, i.e., in equilibrium. At T higher than T*, SGshort-range clusters grown in regions with less frustration are
thermally blocked in a limited time scale of tsty. As T isfurther decreased, clusters of smaller sizes become thermally
blocked, and at T around T* these clusters (of mean linear
size Lcl) become in touch with others. Then spins on cluster
boundaries feel new effective fields and they tend to align tothe direction which increases the SG short-range order andso decreases FCM. This is our scenario [3] to interpret FCM
decrease at T < T*and so a FCM peak at T*. The FCM as wellas HFCM continue to decrease so long as Lcl is significantly
smaller than Lh. Our scenario ends up with speculation that,after the mean size of SG cluster reaches Lh, the equilibrationof an assembly of SG clusters of mean size Lh starts to occur,
making HFCM increase to EQM. Actually, such upturn of
HFCM is observed in real experiments at Thlt below but very
close to T* [3].
References[1] H. Takayama and K. Hukushima, J. Phys. Soc. Jpn. 73, 2077
(2004).
[2] H. Takayama and K. Hukushima, J. Phys. Soc. Jpn. 75, 013702(2007).
[3] P. E. Jönsson and H. Takayama, J. Phys. Soc. Jpn. 74, 1131 (2005).
Authors
H. Takayama and K. Hukushimaa

aUniversity of Tokyo

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31

Asymptotic Dimensional Reduction
near a Quantum Critical Point
Kawashima GroupThe principal effect of frustration is cancellation among
competing couplings. In quasi-two-dimensional systems,
when the frustration exists in inter-layer couplings, it tends toenhance independence of each layer. At finite temperature,
however, this cancellation can never be perfect so as to makethe system purely two-dimensional because thermal fluctuation generates effective net interlayer couplings. Even at zerotemperature, the geometric frustration fails to decouple
layers completely since the zero-point fluctuations restore
the inter-layer coupling as discussed by Maltseva andColeman. As a result, the critical phenomena in any realquasi-two-dimensional system belongs to the universality
class of some three dimensional theory no matter how
perfectly the frustration may seem to cancel the couplings.
However, we find that this does not necessarily apply to thequantum critical point [1]. Namely, as one approaches to thequantum critical point, the fluctuation that introduces effective coupling dies out and pure two-dimensional behaviors
dominate the system. The first experimental evidence of thisphenomenon was found by measuring critical exponents of afield induced QCP in BaCuSi2O6 [2].
BaCuSi2O6 is a spin dimer system whose highly
symmetric crystal structure gives it unique advantages fortackling the fundamental role of dimensionality in the fieldof quantum criticality. The material consists of layers of spin
dimers stacked on top of each other. Each layer is a squarelattice. However, the dimer plaquettes are staggered between
consecutive layers, as shown in Fig. 1. This leads to geometrical frustration of the inter-layer antiferromagnetic interaction. That is, the antiparallel arrangement of spins within andbetween layers conflict. We consider quasi-two-dimensionalanti-ferromagnetic Heisenberg model.

Here, the first term represents the intra-dimer couplingwhereas the second and the third terms the inter-dimer
couplings. The second term is the coupling within a layer
and the third the inter-layer coupling.
We study the quantum critical point by using the spinwave treatment and then examining magnon excitations [1].

Fig. 2. [top] Phase boundary near the QCP measured in BaCuSi2O6 [2]compared to the curves obtained from a MC simulation [1]. [bottom
left] Similar comparison of the magnetization for T = 30 mK andvarying H, and [bottom right] for H = Hc and varying T.

The effective coupling between layers turned out to be
proportional to the number of excited magnons, which meansthat the fluctuation responsible for the effective inter-layer
coupling dies out as we approach zero temperature and thequantum critical point can possess purely two dimensional
characteristics. Interactions between either thermally excitedor quantum condensed bosons induce a crossover to d = 3
away from the QCP. Key to this result is the observation thatzero-point phase fluctuations of the order parameter are
suppressed near a chemical potential tuned BEC. The twodimensional value of the exponent characterizes the phaseboundary near the zero temperature, in agreement with theexperiment [2]. The agreement between the pure two-dimensional behavior and the real experiment is not only qualitative, but it is quantitatively good as can be seen in Fig. 2. The
upper panel shows the critical field measured in the experiment on BaCuSi2O6 plotted with the phase boundary
obtained by quantum Monte Carlo simulation assuming thatthe inter-layer coupling is zero (filled circles) . Both theresults show the two-dimensional quantum criticality that ischaracterized by the asymptotic linear dependence. The
correspondence is further confirmed by other quantities: thefield-dependence of the magnetization at zero (or very low)temperature (lower left panel), and the temperature-dependence of the magnetization at critical magnetic field (lower
right panel).
References[1] C. D. Batista, J. Schmalian, N. Kawashima, P. Sengupta,
S. E. Sebastian, N. Harrison, M. Jaime, and I. R. Fisher, Phys. Rev.
Lett. in press.
[2] S. E. Sebastian, N. Harrison, C. D. Batista, L. Balicas, M. Jaime,
P. A. Sharma, N. Kawashima, and I. R. Fisher, Nature 441, 617 (2006).

Fig. 1. A schematic diagram representing the body-centered tetragonal
BaCuSi2O6 crystal lattice. A dot represent a dimer formed from two
Cu2+ S=1/2 spins. It is apparent that this lattice structure leads togeometrical frustration.

32

ISSP Activity Report 2006

Authors
C. D. Batistaa, J. Schmalianb, N. Kawashima, P. Senguptaa,b, S. E.
Sebastiand,e, N. Harrisonc, M. Jaimec, I. R. Fisherd, L. Balicasf, and P.
A. Sharmac
aTheoretical Division Los Alamos National Laboratory
bIowa State University and Ames Laboatory
cMPA-NHMFL, Los Alamos National Laboratory
dStanford University
eUniversity of Cambridge
fNational High Magnetic Field Laboratory (USA)A New Record of Highest Magnetic
Field, 630 T Generated by theElectro-Magnetic Flux Compression
Takeyama GroupWe have recently improved the primary coil of theelectro-magnetic flux compression (EMFC) system, allowing
it to generate a higher magnetic field with less energy injection and involving a more simplified coil preparation
processes. Thus, we have renewed a record of generating
highest magnetic field to 630 T.
The new coil system has a bent copper sheet inside a
steel coil (Fig. 1). In the new coil system, the copper sheet
bears the main primary current flow. The shape of the copper
sheet is specially designed to improve the electric contact
with the electrode of the current collector plate from thecapacitor bank. The role of the steel coil differs from that in
the old type ones. The steel coil in the new type is designed
to provide sufficient mass to avoid deformation of the copper
sheet during the liner acceleration.
Figure 1 shows the field pulses generated by EMFC,
showing new world record achieving 630 T. The trace of theprevious record is presented for a comparison, where the“feed-gap compensator” was adopted, and 622 T has beenachieved as the indoor world record [1]. Figure 2 shows ahigh-speed photograph of the liner implosion. The cylindrical symmetry of the liner implosion was improved. The
most important factor in generating high magnetic field byflux compression is the velocity of the liner. The velocity of
the liner has also been improved to 2.4 km/s from the old

Fig. 3. The image plot of the calculated temporal evolution of thecurrent density and the temperature distribution in iron (a,b) and copper(c,d), respectively. The injected current J(t) used in the calculation isshown above each image plot.

record; 2.0 km/s. In the new coil system, the efficiency ofthe electric-magnetic energy transfer was estimated to beabout 11 %, again much improved from the old one with
about 5 %. We could have thus improved the efficiency ofthe electric-magnetic energy transfer more than two times.
It is considered that this arises from a difference in thecurrent density distribution between the new coil and the oldone. More specifically, it originates from the difference inthe skin-depth. We have conducted the current density calculation from a combination of Maxwell equations and the heat
conduction taking account of the Joule heating,,.
Here, d denotes the skin depth which is a length representing
a current density distribution.
.

Fig. 1. The traces of magnetic field recently achieved by the EMFC.
The background is a photograph of the new coil. 630 T is the new indoor world record achieved by the new coil.

Fig. 2. A high-speed photograph of the liner implosion process taken bya high-speed camera. (a) An implosion using the new coil, (b) An
implosion using the old coil equipped with a feed-gap compensator.

Here, cp is the specific heat, D is the density, λ is the thermal
conductivity and T is the temperature. We calculate thecurrent density distribution and the temperature distribution
in the case when the external current J(t) is injected to themetal plate. The results of calculation are shown in Fig. 3.
Fig. 3(a) and (c) show the temporal evolution of the current
density inside the primary coil made of iron and copper,
respectively. The upper part of Fig. 3(a)(c) is an illustrationof the current J(t) used in the calculation. Figures 3 (b) and(d) show the temporal evolution of the temperature distribution. It can be seen both from Fig. 3(a) and (c) that thecurrent gradually penetrates deep into the bulk from the innersurface although the current is initially concentrated near thesurface. As the time evolves, the peak of the current density
also moves inside into the material. It can be seen in Fig.
3(b) or (d) that the temperature near the surface is muchhigher than the temperature inside the material. This causes
the increase of resistivity near the surface and the decrease of
the current density there. Thus, the peak of the current
density gradually moves inside. Therefore, the diffusion of
the current density in the iron coil is more prominent thanthat in copper one.

ISSP Activity Report 2006

33

The liner is accelerated within the initial 30 µs according
to the pictures shown in Fig. 2. The current density distribution within this moment plays a crucial role on the liner
motion and the symmetrical shape during the implosion. The
effective feed-gap in the new coil is decreased to 6 mm at
30 µs from 15 mm of the old iron one. It is considered thatthe difference of the current density distribution between ironand copper as shown above resulted in the difference of the
shape of the liner during the implosion in Fig. 2. A roughinsight into the current density distribution in our new
primary coil has been obtained by our simple calculation.
References[1] Y. H. Matsuda et al., Rev. Sci. Inst. 73, 4288 (2002).
Authors
S. Takeyama, E. Kojima, and H. Sawabe

Metamagnetic Transition to
Poor Conductor in BaVS3
Kindo GroupIn the hexagonal perovskite-type sulfide BaVS3,
consisting of one-dimensional (1D) spin chains with S = 1/2
on V4+ along the c axis, several electronic instabilities and aseries of symmetry-breaking have been found in spite of, at afirst sight, simple structural and electronic characteristics atroom temperature. This is due to the interplay among spin,
orbital and charge degrees of freedom accompanied by spin
frustration, orbital degeneracy, low dimensionality, strong
covalency of sulfur, etc. The characteristic temperatures andthe successive phase transitions are summarized as follows:(a) TS = 240 K, structural transition from hexagonal toorthorhombic, (b) TMI = 70 K, metal-insulator (MI) transition
with superlattice formation, (c) TX = 30 K, incommensurate
magnetic ordering. In this report, we present high-field
experiments of single crystalline BaVS3 up to 70 T, which
reveals a metamagnetic transition to a field-induced ferromagnetic state. A field-temperature phase diagram is
proposed for BaVS3 [1].
The high-field magnetizations up to 70 T have beenmeasured by using non-destructive pulse magnets. Figure 1
shows magnetization curves at 4.2 K measured in the fieldparallel and perpendicular to the c axis. The magnetization
increases linearly up to around BM ~ 50 T, and exhibits an

Fig. 1. High-field magnetization curves at 4.2 K in B parallel andperpendicular to the c axis. The low-field part at B < 20 T is magnified
in the inset. A slight change in the curvature is seen around 10 T for
both directions.

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ISSP Activity Report 2006

Fig. 2. B-T phase diagram of BaVS3. AFI and PMI represent theantiferromagnetic insulator and the paramagnetic insulator, respectively. Broken and dashed-dotted lines indicate expected phase boundaries.

abrupt jump with a large hysteresis of about 8 T. At B > BM,
the magnetization shows a plateau with the magnitude ofabout 0.4 µB/f.u., which is approximately half of the saturation magnetization expected for V4+ (3d1) with spin S = 1/2.
A tiny anomaly, i.e., the slight deviation from the linear
relation, is observed around BX ~ 10 T for both directions.
Temperature dependences of field-induced (FI) anomalies
are summarized as the B-T phase diagram shown in Fig. 2.
Being different from the appreciable temperature dependenceof the high-field transition point, the low-field anomaly
shows a weak temperature dependence and suddenly disappears at TX. We believe that the metamagnetic transition at
BM is not of classical spin flip but corresponds to an essentialchange in the electronic state, because (1) the anisotropy inthe transition field is weak, (2) the transition is accompanied
by a large hysteresis, suggesting strong spin-lattice coupling,and (3) the transition is observed even in the spin disorderedstate at TX < T < TMI. Although one may expect that the FI
transition at BM corresponds to the temperature-induced MI
transition, the nature in the field above BM is much different
form that at T > TMI. The moment appeared above BM isnearly half of S = 1/2, which is comparable to the orderedmoment 0.5 µB/f.u. estimated from neutron experiments atzero field [2]. This means that, in contrast to the spin reduction at TMI, the spin retains the magnitude through both thetransitions at BX and BM.
Recently, Kézsmárki et al. [3] proposed that 1D metallic(good) transport of A1g electrons is superimposed on theisotropic hopping (bad) conduction of localized Eg electrons.
One of the possible explanations of the metamagnetic transition at BM is that the gapped A1g state is not affected appreciably by the field of 50 T, while the Eg orbitals are reconstructed to a quasi-degenerate state by the application of the
field. To see the nature of the BM-TMI boundary, we investigated a field dependence of the conductivity and its
anisotropy at 35 and 60 K. The experiment reveals that only
the resistivity in the c plane decreases at BM, however, thereduction is not huge like a MI transition, suggesting that the
FI transition is not a pure MI transition. It is, therefore,reasonable to conclude that the FI state is a poor conductor
only in the Eg bands. This interpretation also intimates thepresence of another phase boundary to a real metallic state athigher fields (broken line in Fig.2). The inside of the BX-TX
boundary should be a long-range-ordered antiferromagnetic
insulating (AFI) phase. It should be noted, however, that theanomaly at BX is faint, and that the entropy is little consumed
at TX. These results indicate that the strong antiferromagnetic
correlation persists even in a paramagnetic insulating (PMI)state outside the BX-TX boundary. On the other hand, it ishardly acceptable that the external field induces the dynamic
PMI state from the AFI state. The width of dM/dB peak at
BM is, in fact, broadened markedly above TX, suggesting thepresence of another unidentified phase boundary or
crossover, which discriminates static and dynamic states(dashed-dotted line in Fig. 2).
In conclusion, we have established the B-T phase
diagram of BaVS3. Being different from the metallic state
above TMI, the FI state above BM is probably a poor
conductor, where the conductivity is recovered only in the Eg
bands and the gapped A1g state remains open. The detectionof the new phase boundary to the real metallic state remains
for further studies.
References[1] Y. Narumi et al., J. Phys. Soc. Jpn. 76, 013706 (2007).
[2] H. Nakamura et al., J. Phys. Soc. Jpn. 69, 2763 (2000).
[3] I. Kézsmárki et al., Phys. Rev. Lett. 96, 186402 (2006).
Authors
Y. Narumi, K. Suga, K. Kindo, T. Yamasakia, M. Shigaa, and H.
Nakamuraa
aKyoto University

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