ISSP
Activity Report
2004
Contents

Pages

Preface1

Research Highlights

2 - 27Highlights of Joint Research

28 - 44

ISSP Workshop

45 - 47

Proposal for Joint Research48 - 79

Publications

80 - 116

ISSP Activity Report 2004

Preface

The ISSP was established in 1957 based on the recommendation of the National Council of Japan to set up a new institutefor condensed matter physics. The idea was to establish a topclass research institute for experimental and theoretical studies of condensed matter and to promote cooperative researchin the field throughout the country. Soon after the inauguration, the name of ISSP became recognized world-wide in thecommunity of condensed matter physics.
Since then there have been two major changes in the historyof the institute. The first was the reorganization of the institutein the mid-eighties. The system of divisions consisting of several groups was introduced andthe project of the "Physics in extreme conditions", which includes Ultra-High Magnetic Fields,
Laser Physics, Surface Physics, Ultra-Low Temperatures, and Very High Pressures, was started. The
second major change was the relocation of the ISSP to the new Kashiwa campus which wascompleted in 2000.
To make the status of the research activities in the ISSP public, new forms were introduced in 2001:
Activity report A for research mostly by the staff of the ISSP and Activity report B for joint research
conducted by the ISSP. This copy is the Activity Report for the fiscal year 2004 in the style of single
volume for all the research activities at the ISSP. The system of national universities in Japan
has made a profound change from April in 2004 by the so called "houjinka", which means thatnational universities, once directly belonged to the ministry of education, have become independentadministrative entities. In the new scheme importance of the Activity Report has been increased.
We are happy to receive any comments on the Report for the possible improvement of the research
activities of the ISSP.

June 18, 2005

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

ISSP Activity Report 20041

Research Highlights"Expansion" of SiO2 Cage Under Pressure
Yagi Group
Many elements and molecules, such as Si, Ge, and H2O,
form clathrate compound, which is composed of cages andincluded host molecules. Numerous studies have been madeto clarify the physical property of these clathrates, because
they exhibit a variety of physical properties depending on the
combination of cage material and the host molecules.
Moreover, one of the H2O clathrates, methane hydrate, isknown as a key material for the energy resource of the next
generation and environmental science of the Earth, and thehigh pressure property of methane hydrate has been studied
intensively to clarify its stability. Melanophlogite is a rare
natural mineral and has a clathrate structure. It is isostructural with type I gas hydrate and the chemical composition isexpressed as 2A : 6B : 46SiO2, where A and B are the hostmolecules in small and large cages, respectively. The cages
are formed by SiO2 network and various gases such as CO2,
CH4, and N2 are included in the cage. Little is known on its
high pressure behavior and we have studied the stability andcompression behavior of this Melanophlogite up to about
25 GPa.
Experiments were made using diamond anvil apparatus,
adopting helium as pressure transmitting medium. Helium isknown to maintain high hydrostaticity, even above its freezing point of 11 GPa at room temperature. Starting material
was a natural sample of melanophlogite from Fortullino,
Italy. The Raman spectrum measured on this sample
indicates that the main host gas of this sample is CO2, in
harmony with the previous report [1]. For some experiments,
this sample was heated in air to 650cC for 7 hours to evacuate the gas from the cage. The intensity of the Raman peak of
CO2 has reduced considerably after this heat treatment, while
no change was observed in the Raman peak of SiO2 network.

Fig. 1. Powder X-ray diffraction patterns of Melanophlogite withincreasing pressure. At 17.5 GPa, new peaks appeared in the low angle
region of each peak and then the original peaks have disappeared at
higher pressures, indicating that isostructural transition has occurred.

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ISSP Activity Report 2004

Fig. 2. Volume compression curve of heated and unheated sample ofMelanophlogite. No phase transition was observed in unheated sample
while sudden increase in volume was found at around 17 GPa in heatedsample. This sudden change in volume occurs reversibly in decreasingpressure cycle at about 11 GPa.

No meaningful change was found in the unit cell volume
also. Small piece of melanophlogite, cut into rectangularshape of about 50µm # 50µm # 30µm, was placed in thesample chamber together with a small piece of ruby crystal,
which works as a pressure marker. Then the chamber was
filled with helium and then compressed. X-ray experiments
were made at the BL13A station of the Photon Factory,
KEK, Tsukuba. Monochromatized synchrotron radiation of
about 30 keV energy was collimated to a thin beam of
30µm # 30µm and irradiated to the sample. Diffracted X-ray
was recorded using Imaging Plate detector. Typical exposure
time was 15 to 30 minutes. Pressure was measured using
ruby fluorescence technique.
Monotonous volume compression was observed up to
25 GPa when non-heated sample was compressed at room
temperature. The volume has decreased more than 20 % at
25 GPa. On the other hand, when the heated sample wascompressed, sudden increase in volume of about 10% was
observed at around 17 GPa. Fig. 1 is the X-ray diffraction
patterns obtained with increasing pressure. When the pressure was increased to 17.5 GPa, new diffraction lines haveappeared in the low angle region of each peak and then theoriginal peaks have disappeared. Diffractions of two phases
coexist in a small pressure range of less than 1 GPa. The diffraction pattern above this transition pressure was identicalwith that before the transition, except the unit cell volume.
This sudden change in volume occurs reversibly with large
hysteresis, and the volume reverts to original compression
curve during decompression at about 11 GPa.
This is a quite unique phase transformation in a sense that
it is an isostructural transition and that the volume increases
under pressure. It is very likely that helium, which was used
as a pressure transmitting medium, have intruded into thecage of melanophlogite and have expanded the volume,
although we do not have any direct evidence at this moment.

In another experiment, we have used CH4 as a pressure transmitting medium and in that case the unit cell volume wascompressed much faster and the crystal has changed into
amorphous below 10 GPa. Helium is much smaller compared to CH4 and probably it can penetrate through the SiO2
network of melanophlogite. The interesting point is that thissudden increase of volume occurs only when the heated sample was used as starting material. In other words, it occurs
when the original molecule in the cage has removed and"vacant" cage was compressed by helium. It is likely that
helium has intruded into the cage at low pressure and thesudden increase in volume is caused by the additional intrusion of helium. Further studies are required to get information about the number of atoms included in the cage underpressure. Present study provides interesting information on
the interaction of cage and host molecule in clathrate compounds.
Reference[1] H. Gies, Z. Krist., 164, 247-257(1983).
Authors
T. Yagi, E. Iidaa, H. Hiraib, N. Miyajima, and T. Kikegawac
a Chuo University
b Tsukuba Universityc Photon Factory, KEK

A New Puzzle about the Magnetization
Plateau in NH4CuCl3
Takigawa GroupIn some quantum spin systems, the magnetization
becomes quantized to a fractional value of the saturation fora finite range of magnetic field. Such magnetization plateaus
indicate Mott localization of bosons carrying the magnetization. When the boson density per unit cell is not an integer, aspin superstructure breaking the translational symmetry is
generally expected. However, this has been observed so far
only in SrCu(BO3)2, a frustrated 2D dimer spin system [1].
Another candidate is NH4CuCl3, a 3D-coupled dimer spin
system, which exhibits magnetization plateaus at 1/4 and 3/4
but not at 1/2 of the full saturation [2]. Since all Cu sites areequivalent in the monoclinic crystal structure (P21/c, Fig. 1),
the plateaus should be associated with symmetry breaking.
However, no evidence has been found for magnetic phasetransitions in the field range of the plateaus.
To explain this puzzle, Matsumoto proposed that three
types of inequivalent Cu dimers are formed due to yet unob-

Fig. 1. Crystal structure of NH4CuCl3 at room temperature (spacegroup P21/c) viewed along the a-axis. Zigzag spin chains run along thea-axis. Locations of the inversion center, the b-screw axes, and thec-glide planes are shown.

Fig. 2. Temperature dependence of (a) Kc* and (b) νc* for the four
inequivalent N sites in the low temperature phase at 7 T.

served structural transition [3] and they saturate successively
with increasing field. The plateaus appear when every dimer
is either in singlet state or completely saturated. Althoughseveral experiments support this model, microscopic originof the plateaus is still unknown.
We have performed 14N nuclear magnetic resonance(NMR) experiments at the field of 7 tesla where the
1/4-plateau appears at low temperatures [4]. Since 14N nuclei
have magnetic dipole and electric quadrupole moments, 14N
NMR is an excellent probe for both the crystal structure andthe spin density distribution. From NMR spectra we obtain
the quadrupole coupling να, which is proportional to theelectric field gradient along the field direction α, and themagnetic hyperfine shift Kα from Cu magnetization.
First we observed a small splitting of ν b below 160 K,
indicating a structural transition marginally breaking theb-screw and the c-glide symmetries but preserving the inversion. Another much more prominent line splitting occurs
below Tc=70 K. Figure 2 shows the four-fold splitting of Kc*
and νc*. While νc* changes rapidly near Tc, substantial variation of Kc* occurs only at low temperatures. This clearly
indicates that what happens at Tc is a structural phase transition, not a magnetic one, most likely associated with orientational order of the NH4 ions. This would modify theexchange scheme, yielding inequivalent Cu sites with distinct magnetic character. Thus our results support the modelby Matsumoto that plateau appears without further symmetry
breaking. Figure 2 also indicates a peculiar feature of doubletransition, which is not understood yet.
For general field directions both Kc* and νc* show eightfold splitting. Their angular dependences are approximately
symmetric about the ac-plane within two degrees, indicatingthat c-glide symmetry is still only marginally broken.
Meanwhile, R up egg et al. observed doubling of the unit cellalong the b-axis by neutron diffraction [5]. A unit cell of the
low temperature phase then contains eight Cu sites, which isthe same as the number of NMR lines for the general field
directions. Thus, the space group should be P1 and the inversion symmetry must be completely broken.

Fig. 3. Possible spin structure for the 1/4-plateau state. The small filled(open) circles indicate the triplet (singlet) Cu sites. Large circles show
the N sites. The dashed lines in (b) and (c) indicate the approximatec-glide planes preserved in each spin configurations.

ISSP Activity Report 20043

We now have ample evidence that in the 1/4-plateau 1/4
of Cu spins form fully polarized triplets while the rest remain
in singlet. The question then is where the triplets are. All previous studies assume the triplets on the nearest neighborpairs as shown in Fig. 3(a). This configuration, however, preserves the inversion but completely destroy the c-glide symmetry inconsistent with NMR results. To satisfy the approximate c-glide symmetry, the triplets must be formed over different chains as shown in Fig. 3 (b) and (c). Such configuration would require extremely peculiar exchange coupling
scheme. Our NMR results have posed a new puzzle about the
spin superstructure in NH4CuCl3.
References[1] K. Kodama et al., Science 298, 395 (2002).
[2] W. Shiramura et al. J. Phys. Soc. Jpn. 67, 1548 (1998).
[3] M. Matsumoto, Phys. Rev. B68, 180403 (2003).
[4] K. Kodama et al., Prog. Theor. Phys. Suppl., submitted.
[5] Ch. Ruegg et al., Phys. Rev. Lett. 93, 037207 (2004).
Authors
K. Kodama, M. Takigawa, and H. Tanakaa
a Tokyo Institute of Technology

Contrasting Spin Dynamics in the
Family of β-Pyrochlore Osmate
Superconductors
Takigawa and Hiroi Groups
Quantum spins on the pyrochlore lattice, a network of
corner-shared tetrahedra, have attracted strong recent interest, because the frustrating interactions may lead to an exotic
RVB spin-liquid ground state. To understand the behavior ofitinerant electrons on such a lattice is an challenging issue.
Anderson's proposal that the RVB-singlet state would evolve
into a superconducting state by carrier doping motivatedexperimental efforts to find superconductors on frustrated
lattices. In 2004, Yonezawa, Muraoka, and Hiroi at ISSP discovered a new family of osmates superconductors AOs2O6(A=K, Rb, Cs), where the Os sites form a pyrochlore lattice[1] (see the ISSP Activity Report 2003). It is particularly
attractive that different alkaline elements lead to large variation of the superconducting transition temperature Tc, whichis 9.6 K, 6.3 K, and 3.3 K for A = K, Rb, and Cs, respective-

Fig. 2. Temperature dependence of 1/T1T in the low temperature region. The pseudo-gap behavior in KOs2O7 is virtually unchanged when
the field is increased. A small Hebel-Slichter beak immediately below
Tc is observed in RbOs2O7 at low field but not in KOs2O7.

ly. This may allow us to identify what determines Tc.
We have performed nuclear magnetic resonance (NMR)
experiments on 39K nuclei in KOs2O7 and on 87Rb nuclei in
RbOs2O7. We found remarkably contrasting magnetic andsuperconducting behavior for the two materials. The temperature dependence of the nuclear spin-lattice relaxation rate
divided by temperature (1/T1T) is shown in Fig. 1. The spinlattice relaxation is dominantly caused by the spin fluctuations of Os-5d electrons. If the spatial spin correlation isabsent, 1/T1T is determined from the static spin susceptibilityor the Knight shift by the Korringa relation. This result isshown by the dashed lines for KOs2O7 (black) andRbOs2O7(red). While the experimental data of 1/T1T is only
modestly enhanced over the Korringa result in RbOs2O7, it isstrongly (more than ten times) enhanced in KOs2O7. We also
note that 1/T1T is strongly temperature dependent only for
KOs2O7 . This temperature dependence is consistent with thespin-fluctuation theory for nearly antiferromagnetic metals inthree dimension (solid line).
Striking difference appears also in the low temperature
behavior as shown in Fig. 2. For KOs2O7, 1/T1T shows apeak near 16 K, which is twice as large as Tc at the magnetic
field of 5 T used for this measurements. This resembles theso-called "pseudo-gap" behavior observed in cuprate hightemperature superconductors in the underdoped region. It
indicates depression of the low energy spin fluctuations as if
singlet pairing takes place even in the normal state.
However, the behavior remains unchanged when the field isincreased to 16 T which reduces Tc to almost a half.
Therefore, superconducting fluctuations must be irrelevant.
Below Tc, a weak Hebel-Slichter peak is observed in
RbOs2O7 but not in KOs2O7.
A possible origin for the contrasting properties of the twocompounds is the subtle difference in the shape of Fermi surface and different degree of nesting as suggested from theband structure calculation by Harima.
References[1] S. Yonezawa et al., J. Phys.: Condens. Matter 16, L9 (2004).
[2] K. Arai et al., cond-mat/0411460.

Fig. 1. Temperture dependence of 1/T1T in RbOs2O6 (red symbol, scaleon the left axis) and KOs2O6 (black symbol, scale on the right axis).
Dashed lines show the Korringa results obtained from the Knight shift
data. The solid line shows the fitting to the spin fluctuation theory fornearly antiferromagnetic metals.

4

ISSP Activity Report 2004

Authors
K. Arai, J. Kikuchi, K. Kodama, M. Takigawa, S. Yonezawa,Y. Muraoka, and Z. Hiroi

Modulated Quadrupole Ordering in PrPb3
Sakakibara and Yoshizawa Groups
Because of a strong intra-atomic spin-orbit coupling, theorbital degeneracy in f electron systems is described in termsof quadrupole moments which are rank-2 tensor operators of
the total angular momentum J. The orbital ordering in f electron systems, i.e., a spontaneous lifting of the orbital degeneracy by interactions, therefore is a phase transition ofquadrupole moments. Following magnetic orderings, onerefers to uniform alignment of the quadrupole moment as aferroquadrupolar state, whereas an ordering having a staggered quadrupolar component is called an antiferroquadrupolar (AFQ) state.
The AFQ ordering systems whose ordering structure has
been identified are still limited in number. All the AFQ
structures known so far have simple q vectors (simple alternations of the quadrupole moments). One of the reasons forthis could be the short-range nature of quadrupolar interactions. Nevertheless, it has been argued theoretically that anindirect quadrupolar interaction of RKKY-type might exist
in some intermetallic systems; the existence of long-range
quadrupolar interactions would not rule out the possibility ofmodulated or incommensurate AFQ structures.
Here we report on our recent study on the AFQ structureof the cubic intermetallic compound PrPb3. The 4f electronsof this compound are almost localized, having an orbitally
degenerate level in the crystalline electric field ground state.
PrPb3 is known to exhibit a second order phase transition at
0.4 K, which has been considered (but not confirmed microscopically) as an AFQ transition. Very recently, we havesucceeded in identifying the AFQ structure of PrPb3 bymeans of neutron scattering measurements in a magnetic
field [1]. To our surprise, the ordered state turns out to be along-period modulated quadrupole phase.
Figure 1 shows the results of the Q scans performed
in a field of H=4 T applied parallel to the [001] axis.
Clear magnetic superlattice reflections are observed at
q=(1/2 !δ 1/2 0), δ~0.125 below the AFQ transition temperature TQ~0.65 K. These reflections are distinguished from theordinary antiferromagnetic ones because their intensity continuously decreased with decreasing field and vanished at

Fig. 2. Obtained magnetic (arrows) and quadrupole (ovals) structures of
PrPb3. The antiferromagnetic moment appears when an external field isapplied to the AFQ state. AFQ-I is the modulated structure observed
below TQ, which turns into the antiphase structure (AFQ-II) at low temperatures in a field above 1 T.

H=0. This behavior is exactly what one would expect for the
AFQ phase. Below ~0.4 K, 3rd harmonic reflections areobserved at q'=(1/2 !3δ 1/2 0), indicating the structure turns
into a square shape at low T.
The magnetic and quadrupole structures of PrPb3
obtained from the present measurements are shown in Fig. 2,
where the ovals and the arrows indicate the expected quadrupole and induced magnetic structures, respectively.
Generally, in an AFQ phase, an antiferromagnetic (AF) component compatible to the AFQ order parameter (OP) is
induced in a field and gives rise to magnetic superlatticereflections. Below TQ, the induced AF moment parallel to[001] has the long-period modulated structure as shown in
Fig. 2(a) (AFQ-I). The underlying AFQ moment is also
expected to have the long-period modulation. This is the firstobservation of the quadrupolar ordering with a long-wavelength amplitude modulation. The AFQ OP is determined to
be of O20 type (=2Jz2-Jx2-Jy2) from the polarization of theinduced AF component. At lower temperatures and above
1 T, this state undergoes a 1st-order transition to an antiphasesquare structure denoted as AFQ-II in Fig. 2(b). Very interestingly, the non-square modulated phase AFQ-I is found to
persist as T " 0 at low field below 1 T. This observation suggests that the local quadrupole moment of Pr3+ is partiallyquenched by hybridization with conduction electrons(quadrupole Kondo effect).
Reference[1] T. Onimaru et al., Phys. Rev. Lett. 94, 197201 (2005).
Authors
T. Onimaru, T. Sakakibara, N. Aso, H. Yoshizawa, H.S. Suzukia, andT. Takeuchiba National Institute of Materals Scienceb Osaka University

Studies on BIODE(Biomolecular light-emitting diode)
Tajima and Akiyama Groups
Fig. 1. Results of the (h 1/2 0) scans in a field of 4 T applied parallel tothe [001] axis. Inset shows the magnetic (solid circles) and nuclear(open circles) reflections observed at 0.125 K.

A thin-film organic light-emitting diode (OLED) was
fabricated by Vincett et al for the first time [1]. This device
is of interest not only for the practical use but also for the

ISSP Activity Report 20045

basic research. By fabricating this device, we can measure
the electro-luminescence (EL) spectra of the insulatingorganic compounds, and can study their electrical and opticalproperties simultaneously.
Recently, we have succeeded in fabricating OLED's fromheme proteins (cytochrome c and myoglobin) [2, 3] andchlorophyll a [4]. This is the first report on the fabrication of
the biomolecular light-emitting diode ("BIODE"). Heme proteins do not or hardly exhibit photo luminescence. In thisrespect, the mechanism of EL in heme proteins are of interested not only for bioscience but also for physics.
Figure 1a and 1b show the current and EL intensity
simultaneously measured for a cytochrome-c BIODE. The
two characteristics are plotted as a function of the biased
voltages applied to the electrodes. The voltage was increased
from zero to ~14 V and then reduced to zero in one cycle.
This voltage cycle was repeated four times. As can be seenfrom the figure, both the current and EL intensity are not
reproducible even for one sample. The external quantum
efficiency of the El spectrum is defined as a probability thata single photon is emitted when a single electron pass acrossthe OLED junction. This quantity is proportional to the EL
intensity and inversely proportional to the current. Figure 1cshows the quantum efficiency, thus obtained, plotted as afunction of the applied voltages. Interestingly, the external
quantum efficiencies evaluated from the current and EL
intensity shown in Figs. 1a and 1b are reproducible for all thevoltage cycles. Moreover, this quantity is consistent within
20 % error in all the cytochrome c devices measured.
Figure 2 shows a schematic energy diagram of the /ITO/
ferricytochrome c/Al/ junction determined from UPS and
optical measurements. The oxidized cytochrome c has a holein the dyz orbital, while the lower dxy and dxz orbitals are
fully occupied. This energy diagram shows that the Al cathode has closer work function to the Fe dz2 orbital than to theFig. 2. The Schematic energy diagram of /ITO/ferricytochrome c/Al/
junction. [B: Soret band; Q: Q band; 690: 690 nm band; 1750: 1750 nm
band (MCD)] The energy levels of eg(π), dz2, dyz, a2u(π), a1u(π) are estimated on the basis of the work function given by UPS spectroscopy andassignment of optical transitions. The levels of dx2-y2, dxz, dxy are notdetermined.

eg(π) orbital. In such a case, electrons are expected to be
more easily injected into the dz2 orbital than to the eg(π)
orbital, when a voltage is applied. Thus the emission intensity of the π-d transition (690 nm band) is enhanced in the EL
spectra. Contrary to the EL spectra, the PL and the absorption spectra directly reflect the optical transition probability.
The eg " dz2 transition is a forbidden transition in the D4hsymmetry. Thus the electrons excited into the eg(π) orbital(by the Soret- or Q-band excitation) hardly fall into the dz2
orbital. This explains why the 690 nm emission is absent in
the PL spectra previously reported.
The EL spectroscopy is a promising technique in the
study of complicated molecules such as biomolecules. This
technique enables one to inject carriers directly to molecules,
and to detect an optical transition that is not easily detectedby ordinary spectroscopic methods. Moreover, it gives us
electrical information as well as optical information.
References[1] P. S. Vincett, W. A. Barlow, R. A. Hann, and G. G. Roberts, Thin
Solid Films 94, 171 (1982).
[2] H. Tajima, S. Ikeda, M. Matsuda, N. Hanasaki, J. Oh, and H.
Akiyama, Solid Sates Commun. 126, 579 (2003).
[3] H. Tajima, S. Ikeda, K. Shimatani, M. Matsuda, Y. Ando, J. Oh,
and H. Akiyama, Synthetic Metals, in press.
[4] K. Shimatani, H. Tajima, T. Komino, S. Ikeda, M. Matsuda, Y.
Ando, and H. Akiyama, Chem. Lett., in press.
[5] S. Ikeda, H. Tajima, M. Matsuda, Y. Ando, and H Akiyama, Bull.
Chem. Soc. Jpn., in press.
Authors
H. Tajima, S. Ikeda, K. Shimatani, T. Komino, M. Matsuda, Y. Ando,and H. Akiyama

New organic superconductor,
β -(meso-DMBEDT-TTF)2PF6
Mori Group

Fig. 1. Current-voltage characteristics (a), EL intensity-voltage characteristics (b), and quantum efficiencies (c) for cytochrome c. Quantumn
efficiencies are evaluated from the data shown in (a) and (b). The inset
shows the molecular structure of cytochrome c.

6

ISSP Activity Report 2004

It is of importance to investigate the electronic propertiesof organic (super)conductors by tuning the nanomolecular
structure. Usually, the critical temperature of organic superconductor (Tc) decreases with applying pressure by-1 K/kbar. In order to apply the negative chemical pressure
with the aim of the increase of Tc, the new donor molecule,meso-DMBEDT-TTF, by the introduction of the methylgroups to BEDT-TTF as a steric hindrance (Fig. 1) was syn-Fig. 1. Molecular structure of meso-DMBEDT-TTF

magnetic susceptibility in the metallic region above 90 K
follows the Heisenberg model with J = 100 K without theanomaly at 90 K. Even below 90 K, the susceptibility isretained and no magnetic order was observed down to
4 K. The phase diagram of β-(meso-DMBEDT-TTF)2PF6
demonstrates the wide range of the superconducting statewith ∆ Tc /∆ P = -0.6 K as shown in Fig. 3.
The further study of the competition of the superconducting and charge ordering state in the hatched region (Fig. 3) isunder progress.
Reference[1] S. Kimura, T. Maejima, H. Suzuki, R. Chiba, H. Mori, T.
Kawamoto, T. Mori, H. Moriyama, Y. Nishio, and K. Kajita, Chem.
Commun. 21, 2454-2455 (2004).
Authors
S. Kimura, T. Maejima, H. Suzuki, R. Chiba, T. Kawamotoa, T. Moria,
H. Moriyamab, Y. Nishiob, K. Kajitab, and H. MoriaTIT
bToho University

Theory of Fano-Kondo Effect of Transport
Properties through Quantum Dots
K. Ueda Groupthesized. Consequently, the new donor afforded the neworganic superconductor β-(meso-DMBEDT-TTF)2PF6 [1].
The crystal structure analysis and band calculation ofβ-(meso-DMBEDT-TTF)2PF6 indicate two-dimensional
dimerized structure similar to those of β-(BEDT-TTF)2X.
Even though the salt has the dimerized donor arrangement,
the distinct metal-insulator (MI) transition is observed at
90 K under an ambient pressure. Below 90 K, the checkerboard-type charge ordering state appears by the observationof the crystal structure analysis, confirmed by the Raman
spectroscopy measurement. By applying the pressure, theMI transition is suppressed and the superconducting
state was exhibited (Fig. 2). The temperature dependence of

Recently, coexistence of the Fano effect and the Kondo
effect, i.e. the Fano-Kondo effect, was experimentallyobserved on transport properties through T-shaped quantum
dots (QDs) [1]. The line-shape of conductance was modified
due to the Fano effect and became a dip in this experiment.
Conductance decreases at low temperatures due to the Kondo
effect when the number of electrons in QD is odd. Since thisbehavior is completely opposite to the usual Kondo effect,
this phenomenon is called as the anti-Kondo effect.
Theoretically, the Fano-Kondo effect in the T-shaped geometry studied so far [2,3,4] has been limited to the anti-Kondocase and its theoretical treatment is not general enough incontrast to the case of closed Aharonov-Bohm interferometer(AB-QD) [5].
We have investigated the Fano-Kondo effect with thetight-binding model for the T-shaped QD with an additional
gate voltage Vq on the central site as shown schematically in
the inset of Fig. 1. The existence of Vq is a key to realize
general Fano effect in this model because the line-shape canFig. 3. Phase diagram of β-(meso-DMBEDT-TTF)2PF6. The competition between superconducting and charge ordering states is observed in
the hatched region.

Fig. 1. (a) Number of electrons in QD as a test function of the gate voltage Vg. U on the x-axis is the Coulomb energy in the QD. (b)Conductance g in unit 2e2/h obtained by Zubarev's approximation for
the number of electrons defined in (a), which shows a characteristic
behavior in the high-temperature regime. (c) Conductance in the lowtemperature regime given by the Friedel sum rule for the number ofelectrons defined in (a). (Inset) Schematic figure of our model.

Fig. 2. The metal-insulator transition occurs at 90 K at ambient pressure. By the suppression of the transition, superconductivity isobserved at Tc = 4.3 K under 4.0 kbar for β-(meso-DMBEDTTTF)2PF6.

ISSP Activity Report 20047

References[1] M. Sato, H. Aikawa, K. Kobayashi, S. Katsumoto, and Y. Iye,
cond-mat/0410062.
[2] M. E. Trio, K. Hallberg, A. H. Ceccatto, and C. R. Proetto, Phys.
Rev. B 65, 085302 (2002).
[3] K. Kang, S. Y. Cho, J. Kim, and S. Shin, Phys. Rev. B 63, 113304(2001) .
[4] A. Aligia and C. Proetto, Phys. Rev. B 65, 165305 (2002).
[5] W. Hofstetter, J. K op nonig, and H. Schoeller, Phys. Rev. Lett. 87,
156803 (2001).
[6] I. Maruyama, N. Shibata, and K. Ueda, J. Phys. Soc. Jpn. 73, 3239
(2004).
Authors
I. Maruyama, N. Shibataa, and K. Uedaa Dept. of Basic Science, University of Tokyo

Importance of Dynamical Electron
Correlation in the Stopping Power ofSolids for Slow Ions
Takada Group

Fig. 2. Numerical results of conductance for the Coulomb interaction
U = 4 by the FT-DMRG from a high-temperature region (red line) to azero-temperature limit (blue line). g 0 is the conductance of the background defined at Vg = 3 limit. We note that this figure is reproduced from Fig. 6 in [6] with smoothing the points of numerical data.

be modified by Vq and in particular the conductance shows a
dip structure due to the Fano effect in the Vq = 0 case. With
this theoretical model we have shown that for a wide class ofmodels for quantum dots there is a unified formula of conductance having a Fano asymmetric parameter. We do notwrite down the formula but show typical line shapes of conductance valid at high temperatures in Fig. 1 (b) and also at
low temperatures in Fig. 1 (c) , where it is assumed that the
number of electrons at the dot sites changes as a function of
the gate voltage as shown in Fig. 1 (a). With varying temperature the conductance shows a crossover between the highand low-temperature regions compared with the Kondo
temperature TK. Behavior of the conductance at high temperatures is characterized by two Fano asymmetric line-shapes
while at low temperatures by a Fano-Kondo plateau inside a
Fano structure.
In order to calculate temperature dependence of the conductance explicitly we used the finite temperature density
matrix renormalization group (FT-DMRG) method, by which
one can calculate a thermal Green's function. The Green's
function in frequency space can be obtained from the thermal
Green's function by a numerical analytic continuation. The
results of the numerical calculations in Fig. 2 show smoothcrossover from hightemperature regime to the zero-temperature limit in (a) the anti-Kondo case by putting Vq = 0 and in(b) a general Fano-Kondo case (Vq ! 0). Since relatively
small U is used in these examples, two Fano peaks athigh temperatures are not well separated from each othercompared with Fig. 1. Actually, the experimental situation
for the anti-Kondo effect [1] corresponds to a relatively
small U, the results of which are qualitatively consistent with
Fig. 2 (a). It is desired that a general Fano-Kondo effectshown in Fig. 2 (b) will be reported experimentally in near
future.

8

ISSP Activity Report 2004

The concept of electron correlation permeates many
subfields of condensed matter physics, but it is rather
difficult to find good examples of its clear manifestation. It iseven more difficult to illustrate its importance in dynamical(i.e., time-dependent) processes. Here we report a success in
obtaining a clean theoretical formula directly relating
dynamical correlation, as characterized by fxc the frequencydependent exchange-correlation kernel of the time-dependent
density-functional theory (TDDFT), with the experimentallyobserved stopping power, -dE/dx, of solids for slow ions.
The nonperturbative theory of scattering of noninteracting electrons by a central potential combined with the density-functional theory (DFT) has proved successful in calculating -dE/dx of an electron gas for slow ions. This scheme
accurately reproduces the measured energy loss of slow protons and antiprotons in a variety of solids. To some extent it
also successfully accounts for the observed behavior in theenergy loss of slow ions with increasing charge Z1e. Thiskind of calculations were extended to projectile velocities
v approaching the Fermi velocity of the target, but so far only
few attempts have been made to include the effect of manybody dynamical correlation that is absent in the existing
DFT-based schemes.
We approach the problem of including dynamical correlation in the framework of the TDDFT and have succeeded
in deriving a rigorous formula for -dE/dx of an interacting
electron gas for ions in the limit of low projectile velocities(v "0) [1]. Depending on whether fxc is involved or not, thisformula can be rewritten as a sum of two terms: The first one
without fxc is shown to yield the conventional result for-dE/dx of noninteracting Kohn-Sham electrons first reported
by Echenique et al. [2]. The second one represents a new andimportant result fully incorporating the effect of dynamicalcorrelation through the imaginary part of fxc.
In the spirit of the widely used local-density approximation (LDA), we can think of an appropriate approximation to
fxc in such a form as the one proposed by Qian and Vignale[3]. Then our formula enables us to make a reasonably accurate evaluation of the friction coefficient (-dE/dx divided by
v in the limit of v " 0) of an interacting electron gas as afunction of the atomic number of the projectile ion Z1. Anexample of the results obtained by such calculations for the
homogeneous electron gas is shown in Fig. 1 in which theNovel Low-Field MagnetoresistanceOscillation Effect in Short Period
Unidirectional Lateral Superlattices
Iye Group

Fig. 1. Comparison between experiment on the stopping power, -dE/dx,
of carbon films for slow ions (characterized by the atomic number Z1)
with three different values of the velocities v and theory on the same
quantity of the homogeneous electron gas with the electron density
equivalent to the valence-electron density in carbon in the limit of zeroion velocity (v " 0).

solid and dashed curves correspond, respectively, to thecases with and without dynamical correlation (or the second
term in our formula). Here the electron density characterized
by the conventional density parameter rs is taken to be 1.59
which is just the same density as the one for valence electrons in carbon films. In view of the fact that at low velocities the energy loss of ions in solids is due mainly to thestopping power of valence electrons, our present theoreticalresults can be directly compared with the experimental onesin carbon.
As can be seen in Fig. 1, experiment exhibits an oscillatory behavior in the friction coefficient with increasing Z1.
This behavior, which is attributed to the shell structure ofvalence electrons around the ion, is reproduced rather well in
the DFT-based calculation (as labeled by LDA), but a better
quantitative agreement with experiment is obtained by ourtheory based on the TDDFT, thanks to the enhancement of-dE/dx by dynamical correlation, or more specifically, theexcitonic effect which has recently been revealed to be very
prominent through the accurate calculation of the dynamical
structure factor of the homogeneous electron gas [4].
Agreement between theory and experiment becomesunsatisfactory for Z1 larger than about 20, indicating a needto go beyond the LDA-type approximation in the descriptionof fxc.

Electron dynamics in a periodic potential is at the heart ofsolid state physics. For the experimental exploration of various model cases, high mobility two-dimensional electron gas(2DEG) in a GaAs/AlGaAs device with an appropriately
microfabricated superstructure is most suitable. In such asystem, electrons move ballistically in an artificial potential
landscape with minimal influence from disorder potential.
Application of perpendicular magnetic field adds an extra
ingredient to the electron dynamics.
A typical example, which is also the situation for thepresent study, is a unidirectional lateral superlattice, where
2D electrons are subjected to a one-dimensional periodic
modulation of potential. For this system, two characteristic
magnetotransport features have been elucidated: One is theso-called commensurability oscillation (CO) effect, and theother is the positive magnetoresistance (PMR) effect. The
former arises from commensurability, or geometrical resonance, between the cyclotron radius Rc = 'kF eB and themodulation period a. The latter is associated with the socalled snake orbit. These effects are basically understoodwithin a semiclassical magnetotransport picture and do not
explicitly require miniband formation. In the present study,
we have observed a novel oscillatory magnetoresistanceeffect which is attributed to geometrical resonance of open
orbits created by miniband formation.
The structure of the sample is schematically shown on
the top panel of Fig.1. A comb-like pattern of resist on thesurface of the GaAs/AlGaAs device creates a weak unidirectional potential modulation at the 2DEG plane with periodranging from 138 to 184 nm. The amplitude of the potential
modulation is typically 0.01 to 0.03 of the Fermi energy EFof the 2DEG.
The main panel of Fig.1 shows the magnetoresistance

References[1] V. U. Nazarov, J. M. Pitarke, C. S. Kim, and Y. Takada, Phys. Rev.
B 71, 121106(R) (2005).
[2] P. M. Echenique, R. M. Nieminen, and R. H. Ritchie, Solid State
Commun. 37, 779 (1981).
[3] Z. Qian and G. Vignale, Phys. Rev. B 65, 235121 (2002).
[4] Y. Takada and H. Yasuhara, Phys. Rev. Lett. 89, 216402 (2002).
Authors
V. U. Nazarova, J. M. Pitarkeb, C. S. Kima, and Y. Takada
a Chonnam National University
b Euskal Herriko Unibertsitatea

Fig. 1. Top panel: schematic drawing of the structure and an SEM picture of the sample. Main panel: Magnetoresistance trace in the low field
range showing the PMR and CO features. Small amplitude oscillation
seen in the PMR region is made conspicuous in the second derivative
trace.

ISSP Activity Report 2004

9

Fig. 2. Left panel: Energy bands of 2DEG reconstructed by the superlattice potential. Right panel: Open orbits created by the Bragg reflection. The corresponding trajectories in real space are isomorphic tothese but rotated by 90 degrees.

trace in the low field range. The two characteristic features
mentioned above, i.e. PMR and CO, are prominent. We havefound that a small amplitude oscillation is superposed on the
trace in the PMR region. The oscillation can be made conspicuous by taking the second derivative of the trace, whichis shown in the same figure. The oscillation gradually diesaway at higher temperatures.
In Fig.2, the energy bands of 2DEG reconstructed in thepresence of periodic potential and the open orbits thereby are
shown. The electron trajectories in real space are isomorphic
to these except they are rotated by 90 degrees. The widths ofthese wiggling trajectories in real space, which we denote bybi,j for the trajectory (i, j), can be calculated from an elementary geometry as
R
V
2
2
W
'kF S
r
j
S 1 - c ir m - 1 - e
Wbi, j =
(1)
o
akF
akF W
e B SSW
T
X bi,j 's coinGeometrical resonanceoccurs when one of thecides with integer multiples of a. The calculated values ofthe magnetic field show excellent agreement with theobserved positions of resistance maxima. Their changes withthe electron density and the modulation period are also consistently explained by the model, rendering a strong support.
It is stressed that the newly observed oscillatory magnetoresistance effect, which is attributed to geometrical resonanceof the open orbit trajectory and the superlattice periodicity, isa manifestation of the miniband formation.

nance with the Fermi energy when the local spin is 1/2. The
Kondo state should thus be a coherent state though it is anentanglement of huge number of freedoms.
The best way to check this property is to see the quantum
interference of a Kondo state. For this purpose, we haveadopted a quantum wire with a side-coupled quantum dot,
which is schematically shown in the inset of Fig.1. Here theinterference occurs between the direct path and the path witha bounce to the quantum dot. The potential of the dot can bevaried with the gate voltage Vg and the coupling strength canalso be tuned by the voltage Vm.
In this structure ordinal Coulomb peaks of the conductance trough the dot versus gate voltage appear as dips in the
conductance through the wire due to the interference. This isa kind of the Fano effect [2] and the lineshape shown in the
topmost of Fig.1 really has Fano distortion. With increasing
the coupling strength, the conductance between two adjacentdips decreases alternatively as marked A and B. At lastneighboring dips are combined into single dips. This is aclear sign of the Kondo resonance. We have thus observed
the Fano-Kondo effect.
From the observation we can clearly say that the Kondo
state is quantum mechanically coherent. An interestingobservation is that the direction of the distortion in thelineshape rapidly changes with the growth of the Kondo state(in the case of dip A, around cyan-colored curve). This is dueto the enhancement of coupling strength from asymptotic
strong coupling nature of the Kondo effect.
References[1] H. Aikawa, K. Kobayashi, A. Sano, S. Katsumoto, and Y. Iye,
Phys. Rev. Lett. 92, 176802 (2004).
[2] K. Kobayashi, H. Aikawa, S. Katsumoto, and Y. Iye, Phys. Rev.
Lett. 88, 256806 (2002).
Authors
S. Katsumoto, M. Sato, H. Aikawa, and Y. Iye

Reference[1] A. Endo and Y. Iye, Phys. Rev. B 71, 081303(R) (2005).
Authors
A. Endo and Y. Iye

The Kondo Effect Recovers Quantum
Coherence
Katsumoto Group
It is well known that quantum coherence of an electron
traversing in the vacuum or a metal is lost when it is"observed". The observation process can be viewed as formation of entanglement with "environment" of infinite numbers of freedom. Such a process can be triggered by various
origins, e.g., spin-scattering with local magnetic impurities.
We have confirmed that by observing an amplitude of
Aharonov-Bohm (AB) oscillation in a ring with a quantum
dot, which can work as a magnetic impurity[1].
At the same time, however, the spin-scattering is the elementary process to form a Kondo state, which is in reso-

10

ISSP Activity Report 2004

Fig. 1. Conductance of the wire versus the gate voltage Vg of theside-coupled dot measured at several coupling strength tuned by the
coupling gate voltage Vm. The step in Vm is 6mV and the data areoffset by 0.3e2/h for clarity. The inset shows a schematic diagram ofthe sample.

Direct Observation of Strain-Induced
Change in Surface Electronic Structureon the Cu(001) Surface
Komori Group
Recent experimental studies have reported that chemical
reactions on d-electron metal surfaces can be modified by theexternal strain [1]. Theoretical attempts to interpret these
phenomena have been done on the basis of first-principles
calculations of electronic structure, whereas there have been
little experimental efforts to examine the electronic structure
itself. We have adopted the Cu(001) surface as a model system, whose "natural" electronic structure is well-known, anddemonstrated [2] the direct evidence of the strain-inducedchange of individual Cu3d states by angle-resolved photoemission spectroscopy.
We introduced a local strain field on the Cu(001) surface
by nitrogen partial adsorption. At 0.3 mono-layer (ML) of
the averaged nitrogen coverage, the surface is covered with awell-ordered array of the N-adsorbed domain as shown in

Fig. 1. The clean Cu region imaged brightly is compressed
because of the large lattice constant on the N-adsorbed
domain. In a simple approximation, the Cu lattice deformation on average will be proportional to the nitrogen coverage.
Figure 2 shows the observed band dispersions around the
M point on the clean, 0.1 ML, 0.2 ML and 0.3 ML nitrogenadsorbed surfaces. In these figures, we can see two kinds ofelectronic structure change: (1) the binding energy of theTamm state decreases with increasing the nitrogen coverage,ie., the increase of the strain at the clean region, (2) the folding point of the Tamm state moves to a larger k-point withincreasing the strain.
The experimental results were compared with the firstprinciples calculations with a symmetric slab model. A uniform compressive stress was given to the slabs laterally
while interlayer relaxation due to the stress was allowed forthe simulation of the strained Cu surface. The calculated
energy shifts of the surface states excellently agree with theobservation. Moreover, the shift of the folding point of theTamm state is quantitatively explained by the sum of theBrillouin zone extension and the change of the work function
due to the lattice constant reduction.
References[1] For example, M. Gsell et al., Science 280, 717 (1998).
[2] D. Sekiba, et al., Phys. Rev. Lett. 94, 016808 (2005).
Authors
D. Sekiba, K.Nakatsuji, Y. Yoshimoto, and F. Komori

Direct Observation of a Precursor State for
Chemisorption: Ethylene on Si(100)c(4#2)
Fig. 1. (a) STM image of the partially nitrogen-adsorbed Cu(001) surface. The Dark area is the N-adsorbed domain, and the bright part corresponds to the remaining clean region. The nitrogen coverage is 0.3 ML,
and the image size is 100 nm # 100 nm. (b) Magnification of the STM
image. The arrows schematically indicate the direction of atom replacement in the N-adsorbed domain. The clean surface is thus compressed.

Fig. 2. Band dispersions experimentally observed on the clean (a), 0.1
ML N-adsorbed (b), 0.2 ML (c) and 0.3 ML (d) surfaces. The solid
curves in (a-c) and the white curves in (c,d) indicates the band dispersion of the Tamm state, a surface state, determined on the clean surface.
The solid curve in (d) show the Tamm state band dispersion. The whiteline in each figure represents the folding point of the Tamm state on theclean surface, and the white dotted line in (d) shows the new folding
point. Both the energy at M and the wave vector of the folding point
shift with increasing the N-coverage.

Yoshinobu GroupThe cycloaddition reaction of alkene to the Si(100) surface is one of the promising methods to link an organic
molecule to the substrate via stable di-σ SiC bonds. Since
the discovery of the di-σ bonded ethylene on Si(100), many
experimental and theoretical studies have been reported on
the chemisorbed state. In addition, the reaction mechanism
has been challenged because the frontier orbital theory
requests that the [2+2] cycloaddition reaction between
ethylene and the symmetric dimer on Si(100) is apparently
forbidden in thermal reaction. However, only a few studies
have been reported on the reaction mechanism of di-σ bondformation. In this study, we report the direct observation of aprecursor state for the di-σ bond formation of ethylene on
Si(100)c(4 #2) using high-resolution electron energy loss
spectroscopy (HREELS) at low temperature, and the stableprecursor state is identified as a weakly adsorbed π-complex
type for the first time.
In Figure 1, EELS spectra at 48 K show several new
peaks in addition to the vibrational peaks originating fromthe di-σ ethylene species. Judging from the CH stretching(3051 cm-1) and the CC stretching (1522 cm-1) modes andthe strong CH2 wagging mode at 968 cm-1, these peaks originate from a weakly adsorbed ethylene species. By comparing
the vibrational spectra of chemisorbed ethylene with ethylene-transition metal complex, we conclude that this weakly
adsorbed ethylene is a π-complex type. Using the empirical
relation between the CC stretching energy and the CC bondlength for adsorbed ethylene species, the CC bond lengthof this species is estimated to be 1.38 Å.

ISSP Activity Report 200411

Development of Novel Probes for
Atomic Force Microscopy
Hasegawa Group

Fig. 1. HREELS spectra of (a) C2H4 and (b) C2D4 on Si(100)c(4 # 2) at
48 K. (c) After exposing C2H4 on Si(100) at 48 K followed by heating
at 150 K, the spectrum was measured at 48 K. All spectra were measured in the specular mode. The primary electron energy, Ep=5.0 eV.

When the Si surface exposed to ethylene at 48 K
was annealed at 150 K, the peaks originating from the
π-complex species disappeared and the peaks originatingfrom the di-σ bond species were increased in intensity.
Therefore, the adsorbed π-complex species is a precursor
state for the di-σ chemisorption. It should be noted that the
π-complex precursor and the di-σ species coexist at low
temperature, and the reaction from the precursor to the di-σ
species does not proceed at 48 K. This suggests that when
gaseous ethylene molecules enter the adsorption potential on
Si(100)c(4 #2) some molecules are trapped in the intrinsic
precursor state, but the other molecules can proceed to thedi-σ bond formation. From the vibrational spectra as a function of temperature, the activation energy Ea is roughly estimated to be 0.2 eV using the Arrhenius formula (see Fig.2).
Reference[1] M. Nagao, H. Umeyama, K. Mukai, Y. Yamashita, J. Yoshinobu,
K. Akagi, and S. Tsuneyuki, J. Am. Chem. Soc. 126, 9922-9923 (2004).
Authors
M. Nagao, H. Umeyama, K. Mukai, Y. Yamashita, J. Yoshinobu,
K. Akagia, and S. Tsuneyukia
a Department of Physics, University of Tokyo

Fig. 2. Potential energy diagram for the chemisorption of ethylene on
Si(100)c(4 # 2). Ea, E1 and E2 represent the activation energy for thedi-σ formation, the adsorption energy of the precursor state and that ofthe di-σ chemisorption state, respectively.

12

ISSP Activity Report 2004

Atomic force microscopy (AFM) has been utilized toimage nano-scale surface topograph including its atomic
structure through a detection of forces on the surface. How to
detect the force, thus, plays a crucial role in spatial resolution
and detection efficiency of AFM. Here we reports two new
AFM probes which show unique performances.
In usual AFM operations, a commercial cantilever with a
tip made of Si is used as a force sensor. In some cases, however, such as magnetic force microscopy and scanning
capacitance microscopy, a metal probe is desired because of
its high conductivity and magnetic properties. Most of themetal probes are produced by coating, making the tip bluntand deteriorating the spatial resolution. In order to avoid
the dull tip without losing a high Q factor of the Si
cantilever, we formed a metal-tip cantilever by attaching a
5µm-diameter tungsten tip to the commercial Si cantileverand sharpening it with focused ion beam (FIB) [1]. TEM
observation revealed that the curvature radius of the W tip isquite small, ca. 3.5nm.
Atomically resolved non-contact (nc) AFM image of theSi(111)-(7 #7) surface was successfully taken with the tip.
This micro-fabrication method is applicable to any conductive wires. So far, we have made a Au-tip cantilever and succeeded in drawing a line of Au with a width of ca. 100nm ona Si substrate by field evaporation from the tip.
In nc-AFM, small oscillation amplitude is desirable for a
detection of short-range forces, which contributes to theatomic resolution [2]. The small oscillation amplitude using
a conventional cantilever, however, causes a snapping of thecantilever into the sample ("jump to contact"). To avoid thetip crash we need to use a sensor with a high stiffness. We
thus developed an AFM probe using a quartz length-extension resonator (LER) whose spring constant is quite high(~540 000 N/m) (Fig. 2(a)). Self-sensing capability of quartz
due to its piezoelectric effect eliminate a need of using cumbersome optical alignments, preferable for low temperatureapplications.
A tungsten tip was attached on the LER rod (Fig. 2(b)),
and was fabricated with FIB as a tip [3] (Fig. 2(c)). Prior to
the AFM experiment, field ion microscopy observation of
the tip was performed in-situ to remove the oxide layer.
Atomically resolved nc-AFM imaging of the Si(111)-7 #7
surface was obtained with the probe (Fig. 2(d)), demontrating

Fig. 1. SEM image of the W tip attached to the Si cantileverFig. 1. Model of a LaTiO3 nanowire array embedded in SrTiO3. The
wire layer has a thickness of one unit cell.

Fig. 2. (a) a quartz LER (b) a SEM image of the front rod with a W tip.
(c) a zoomed SEM image of the W tip fabricated by FIB. (d) an ncAFM image of the Si(111)-7 # 7 surface. The observed area is 19 nm #9 nm.

its high performance.
The TEM and SEM observations and the FIB processing
were performed at Electron Microscope Section of Materials
Design and Characterization Laboratory in ISSP. Technicalassistance by M. Ichihara and F. Sakai is highly appreciated.
References[1] K. Akiyama, T. Eguchi, T. An, Y. Fujikawa, Y. Tamada-Takamura,T. Sakurai, and Y. Hasegawa, Rev. Sci. Instrum. 76, 033705 (2005).
[2] T. Eguchi and Y. Hasegawa, Phys. Rev. Lett. 89, 256105 (2002).
[3] T. An, T. Eguchi, K. Akiyama, and Y. Hasegawa, to be publishedin Appl. Phys. Lett.

effects between the film and the surrounding SrTiO3. The
electronic thickness of the wires is therefore larger than thephysical thickness.
The nanowires were grown by pulsed laser ablation at atemperature of 1000cC in step-flow mode by depositing
approximately half a monolayer of LaTiO3. In this growthmode the newly-deposited material decorates the step edges
of a vicinal substrate surface. The density of wires can thus
be adjusted by changing the miscut angle of the substrate.
We found that wires and thin LaTiO3 layers on an open
substrate surface become insulating, likely because of theformation of depletion layers due to the presence of surface
states. The nanowire arrays were therefore capped with a 10
nm-thick SrTiO3 layer. Contact to the wires was made bybonding wires directly to the crystal surface, penetrating thethin capping layer (Fig. 2,top).
The low-temperature resistivities measured along thedirection of the wires and perpendicular to the wires areshown in Fig. 2. Due to the relatively large contact area of
the wire bonding pads, these measurements reflect the properties of approximately 100 wires connected in parallel. As
shown in Fig. 2, the temperature dependence of resistivity

Authors
K. Akiyama, T. An, T. Eguchi, M. Hamada, and Y. Hasegawa

Metallic Heteroepitaxial
Oxide Nanowire Arrays
Lippmaa Group
Transition metal oxides are well known for a wide varietyof ordering mechanisms and rich electronic phase diagrams.
Our work is focused on understanding the electronic structure and properties of oxides in confined geometries, such as
two-dimensional layers, nanowires, or nanodots. In this work
we have studied the transport properties of LaTiO3 nanowire
arrays embedded in SrTiO3. A typical layer thickness for
LaTiO3 is one unit cell, while the width of the wire is in the
20 to 50 nm range, as shown in Fig. 1.
LaTiO3, in its stoichiometric form, is a Mott insulator.
Metallic conductivity, however, can be easily induced byslight doping. In our case, since the single unit cell-thick
LaTiO3 films are grown on Sr-terminated SrTiO3 substrates
and embedded in SrTiO3, it is more accurate to look at thefilms as consisting of (La,Sr)TiO3, where the actual carrier
concentration in the LaTiO3 layer depends on charge transfer

Fig. 2. (Top) a photo of a transparent metallic nanowire array. The wire
direction is vertical in the image. Bonding wires used for transportmeasurements are visible on the crystal surface. (Bottom) Low-temperature transport measurements of a wire array perpendicular ( =) andparallel (||) to the wire direction.

ISSP Activity Report 200413

along the wires on a T 2 scale is very close to linear, as
expected for doped (La,Sr)TiO3+δ for a carrier concentration
perof .1022 cm-3. In contrast, the resistivity along thependicular direction is nearly three orders of magnitude
higher and shows a nonlinear behavior on a T 2 scale. Thisbehavior and resistivity values are characteristic of lightlydoped SrTiO3, with a carrier concentration in the 1019 cm-3
range. Note that carrier localization appears to occur in thisdirection at temperatures below about 50 K.
The wire array has a very regular structure and we haveconfirmed wire conductivity over a distance of several millimeters. The uniformity of the wire array depends mainly on
the quality of the substrate polishing, since the wires follow
the surface steps without breaking.
Reference[1] T. Mihara, K. Shibuya, T. Ohnishi, H. Koinuma, and M. Lippmaa,
Thin Solid Films, in print.
Authors
M. Lippmaa, T. Ohnishi, T. Mihara, and K. Shibuya

Successive Transitions in Charge Density
Wave State under High Magnetic Fields
Osada group
We have studied possible phase diagram of charge density wave (CDW) state under magnetic fields by analyzing thedensity response function, and successfully explained mysterious successive transitions observed in high-field CDW
state in organic conductors α-(BEDT-TTF)2KHg(SCN)4 and(Per)2M(mnt)2 (M=Au, Pt).
Generally, the zero-field insulating CDW phase (CDW0)
is destabilized due to Zeeman effect in sufficiently strong
magnetic fields. In higher magnetic fields and sufficiently
low temperatures, there appears a new metallic CDW phase(CDWx) where only one spin-subband contributes todensity wave formation. The high-field "kink transition" in
α-(BEDT-TTF)2KHg(SCN)4 is considered as CDW0-CDWx
transition. In this compound, however, irregular successive

Fig. 2. Wave-number dependence of the density response function ofhighly anisotropic 2D electron system under magnetic fields. Peaks
indicated by arrows correspond to possible CDW channels. Nesting
imperfection could stabilize semimetallic CDWy subphases whichcause successive transitions between CDW0 and CDWx.

phase transitions have recently been discovered between
CDW0 and CDWx phases under tilted magnetic fields[D. Andres et al., PRB68, 201101 (2003)]. We haveconfirmed these phenomena in wider field range using
50T-class non-destructive pulsed high magnetic fields asshown in Fig.1. The successive transitions extend over thewide field range from 12T to 40T, which is hardly explainedby the quantized changes of the CDW wave number in
CDW0-CDWx transition as suggested by Andres et al.
In order to clarify the origin of the successive phase transitions, we have investigated the CDW instability of highly
anisotropic two-dimensional electron systems by evaluating
the density response function under magnetic fields. We
have found that nesting imperfection of two Fermi sheets
could stabilize semimetallic CDW state (CDWy) in higher
field region of CDW0 even in the systems without any
CDW-SDW mixing as assumed by Zanchi et al. [PRB53,
1240 (1996)]. In the CDWy state, both up-spin and downspin subbands show imperfect nesting leaving electron andhole pockets, respectively. Moreover, the CDWy state could
be divided into subphases at low temperatures resulting fromthe quantized nesting effect in the similar manner as thefield-induced SDW case. In each subphase, one of electron
and hole pockets is fully quantized by breaking electron-holecompensation. It causes finite Hall effect. After successive
transitions among the CDWy subphases, finally, the system
undergoes the transition into a metallic CDWx state.
The above picture well explains the phase transitions
observed in α-(BEDT-TTF)2KHg(SCN)4, and it might bealso applicable to the successive field-induced CDW transitions in (Per)2M(mnt)2 (M=Au, Pt).
Authors
T. Osada, E. Ohmichi, M. Kobayakawa, A. Ogasawara, K. Kajiki, and
H. Imamura

Pressure Induced Metal-Insulator
Transition in CeRu4Sb12 Single Crystal
Uwatoko Group
Fig. 1. Interayer magnetoresistance of an organic conductor
α-(BEDT-TTF)2KHg(SCN)4 for different field orientations measured
by the polar angle θ from the normal of 2D plane. As indicated byarrows, several transitions appear between the low-field CDW phase(CDW0) and the high-field one (CDWx) under tilted magnetic fields.

14

ISSP Activity Report 2004

Ternary intermetallic compounds RT4X12 (R=rare earth;
T=Fe, Ru, Os; X=P, As, Sb) with the skutterudite structure
have various physical properties [1]. Among them, most of

the Ce-based compounds CeT4X12 show semiconductive
behavior which might be attributed to the large c-f hybridization strength originated from the unique structure. Actually,magnitude of energy gap increases with decreasing lattice
constant in CeT4X12 [2]. CeRu4Sb12 is, exceptionally, metallic not explained by the band calculation which predicts it
semiconducting. Moreover, CeRu4Sb12 shows anomalous
behaviors such as heavy fermion and non-Fermi-liquid(NFL)[3,4] at ambient pressure. Recently, Kobayashi et al. reported
NFL behavior is suppressed with an extension of Fermi
liquid regime under P = 1.3 GPa [4]. These features must be
related to the delicate electronic structure in the vicinity of
the Fermi level. This is supported by Shubnikov-de Haas andthe de Haas-van Alphen experiments in CeRu4Sb12 whichclarified small Fermi surface at low temperature with highly
enhanced effective mass [5]. It is expected that c-f hybridization plays important roles in the electrical mechanism of
CeRu4Sb12. Therefore, we have studied the transport properties under applied high pressure which can have a profoundeffect on the c-f hybridization.
Figure 1 shows a temperature dependence of the electrical resistivity of CeRu4Sb12 under several constant pressures,
1.5 - 8 GPa, respectively. In order to obtain the magnitude ofthe energy gap at low temperature, we assume CeRu4Sb12
being simple activated semiconductor and plot lnρ as a function of 1/T. A linear relation is observed in a narrow and limited temperature range. Fits to an activated conduction law

ρ = exp(∆ / T) yield small band gap ∆ = 0.3 K, 9.1K, 24.7 K
under P = 6 GPa, 7 GPa, 8 GPa, respectively. A pressure
coefficient of the energy gap d∆ /dP is about 12.2 K/GPa.
The magnitude of ∆ under high pressure is close to that of
CeOs4Sb12, ∆~10 K[6]. This is consistent with the relationship between ∆ and a lattice constant in CeT4X12 as mentioned at the beginning. The problem is that Tmax is slightly
shifted to lower temperature side as increasing pressures.
This fact is contrary to effect of pressure on the typicalCe-based Kondo compounds, though no clear explanation
has been made. Investigation of the physical properties underhigher pressure can lead us to solving them. The electricalmechanism of CeRu4Sb12 being small band-gap semiconductor under high pressure may have the same origin as otherCe-based skutterudite compounds which show semiconducting behaviors. More researches, such as Hall Effect measurement under high pressure, are necessary to understand theelectrical mechanism of CeRu4Sb12 more precisely and arein progress.
References[1] H. Sato et al, Phys. Rev. B 62, 15125 (2000)[2] I. Shirotani et al., J. Solid State Chem. 142, 146 (1999)[3] N. Takeda et al, J. Phys. Conds. Matter. 13, 5971 (2001).
[4] M. Kobayashi et al., Physica B 329-333, 605 (2003).
[5] K. Abe et al., Physica B 312-313, 256 (2002).
[6] E.D. Bauer et al., J. Phys. Cond. Matter 13, 4495 (2001)
Authors
N. Kurita, M. Hedo, Y. Uwatoko, M. Kobayashia, H. Sugawaraa, and
H. Satoaa Tokyo Metropolitan University

Nonlinear Optics in the Extreme Ultraviolet
Watanabe Group

Fig. 1. Temperature dependence of electrical resistivity in CeRu4Sb12
under several constant pressures, 1.5-8 GPa. The inset in figure 1 shows
the resistivity at low temperature under 6 GPa. At 1.5 GPa, the resistivity exhibits a maximum (ρmax) around 80 K (Tmax) above which itshows -logT dependence due to Kondo scattering, based on the samemechanism at ambient pressure. Below 80 K it decreases rapidly. The
resistivity has small pressure dependence, up to 5 GPa, except that
ρmax tends to be gradually remarkable with increasing pressure. As
shown in Fig.1 inset, the resistivity under 6 GPa has a small maximum
around 8 K. With further increasing pressures, the resistivity isenhanced as a whole and drastically at the low temperature regime.
Semiconducting behavior was stabilized under high pressure in
CeRu4Sb12. While, the resistivity at 8 GPa shows a tendency to saturatewith decreasing temperature.

Attosecond pulse generation was realized through highharmonic generation and has made it possible to observe
attosecond phenomena [1]. Another interesting application of
attosecond pulses is nonlinear optics in the XUV and soft
x-ray regions, demanding the spatial and temporal coherenceof light sources. So nonlinear optical phenomena have neverbeen observed by using synchrotron orbital radiation andhigh harmonics are promising for such investigations.
However, the pulse energy of attosecond pulses available so
far does not appear to be intense enough to cause nonlinear
optical phenomena. In this work, intense attosecond pulses
were nonadiabatically generated by using 8-fs blue laser
pulses. We have observed the above-threshold ionization(ATI) of rare gases by XUV pulses [2] and demonstrate theautocorrelation measurement of isolated attosecond pulses.
8-fs blue laser pulses producing high harmonics weregenerated by broadband frequency doubling of Ti:sapphirelaser pulses (photon energy 1.55 eV) [3]. Broadband frequency doubling enables us to shorten the pulse duration
with high efficiency. The blue laser induces relatively larger
nonlinear dipole moment. At the same time, high harmonic
pulses rises rapidly nonadiabatically and the field ionization
shuts off the harmonic generation, producing intense isolated
attosecond pulses.
The ninth harmonic pulses of the blue laser (photon energy 27.9 eV) were generated by focusing the blue laser pulses
into Ar gas and were characterized by the autocorrelation
technique, in which the two-photon ATI process in helium
atoms was used. The ATI process was confirmed by observ-

ISSP Activity Report 2004

15

Fig. 1. Autocorrelation traces of the ninth harmonic pulses (27.9 eV):The pulse durations of the driving laser were 8.3 fs (a) and 12 fs (b).
The fitting results to the Gaussian function are shown in the red lines.
The blue lines are the calculated autocorrelation functions from thespectra.

ing the photoelectrons ejected from helium atoms. The
photoelectron yields were recorded as a function of therelative delay time of two high harmonic pulses to form an
autocorrelation trace. The results are shown in Fig. 1. The
shortest pulse duration obtained was 950 as with pulse energy of 2 nJ [4].
The autocorrelation measurement is the simplest type of
pump-probe experiment and is easily extendable to timeresolved spectroscopy. Attosecond time-resolved spectroscopy in the XUV region is expected in the near future.

examples is superconductivity, in which the pairing of twoelectrons makes a tiny energy gap at EF, leading to unexpected physical properties. Photoemission spectroscopy is a powerful tool to measure the electronic structures. However,being sensitive to surface, such studies have not been able toprovide reliable data for f-electron superconductors which
exhibit a surface electronic structure different from thebulk[1]. Nonetheless, they exhibit intrinsic but anomalous
normal- and superconducting-state properties. Moreover, theextremely small energy scale of the f-electron superconductors has made it difficult to be studied in detail. Escape depthof photoelectron shows strong photon energy dependence
and the increase of escape depth using soft x rays has been
demonstrated with a resolution of ~100 meV. But the value
is 100 times larger than the energy scale of superconducting
gap. However, as shown in known dependence (universal
curve[2]), bulk sensitivity can be achieved to use much lower
photon energy (<10 eV). And to get higher energy resolution
than He discharge lamp (line width of ~1.0 meV) is required
to observe the superconducting gap of f-electron systems. To
overcome these limitations, we have constructed a new photoemission spectrometer using vacuum-ultraviolet laser as aphoton source, as illustrated in Fig. 1(a). The spectrometersystem is built using a newly-developed hemisphericalelectron analyzer (GAMMADATA-SCIENTA R4000) andlaser system producing a high-flux (2.2 #~ 1015 photons/sec)quasi-continuous-wave (quasi-CW, a repetition rate of
80 MHz) with the highest energy of 6.994 eV developed by
Watanabe group (ISSP)[3]. The use of the quasi-CW laser isessential for preventing space-charging effects that broadenenergies of photoelectrons. As shown in Fig. 1 (b), an
ultrahigh-resolution measurement of a gold Fermi-edge
spectrum measured at 2.9 K shows a good correspondence tothe convolved Fermi-Dirac function, indicating the energy
resolution of 360 µeV.
Using the newly developed system, we have measured an

References[1] E. Goulielmakis, M. Uiberacker, R. Kienberger, A. Baltuska,
V. Yakovlev, A. Scrinzi, T. Westerwalbesloh, U. Kleineberg,
U. Heinzmann, M. Drescher, and F. Krausz, Science 305, 1267 (2004).
[2] N. Miyamoto, M. Kamei, D. Yoshitomi, T. Kanai, T. Sekikawa, T.
Nakajima, and S. Watanabe, Phys. Rev. Lett. 93, 083903 (2004).
[3] T. Kanai, X. Zhou, T. Sekikawa, S. Watanabe, and T. Togashi, Opt.
Lett. 28, 1484 (2003).
[4] T. Sekikawa, A. Kosuge, T. Kanai, and S. Watanabe, Nature 432,
605 (2004).
Authors
T. Sekikawa, A. Kosuge, T. Kanai, and S. Watanabe

Development of Laser Excited Ultrahigh
Resolution Photoemission Spectrometerand the Study of Unconventional
Superconductor CeRu2
Shin Group
It is well known that the variety of properties that conducting materials display stems from the electronic structure
at and near Fermi level (EF). One of the most fascinating

16

ISSP Activity Report 2004

Fig. 1. (a) Schematic diagram of the PES spectrometer system using alaser as a photon source (Laser-PES) and a demonstration of the submeV energy resolution. The 2nd harmonic of a quasi-CW frequencytripled Nd:YVO3 laser by using an optically-contacted prism-coupled
KBe2BO3F2 (KBBF) crystal is focused on a sample with a CaF2 lens[3]. The kinetic energies of electrons emitted from the sample are measured with a high-precision hemispherical electron analyzer (GAMMADATA-SCIENTA R-4000). The PES spectrometer and the laser systems are vacuum-separated by a CaF2 view port, through which the
produced 6.994 eV light can be transmitted. (b) Ultrahigh-resolution
PES spectrum of an evaporated gold film measured at 2.9 K (red circles), together with the FD function at 2.9 K convolved by a Gaussianwith full width at half maximum of 360 µeV (a blue line). Total energy
resolution of 360µeV was confirmed from the very good match
between the experimental and calculated spectra.

1.35 meV with a leading-edge shift to higher binding energy,
indicative of the opening of superconducting gap. The formation of the gap across Tc is more clearly seen in the inset
showing symmetrized spectra, which removes the effects of
the Fermi-Dirac function. These results constitute the first
PES measurement of superconducting electronic structuresof an f-electron superconductor. Figure 2(b) shows thenormalized superconducting spectrum, compared with threecalculated spectra with (∆max, ∆min, Γ) = (0.98, 0.98, 0.13;
isotropic case), (1.12, 0.50, 0.90; anisotropic case),[1.18, 0.00, 0.001; maximum anisotropy (nodal) case] inunits of meV. Calculated spectrum using an anisotropic gap
reproduces the experimental spectrum very well. These
analyses indicate the superconducting gap of CeRu2 has ananisotropy with ∆min/∆max ratio of 0.446.
The present success in probing the bulk superconducting
electronic structure of an f-electron superconductor, CeRu2
having a delicate surface, using laser PES opens up important opportunities in solid-state physics. First, the technique
can be applied to any other material displaying differing surface and bulk electronic states. Second, the laser has a great
advantage in the small natural line width (260 µeV) without
sacrificing total photon numbers, indicating a great potential
for further increase of the energy resolution. This feature isin contrast to other light sources where increasing resolution
inherently reduces the intensity. Third, utilizing k-resolving
capability of PES, one can measure k-dependent electronic
structures, especially for the k-dependent superconducting
gap. These characteristics indicate that Laser-PES is a uniqueexperimental probe to study the anisotropic superconducting
order parameter of correlated materials. Such studies will
reveal the origin of unconventional superconductivity originating in the interplay of localized and itinerant electrons,
which is one of the fundamental questions in the condensed
matter physics.
References[1] A. Sekiyama et al., Solid State Commun. 121, 561 (2002).
[2] M. P. Seah and W. A. Dench, Surf. Interface Anal. 1, 2 (1979).
[3] T. Togashi et al., Opt. Lett. 28, 254 (2003).
[4] T. Kiss et al., Phys. Rev. Lett. 94, 057001 (2005).
Authors
S. Shin, K. Ishizaka, T. Kiss, T. Yokoya, C. Chen, T. Togashi, andS. Watanabe
Fig. 2. Ultrahigh-resolution PES data of CeRu2. (a) T-dependent ultrahigh-resolution spectra near EF with an inset showing the symmetrized
spectra from the same data. (b) Normalized SC DOS at 3.9 K comparedwith calculated spectra. Error bars correspond to square root of intensity. Blue, red, and green lines are results of fittings with anisotropic gapsof a form ∆(θ) = ∆min+(∆max - ∆min) # cos(2θ) of (∆max, ∆min, Γ) =(0.98, 0.98, 0.13), (1.12, 0.50, 0.07), and (1.18, 0.00, 0.001) in units ofmeV, where Γ is a thermal broadening parameter and θ the polar angle(0 E θ E π/4). The inset schematically illustrates the anisotropic gap,
where a red broken circle represents FS and the distance between bluelines correspond to a gap value.

f-electron superconductor CeRu2[4], which has the highest
Tc (6.2 K) among the Ce compound. High-quality single
crystals of CeRu2 with the residual resistivity ratio of ~270
were provided by Prof. Y. Onuki (Osaka Univ.). The totalenergy resolution for the measurement was set to 520 µeV.
Clean surfaces were obtained by fracturing samples in situ(< 2 # 10-11 Torr) at 3.8 K.
In Fig. 2(a), temperature dependent ultrahigh resolution
PES spectra across Tc measured with a total energy resolution of 520 µeV are shown. While the spectrum at 8.0 K,
normal phase, has a clear Fermi edge, the spectrum at 3.8 K
in the superconducting phase shows a sharp peak at

Si(111)-6 #1-Ag Structure Studied by
Surface X-Ray Diffraction
T. Takahashi Group
Metal induced reconstruction of silicon surface attracts
much attention from viewpoints of physical interests andapplications. In the case of silver on Si(111), it is well known
the 3 # 3 -Ag surface is formed when one monolayer ofsilver is deposited on the surface at temperature about
550cC. The 3 #1-Ag surface appears after further heating the3 # 3 -Ag surface at about 600cC. The 3 #1-Ag structure
reversely changes to the 6 #1-Ag structure at room temperature. Whereas the 3 # 3 -Ag structure is well establishedafter a long controversy [1], the structure of the 3 #1 and 6 #1
[2] remains ambiguities.
Metal induced 3 #1 structures appear on Si(111) for alkali metals such as Li, Na, K and Mg. The honeycomb-chain
channel model [3] is most probable for those 3 #1 structures

ISSP Activity Report 2004

17

Authors
K. Sumitani, T. Hoshino, K. Masuzawa, Y. Nodumi, S. Nakatani,
H. Tajiria, K. Akimotob, H. Sugiyamac, X. Zhangc, and H. Kawataca JASRI/Spring-8
b Nagoya University
c KEK-PF

Strong Optical Absorption
by a Single Quantum Wire
Akiyama Group

Fig. 1. Intensity along 00 rod from the Si(111)-6 # 1-Ag surface. Circles
indicate measured intensities, and solid curve is the calculated one forthe best fit model shown in Fig. 2.

where metals are arranged in a chain, but the atomic arrangement of the 3 #1 structure for Ag is not studied compared tothose alkali metal induced surfaces.
In this work we study the perpendicular structure of the
6 #1-Ag surface by the measurement of X-ray crystal truncation rod scattering. Experiments were made using the six-circle diffractometer install in the beam line 15B2 of the Photon
Factory. It is essential to measure the diffraction intensity atpoints with large perpendicular momentum transfer to determine the structure with high resolution. Figure 1 shows thediffraction intensities measured along the 00 rod. Sharp
peaks at L=3 and 9 correspond 111 to and 333 Bragg reflections, respectively. The intensity change shows a strong
asymmetry around 333 Bragg reflection. Intensities at somerange between Bragg points were less than the backgroundlevel. This weak intensity comes from the destructive interference between surface layers and the substrate, and thus
suggests that the perpendicular structure is rather simple.
The analysis was done based on the 3 #1 structure
because the perpendicular structures are expected to change
little in the phase transition between 3 #1 and 6 #1 structures.
In the analysis we tried some models with different Si density. Least squares fits were done for each model by changing
the height of Ag and Si atoms where the coverage of Ag was
fixed to 1/3. The best fit was obtained for the model wherefour Si atoms in the 3 #1 structure are located almost in
the same height as illustrated in Fig. 2. The height ofreconstructed Ag and Si atoms from the substrate was 3.0 Å
and 2.3 Å, respectively. These values are almost the same
those obtained for the 3 # 3 -Ag structure. This meansthat the perpendicular structure changes little as the structuralchanges from 3 # 3 to 3 #1.

A direct measurement of optical absorption by a nanometer-scale quantum wire has been one of the major challenges
in nano-science. We have recently performed such an experiment, and observed strong optical absorption by a single
quantum wire: About 98% of incident light is absorbed by a
14nm # 6nm single GaAs quantum wire of 500µm-length,
though the wire is much smaller than a floating dust in air!
A quantum wire (T-wire) is formed at a T-shaped intersection of two GaAs/AlGaAs quantum wells, as shown in
inset of Fig. 1(a). The red contour curves show the wavefunction of electrons confined in a 14 nm # 6 nm lateralsized wire. The T-wire is embedded coaxially in the center ofan optical waveguide. Length of the T-wire and the waveguide is 500µm. In the absorption measurement, incident
light goes into the waveguide from one end, travels 500µm
along the T-wire, and comes out from the other end of the
waveguide. Coupling efficiency of the incident light is 24%,and the T-wire optical-confinement factor Γ = 4.6 # 10-4.
Figure 1(a) shows the transmittance spectrum for the
single T-wire around the band gap at 5 K, where strong

References[1] T. Takahashi and S. Nakatani, Surf. Sci. 282, 17 (1993).
[2] C. Collazo-Davila et al., Phys. Rev. Lett. 80, 1678 (1998).
[3] S. C. Erwin, Phys. Rev. Lett. 75, 1973 (1995).

Fig. 2. Side view of the best fit model for the Si(111)-6 # 1-Ag surface.

18

ISSP Activity Report 2004

Fig. 1. (a) The transmittance spectrum for a single T-wire at 5 K. The
inset shows the T-wire structure and transmission light with horizontal
polarization. (b) The absorption spectrum derived from the transmittance spectrum. The 1D exciton shows a strong and narrow absorption
peak.

attenuation is observed at 1.5828 eV and above 1.600 eV. In
the other region, we observe clear Fabry-Perot oscillations.
Figure 1(b) shows the absorption spectrum for the single
T-wire derived from the transmittance spectrum. Threeabsorption structures of the 1D exciton, the excited stateof 1D exciton, and the 1D continuum states, are resolved.
The absorption peak of 1D exciton has a width of 1.6 meV.
The absolute value of absorption for 1D exciton shows
α =80 cm-1. This value gives e-αL = 0.018 for waveguidelength L=500 µm, which means that 98% transmission light
is absorbed by a single pass of the 500-µm-long waveguidein spite of the small confinement factor Γ. This demonstrates
strong excitonic absorption of the single T-wire.
Reference[1] Y. Takahashi, et al., Appl. Phys. Lett. 86, 243101 (2005);
CLEO/QELS2005, Baltimore, USA, May 2005.
Authors
Y. Takahashi, Y. Hayamizu, H. Itoh, M. Yoshita, H. Akiyama,
L. Pfeiffera, and K. Westaa Bell laboratories, Lucent Technologies, USA

Optical Study ofa Single-Quantum-Wire FET
Akiyama Group
Optical investigations of a single-quantum-wire fieldeffect transistor (FET), that is an n-type modulation-doped
single T-shaped quantum wire with tunable 1D electron density via gate voltages, are made via photoluminescence (PL)and PL excitation (PLE) spectroscopy at 5 K. The sample
structure and experimental setup are schematically shown in
Fig. 1.
The measured PL and PLE spectra at various gate voltages (Vg) are shown in Fig.2 (a). The evolution of theseFig. 2. Experimental (a) and theoretical (b) spectra of PL (red) and PLE(blue), where 1D electron density increases with the applied gate voltage. Each spectrum is normalized by peak intensity. The evolution of
the spectra shows metal-insulator crossover, where the peaks of excitons (X) and trions (X-) evolve to band-to-band transitions in the presence of an electron plasma. The theoretical curves are calculated withthe free-particle approximation.

spectra shows the metal-insulator crossover from excitonic
discrete peaks to band-to-band transitions with large
Burstein-Moss shifts. The PL and PLE spectra in the
band-to-band transition regime are compared with a calculation based on the free-particle approximation, as shown in
Fig. 2 (b).
In low-density regime (Vg=0-0.2V), we observed complete oscillator-strength transfer from excitons (X) to trions(X-). At high-density regime (Vg=0.5-0.7V), a broad absorption onset corresponding to the Fermi edge shows blue shift
with increased density. In the crossover region from trion to
band-to-band transition (Vg=0.3-0.4V), we found a double
peak structure corresponding to the Fermi edge and squareroot divergence of 1D density of states. These spectralfeatures demonstrate bound-state formation and carrier
population in inverse-square-root density of states inherent to
1D systems.
Reference[1] T. Ihara et al, The 27th international conference on the physics ofsemiconductors (ICPS-27), Flagstaff, USA, July 2004.

Fig. 1. Schematic structure of n-type doped T-shaped quantum wire.
The cross-sectional region of 14nm quantum well (stem well) and 6nm
quantum well (arm well) works as a 1D wire. The stem well is modulation doped and connected to the ground level. We tune 1D electron
density in the wire by applying gate voltage (Vg). Excitation and detection are perpendicular to each other both in light directions and inpolarizations, to eliminate intense laser-scattering noises.

Authors
T. Ihara, Y. Hayamizu, M. Yoshita, H. Akiyama, L. N. Pfeiffera, andK. W. Westaa Bell laboratories, Lucent Technologies, USA

ISSP Activity Report 2004

19

Magnetic Dead Layers Induced by
Strain at Fct Fe/Rh(001) Interface
Kakizaki group
It is known that in Fe films grown epitaxially on a substrate with a large lattice misfit, a strain at the interface causes the elastic deformation of the films, i.e. the Poisson effect,
and leads to face-centered-tetragonal (fct) structure. The
strain at the interface is released during the film growth by
the change of the lattice constant or by the dislocation at theinterface, which modifies the structural and magnetic phasesof the Fe films. It has been theoretically shown that fcc andfct Fe manifest diverse magnetic phases depending on thelattice constants [1-3]. Recently we studied the structure andmagnetism of the Fe films grown on the Rh(001) substrate(a0=3.80 Å) with a relatively large lattice misfit [4, 5]. The
structure of the Fe film reveals fct(001) compressed along
the direction perpendicular to the surface in low coverageregion and gradually changes to distorted bcc(110) as thefilm thickness increases. To investigate the relationshipbetween the structure and magnetism during the film growth,

Table 1. Total energy minimum, E, for the NM, AF, and FM phases offcc, and fct Fe. The values are presented after subtracting the minimum
energy for fcc FM Fe, Efcc FM Fe. Lattice constants (a, b, and c) andratio of c/a at the minimum energies are also shown.

Fig. 2. Thickness dependence of XMCD signals of the Fe/Rh(001) system measured at Fe 2p3/2 absorption edges. The inserted figure illustrates the geometry of the XMCD experiments.

we have made band structure calculations, spin- and angleresolved photoelectron spectroscopy (SARPES) and soft
X-ray magnetic circular dichroism (XMCD) experiments.
We have calculated the total energies of three different
magnetic phases, i.e. nonmagnetic (NM), antiferromagnetic(AF) and ferromagnetic (FM) phases, as a function of an
out-of-plane lattice constant c. The results show that the totalenergy minimum for FM phase is smaller than those for AF
and NM phases. Since the ratio of c/a at this equilibrium
state is very close to those of 4 ML and 5 ML Fe films(c/a=0.84), which were evaluated by the X-ray photoelectron
diffraction experiments [4], we conclude that 4 ML and 5
ML Fe films on the Rh(001) surface are in a fct structure and
FM. We have measured the SARPES spectra of the Fe films
with various film thicknesses. It was observed that in Fe
films over 3 ML, the distinct majority and minority spin
spectra are observed even at RT, which reveals that Fe films
are magnetically live layers, i.e. FM. The TC of the film,
which is estimated by temperature dependence of the spinpolarization of the spectral features, increases with film
thickness and reaches to the value of bulk bcc Fe (1043 K) at
about 6 ML. We also found that TC falls to 0 K in 2 ML film
thickness, which implies that the Fe films below 2 ML aremagnetic dead layers. To confirm above observation we havemeasured the thickness dependence of the XMCD signals at
Fe 2p3/2 absorption edges, and found that the XMCD signals
observed for Fe films below 2 ML are smaller than the detection limit. The results are consistent with those observed in
the thickness dependence of TC and confirms the existence ofthe magnetic dead layers in the Fe films induced by strain at
the interface.
References[1] V. L. Moruzzi, P. M. Marcus, and J. K up bler, Phys. Rev. B 39, 6957
(1989).
[2] T. Asada and K. Terakura, Phys. Rev. B 46, 13599 (1992).
[3] S. L. Qiu and P. M. Marcus, Phys. Rev. B 60, 14533 (1999).
[4] K. Hayashi, M. Sawada, A. Harasawa, A. Kimura, and A. Kakizaki,
Phys. Rev. B 64, 054417 (2001).
[5] K. Hayashi, M. Sawada, H. Yamagami, and A. Kakizaki,
J. Phys. Soc. Jpn. 73, 2550 (2004).
Authors
K. Hayashi, M. Sawada, H. Yamagami, and A. Kakizaki

Fig. 1. SARPES of 3ML Fe/Rh(001) at 100K and at RT. The binding
energies of the spectral features in the majority and minority spin spectra are indicated by vertical bars with corresponding valence bands.

20

ISSP Activity Report 2004

New Orbit Correction Method for Beam
Stabilization in High-Brilliance
Synchrotron Light Sources
Nakamura GroupA new orbit correction method that we proposed and formulated, the eigenvector method with constraints (EVC) [1],is very suitable for orbit feedback scheme of beam stabilization in high-brilliance synchrotron light sources and has
many advantages. The EVC can more tightly fix beam positions at light source points such as insertion devices than theordinary eigenvector method (EV) and also reduce the closed
orbit distortion (COD) of the whole ring like the EV. Namely
the EVC has both functions of exact local orbit correction forlight source points and global orbit correction for the wholering.
Computer simulations successfully demonstrate the features of the EVC. Figure 1 shows simulated vertical CODsof the Super-SOR ring before and after correction by the
EVC and EV. Both methods much reduce the global COD of
the ring. The constraint conditions of the EVC in this simulation are that the beam positions at 28 beam position monitors(BPMs) on both sides of 14 long straight sections used
for insertion devices are zero. Figure 1a shows that the EVC
corrects all the beam positions and angles of the 14 lightsource points to zero, while the EV cannot completely correct the COD at the 28 BPMs, as shown in Fig. 1b. The EVC
can also be applied to the stabilization of the photon beam
positions at the SR beamlines. The EVC requires only one

feedback loop or system for the beam stabilization at manylight source points. Thus the cost and manpower for the
beam stabilization can be much reduced by adopting the
EVC.
The EVC was experimentally studied in the PF ring and
the PF-AR ring at KEK in order to demonstrate the effectiveness of the EVC in "real machines" with various errors andnonlinearities. The PF ring has 65 BPMs for COD measurement, 42 vertical dipoles(VDs) for usual vertical orbit
correction and 28 fast vertical steering magnets(FVSs) for
global orbit feedback. The vertical COD generated by eachof 42 VDs was corrected by 28 FVSs in this study. The constraint conditions of the EVC are that the beam positions at 4
BPMs on both sides of two insertion devices are zero. The
study results of the PF ring are shown in Fig. 2. Figure 2aclearly shows that the EVC corrects all of the RMS(rootmean square) CODs for the constrained 4 BPMs to a submicron level, while the EV cannot reduce all of the RMS
CODs to this level. On the other hand, there is no clear difference between the EVC and EV in the RMS CODs for all
the BPMs, as shown in Fig. 2b. Similar results were also
obtained in the machine study of the PF-AR ring. The EVC
worked well in these real machines in spite of the various
existing errors and nonlinearities. These results demonstratethat the EVC is very effective and useful for orbit correction
and beam stabilization in synchrotron light sources.
Reference[1] M. Satoh, Y. Kamiya, and N. Nakamura, Proc. of EPAC98,
Stockholm, 1998, pp.1723-1725.; Y. Kamiya, N. Nakamura, and
M. Satoh, Proc. of PAC99, New York, 1999, pp.1174-1176.
Authors
N. Nakamura, H. Takaki, H. Sakai, K. Haradaa, and T. Obinaa
a High Energy Accelerator Research Organization(KEK)

Small-angle Neutron Scattering Studyon Pressure-Sensitive Polymer Gels
Shibayama Group

Fig. 1. Vertical closed orbits of the Super-SOR ring before and after
correction by (a) the EVC and (b) the EV. The solid circles indicate 28
beam positions at the constrained BPMs after correction.

Fig. 2. The RMS COD values for (a) the constrained four BPMs and (b)all the BPMs after correction by the EVC and the EV at the PF ring.

Poly(N-isopropyl acrylamide) (PNIPA) hydrogel is oneof the most well-known thermosensitive gels that undergo avolume phase transition at 34cC [1, 2] The temperature sensitivity of the PNIPA gel is ascribed to the hydrophobicity of
the N-isopropyl groups located on the side chain. At low
temperatures, PNIPA chains are solvated in water owing to"iceberg structure" formation of water surrounding the

Fig. 1. (a) P−T phase diagram of a 7 wt % PNIPA-AAc weakly
charged hydrogels. (b) and (c) are the stone-and-sand model explaining
the origin of convexity of the phase behavior.

ISSP Activity Report 200421

peak disappeared at P - 100MPa, and then reappeared byfurther increase in P. The scattering functions were successfully reproduced with the Panyukov-Rabin theory for polymer gels with frozen-inhomogeneities[5].
The analysis indicated that the convexity is due to thequadratic nature of the polymer-solvent interaction withrespect to P as a result of strong pressure dependence of
iceberg formation.
References[1] Y. Hirokawa and T. Tanaka, J. Chem. Phys. 81, 6379 (1984).
[2] M. Shibayama, T. Tanaka, and C. C. Han, Macromolecules 97,
6829 (1992).
[3] I. Nasimova, T. Karino, S. Okabe, M. Nagao, and M. Shibayama,
Macromolecules 37, 8721 (2004).
[4] I. Nasimova, T. Karino, S. Okabe, M. Nagao, and M. Shibayama,J. Chem. Phys. 121, 9708 (2004).
[5] S. Panyukov and Y. Rabin, Phys. Rep. 269, 1 (1996).
Fig. 2. SANS intensity functions of PNIPA-AAc hydrogels at 50 cC
obtained at various pressures. Note that I(q) has a peak at q - 0.02 Å-1
and the peak shifts toward higher q and decreases with increasing P up
to Pc and increases with further increasing P. The solid lines denote thecalculated scattering functions with Panyukov-Rabin theory.

hydrophobic N-isopropyl groups. An increase of temperatureleads to melting of the iceberg structure, resulting in aggregation of PNIPA chains followed by gel collapsing.
Introduction of charges to PNIPA chains leads to an increasein the transition temperature as well as an increase of thegap between the swollen and collapsed phases more than afactor of 100 folds by volume. Because of these unique properties, various applications have been proposed, such as
intelligent drug delivery systems, sensors of various diseases,
recollecting agents of rare and/or toxic metals, etc. Since iceberg formation accompanies a positive volume change, thetransition is expected to be pressure sensitive. We carriedout small-angle neutron scattering (SANS) experiments
in order to investigate the microscopic structure ofhydrogels undergoing volume phase transition by temperature and/or pressure. SANS studies were carried out on
weakly charged poly(N-isopropyl acrylamide-co-acrylicacid) (PNIPA-AAc) gels. The phase diagram was determinedin the pressure-temperature (P−T) plane by light scattering.
Fig.1(a) shows the P−T phase diagram of a 7 wt%
PNIPA-AAc gel. The phase diagram is a parabolic function,
having maximum at (Pc = 100.2 MPa, Tc = 54.7 cC). It isquite interesting that the P− T curve has a maximum.
Fig.1(b) and (c) show the stone-and-sand model that
accounts for the convexity. At atmospheric pressure, anegative volume change of mixing, ∆Vm < 0, was obtained.
Hence, the following inequality is deduced, ∆Vm & ∆Vm,ice,where ∆Vm,ice is the volume change associated with iceberg
formation. Since, the iceberg formation is exothermic and theenthalpy of mixing, ∆Hm, is negative, dT/dP = TdVm/dHm ispositive. This is why the phase diagram has a positive slope.
However, ∆Vm is expected to be positive at high pressures,
resulting in a change of the slope to negative above Pc[3].
SANS experiments were carried out according to theP− T phase diagram. At low temperature and pressure
region, where the systems were in one phase, the scattering
intensity functions, I(q)s, were well described with OrnsteinZernike functions. However, when SANS intensity functions
were collected at 50cC along the horizontal dashed line
shown in Fig.1(a), an re-entrant behavior was observed. As
shown in Fig.2 a scattering maximum appeared in I(q)s at
P = 0.1MPa. This scattering maximum is due to microphaseseparation as a result of antagonism between the electrostaticand hydrophobic interactions. However, as P increased, the22

ISSP Activity Report 2004

Authors
I. Nasimovaa, T. Karino, S. Okabe, M. Nagao, and M. Shibayama
a Moscow State University

Glass Transitions of Room-temperature
Ionic Liquids
Yamamuro Group
Ionic compounds usually have melting temperatures Tmhigher than room temperature because of their strong andlong-range interionic interactions. Recently, it was found thata series of organic ionic compounds have Tm lower thanroom temperature and they are called "room temperature
ionic liquids" or simply "ionic liquids". These compounds
have various useful properties as a solvent, e.g., low vaporpressure, high solubility for both hydrophilic and hydrophobic solutes, etc. In spite of a number of research focusing
attention on their application, few studies on their basic
physical properties have been done. We have measured heatFig. 1. Molar heat capacities of [bmim]Cl and [bmim]FeCl4 measured
with an adiabatic calorimeter in Yamamuro laboratory. Open symbols
denote the data of glassy and liquid states and closed symbols those of
crystalline one. Tg and Tfus represent glass transition and fusion temperatures, respectively. The inset shows the structure of [bmim]+ ion.

Slow Dynamics in Superparamagnets,
Superspin Glasses, and Spin Glasses
Takayama Group

Fig. 2. Quasielastic neutron scattering spectra of [bmim]Cl measured at
353 K for several momentum transfer values on AGNES spectrometerof ISSP. The data are fitted to Lorentzian functions to obtain self-diffusion coefficients. The inset gives the temperature dependence of themean square displacements divided by the value at 20 K.

capacities and neutron scattering spectra of some salts of
1-butyl-3-methylimidazolium ion [bmim]+, which is a typicalcation of ionic liquids [1].
Fig. 1 shows the heat capacities Cp of [bmim]Cl and[bmim]FeCl4. The former compound is important as a prototype ionic liquid and the latter is recently remarked as a magnetic ionic liquid [2]. A glass transition with a large Cp jump
was found at 225 K in [bmim]Cl and 182 K in [bmim]FeCl4.
It is of interest that the fusion of [bmim]FeCl4 is much sharper than that of [bmim]Cl. The temperature dependence ofconfigurational entropy, obtained by integrating Cp/T with T,
demonstrated that both ionic liquids are of "fragile liquids"(liquids with highly disordered structures largely changing
with temperature) as molecular and polymer liquids. This
result was quite unexpected since the ionic interaction generally causes ordering of liquid structure.
In both ionic liquids, quasielastic neutron scattering(Q-dependent broadening of elastic peaks) was observed asshown in Fig. 2. The inset reveals an anomalous increase of<u2> showing an onset of some fast relaxation process
around Tg. Self-diffusion coefficients Ds were derived fromthe data in Fig.2. The activation energy calculated from thetemperature dependence of Ds is 11 kJ/mol, which is considerably smaller than that of the intramolecular rotation ofbutyl-group in organic molecules, 13-15 kJ/mol.
Both calorimetric and neutron scattering data clearly
revealed that the butyl-group of [bmim]+ ion is highly disordered and flexible, and entropically stabilizing ionic liquid
phase. This can be an origin of low melting temperatures ofionic liquids which is the most significant question in thescience of ionic liquids.
References[1] O.yamamuro, Y. Inamura, S. Hayashi, and H. Hamaguchi, J. Chem.
Phys., submitted.
[2] S. Hayashi and H. Hamaguchi, Chem. Lett. 33, 1590 (2004).
Authors

O. Yamamuro, Y. Inamura, S. Hayashia, and H. Hamaguchia
a Graduate School of Science, University of Tokyo

Relatively dense ferromagnetic nanoparticle systems,
now called superspin glasses, exhibit slow dynamics whichis qualitatively indistinguishable from that observed in(atomic) spin glasses and its origin is attributed to frustrating
and random dipole interactions among particle moments(supersins). However, even in dilute nanoparticle systems
where the dipole interactions are vanishingly small (superparamagnets), rather similar slow dynamics is often
observed. We have argued that such slow dynamics originates solely from a broad distribution of relaxation times
which comes from that of the anisotropy energy barriers
against superspins, which in turn are assumed to be proportional to nanoparticle volume V. In fact, the master equationfor each supersin magnetization M(t;V) is easily solved
including its history of temperature T[t] and field h[t] up to
time t, and peculiar slow dynamics, such as the memory
effect shown in Fig. 1, can be reproduced by integrating
M(t;V) with an appropriate V-distribution [1,2].
A qualitative difference in slow dynamics in superparamagnets and superspin glasses is clearly seen in thezero-field-cooled (ZFC) process, since M(t;V) remains zeroin the former when it starts with M(0;V) = 0, while the spinglass (SG) order grows in the latter whose effect can beobserved after the process. Another interesting difference isseen in the field-cooled magnetization (FCM) at T lower thanthe (rounded) cusp temperature: FCM of superparamagnets
continues to increase by further decreasing T (Fig. 1), while
FCM of most of (super)spin glasses tends to decrease thougha little in magnitude. The latter feature, we have argued [3],is interpreted as an indication of non-equilibrium nature of
the FC process which has been roughly considered to beequilibrium one as compared with the non-equilibrium ZFC
process.
Between slow dynamics in superspin glasses and spinglasses there exits the following difference. We have tried toobserve experimentally the rejuvenation effect, i.e., thesensitivity of the SG state on a T-change seen in spin glasses,
in a superspin glass without success on proving its presence

Fig. 1. Memory effect of a superparamagnet which is field-cooled with
two intermittent stops when h is set zero. In the spectrum of M(t;V) at
the lowest temperature the effect of the stops are clearly imprinted(lower-left inset), and its magnetization nearly follows the coolingcurve when the system is reheated. In the upper-right inset the FCM
and ZFCM of the system are shown.

ISSP Activity Report 200423

[4]. The result is attributed to a quantitative difference intime scales of experiments tobs relative to a microscopic(super)spin flip time t0. Namely, since t0's of superspins,
which are governed by thermal activated processes, are much
longer than t0's of atomic spins, in order to observe therejuvenation effect within a common magnitudes ofT-change, we have to set tobs much longer than the experimental time that we could use.
The qualitative and quantitative differences above
discussed are important to get a proper understanding ofslow dynamics in SG-related materials and other complex
systems as well.
References[1] M. Sasaki et al., Phys. Rev. Lett. 93 139701 (2004).
[2] M. Sasaki et al., Phys. Rev. B 71 104405 (2005).
[3] P. E. J op nsson and H. Takayama, J. Phys. Soc. Jpn. 74 1131 (2005).
[4] P. E. J op nsson et al., Phys. Rev. B 71 104404 (2005).
Authors
M. Sasaki, P. E. J op nsson, P. Nordblad, H. Mamiya, H. Yoshino, and
H. Takayama

Room Temperature CMR beyond 1000%in the A-site Ordered Perovskite Manganite
Y. Ueda GroupWe have successfully achieved colossal magnetoresistance (CMR) beyond 1000% at room temperature in perovskite manganese oxides, as shown in Fig. 1. For the last
decade, magnetoresistance (MR) effect has been extensively
investigated from various view points of not only fundamental solid state physics but also application in electronic
devices. Many MR materials have been discovered as ingranular films, tunneling junctions and transition metaloxides. Especially, the research of MR oxides has been
focused on the perovskite manganites, R1-xAxMnO3 (R = rareearth elements and A = Ca, Sr and Ba), because they exhibit
CMR associated with the competition between charge/orbital

order (CO) and ferromagnetic metal (FM) generated by the
double exchange interaction. However, the manganites with
CMR such as La1-xCaxMnO3 and (Nd,Sm)0.5Sr0.5MnO3 havea disadvantage of application; the CMR appears far below
room temperature. Therefore, it has been desired to explore
new materials with CMR around room temperature.
Recently we successfully synthesized the A-site orderedBa-based manganites, RBaMn2O6 [1-3]. RBaMn2O6 has alayer-type order of R3+ and Ba2+ ions in the structure and as
a result the MnO2 square-lattice is sandwiched by two typesof rock-salt layers, RO and BaO, with different sizes. The
FM is dominant in RBaMn2O6 (R = La, Pr and Nd) withrelatively small mismatch between RO and BaO lattices,while in RBaMn2O6 (R = Sm ~ Ho, Y) with large mismatch,
the CE-type CO insulator (COI) is stabilized at the remarkably high temperatures (TCO) far above 300 K. Such high
TCO could mainly originate from the absence of electrostatic
potential disorder at the A-sites in the layer-type order of
R3+ and Ba2+. Since CMR is related to the conversion of COI
phase to FM phase under magnetic fields, MR effect above
room temperature is expected in RBaMn2O6. SmBaMn2O6
with TCO = 375 K is a candidate because it is located near
FM region in the electronic phase diagram of RBaMn2O6.
However the COI phase itself in SmBaMn2O6 is very toughand MR effect is not observed even at 9 T. We have engaged
in the realization of CMR at room temperature by introducing some sophisticated disorder to SmBaMn2O6. We haveachieved the record of MR effect over 340% in the sintered
Sm0.90La0.24Ba0.86Mn2O6 and beyond 1000% in its single
crystal under 9 T at room temperature, where MR(%) is given
by MR(%) = {[ρ(0) - ρ(H)] / ρ(H)} # 100%; ρ(H) and ρ(0) are
the resistivity under 9 T and zero magnetic field, respectively. The MR effect about 340 % is, to the best of our knowledge, the highest among MR(%) reported in transition metal
CMR oxides. In the single crystal, the COI is perfectly converted to FM, showing the remarkable CMR beyond 1000%
at room temperature. The MR efficiency under much lower
magnetic fields can be expected in thin films of the system.
References[1] T. Nakajima, H. Kageyama, H. Yoshizawa, and Y. Ueda, J. Phys.
Soc. Jpn. 71, 2843 (2002).
[2] T. Nakajima, H. Kageyama, H. Yoshizawa, K. Ohyama, andY. Ueda, J. Phys. Soc. Jpn. 72, 3237 (2003).
[3] T. Nakajima, H. Yoshizawa, and Y. Ueda, J. Phys. Soc. Jpn. 73,2283 (2004).
Authors
T. Nakajima and Y. Ueda

Discovery of Metal-Insulator Transitionin Hollandite Vanadium Oxide, K2V8O16
Y. Ueda and Yagi Groups

Fig. 1. Temperature dependences of resistivity under various magnetic
fields and magnetoresistance MR(%) at 9 T in single crystal of
Sm0.90La0.24Ba0.86Mn2O6. The system shows a metal-insulator transition around 320 K under no magnetic field. The resistivity of the insulator phase decreases with increasing magnetic field and the system
becomes metallic at 9 T in the whole temperature range. The inset
shows MR(%) at 9 T around room temperature. The MR(%) is beyond
1000% at room temperature. This is the record of MR effect in transition metal CMR oxides.

24

ISSP Activity Report 2004

We newly found a metal-insulator (MI) transition as afunction of temperature in hollandite vanadium oxide,
K2V8O16. The MI transition has been a highlight in thestrongly correlated electron system. Many compounds in thebinary V-O system show MI transitions accompanied by the
charge ordering and the formation of spin-singlet V4+-V4+
pairs. In the ternary systems, we recently discovered thecharge order transition accompanied by the formation of spin
gap and the devil's flower type phase diagram under highpressure in the 1/4-filled ladder insulator, NaV2O5 [1].

shown in Fig. 2, which satisfies a ratio of V4+/V3+ = 3/1 anda superlattice of 2 # 2 in the a-b plane. The V4+-V4+ andV3+-V3+ pairs are formed in each chain along the c-axis,
resulting in a duplication of the c-axis. The origin of the twostage transition has been open question. We also succeeded
in synthesizing RbV8O16 under the same conditions.
RbV8O16 shows a similar MI transition around 220 K
accompanied by a similar reduction of magnetic susceptibility, but we have not observed any superlatttice in RbV8O16.
References[1] K. Ohwada et al., Phys. Rev. Lett. 87, 086402 (2001).
[2] T. Yamauchi, Y. Ueda, and N. Mori, Phys. Rev. Lett. 89, 057002(2002) .
Authors
M. Isobe, N. Kouno, T. Yamauchi, J. Yamaura, H. Ueda, Y. Ueda,
H. Gotou, and T. Yagi.
Fig. 1. Temperature dependences of resistivity and magnetic susceptibility for K2V8O16.

Furthermore we found the MI transition as a function oftemperature and the pressure-induced superconductivity in
β-vanadium bronzes, A0.33V2O5 (A = Li, Na, Ag, Ca, Sr) [2].
Vanadium oxides have been one of central materials in the
study of strongly correlated electron system, as mentioned
above.
We have engaged in the development of new vanadiumoxides. The hollandite vanadium oxide, K2V8O16 has beensuccessfully synthesized by a solid state reaction under
4 GPa at 1473 K for 1 hour. Small crystals were also foundin the sintered sample. By using single crystals we firstfound a metal-insulator (MI) transition with the two step
jumps of resistivity about three orders around 170 K, as
shown in Fig. 1. The transition is accompanied by a large
reduction of magnetic susceptibility and a structure change
from tetragonal to orthorhombic (or monoclinic). Single
crystal X-ray diffraction study reveals a superlattice of2 # 2 # 2 in the low-temperature insulator phase, suggesting a charge ordering. The reduction of magnetic susceptibility at the MI transition implies the formation of spin-singlet V4+-V4+ pairs and V3+-V3+ pairs. The structure of
K2V8O16 consists of V8O16-framework and K+ located at thetunnel sites of V8O16-framework, as shown in Fig.2. The
V8O16-framework is constructed from the double chains
formed by sharing the edges of VO6 octahedra. K2V8O16 is amixed valent oxide with a ratio of V4+/V3+ = 3/1. We propose a possible charge ordered manner shown in Fig. 2. In
this model, there exist two kinds of double-chain; the red one
formed by V3+ and V4+ chains and the blue one formed byonly V4+ chains. Each double-chain forms two kinds of tube
with K+ within each tube. The two tubes order in a mannerComplete Phase Diagram of Field
Stabilized Novel Ferrimagnetic States
Found in New Chromium Spinel HgCr2O4
Y. Ueda and Takeyama Groups
HgCr2O4 is a new member of chromium spinels. We
found that this material is an antiferromagnet with a relatively small exchange constant. By applying magnetic field, themagnetic structure is easily adjusted; from antiferromagnet
to ferrimagntic state with three-up and one-down spin configuration, and then finally to ferromagnet.
Cubic normal spinels AB2X4 have a pylochlore lattice as a
B-sublattice. It is well-known that antiferromagnet on apyrochlore lattice is a subject of strong spin frustration. For
chromium spinels, magnetic Cr3+ ions have no orbital degen-Fig.2. Schematic drawing of the crystal structure and the chargeordered pattern for hollandite vanadium oxide, K2V8O16.

Fig. 1. Temperature dependence of magnetization curve for HgCr2O4.
At low temperatures, a metamagnetic jump at 10T followed by aplateau region with one half moment and a kink about 35T are
observed. Magnetization plateau phase with a half moment corresponds
to three-up and one-down ferrimagnetic state, and the final state is ferromagnetic state. The inset shows complete H-T phase diagram of
HgCr2O4. By applying high magnetic field, antiferromagnetic stategoes into ferrimagnetic state and then ferromagnetic state.

ISSP Activity Report 200425

eracy, indicating they are pure S = 3 2 Heisenberg spin
system on a pylochlore lattice. At low temperatures, antiferromagnetic orderings are marginally achieved with a help ofstructural distortion for ACr2O4 (A = Mg, Zn, Cd and Hg).
We might expect the formation of completely different spinordering patterns and then different lattice distortions are
realized by applying high magnetic field.
We have found a novel metamagnetic transition for
CdCr2O4 at 28 T[1]. This transition field is too high forvarious measurements. Because of relatively small magnetic
exchange energy for HgCr2O4, the whole feature of magnetization process is expected to be obtained. Magnetization
curve of HgCr2O4 at 1.8K shows a discontinuous jumparound 10 T, followed by a magnetization plateau as shownin the figure. A magnetization jump shows a hysteresis,
indicating a first order transition, which is likely with astructural transition. The magnetic moment at the plateau is
1.5µB per Cr3+, which is exactly one half of the full momentof S = 3 2 Cr3+. One half magnetization plateau can be
assigned to a collinear three-up and one-down ferrimagnetic
configuration. In addition to the half magnetization plateau,
we found a transition to ferromagnetic state as a kink round
35T with hysteresis, indicating a second structural change.
These features can be visually summarized as a H-Tphase diagram in the inset of the figure. Below TN = 5.8 K,
the range of ferrimagnetic state gradually expands, and thenquickly diminishes above TN. The robustness against temperature fluctuation suggests the collinear spin structure. Finite
stable region at T = 0K suggests spin-lattice coupling helps
to stabilize this phase.
In summary, we can draw a complete H-T phase diagramof HgCr2O4, which indicates spin-lattice couplings and thermal fluctuation stabilize the novel three-up and one-down
ferrimagnetic state.
Reference[1] H. Ueda, et. al., Phys. Rev. Lett. 94, 047202 (2005).
Authors
H. Ueda, H. Mitamura, T. Goto, and Y. Ueda

3D Superfluidity and Vortex Physics ofThin He Films Formed on Prous Media
under Fast Rotation
Kubota Group
Non-charged superfluids demand us special technology
to study their unique properties. Recently we came acrossreports of new superfluidity found in quantum solid 4He[1]and possibly in other systems[2]. They are studied using amechanical method, torsional oscillator technique, to detect
the effective mass change of the container, which holds the
sample. One of the common features in such systems as
supersolid is their small superfluid density. This reflects
rather long 3D coherence length of the system.
Submonolayer superfluid 4He films condensed on flat substrates show ideal 2D KT transitions, whereas such films
formed on 3D connected surfaces of porous media show also
3D transitions with rather small 3D superfluid density[3,4].
We have been studying superflow property[5] as well as
torsional oscillator responses[6]. 3D vortex lines have been
detected by our detailed torsional oscillator study[6]. They
are signaled by the circulating superflow around pore vortex

26

ISSP Activity Report 2004

Fig. 1. Dissipation peaks and superfluid density for a He submonolayer
superfluid film on 1 µm pore porous glass substrate, expressed in realand imaginary part of ε under rotation. The numbers refer to rev/sec.

Fig. 2. Non-linear dependence of the rotation induced peak for 10 µm
pore system, compared with 1µm pore system linear dependence.

core. The flow field interacts with 2D vortices and dissipatesenergy and this gives information on the vortex line. The
vortex core is modified by the pore size and we may expect
non-linear behavior because of overlap of the cores underexperimentally achievable conditions[7], which are impossible for the bulk superfluids, 4He and 3He.
Fig.1 shows TO result for submonolayer superfluid He
film condensed on the pore surface of porous glass with controlled pore size of 1 µm, expressed in terms of imaginary
part as well as real part of "dielectric permeability" ε, whichis the ratio of the effectivemass of the superfluid component
participating in the oscillation to the total or "bare" superfluid mass. The imaginary component indicates the energy dissipation caused by 2D vortices, compared with the real part,which is proportional to the superfluid mass. Under rotation
we observe a new dissipation peak, whose height changes
linearly with the rotational speed. This linear dependence isdisplayed in Fig. 2 for 1µm system, whereas data for
10µm pore diameter substrate with the same film thickness
deviates from the linear dependence at rotational speed
beyond 1rev/sec, namely a non-linear behavior for the caseof 10µm pore system. We estimate the vortex line spacing
and find out that this is actually happening because vortex
lattice spacing approaches finite multiple of pore lattice average spacing.
We believe this is the first non-linear vortex phenomenon
approaching Ωc2, observed in a superfluid under rotation.
References[1] E. Kim and M. Chan, Nature 427, 225 (2004); Science 305, 1941
(2004).
[2] see for example, H. Araki et al., J. Low Temp. Phys. 134, 1145
(2004).
[3] T. Minoguchi and Y. Nagaoka, Prog. Theor. Phys. 80, 397 (1988).
[4] T. Obata and M. Kubota, Phys. Rev. B 66, 140506(R) (2002).
[5] M. Kubota et al., J. Low Temp. Phys. 113, 429 (1998).

[6] M. Fukuda, M.K. Zalalutdinov, V. Kovacik, T. Minoguchi,
T. Obata1, M. Kubota, and E.B. Sonin, Phys. Rev. B 71, 212502,(2005).
[7] T. Obata, M. Fukuda, N. Mikhin, J. D. Reppy, and M. Kubota, J.
Low Temp. Phys. 134, 559-565 (2004).
[8] T. Obata, I. Tanuma, T. Igarashi, and M. Kubota, J. Low Temp.
Phys. 138, 929-934 (2005); and to be published.
Authors
T. Obata, I. Tanuma, Y. Ito, T. Igarashi, E. Sonina, T. Minoguchib, and
M. Kubota
a Racah Institute of Physics, Hebrew University of Jenusalemb Inst.on Physics. University of Tokyo, Komaba, Tokyo

Vortex State of 3He-A under Rotation
up to 1 rev/sec Speed, Studied by theMain NMR Line Width Change
Kubota Group
Vortex state of the unique system of superfluid 3He has
been studied under rotation for many years. In the anisotropic superfluid 3He-A, there have been four different vortex
states experimentally reported. The vortex state has beenstudied most commonly by torsional oscillator technique and
by NMR techniques. Since the earliest NMR experiment, in
addition to the appearance of vortex satellite peak, the main
NMR line width in the A phase was realized to be broadened under rotation. A unique theoretical work by Fomin and
Kamenskii(1982) predicted three different temperature
ranges where NMR line width change under rotation to show
different dependences on the reduced temperature by considering spin diffusion constant change in the vicinity of Tc andspin wave velocity at the lowest temperature range. In
between they predicted a plateau of the line width change asa function of the reduced temperature. Experimental work
has been reported by Helsinki Group, Eltsov et al.(2001) andreferences therein. As is discussed by Eltsov et al., the mostcommon vortex in 3He-A is Continuously Unlocked Vortex(CUV). So far the predicted three temperature ranges have
not observed in experimental study.
We report our NMR line width measurements and discuss the three distinct temperature ranges where we could
obtain different features under rotation up to 1 rev/sec speed,which had not been realized till present study.
Fig. 1. illustrates an example of temperature sweep under
the highest rotational speed of 6.28 rad/sec. We made such

Fig.2. 3He-A main line width change as a function of reduced temperature for different rotational speed given in rad/sec.

measurements also at 4.5, 3, 2, 1rad/sec speed and further wemade rotational speed Ω sweeps at fixed temperatures. Fig. 2.
indicates rather clearly the different reduced temperature
τ= 1-T/Tc dependence changes predicted in the old theory[1],but the linear rotational speed dependence was realized onlyat the lowest temperature (highest τ) region. Since the NMR
line width is determined by dynamical features of spin(d) as
well as mass(angular momentum l) freedom of the orderparameter of 3He-A, further related study is necessary.
References[1] I.A. Fomin and V.G. Kamenskii, JETP Lett. 35, 302 (1982).
[2] V.B. Eltsov, R. Blaauwgeers, M. Krusius, J.J. Ruohio, and
R. Schanen, J. Low Temp. Phys. 124, 123 (2001).
Authors
Y. Kataoka, M. Yamashitaa, K. Izumina, O. Ishikawab, and M. Kubota
a Grad School, Phys.Kyoto Univ.
b Phys. Dept. Osaka City Univ

Fig. 1. NMR spectrum (as a function of frequency shift in kHz from
Larmor frequency.) of superfluid 3He-A under the world record rotational speed of 6.28 rad/sec for different temperatures over the A phase
at P=30 bar. We observe the frequency shift and line broadening as
temperature decreases. In addition, another satellite peak(so called vortex peak) appears at a constant frequency.

ISSP Activity Report 200427