ISSP

Activity Report
2008

Contents

Pages

Preface		1

Research Highlights		

2 - 29Highlights of Joint Research

30 - 43

International Conferences
and Workshops

44 - 45

ISSP Workshops		

46 - 47

Subjects of Joint Research48 - 113

Publications

114 - 155

ISSP
ISSPActivity
ActivityReport
Report2008
2008

Preface

This booklet constitutes the annual report of the activity of
the Institute for Solid State Physics (ISSP) of the University ofTokyo for the fiscal year 2008.
ISSP was established in 1957 as a joint-use central research
institution in the field of basic materials science. After its 40
years of operation at the Roppongi campus, ISSP relocateditself to a new campus in Kashiwa city and started afresh in
April 2000. In 2005, ISSP underwent an external evaluation
by an international review committee. In the review, evaluation
was given on the research accomplishments, especially thoseutilizing new facilities installed at the time of relocation. The
scientific achievements during the first 5 years of research
activity in Kashiwa were highly praised. Valuable comments and suggestions were also given on
the future research directions and the betterment of the ISSP management. In November 2007,
ISSP celebrated its 50th anniversary and held a commemorative symposium at the former Roppongi
campus, which has been renovated as the National Art Center and the National Graduate Institute for
Policy Studies.
ISSP is thus moving forward in the second 50 years with its mission as a hub of the basic materials
science research in Japan. In addition to the various on-going researches summarized in this booklet,
there are several important projects whose status range from actual construction stage to planning/budget-seeking stage. The long-pulse high magnetic field laboratory shall offer a versatile platform forexperiments in high magnetic field. Together with the short-pulse megagauss field installation alreadyin operation, it constitutes the International MegaGauss Science Laboratory. A new synchrotron
radiation beam-line at the SPring-8 facility shall be in operation in autumn this year and a new neutron
beam-line is under construction at the J-PARC MLF facility. Other projects vigorously promoted
in collaboration with the respective communities include, an advanced coherent light laboratoryenvisaged as a state-of-the-art facility of ultrafast, short-wavelength and high-peak-power lasers,
and a computational materials science project planned in conjunction with the Next-Generation
Supercomputer.

June, 2009

Yasuhiro Iye
Director
Institute for Solid State Physics
The University of Tokyo

ISSP Activity Report 2008Research Highlights

References[1] Z. Hiroi, N. Kobayashi, M. Hanawa, M. Nohara, H. Takagi, Y. Kato,and M. Takigawa, J. Phys. Soc. Jpn. 70, 3377 (2001).
[2] H. Yoshida, Y. Okamoto, T. Tayama, T. Sakakibara, M. Yokunaga,
A. Matsuo, Y. Narumi, K. Kindo, M. Yoshida, M. Takigawa, and Z.
Hiroi, J. Phys. Soc. Jpn. 78, 043704 (2009).
[3] F. Bert, D. Bono, P. Mendels, F. Ladieu, F. Duc, J.-C.Trombe, and P.
Millet, Phys. Rev. Lett. 95, 087203 (2005).

Phase Diagram and Spin Dynamics inVolborthite with a distorted Kagomé Lattice
Takigawa and Hiroi Groups
The search for exotic ground states in two-dimensional(2D) spin systems with frustrated interactions is a challenge
in condensed matter physics. In particular, the spin-1/2
antiferromagnetic Heisenberg model on a kagomé lattice,a 2D network of corner-sharing equilateral triangles, isbelieved to have spin-liquid ground state with no broken
symmetry. All candidate materials known to date, however,depart from the ideal kagomé model in one way or anotherdue to disorder, structural distortion, anisotropy, or longer
range interactions. Vorlborthite Cu3V2O7(OH)2·2H2O isan example, which, unlike the ideal model, has kagomélike layers formed by isosceles triangles. Consequently, it
has two Cu sites and two kinds of exchange interactions(Fig. 1, inset). Nevertheless, the absence of magnetic order
down to 2 K, much lower than the Curie-Weiss temperature
115 K, indicates strong effects of frustration [1]. Recently,
Yoshida et al. have significantly improved sample quality
by hyfrothermal annealing, allowing us to study intrinsic
properties at lower temperatures [2].
We have performed 51 V-NMR experiments on ahigh-quality powder sample of volborthite. The NMR
spectrum at B = 1 T shows a sudden broadening near T =1 K (Fig. 2) accompanied with a sharp peak in the nuclear
spin-lattice relaxation rate 1/T 1 (Fig. 3), indicating a

Fig. 1. (Inset) The crystal structure of Cu3V2O7(OH)2·2H2O. (mainpanel) The phase diagram proposed by the present study. The squares(triangles) represent the phase boundaries determined from the NMR
spectra as function of temperature (Fig. 2) (magnetic field). The circles
and asterisks indicate the peak of 1/T1 (Fig. 3) and the onset of thehysteresis in the susceptibility [2].

ISSP Activity Report 2008

remains still open in our case whether the order is truly staticin the sense that G(t) remains finite in the limit of t → ∞.
We found a sudden change of the NMR spectra when
the field exceeds 4.5 T, where the first magnetization step
was observed [2]. The behavior of 1/T1 also changes qualitatively (Fig. 3), indicating a second magnetic phase with
distinct spin structure and dynamics at high B. The B-T phase
diagram established by our work is shown in Fig. 1. We
expect more phases will be identified by future work.

Authors
M. Yoshida, M. Takigawa, H. Yoshida, Y. Okamoto, and Z. Hiroi

Antiferromagnetism in AFe2As2(A=Ba, Sr): the Parent Compounds of the
New Iron Pnictide Superconductors
Fig. 2. T-dependence of the line width (FWHM) at various magnetic
fields. (B’ stands for the center of gravity of the spectrum.) The inset
shows the NMR spectra at 11.243 MHz (B’=1 T).

magnetic transition. Similar but weaker anomalies have beenreported previously [3], which coincided with the hysteresis of the susceptibility. Since the hysteresis in our sample
is suppressed below 0.3 K at 1 T, the magnetic transition
has nothing to do with the spin-glass like behavior. The
dynamics in the low temperature phase is quite unusual.
The T-linear behavior of 1/T1 (Fig. 3) below 0.6 K indicateshighly enhanced low-energy fluctuations incompatible withthe spin-wave theory. The spin-echo decay rate 1/T2 also
shows anomalous enhancement and reaches a finite value at
T = 0, indicating extremely slow fluctuations. This means
the power spectrum of the local spin correlation G(t) =<S(t)·S(0)> has at least two frequency scales as illustrated
in Fig. 3. Although line broadening and a peak in 1/T1
are normally considered evidence for a magnetic order, it

Fig. 3. T-dependence of 1/T1 at various magnetic fields B. The asterisks represent the T-dependence of 1/T2 at 1 T. The solid (dotted) line
shows the power law dependence Tα with α = 1 (1.5). The inset showsa schematic illustration of G(ϖ), the power spectrum of the local spincorrelation function, with two distinct frequency scales.

Takigawa and Ohgushi Groups
The discovery of high temperature superconductivity inthe layered iron-pnictide compounds, first in RFeAsO1-xFx(R: La or other rare earth elements) [1] and subsequently in
A1-xBxFe2As2 (A=Ba, Sr, Ca; B=K, Na), with the superconducting transition temperature exceeding 50 K has opened
new opportunities to investigate the interplay between
magnetism and superconductivity. The undoped parent
compounds (RFeAsO and AFe2As2) are compensated metals
and undergo an antiferromagnetic (AF) transition. The superconductivity appears upon elemental substitution or, in thecase of AFe2As2, by applying pressure.
We have performed the nuclear magnetic resonance(NMR) experiments on 75As nuclei using high quality single

Fig. 1. 75As-NMR spectra in SrFe2As2 at 48.31 MHz obtained bysweeping the magnetic field along the c-axis (red) or along the a- or
b- axis (blue). A single set of quadrupole-split three lines is observedin the paramagnetic state (205 K). Below Tt = 199K symmetric line
splitting occurs for H // c, while the lines shift without splitting forH ⊥ c, indicating that the staggered fields due to AF order is parallel tothe c-axis. This in turn determines the stripe-type spin structure shownin the left inset. The right inset shows the angular variation of the NMR
spectra at T = 20 K with the field rotated in the ab-plane.

Fig. 2. T-dependence of the hyperfine field Δ due to AF order in
BaFe2As2 and SrFe2As2. Δ is determined from the splitting of thecentral line for H // c (crosshairs) or from the line shift for H ⊥ c. The
T-dependences of the intensity of the central line at the paramagnetic
resonance fields shown in the insets provide clear evidence for the firstorder AF transitions with hysteresis.

crystals of BaFe2As2 and SrFe2As2. The NMR spectra shownin Fig. 1 split symmetrically upon entering into the AF phasefor the field along the c-axis but not for the field along thea- or b- axis. This result combined with the symmetry of thehyperfine interaction between As nuclei and Fe moments
imposed by the crystal structure allowed us to select uniquely
the stripe-type AF spin structure shown in the left inset of
Fig. 1. Evidence for the orthorhombic crystal distortion in
the AF phase was provided by the two sets of quadrupole
satellite lines. The splitting of each set shows identicalangular variation in the ab-plane with a phase shift of 90
degrees (Fig. 1 right inset). Similar results were obtained
also for BaFe2As2.
Figure 2 shows the T-dependence of the hyperfine field
due to AF moments, which is proportional to the staggered
moments. The discontinuous jump at the transition temperature indicates first order transitions for both materials. This isfurther confirmed by the hysteresis in the temperature dependence of the intensity of the central line at the paramagnetic
resonance field (inset in Fig. 2).
Figure 3 shows the T-dependence of the nuclear
spin-lattice relaxation rate divided by temperature 1/(T1T).
It shows substantial enhancement in the paramagnetic phasenear the transition temperature, indicating developmentof the AF spin fluctuations. In particular, BaFe2As2 shows
strong enhancement only for H // c. Such anisotropy may
be due to orbital-selective spin fluctuations through thespin-orbit coupling. A finite value of 1/(T1T) at low temperatures indicates that a part of the Fermi surface survives in the
AF phase. Work is now in progress to investigate pressure
induced superconductivity.

Fig. 3. T-dependence of the nuclear spin-lattice relaxation rate divided
by temperature (T1T)-1 in BaFe2As2 and SrFe2As2 for different fieldorientations. In BaFe2As2 strong enhancement of (T1T)-1 near Tt isobserved only for H // c, indicating development of anisotropic spin
fluctuations at the stripe wave vector.

ISSP Activity Report 2008

Magnetic Torque Studies on d-π electron
System TPP[Fe(Pc)(CN)2]2

References[1] Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am.
Chem. Soc. 130, 3296 (2008).
[2] K. Kitagawa, N, Katayama, K. Ohgushi, M. Yoshida, and M.
Takigawa, J. Phys. Soc. Jpn. 77, 114709 (2008).
[3] K. Kitagawa, N, Katayama, K. Ohgushi, and M. Takigawa, J. Phys.
Soc. Jpn. 78, (2009) in press.

Tajima Group

Authors
K. Kitagawa, N. Katayama, K. Ohgushi, M. Yoshida, and M. Takigawa

Low-Temperature Magnetizationof a non-Kramers doublet
ground state system PrMg3:
Evidence of the quadrupole Kondo effectSakakibara GroupIn the cubic compound PrMg3, the J = 4 multiplet of
Pr (4f2) ion splits into four levels due to the crystalline
electric field (CEF) effect, and the ground state becomes aΓ3 non-Kramers doublet. Interestingly, this doublet has nomagnetic dipole moment; it carries purely multipole (twoelectric quadrupoles and one magnetic octupole) degrees of
freedom. Previous specific heat C(T) measurements revealed
no signature of a phase transition above 0.5 K [1]. How thedegeneracy of the Γ3 doublet is lifted as T → 0 has beena matter of considerable interest. The reported C/T value
reaches a huge value of about 2.8 J/mol K2 at 0.54 K [1],suggesting a formation of a heavy fermion state arising fromthe purely multipole degrees of freedom.
Unfortunately, the crystal structure of PrMg 3 is of
Heusler type in which a random site exchange between Pr
and Mg atoms may occur. The site disorder locally breaks
cubic symmetry around Pr sites, and would weakly split theΓ3 doublet with an amplitude that may have some distribution. This effect causes a continuous releasing of the entropyof the doublet, resulting in a large low-T specific heat. Thus
the specific heat data alone are not convincing to assert
the heavy fermion state whose entropy releasing is due toquantum fluctuations. In order to explore evidence of thepossible Kondo effect, we have performed magnetization
3+

Fig. 2 Log-T plot of χ4f below 30 K. The inset shows a linear plot up to
150 K. Solid lines represent the behavior expected from the CEF calculation. Dashed line shows a –T1/2 dependence predicted for a quadrupole Kondo model.

measurements on a single crystal of PrMg3 down to 60 mK
in fields up to 8.5 T [2]. It can be shown that the magnetic
susceptibility of the Γ3 doublet system is much less affectedby a site disorder than the specific heat.
Figure 1 shows the 4f electronic part of the susceptibility
χ4f obtained in a field of 1 T applied along [001] (circles)and [110] (squares) directions. Here the Pr nuclear Curie
term is subtracted from the data. For both directions, acontinuous increase in χ4f is observed on cooling below 3 K.
We found that this increase is not suppressed even in a strong
field of 7 T, ruling out the effect of magnetic impurities.
Figure 2 shows χ4f(T) in a wider temperature range.
We found that χ4f(T) below 10 K is significantly enhanced
over the T-independent Van-Vleck behavior calculated for
the CEF level scheme of the present system (solid lines),and continues to increase down to the lowest temperature.
This anomalous behavior of χ4f(T) is intrinsic to PrMg3. We
consider that the low-T enhancement of χ4f(T) comes fromhybridization between 4f and conduction electrons. The
dotted line in Fig.2 is a –T1/2 behavior theoretically predicted
for the quadrupole Kondo effect in a Γ3 doublet groundstate system [3]. Our data well obey this –T1/2 behavior in atemperature interval 0.3 < T < 2.5 K. A deviation from the–T1/2 dependence is observed in χ4f(T) at very low temperatures below 0.3 K, which is reasonable since the theory
is based on an impurity model whereas PrMg3 is a latticesystem. The present experimental results strongly support theoccurrence of the quadrupole Kondo effect in PrMg3.

The title compound TPP[Fe(Pc)(CN)2]2 is a one-dimensional molecular conductor that exhibits giant negative
magnetoresistance (GNMR) [1]. Here, Pc and TPP denotephthalocyanine and tetraphenylphosphonium, respectively.
This GNMR is an interesting phenomenon, since it is notcaused by a sharp transition such as the field-induced
metal-insulator transition. It is highly anisotropic for themagnetic field direction, reflecting molecular orientation of[Fe(Pc)(CN)2].
F igu r e 1(a) shows t he cr ysta l st r uct u r e of
TPP[Fe(Pc)(CN)2]2. The Pc ring in [Fe(Pc)(CN)2] affords
π electrons that form a conduction band in the solid state.
The Fe(III) at the center of the ring is in the low-spinstate with S = 1/2. Because of the four-fold symmetry of[Fe(Pc)(CN)2], orbital magnetic moment of Fe(III) is notcompletely quenched. Consequently, Fe(III) behaves as alocal magnetic moment having highly anisotropic g factors(gxx = 0.52, gyy = 1.11, gzz = 3.62) [2]. There are strong
interactions between the π electrons in the Pc ring and the delectrons of Fe(III). This intra-molecular d-π interaction is
the origin of the GNMR phenomena in molecular conductorsof [Fe(Pc)(CN)2]. In order to study this GNMR phenomena
in detail, magnetic states of both d and π electron systems
in [Fe(Pc)(CN)2] salts should be clarified on the basis of
microscopic pictures. However, attempts to do so had not
succeeded.
Recently we have measured the magnetic torque andheat capacity of TPP[Fe(Pc)(CN)2]2 salt and analyzed them
together with magnetic susceptibility data [3]. Our study
revealed that: i) the peak in the magnetic susceptibility data

References[1] H. Tanida, H.S. Suzuki, S. Takagi, H. Onodera, and K. Tanigaki,
J. Phys. Soc. Jpn. 75 073705 (2006).
[2] T. Morie, T. Sakakibara, H.S. Suzuki, H. Tanida, and S. Takagi,
J. Phys. Soc. Jpn. 78 033705 (2009).
[3] D.L. Cox and M. Makivic: Physica B 199-200 391 (1994).
Authors
T. Morie, T. Sakakibara, H.S. Suzukia, H. Tanidab, and S. TakagibaNational Institute for Material Scienceb
Tohoku University

Fig.1 4f-electronic part of the magnetic susceptibility of PrMg 3
obtained in a field of 1 T. Crosses denote the raw data including the Pr
nuclear contribution.



ISSP Activity Report 2008

Fig. 1. (a) The definition of the field direction and crystal structure of
TPP[Fe(Pc)(CN)2]2. (b),(c): Field dependence of the magnetic torque atθ = 22.5 deg. (d)-(g): Magnetic torque curves at 8, 10, 12, 13 K.

Fig. 2. (a) The simulation of the torque curves based on Eqs. (2) and (3)at Hc / H0 = 0.3. (b) Field dependence of the torque amplitude calculated using Eqs. (2) and (3).

around 25 K is due to the antiferromagnetic short-rangeorder of d-electrons; ii) π electrons fall into an antiferromagnetic state below 13 K, with fluctuations of state appearing
even at 17 K; and iii) an anomalously large spin-flop field(80 kOe at 9 K) is observed for the π-electron antiferromagnetic state. In this paper we report the summary of our study.
Figures 1(b) & (c) show the field-strength dependenceof the magnetic torque at θ = 22.5° below 17 K and above
23 K, respectively. The magnetic torque in the lowest field
is negative below 17 K. As the field increases, the torquereaches a minimum and abruptly increases, finally becoming
positive. In the temperature range above 23 K, the torque isalmost proportional to H4 on both the positive and negativesides. Inversion of the torque curve occurs around 27 K.
Figures 1(d)~(g) show the toque curves measured at 8, 10 12,
13 K. Interestingly, very complicated torque curves appear at
12 and 13 K.
In order to analyze the results, we numerically calculated
magnetic torque due to d electrons using the anisotropic
Heisenberg model in one dimension. The calculation was
performed in the range of H < 100 kOe and T > 10 K using
the exchange interactions Jx,y/kB = 16 K and Jz/kB = 32 K.
Based on this calculation, we found the torque curves areapproximated by the following equation:
τd (H, θ, T) = A(T)H4 sin(4θ)(1)
A(T) > 0 ( T ≤ 30 K); A(T) < 0 ( T ≥ 30 K)
This equation well explains the torque-curve inversion experimentally observed around T=27 K.
The complicated torque curves below 13 K cannot be
explained in this model. In order to interpret them, we added
the torque component of the antiferromagnetic π electrons inthis way:τ (H, θ ) = τd (H, θ ) + {τC (H, θ) +τD (H, θ) }/ 2.
(2)Here, τC (H, θ) and τD (H, θ) respectively represent thetorque component for the antiferromagnetic chains of π spins
formed by molecules C and D (see Fig. 1a). This contribution is given by⎞χ⊥ − χ// 2 ⎛sin2θDτD (H, θ )= − ―――
H sin ⎨arctan ――――――――⎬ cosφD , (3)2cos2θD − (H / Hc)2 ⎠⎝where χ// and χ⊥ are the magnetic susceptibilities of π spinsin the antiferromagnetic state for the fields parallel andperpendicular to the easy axis, Hc is the spin-flop field of theπ electrons, θD is the angle between the magnetic field andthe direction of easy axis of molecule D, φD is the tilt of thetorque vector of molecule D from the rotational axis. Similar
expression can be defined for τC (H, θ).
Figure 2 shows a simulation based on Eqs. (2) and (3).
The consistency between the observation and calculation issatisfactory. The calculation demonstrates that the spin-flop
field Hc is approximately given by the field where the torqueat θ = 22.5º exhibits a minimum. Experimentally, this field is~ 80 kOe at T = 9 K.

ISSP Activity Report 2008



References[1] N. Hanasaki et al., Phys. Rev. B, 62, 5839 (2000).
[2] N. Hanasaki et al., J. Phys. Soc. Jpn. 72, 3226 (2003).
[3] H. Tajima et al., Phys. Rev. B, 78, 064424 (2008).
Authors
H. Tajima, G. Yoshida, M. Matsuda, K. Naraa, K. Kajitaa, Y. Nishioa,
N. Hanasakib, T. Naitoc, and T. InabecaToho Universityb
Okayama Universityc
Hokkaido University

Metallic Behavior Achieved by aSupramolecular Copper Complex with a
Coordination of Oxidized Pyrazino-Fused
Donors and Mixed Halide Anions as
Ligands, [Cu(I)Cl0.2Br1.3(pyra-TTF)0.5+]
Mori GroupThe functional metal complexes to exhibit conductivity, magnetism, permittivity, optical response, and so on
have attracted a great deal of interest. In particular, metal
complexes with a coordination of functional molecularligands have been intensively studied in view of multifunctionality for the magnetism of metal anions and the conductivity of redox ligands such as magnetic conductors. In
order to explore more varieties beyond the limited acceptor
ligands and broaden the range of d-π interactions, metal
complexes with a coordination of “donor” ligands havebeen synthesized [1-4]. Herein, we report the successfully
prepared supramolecular copper complex with a coordinationof oxidized pyrazino-fused donors and mixed halide anions
as ligands, [Cu(I)Cl0.2Br1.3(pyra-TTF)0.5+] (1) (pyra-TTF= pyrazino-tetrathiafulvalne) which reveals the metallic
behavior down to 250 K.
Figure 1 shows the X-ray single crystal structure of 1;
Cu+ and (Cl-0.2Br-1.3) construct 1D supramolecular chains
along the c axis, where a Cu(I) cation is directly coordinatedby cationic pyra-TTF0.5+. The mixed halides (Cl-0.2Br-1.3)
are located at a normal position and an inversion center withpositionally disorder in the complex. The compositions of
the three samples investigated by crystal structure analyses
are Cu(I)ClxBry(pyra-TTF) [x = 0.160(6), y = 1.339(6);0.207(8), 1.293(8); 0.21(1), 1.29(1)], namely Cu(I)Cl0.2Br1.3(pyra-TTF). This framework of supramolecular mixed
halide chains [CuCl0.2Br1.3]n constructs assembled donor
ligand columns of 1. The overlap integrals calculated by
WinMOPAC (ver. 3.9.0, Fujitsu Limited) and band calculations suggest that this complex is a pseudo-one-dimensional
conductor composed of dimerized donor ligands.

Fig. 2. Temperature dependence of resistivity for 1.

The resistivity measurement was performed on single
crystals with carbon paint as four-probe contacts. As shownin Fig. 2, 1 shows metallic behavior down to 250 K withσrt = 200 Scm-1. The disorder of mixed halide anions mightease the dimerization of donor ligands and afford the metallic
conduction paths; the average half-width of Bragg peak forthe three metallic samples of 1, Δϖ = 0.49 º. The metallic
behavior of 1 is first achieved among supramolecular metal
complexes with a coordination of donor ligands. With
lowering temperature, the resistivity gradually increases
below 250 K (Ea = 6 meV), and drastically goes up at 190
K (Ea = 68 meV). The metal-insulator transition at 190 K isrelated to the low dimensionality of donor ligand columns.
ESR was measured with a Bruker EMS 9.3 GHz X-band
ESR spectrometer at 300, 250 200, 150, 100, 50, and 4 K
to investigate the oxidation state and the copper spin stateof 1. The g values are 2.013-2.014 from 300 K to 4 K; thecomplex has Cu(I) cations and the oxidized pyra-TTF. The
decreases in the ESR line width ΔH and the intensity I(∝ ΔH2h: h; height of estimated Lorentzian signals) wereobserved below 200 K.
To investigate the magnetic properties in detail, thetemperature dependence of magnetic susceptibility for 1
was investigated by a Quantum Design MPMS-XL SQUID
magnetometer under 10,000 Oe. The magnetic susceptibility
χ after the subtraction of the Pascal diamagnetic contribution at 300 K is 1.26 × 10-4 emu mol-1. From 300 K to 190
K, χ is constant, indicating the Pauli paramagnetism of themetallic characteristic, and below 190 K, χ decreases gradually to 0.63 x 10-4 emu mol-1 at 100 K, but remains half ofχ at room temperature (Fig. 3). This “imperfect” singlet
state occurred because the instability of the low dimensionality is suppressed by the linked rigid supramolecular(CuCl0.2Br1.3)n framework.
X-ray crystal structure analyses of 1 at low temperatures,
170 K and 120 K, were carried out to investigate the anomalyof the resistivity and magnetic susceptibility at 190 K. There
were no drastic changes in either cell parameters and overlap
integrals from 293 K, 170 K, to 120 K, or in superlattice
below 190 K, indicating that the CDW (charge density wave)
did not appear. The structures at every temperature areisostructural, where one donor and one unit of (CuCl0.2Br1.3)
are crystallographically independent, suggesting no occurrence of a charge disproportion of donors. Moreover, theabsence of a rapid increase in ΔH below 190 K, together
with the constant g values of ESR signals at all temperatures,indicate no appearance of antiferromagnetic order. Therefore,
the metal-insulator transition at 190 K might be caused by
the incoherent tetramerization of donors to be partial singlet
state. It is characteristic of supramolecular metal complexes
that the coherent electronic and lattice instabilities to acomplete singlet state are prohibited by the framework of[CuCl0.2Br1.3] chains.
In conclusion, we succeeded in synthesizing a metallic
supramolecular copper complex with a coordination tooxidized pyra-TTF and the mixed Cl- and Br– anions as
ligands, [Cu(I)Cl0.2Br1.3(pyra-TTF)0.5+] (1). The assembled oxidized donor ligands create pseudo-1D conduction
paths, since the dimerization of donor ligands is eased by
the disorder of mixed halide anions. Then, 1 shows themetallic behavior down to 250 K and the increase in resistivity down to 190 K. Below 190 K, 1 is transformed intoa semiconductor toward the partial singlet state, but theminimum susceptibility χ at 50 K remains half of χ at room
temperature owing to the rigid framework of supramolecularcopper halide chains [CuCl0.2Br1.3]n. The introduction ofa large variety of functionalities for coordinated ligands
promises the development of new trends in functional metal
complexes.

phenomena are subtle and highly sensitive to sample purity
and homogeneity of the environment, and thus the studyof highly clean system under ambient conditions is ideal.
For the study of such exotic phenomena, heavy-electron
metals of 4f intermetallic compounds have provided prototype materials. An important issue concerning 4f intermetallics is the possibility of the parallelism between thephysical properties of electron-like (4f1 Ce) and hole-like(4f13 Yb) compounds. The study of Ce and Yb materials
indeed finds many similarities deriving from the antiferromagnetic coupling between the 4f magnetic moments
and the conduction electrons (the Kondo screening) whichleads to highly enhanced electronic effective masses at
low temperatures. There has been however, a remarkabledifference among their superconducting properties. Whilea number of Ce-based unconventional superconductors isfound in the vicinity of a quantum critical point, in Yb-based
heavy fermion compounds superconductivity has never beenreported.
Recently, we have succeeded in synthesizing a new
structural form of YbAlB4, β-YbAlB4 [1]. Our detailed
work on the low temperature transport and thermodynamic
properties of β-YbAlB4 has revealed that this material is arare example of a highly clean metal that displays quantum
critical behavior at ambient pressure and under no appliedmagnetic fields [2]. Furthermore, we have discovered superconductivity in the quantum critical regime characterized bypronounced non-Fermi-liquid response [2,3].
The superconductivity is observed below Tc = 80 mK(Fig. 1). Interestingly, the system exhibits pronounced
non-Fermi-liquid behavior above Tc, which is characterized by T1.5 dependence of the resistivity (Fig. 1), T-1/3
dependence of the susceptibility and logarithmic divergent electronic specific heat coefficient [2]. Furthermore,
the magnetic field dependence of the non-Fermi-liquidbehavior in Fig. 1 indicates that β-YbAlB4 is a rare example
of a pure metal that is quantum critical without tuning any
physical parameter, i.e. without doping, applied pressure
and magnetic field [2]. Our study using high-purity single

References[1] S. Ichikawa, S. Kimura, H. Mori, G. Yoshida, and H. Tajima, Inorg.
Chem. 45, 7575 (2006).
[2] S. Ichikawa, S. Kimura, K. Takahashi, H. Mori, G. Yoshida, Y.
Manabe, M. Matsuda, H. Tajima, and J. Yamaura, Inorg. Chem. 47,
4140 (2008).
[3] S. Ichikawa, H. Mori, K. Takahashi, and J. Yamaura, Solid State
Sci. 10, 1724 (2008).
[4] S. Ichikawa and H. Mori, Inorg. Chem., to be publishedcv.
Authors
S. Ichikawa, K. Takahashi, and H. Mori

Quantum Criticality and Superconductivity in
the New Heavy Fermion System β-YbAlB4
Nakatsuji, Sakakibara, and Ishimoto Groups

Fig. 1. Crystal structure of 1 (blue dotted line; (Br1,Cl1)…H1 contacts).

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ISSP Activity Report 2008

Fig. 3. Temperature dependence of magnetic susceptibility of 1.

A challenge for the standard model known as Fermi
liquid theory is anomalous (non-Fermi-liquid) propertiesof metals and still rare but increasing number of unconventional superconductivity, both of which are often found in
the vicinity of the quantum critical point, where a magnetic
ordering temperature is driven to absolute zero by tuninga physical control parameter. These nontrivial collective

Fig. 1. Contour plot of the resistivity exponent α defined by Δρ =(ρ(T) − ρ(T)(0)) ~ Tα in the temperature-field phase diagram [2]. Inset:
Zero-field in-plane resistivity ρab vs. T for various single crystals withdifferent qualities, showing high sensitivity of Tc to sample purity.
This indicates strong pair-breaking effects due to impurities, probably
of nonmagnetic type, and suggests an unconventional character of thesuperconductivity [3].

ISSP Activity Report 2008



Fig. 1. Temperature dependence of the spectral weight of an oscillator
in a quartic potential.

Fig. 2 Temperature dependence of the upper critical field Bc2 along theab-plane (square, blue) and the c-axis (circle, red)[3]. Different single
crystals with similar Tc were used for each field direction measurement.
Broken lines represent the curves obtained by fitting to the experimental
results near Tc using the WHH model. Inset, the crystallographic unit
cell of β-YbAlB4 [1].

crystals indicates that the superconductivity is in the clean
limit and most likely involves heavy quasi-particles [3].
Upper critical fields are anisotropic, and strongly suppressed
for the field along the c-axis, possibly because of theparamagnetic effect due to the divergent c-axis susceptibility(Fig. 2). Strong sensitivity of Tc to sample purity suggeststhat the superconductivity is of an unconventional, non-swave type (Fig.1 inset).
References[1] R. T. Macaluso, S. Nakatsuji, K. Kuga, E. L. Thomas, Y. Machida,
Y. Maeno, Z. Fisk, and J. Y. Chan, Chem. Mater. 19, 1918 (2007).
[2] S. Nakatsuji, K. Kuga, Y. Machida, T. Tayama, T. Sakakibara,
Y. Karaki, H. Ishimoto, S. Yonezawa, Y. Maeno, E. Pearson, G. G.
Lonzarich, L. Balicas, H. Lee, and Z. Fisk, Nature Phys. 4, 603 (2008).
[3] K. Kuga, Y. Karaki, Y. Matsumoto, Y. Machida, and S. Nakatsuji,
Phys. Rev. Lett. 101, 137004 (2008).
Authors
S. Nakatsuji, K. Kuga, Y. Matsumoto, Y. Machida, T. Tayama,
T. Sakakibara, Y. Karaki, and H. Ishimoto

Frequency Spectrum of an
Anharmonic Ionic Vibration
K. Ueda Group
There is now growing interest in the anharmonicionic vibrations in various materials which include thefilled skutterudites, the intermetallic clathrates and the
β-pyrochlore oxides. The anharmonic ionic vibrations arecalled rattling modes when the guest ions are oscillatingin oversized cages. We are concerned with various unusual
properties of the recently found superconductors in the
β-pyrochlore compounds AOs2O6 (A = K, Rb, Cs).
As the size of the guest ions becomes smaller, the vibrational modes of alkaline ions show stronger anharmonicity
which results in the lower frequency for KOs2O6 comparedwith RbOs2O6 and CsOs2O6. It is interesting to note thatthe transition temperature of the superconductivity is theISSP Activity Report 2008

highest for the K compound. The temperature dependence ofthe resistivity of the K compounds shows a strong concavedownward temperature dependence in contrast to the twoother compounds where a usual T 2 dependence is observed.
Recent experiments of NMR relaxation rates at the Ksite have been demonstrated to be entirely dominated by thevibrations of the K ion. Furthermore its temperature dependence has been found to be very anomalous, showing a peakat low temperatures and decreases towards high temperatures. To understand these unusual properties we studied
the anharmonic lattice vibration coupled with conduction
electrons by using the self-consistent harmonic approximation [1]. Subsequently, it was pointed out that the essence of
the unusual behaviors lie in the fact that the spectral weightof vibrational modes shifts to higher energy side as thetemperature is increased [2].
However the two treatments mentioned above assumed
the phenomenological damping of the vibrational modes
which may be justified only within the framework of theself-consistent harmonic approximation. In the present study
we have investigated the phonon Green’s function for any
strength of anharmonicity within the second order perturbation theory with respect to the electron-phonon coupling. It isshown that for any type of anharmonicity the phonon Green’s function is expressed by introducing a self-energy, a vertex
correction and a renormalization constant for each transition
between the two energy levels of the oscillator [3].
References[1] T. Dahm and K. Ueda, Phys. Rev. Lett. 99 187003 (2007).
[2] T. Dahm and K. Ueda, J. Phys. Chem. Solids 69 3160 (2008).
[3] M. Takechi and K. Ueda, J. Phys. Soc. Jpn 78 024604 (2009).
Authors
M. Takechi and K. Ueda

Recently shot noise in the Kondo regime has beenstudied. Especially in Refs.[1,2], the Fermi-liquid nature has
been reexamined. They have considered the "backscattering
current" Ib which expresses a reduction from the perfecttransmission for the π/2 phase shift. It has been shown that
the Fano factor Fb = S/2eI results in a universal fractional
value 5/3 up to V3. This universal feature has stimulated
further studies. A shot-noise measurement has been reported
on 5/3 [3]. In the context of the full counting statistics[4] thisresult has been reproduced.
We have extended this result into any strength of
Coulomb interaction by using the renormalization perturbation theory (RPT). Concerning shot noise, we have used the
new formula of shot noise proposed in our recent study[5].
There, we have an expression of differential conductance:
G = βS/4 -βSh/4 where S is the current-current correlation
function, and Sh is the non-trivial current-charge correlation function and β = 1/kBT. We have called it the nonequilibrium Kubo formula. This is written into Sh = S-4kBTG.
At zero temperature Sh equals the conventional shot noise
S. In the linear response regime Sh is proven to vanish, andthe Nyquist-Johnson relation is reproduced. Furthermore
in noninteracting systems S0-4kBTG0 expresses the known
result of the shot noise. Therefore we have proposed thatSh gives a definition of shot noise at any temperature incorrelated systems. Thus theoretically calculated Sh can becompared with S-4kBTG using S and G measured in experiments. Therefore, the nonequilibrium Kubo formula enables
us to address shot noise at any temperature in correlated
systems [6]. We have applied this approach to the shot noise
in a quantum dot. Then, the Fano factor has been estimated as
Fb = 1+4(R-1)2/[ 1+5(R-1) 2 ]where R is the Wilson ratio. Using limiting values of the
Wilson ratio R = 2 in the Kondo limit, Fb = 5/3 is correctly
derived. Therefore this expression gives the general expression in any Coulomb interaction strength [6].
References[1] E. Sela, Y. Oreg, F von Oppen, and J. Koch,Phys. Rev. Lett. 97, 086601 (2006).
[2] A. Golub, Phys. Rev. B 73, 233310 (2006); ibid. 75, 155313 (2007).
[3] O. Zarchin, M. Zaffalon, M. Heiblum, D. Mahalu, and V. Umansky,
Phys. Rev. B 77, 241303(R) (2008).
[4] A. O. Gogolin and A. Komnik,
Phys. Rev. Lett. 97, 016602 (2006).
[5] T. Fujii, J. Phys. Soc. Jpn. 76, 044709 (2007).
[6] T. Fujii, submitted to J. Phys. Soc. Jpn. (2009).

Fig. 1. In the GICs denoted by MCx, the metal element M is ionized
into MZ+ with Z the valence. The valence electrons released from M go
into either the two-dimensional (2D) π-bands on the graphite layers or
the three-dimensional (3D) interlayer band with the branching ratio off : (1 - f). The 3D electrons with the effective mass m* superconduct
with the formation of the Cooper pairs mediated by the polarization
waves of ions.

room-temperature superconductor.
In pursuit of the latter goal, a proposal was made in 1978
for a scheme of calculating Tc directly from the dynamic
electron-electron effective interaction V(q, ϖ), including
both the phonon-mediated attraction and the Coulomb repulsion on the same footing [1]. The scheme was successfully
applied to the n-type semiconducting SrTiO3 in which polarcoupled phonons leading to the stress-induced ferroelectrics
were clarified to play a crucial role [2]. This success has
assured us of both usefulness of this scheme and adequacy
to employ the random-phase approximation in determining
V(q, ϖ) in the polar-phonon mechanism. In 1982 this scheme
was also successfully applied to the alkali-doped GICs
such as KC8 and RbC8 with Tc in the range 0.01-0.1 K byadopting such a model for the GICs as schematically shownin Fig. 1 [3].
In 2005, a breakthrough occurred in the field of the GIC
superconductors. By changing alkali dopants into alkalineearth ones in synthesizing the GICs, Tc was found to be verymuch enhanced. In CaC6, for example, Tc = 11.5 K, which
was increased further to 15.1K under pressures, and in YbC6,
Tc = 6.5 K. Note that Tc in CaC6 is about a hundred times
higher than that in KC8, though the ion masses are about thesame.
In order to check whether the same model in Fig. 1

AuthorT. Fujii

Mechanism of Superconductivity inGraphite Intercalation Compounds (GICs)

Effect of Coulomb Interaction for
Shot Noise in a Quantum Dot
K. Ueda Group
Recent development of nanotechnology has enabled us to
study the correlated transport in mesoscopic devices. Kondo
effect in a quantum dot is a fascinating example. In this year,
our group has investigated the shot noise for the Kondo effectin a quantum dot.

Takada GroupIn the study of the superconducting transition temperature
Tc by ab initio calculations, the ultimate goal is to predict
Tc by using only the information on elements composing
superconductors. A less ambitious and yet very important
goal is to make an accurate estimation of Tc directly froma microscopic Hamiltonian (in particular, without resort
to the concept of the Coulomb pseudopotential μ*), whichhelps us not only better understand the mechanism of superconductivity but also obtain valuable hints to synthesize aFig. 2. Calculated Tc as a function of the branching ratio f. The calculation is done from first principles based on the model in Fig. 1 withusing the band mass of the 3D electrons for m*. The experimental
result of Tc for each GIC is plotted at the value of f determined by the
band-structure calculation.

ISSP Activity Report 2008

applies to the newly-synthesized alkaline-earth-doped GIC
superconductors or not, we have recently calculated Tc basedon exactly the same scheme [4]. The parameters in the model
have been changed appropriately for the new superconductors; for example, we have used Z = 2 instead of Z = 1,m*~3me (due to the mixing of d electrons with the interlayerband) instead of m* = me (me: the mass of a free electron)and f = 0.16 (reflecting the stronger attractive potential of
M2+ for the interlayer band) instead of f = 0.6. As shown in
Fig. 2, we find that our calculations reproduce all the experimental results quite well, implying that our model must be astandard one representing all GIC superconductors with Tc
ranging more than three orders of magnitude. Our calculations also reveal that the key parameters to control Tc are
Z and m*; the difference in Tc by two orders between KC8
and CaC6 can be accounted for by (1) doubling Z, which
enhances Tc by one order, and (2) tripling m*, which alsoenhances Tc by one order.
Encouraged by this success and with confidence in thepredictive power of our standard model, we have estimatedthe optimum Tc in the GIC superconductors to find that Tc
may become much higher than 10K, if M is chosen as Ti or
V. A further enhancement of Tc, which might enter the range
50 - 100K, can be expected, if two or more kinds of Ms are
suitably intercalated, so that both m* and Z are made large
enough and the polar-phonon energy is made as large as
possible.
References[1] Y. Takada, J. Phys. Soc. Jpn. 45, 786 (1978).
[2] Y. Takada, J. Phys. Soc. Jpn. 49, 1267 (1980).
[3] Y. Takada, J. Phys. Soc. Jpn. 51, 63 (1982).
[4] Y. Takada, J. Phys. Soc. Jpn. 78, 013703 (2009).
AuthorY. Takada

Pump Current Problem in Classical
Stochastic Systems
Oshikawa Group
Fluctuations are abundant in our daily life. They areused efficiently in living matter in order to transmit information and substances. One of the famous examples of suchphenomena is a Brownian motor or ratchet, in which thestochasticity plays an important role to obtain a unidirectional current of particles. In addition, understanding of theFig. 1. An illustrative example of pump current problem induced bycyclic perturbations. Many ions are absorbed into or emitted from a red
blood cell through ion pumps, and the ion concentrations in the cellare maintained constant. When a cyclic perturbation (electric field) is
applied, the pumping mode is activated, and the ion concentrations in
the cell are changed.

10

ISSP Activity Report 2008

shown in Fig. 2. While the model parameters come back to
the initial value after one cycle, the final state vector after
the cycle is in general different from the initial one; the statevector obtains an additional phase after the cycle, as in thecase of quantum mechanics. The additional phase gives all
the statistics of the pump current. In addition, the area in theparameter space enclosed by the loop is related to the magnitude of the pump current. This implies that there is no pumpcurrent when there is only one parameter change; in thiscase, a line is depicted in the parameter space, and the area ofthe line is zero.
The key point of this work is the analogy to the geometrical phase in quantum mechanics. Precisely speaking, ourmodel does not map to a standard quantum mechanicalproblem, since the ‘Hamiltonian’ is non-Hermitian. Nevertheless, many techniques and concepts developed in quantum
mechanics and condensed matter physics can be applied. The
present approach opens a path to deeper understanding ofclassical stochastic systems in the future.
Fig. 2. Phase shift due to the cyclic motion in the parameter space.
Cyclic perturbations make a closed loop in the parameter space. In
contrast, the state vector obtains an additional phase μ after one cycleof the perturbations. This phase gives a net current of the system. In
addition, the area of the red circle in the parameter space is related tothe magnitude of the pump current.

stochasticity in microscopic or mesoscopic systems would
greatly benefit future developments of nanotechnology.
A net current induced by periodic perturbations is known
as a pump current, which is a typical problem of nonequilibrium physics. There are several experiments for the pumpcurrent problem. In one of the experiments, red blood cells
are exposed to an oscillating electric field. Figure 1 shows anillustrative explanation for the experiments. The red blood
cell maintains its intracellular sodium and potassium ioncomposition relatively constant because of the ion transfer
activity of an enzyme called (Na, K)-ATPase in the cell
membrane. It has been shown that the pumping function of
the enzyme is activated due to the oscillating electric field.
The ion transfer is in fact a stochastic phenomenon, andthe experiments imply that cyclic perturbations can control
particle currents using the stochasticity of the system.
A simple stochastic model with particle hopping has
already been proposed, which shows a pump current due toperiodic perturbations to transition rates. A pump current
and its fluctuations have been calculated in an adiabatic
regime, where the periodic perturbations change slowly.
However, the experiments are actually carried out also in
the non-adiabatic regime, where the time scales of particle
movements and external perturbations are comparable. This
calls for theoretical advancements on the non-adiabatic
regime.
We developed a new theoretical approach based on amathematical analogy to quantum mechanics, although westudy a classical stochastic model. By using a generalization of the non-adiabatic geometrical phase in quantum
mechanics called as Aharonov-Anandan phase, we succeededin calculation of all the current statistics even in the
non-adiabatic regime [1, 2]. It was shown that the qualitative
behavior of the average current is consistent with the experimental one; the current shows stochastic resonance behavior
and power-law decay with respect to the frequency of theexternal perturbations. The geometrical phase interpretation
also leads to an intuitive understanding of the condition toobserve a non-vanishing pump current. A periodic change
in time of the model parameters, such as hopping rates, canbe represented by a closed loop in the parameter space, as

References[1] J. Ohkubo, J. Stat. Mech., P02011 (2008).
[2] J. Ohkubo, J. Chem. Phys. 129, 205102 (2008).
AuthorJ. Ohkubo

Nonadiabatic Electron-Nucleus
Dynamics from Time Dependent
Density Functional Theory
Sugino Group
Modern electronic structure theories are mostly basedon the Born-Oppenheimer (BO) approximation to decouple
the electrons and nuclei, but even with their large mass
asymmetry this approximation breaks down when nearly
degenerate adiabatic states strongly couple with each other.
The coupled electrons and nuclei exhibit nonadiabatic
dynamics, which is a hot topic of ultrafast laser science. The
dynamics is described by the BO surfaces and their nonadiabatic couplings (NACs), and thus their accurate evaluation
is a starting point of theoretical study. Here we have developed a feasible scheme to compute the first order NAC, or
the NAC vector, which had been available only through atime-consuming quantum chemical method. Using the NAC
vectors calculated for the formaldimine molecule (CH2=NH)
we have simulated the photo-isomerization process andobtained the quantum yield.
Our method is formulated within the time-dependent
density functional theory (TD-DFT) and is based on the
adiabatic local density approximation (ALDA) for theexchange and correlation of the electrons, but no further
approximations are used for the electrons contrary to similar
existing methods. TD-DFT is a scheme to compute theresponse of the electrons to an external perturbation, andwhen the electrons are perturbed by displacing a nuclear
position, the response contains information on the NAC; thisprovides a means to compute NAC from TD-DFT. When an
algebraic equation is derived to obtain NAC, the calculation
has made extremely simple and robust [1]. Indeed, NAC
of the formaldimine molecule could be computed on meshpoints of nuclear coordinate (Fig. 1) and along the nuclear

Fig. 1. Born-Oppenheimer surfaces of formaldimine (CH2=NH) in thecoordinate space (φ, θ) , for the ground state (S0) and the first excitedstate (S1). The First order nonadiabatic couplings (NACs) are also
shown at the bottom.

trajectory allowing, respectively, full quantum wavepacket
simulations and semiclassical surface hopping simulation.
Figure 1 shows the potential energy surface of the groundstate (S0) and the first excited state (S1) of the formaldimine. Other excited states are separated in energy and thus
low energy dynamics concerns these surfaces only. Becauseof this property, the second order NAC can be obtained
from the first order NAC [1], which enables to conduct a
legitimate full quantum wavepacket simulation. As seen in
the figure, value of the first-order NAC is much larger forthe twisting direction (φ) than for the inversion direction(θ). With this asymmetry in mind the wavepacket simulation was conducted along φ. As φ increases from 0°, whichcorresponds to the Frank-Condon excited geometry, to 90°,
which corresponds the pseudocrossing point which is locatedat a midway between the initial and isomerized geometries,
a nonadiabatic transition occurs. The branching ratio, or
the probability of being on the ground state versus thatremaining on the exited state after the transition, is 0.53. The
component of the wavepacket remained on the excited stateproceeds further in the φ direction but the group velocity isreversed at the classical turning point and the second transition occurs at 90°. After these transitions, 70% of the stateis found isomerized. When the simulation is done using fulldegrees of freedom semiclassically, the dynamics is characterized by two dimensional motion in (φ, θ), and the quantum
yield becomes finally 0.47
The application is presently limited to the transition
between the ground state and an excited state (DFT-restriction) and the excitation should be of one-electron character(ALDA-restriction). The quantum dissipation effect needs
to be accurately described as well. In spite of them, oursimulation has advance a step towards a realistic excitedstate dynamics which has been very rarely studied from firstprinciples.
References[1] C. P. Hu, H. Hirai, and O. Sugino, J. Chem. Phys. 127, 064103(2007).
[2] H. Hirai and O. Sugino, Phys. Chem. Chem. Phys. 11, 4570 (2009).
Authors
H. Hirai, CP. Hua, and O. SuginoaNational Institute for Materials Science

ISSP Activity Report 200811

Enhanced Spin Susceptibility toward
the Charge-Ordering Transition
Kato Group
Charge-ordering (CO) transition in organic conductors has attracted much interest as emergence of strong
correlation effect induced by long-range Coulomb interaction. Many CO materials show a sharp metal-insulatortransition characterized by a rapid exponential increase ofresistivity below the metal-insulator transition temperature
TMI. In contrast to remarkable change in the charge degreeof freedom, the spin susceptibility is frequently less sensitive to the CO transition, but exhibits non-trivial dependence on temperature. For example, the spin susceptibilityof θ-(BEDT-TTF)2RbZn(SCN)4, which is a typical COmaterial, has a broad peak around TMI without any sign ofsingularity. In the weak-coupling theory like random phaseapproximation (RPA), the CO transition is described fromthe metallic side by divergent development of charge fluctuation at a finite wave number corresponding to charge patternin the CO phase. Is it possible to understand gradual increaseof the spin susceptibility toward the CO transition by
development of charge fluctuation? In order to answer thisquestion, RPA is unsuitable because the spin susceptibility
in RPA is not affected at all by charge fluctuation. Therefore, we have to go beyond RPA by considering the vertex
corrections for describing coupling between charge and spin
fluctuations.
We have studied response functions up to leading contribution of vertex corrections in a two-dimensional extended
Hubbard model [1, 2]. We have formulated an iterative
method for producing a series of the non-skeleton diagrammatic expansions within the framework of Baym-Kadanoff
conserving approximation, and have used the first step of thisnon-skeleton conserving approximation (1NSCA). In Fig. 1,
we show the spin susceptibility as a function of the nearestneighbor Coulomb interaction V. The temperature and theon-site Coulomb interaction are taken as T = 0.1 and U = 3,
respectively. The spin susceptibility is gradually enhanced
toward the CO transition point at V = VCO. 1NSCA consistsof a sum of three diagrams for the irreducible response
function as shown in the figure: The self-energy correction(SC), the Maki-Thompson vertex correction (MT), and theAsmalazov-Larkin (AL) vertex correction. In our calculation,
the AL vertex correction is responsible for enhancement of
the spin susceptibility.

Quantum Dot Spin Detector

Our result is consistent with experimental measurement of the spin susceptibility for the organic CO materials,
θ-(BEDT-TTF) 2 RbZn(SCN) 4 and β-(meso-DMBEDTTTF)2PF6. We also speculate that the spin susceptibilityof inorganic CO materials such as β-Na0.33V2O5 may be
explained by our theory. Detailed comparison between
theory and experiment is a future problem. We note that the
spin susceptibility is suppressed by antiferromagnetic spin
fluctuation [3]. Our study of the spin susceptibility in the COmaterials will give a fundamental insight to the interplay ofthe spin and charge degree of freedom in strongly interacting
electron systems.
References[1] K. Yoshimi, Ph. D Thesis, University of Tokyo (2009).
[2] K. Yoshimi, T. Kato, and H. Maebashi, submitted to J. Phys. Soc.
Jpn.
[3] Y. Fuseya, H. Maebashi, S. Yotsuhashi, and K. Miyake, J. Phys.
Soc. Jpn. 69, 2158 (2000).

Katsumoto Group

Fig. 2. Change of the average local magnetic induction in each of the 3× 3 sub-areas of the Hall cross. The steps in the traces signal changes ofthe vortex configuration in the mesoscopic superconductor.

Authors
K. Yoshimia, T. Kato, H. MaebashiaCollege of Liberal Arts and Sciences, Tokyo Medical and Dental
University

Micro-Hall Magnetometry for
the Study of Vortex State in
Mesoscopic Superconductor
Iye Group
When a magnetic field is applied to a clean bulk type-II
superconductor, quantized vortices form the so-called
Abrikosov lattice. In a mesoscopic superconductor withdimensions comparable to the characteristic length scalesof the superconductor, the vortex state is determined by thesubtle interplay among the vortex interactions with othervortices, with the system boundaries and possibly withdefects. Theoretically, this constitutes an interesting andwell-defined oligo-body problem.
We have developed a multi-probe micro-Hall device
fabricated from a GaAs/AlGaAs two-dimensional electrongas (shown in Fig. 1) that can be used for non-invasive local
magnetometry. The Hall cross consisted of channels in a 3
× 3 matrix form. The superconducting sample to be investigated is placed at an appropriate position near the Hall cross.
Measurement of the Hall resistances Rxy for different
combinations of the current- and voltage-leads provides
information on the local magnetic field distribution.
A mesoscopic superconductor (a square sheet of Al with
4 holes at the corners) was placed on top of the Hall cross.
The purpose of 4 holes was to stabilize the configurationof 4 vortices and to explore the possible occurrence of an

Fig.3 Sequencial changes of the magnetic field distribution. The smaller
figures represent the difference between two successive states. The
vortex configurations shown with the solid dots are reconstructed fromthe data.

antivortex. Traces in Fig. 2 show the changes in the local
magnetic induction averaged over each of the 3 × 3 sub-areasof the Hall cross as the external field is swept up from anegative value. Steps in these traces signal the change in thevortex state. Starting from the Meissner state near H = 0,vortices enter one by one.
Figure 3 shows the evolution of the vortex configuration
reconstructed from the data in Fig. 2. The first four vortices
are trapped at the holes near the corners. The stability of the4- vortex state is reflected in the width of the corresponding
plateaus in Fig. 2. It is interesting that there occurs reconfiguration of the 3-vortex state at H = 0.442mT . Similar reconfiguration is also seen for the 7-vortex state at H = 1.767mT.
To summarize, we have developed a multi-probe microHall device with decent spatial resolution that enables us
to trace the evolution of the vortex state. With the sufficient
detection sensitivity, the occurrence of an antivortex in thepresent sample was ruled out.

Electron spin is a promised candidate of a freedom to
be used in next-generation devices such as quantum-bits
or “spintronics” devices. A key technique to realize suchdevices is highly sensitive detection of electron spin polarization. However compared with charge (monopole) imbalance,
which can be easily detected through the flinging electricfield, detection of spin polarization through the flinging
magnetic field is very difficult due to the dipole nature of
spin. Hence, usually spin-polarization in solids is detected
through electric transport entangled with spin polarization, which inevitably affects the spin sources. Such strong
disturbance is usually fatal for fragile spin sources such as
spin-orbit polarization creators in non-magnetic semiconductors. We have developed a highly sensitive method of
spin-polarization detection through excited state spectroscopy with least disturbance to target devices.
Figure 1 is displaying the device setup on a scanningelectron micrograph. A quantum dot (QD) with a plunger
gate is coupled to a side of a quantum wire and the flinging
electric field from this set is detected by a quantum point
contact (QPC) put just aside of the dot. In measurements,
the electro-chemical potential of the dot is periodicallyswung up and down by the plunger gate voltage (Vp). Thisoscillation of the gate causes that of electric field which issynchronously detected by the QPC. When the base levelof Vp is shifted and a discrete energy level comes into theoscillation window, single electrons begin shuttling between
the wire and the dot through quantum tunneling. The shuttle
motion screens the oscillation of the fringe field resulting in
the diminishment of the signal, through which the tunnelingprobability as well as the level position can be detected.
Now we prepare an electronic state which can accommodate only an electron with a spin of certain direction, then thetunneling probability is determined by the product of orbital
wavefunction overlapping and spin-direction projection due
to the Pauli principle. Such preparation can be performedby various methods, e.g., by using two sequential pulses on
Vp with different depths yields signal, which has monotonic
correspondence with spin polarization.

Reference[1] H. Sano, A. Endo, S. Katsumoto, and Y. Iye, submitted to Appl.
Phys. Lett.

Fig. 1. Spin susceptibility as a function of the nearest-neighbor
Coulomb interaction V for T = 0.1 and U = 3. Susceptibility is normalized by the Pauli susceptibility of noninteracting electron systems.
Three types of diagrams treated in the present calculation are also
shown.

12

ISSP Activity Report 2008

Authors
H. Sano, A. Endo, S. Katsumoto, and Y. Iye
Fig. 1. (a) Schematic drawing of the multi-probe micro-Hall device.
The superconducting sample to be studied is placed on top. (b) SEM
image of the sample.

Fig. 1. Scanning electron micrograph of the device setup showing QD(quantum dot), QW (quantum wire), QPC (quantum point contact). P is
the plunger gate, to which rectangular wave voltage is applied.

ISSP Activity Report 200813

dimer with one Si or Sn atoms in the differential conductanceimages at various sample-bias voltages.
Figure 1 shows the topographic STM images of the
Si-Ge dimers on the surface at 80 K. Two kinds of buckled
dimers with a Si atom at either the upper or lower position(Si U- and L-dimers) are made by deposition of Si atoms
on the clean surface at room temperature as in the models.
These dimers can be distinguished by observing theirsample-bias dependence. Standing waves appear and decay
from the both kinds of the impurity dimer. The cross sectionsof the dI/dV images from the Si-Ge dimers are shown in
Figs. 2(a). The standing waves due to a Sn-Ge L-dimer
with a Sn atom at the lower position (Sn L-dimer) [1] areshown in Fig. 2(b) for comparison. The scattering phase of
Si L-dimer has the sign opposite to those of Sn L-dimer and
Si U-dimer. The results indicate the scattering potential by
Si L-dimer is attractive while the potentials due to other two
impurity dimers are repulsive. The signs of the scattering
potential are reproduced by first-principles calculations. The
Fig. 2. (a) Measured spin polarization Γ1/Γ2 (red line) and the wire
conductance (blue curve) as a function of wire gate voltage. Filled
triangles indicate conductance plateau with even conductance quanta(n: even) while open ones those with odd. (b) Number of conductancechannels for up (cyan) and down (red) spins.

We have applied the above method to detect spin polarization in quantum wire placed in strong magnetic fieldparallel to the two-dimensional electron gas, which forms
the wire. The parallel field causes the Zeeman splitting both
inside the wire and the dot. Combination of the former split
and the control of the wire gate voltage can switch on andoff the spin polarization in the wire. On the other hand, thelatter split automatically fixes the spin state in QD, hence
simple rectangular wave oscillation of Vp can produce thespin-polarization signal. Figure 2 shows the wire conductance and thus measured spin polarization as a function of
the wire gate voltage. As a consequence of transverse mode
formation, the wire conductance is quantized to n(e2/h),
where n is an integer, e the electron charge, h the Planck
constant. The spin polarization should appear when n is an
odd integer and disappear for evens. Such behavior indeed
appears in Fig.2 confirming that the proposed detection isreally working.

difference between Si and Sn L-dimers is attributed simply
to energy difference of the valence pz states among Si, Ge
and Sn atoms. [2] We are able to transform one kind of the
Si-Ge dimer to the other kind reversibly by changing the bias
polarity together with the buckling orientation of all the Ge
dimers in the dimer row including the impurity dimer. Thus,
using the Si-Ge dimer, we have realized a scattering phaseswitch in an atomic scale.
References[1] K. Tomatsu, K. Nakatsuji, T. Iimori, Y. Takagi, H. Kusuhara,
A. Ishii, and F. Komori, Science 315, 1696 (2007).
[2] K. Tomatsu, B. Yan, C. Wang, M.Yamada, K. Nakatsuji, G. Zhou,
W. Duan, and F. Komori, Phys. Rev. B 78, 081401(R) (2008).
Authors
K. Tomatsu, B. Yana, M. Yamada, K. Nakatsuji, W. Duana, andF. KomoriaTsinghua University

Vortices in Nano-sized Superconductors
and their Size EffectHasegawa Group

Fig. 1. Topographic (A-D) STM images of Si U- and L-dimers on theGe(001) surface at 80 K for two sample-bias voltages ± 0.5 V. At + 0.5
V, the both dimers are lower than the pure Ge dimer while at - 0.5 V, Si
L(U)-dimer is higher than (has the same apparent height as) the pure Ge
dimer. We identified the both dimers by comparing these STM images
with the local electronic states obtained by DFT calculations for theiroptimized atomic structures. The corresponding schematic models forthe dimers are given in the left column.

When type-II superconductors are subjected to an
external magnetic field, quantized magnetic fluxes orvortices each with a radius of the coherence length ξ areformed. Using a 3He-cooled low-temperature scanningtunneling microscopy (STM), we studied the vortex formation in nano-sized superconductors whose sizes are in anorder of ξ to investigate the size dependence of the vortex
formation [1].
Lead is one of the elemental superconductors, whosecritical temperature is rather high (7.2K). We deposited Pb
on a Si(111) substrate and formed island structures whose
typical sizes are 25-75 nm in radius and 3 nm (9 monolayers)
in thickness with an atomically flat surface. The coherence
length of the Pb thin film is ~ 31 nm at 2.0 K. Because ofa short mean free pass limited by the thickness, the Pb thin

Authors
S. Katsumoto, T. Otsuka, and E. Abe

Atomic Switch for Scattering Phase of
Electrons at Ge (001) Surface
Komori GroupAn electron changes its phase when it is scattered by anatomic potential. The scattering phase shift can be detectedon the metal and semiconductor surfaces by observing theelectron standing wave in the differential conductance (dI/dV) image using scanning tunneling microscopy (STM). We
have studied the reflection phase of one-dimensional (1D)electrons at four kinds of atomic potentials on the clean
Ge(001) surface. The clean Ge surface consists of buckled
dimers, which align in the [110] direction and form dimer
rows. The empty π*-band electrons of the surface Ge dimers
behave like 1D free electrons along the dimer row. We
measured the π*-band standing waves around the buckled

14

ISSP Activity Report 2008

Fig. 2. ZBC measured at two sites A and B marked in Fig. 1(a) undervarious magnetic fields. The jump observed at A corresponds to thevortex formation.

films are type-II superconductors whereas bulk Pb is type-I.
Figure 1 shows an STM image (a) taken on a Pb island
whose radius is ~ 55 nm and tunneling conductance images(b-d) taken at the zero bias voltage under various magnetic
fields perpendicular to the island’s surface. Since thezero-bias conductance (ZBC) is correlated with the densityof states (DOS) at the Fermi level induced by the Cooperpair breaking, higher ZBC indicates lower pair potential(order parameter). Slightly large ZBC area in the peripheralof the Fig. 1(b) image, which was taken at 0.5T, is explainedwith supercurrent circulating there. The ZBC image taken at
0.6 T (c) shows high ZBC area around the center. It is indeed
confirmed as a vortex since its cross-sectional profile waswell fitted with the calculated DOS around a vortex using theEilenberger equation. At 1.0 T (d), the ZBC values are quite
high in the whole area implying complete breakdown of thesuperconductivity in the island.
We also measured ZBC as a function of the magnetic
field at the center of the island (A) and a peripheral site (B)40 nm away from the center. In the profile shown in Fig. 2,
a jump corresponding to the vortex formation is observed
at site A around 0.6 T whereas at site B ZBC increases
monotonically with the field until saturated at 0.9 T. Aroundthe vortex penetration field we also observed a hysteresis of
ZBC and it is explained with an energy barrier, called the
Bean-Livingston barrier, existing in the peripheral of theislands, which the vortex needs to overcome for the penetration (expulsion) into (from) the island.
Since the magnetic fields for the vortex penetration /expulsion can directly be measured for each island from a
ZBC vs. magnetic field curve like the one shown in Fig. 2,
we examined their lateral size dependence. It is found thatlarger islands have lower critical fields and that there is aminimum lateral island size for the vortex penetration. Thesefindings were explained with the theoretical calculations
based on the Ginzburg-Landau equations, and the experimentally obtained minimum size was consistent with theestimated one by the theory [2].
References[1] T. Nishio et al., Phys. Rev. Lett. 101, 167001 (2008).
[2] V. A. Schweigert et al., Phys. Rev. B 57, 13817 (1998).

Fig. 2. Red curves are cross sections of the standing waves of the 1D
π* electrons in the dI/dV images for Si L- and U-dimers (a) and Sn
L-dimer (b) at several sample bias voltages. The data were obtained at
80 K. The dimer is at the origin of the abscissa in each figure. Each blue
curve is the damping oscillation fitted to each red curve. For the fitting,
we used the dispersion relation of the π*-band obtained on the cleansurface.

Fig. 1. (a) STM image of a Pb island whose thickness is 3 nm. (b-d)
ZBC images taken in the black square area of (a) at 2.0K undermagnetic fields below the vortex formation (b, 0.5T), just above thevortex formation (c, 0.6 T), and above the super-normal transition (d,0.9 T). A vortex is clearly visible around the center in (c).

Authors
T. Nishio, T. An, T. Eguchi, H. Sakataa, S. Linb, N. Hayashic,
N. Nakaic, M. Machidac, and Y. HasegawaaTokyo University of Scienceb
National Institute for Materials SciencecJapan Atomic Energy Agency

ISSP Activity Report 200815

Ruddlesden-Popper Quantum Wells
Lippmaa Group
Several different approaches have been proposed for
fabricating two-dimensional oxide quantum wells. In mostcases, the techniques known from traditional semiconductors have been applied to oxides. However, epitaxial oxide
heterojunctions or superlattices do not generally show good
carrier confinement. One of the main reasons for this is
the complicated structural and electronic distortions that
occur at oxide heterointerfaces. Charge transfer across aninterface and the formation of very thick depletion layers
mean that quantum well structures that offer excellent structural confinement do not generally provide good electronic
confinement.
We are attempting to solve this problem by fabricating aquantum well that combines a cubic perovskite lattice with a
Ruddlesden-Popper (RP) phase. This combination is similar
to a bulk RP material with the exception that we embed onlya single RP formula unit in a perovskite matrix, as illustrated
in Fig. 1.
This structure has several advantages over a simple
perovskite heterojunction. The center part of the well structure has the same cation coordination and lattice parameteras the matrix material, but it is isolated from the matrix byrocksalt layers on both sides. The rocksalt layers provide
both electronic confinement and ionic confinement, i.e. thestructure can be thermodynamically stable and thus resistantto cation interdiffusion.
An essential part of growing such structures is the ability
to monitor the growth of each atomic layer. We do this bymeasuring the intensity of the specular reflection in a reflection high-energy electron diffraction pattern. The intensity variation with time (Fig. 2) shows that we can indeed
observe the formation of each perovskite and rocksalt block.

Fig. 2. Specular RHEED intensity during the growth of aLa1.2Sr1.8Mn1.7Ru0.3O7 film. The alternating sequence of rocksalt(La,Sr)O and perovskite (La,Sr)(Ru,Mn)O7 blocks can be seen. In thiscase, the growth started with the formation of a double perovskite blockat the interface.

In this example, a La1.2Sr1.8Mn1.7Ru0.3O7 film was grown on
a SrTiO3 surface. By suitable buffering of SrTiO3 substratesurface with a fractional layer of (La,Sr)O, it is possible tochoose which block forms at the initial interface, either aperovskite unit or a rocksalt unit.
Reference[1] M. Matvejeff, T. Chikyow, and M. Lippmaa, Journal of Crystal
Growth 311, 1201 (2009).
Authors
M. Matvejeff, M. Lippmaa, and T. ChikyowaaNational Institute for Materials Science

Vortex Physics in hcp solid 4He anda new development of Superfluidity Study
Kubota Group

Fig. 1. Structure of a quantum well formed by embedding a single
Ruddlesden-Popper formula unit layer in a perovskite matrix.

16

ISSP Activity Report 2008

The report of the Kubota group on the evidence forquantized vortices and the vortex fluid (VF) state [1] hasbeen stirring the low temperature community. Additional
reports on the possible transition from VF state into a 3D
supersolid(SS) state [2] are further exciting the community,
although there is neither a microscopic theory nor common
understanding of the description of all the experimental
observations reported so far for the solid 4He.
Proposals for the existence of supersolid state in aquantum solid, solid 4He go back to the 1960’s. The reason
for expecting such a peculiar state of coexistence of order inreal space, the lattice structure of the solid, combined withsimultaneous order in momentum space, as required for thesuperfluidity, is based on the fact that disorder, actually,
vacancies and interstitials should exist to some extent evenat absolute zero temperature in some quantum solids on
account of the large zero point motions. Since the time whenthe “classical proposals” were made around 1970 [3], there
have been substantial experimental efforts exerted to discoversuch a state over more than 35 years, before Kim and Chanannounced their first “possible non-classical rotational

inertia”(NCRI) observation in solid 4He samples formedin Vycor porous glass in January 2004 [4] by torsional
oscillator(TO) technique. They reported also results withbulk torous samples [5] with and without a blockage to test
the macroscopic features of the observation. Confirmation of
Kim and Chan’s experimental observations by other groups
took quite some time and it was around the beginning of
2006, when other groups including the Kubota group started
reporting similar results. This was because the features ofthe phenomena are quite sensitive to very small ac excitation
velocities in the TO experiments. As little as 10μm/s linear
speed seemed to cause decrease of the TO signals substantially. It was, however, also becoming problematic to expect
BEC of any quasi particles(vacancies or interstitials) or otherexcitations at 200 mK or so as reported by Kim and Chan[4,5] considering the known densities of these excitations.
It was P. W. Anderson, who proposed the vortex fluid
scenario [6] for the experimental observations reported till
2007. His point is as follows; the reported “onset temperature” of 200mK [4] to 500 mK [1] is too high to expect BEC
to occur from the knowledge of number density of vacancies
or interstitials and reported logVac linear dependence [4,5],
where Vac is the linear velocity at the torsion pendulum edge.
This indicates involvement of vortex lines in the observation.
He also pointed out that the reported TO results would represent non-linear rotational susceptibility(NLRS) of the VF
state [6].
Penzev, et al., [1] reported a clear finding of the onset
temperature To at 500 mK for samples at 32 and 49 barpressure, the highest To ever reported for solid 4He, where
significant Vac dependence starts to appear. The TO responseof solid 4He has a unique feature, which is opposite to the
behavior of superfluid transition of all known superfluids.
Namely, the TO signals become larger when the excitation issmaller over decades of amplitude change. We discuss that it
is fluctuation of the thermally excited quantized vortices inthe VF state [1]. The VF state has been observed and heavily
discussed in various new types of superconductors startingfrom the Cuprate superconductors. Evidence of quantizedvortices has been reported especially for Under-doped
Cuprate superconductors [7]. The VF state in He is expected
to become a 3D supersolid state at lower temperatures below

Fig. 1. “Non Linear Rotational Susceptibility(NLRS)” of the vortex
fluid state (open symbols) and the hysteretic component of NLRS,ΔNLRShys (filled symbols) which we discuss to be supersolid density
ρs, as a function of T. The high temperature part of NLRS behaves as
1/T2, whereas ρs starts from Tc and increases towards T = 0K and thelatter behaves as an order parameter whereas NLRS does not behave as
an order parameter. The NLRS of VF is found to behave as “paramagnetic”, with Langevin function with x ∝ 1/T2. Langevin function
behavior is expected for susceptibility of an ensemble of classicaldipoles, but usually with x ∝ 1/T.

some Tc, because of increase of low D coherence length
towards lower T, which originates in lower dimensional
subsystems in the system and causes a transition to a SS statewith macroscopic coherence.
We have experimentally found a transition from the VF
state to the SS state by observing a hysteresis in the TO
response with changing excitation below a temperature, Tc[2]. Figure 1 shows the T dependence of both NLRS of VF
and the hysteretic component of NLRS, which we argue tobe associated with supersolid density ρs. Both are plotted
as functions of T. This observation is consistent with ourpreliminary results of TO measurements under DC rotation,
where the evidence of vortex lines penetration occurs below
the same Tc ~ 75 mK, far below To.
References[1] A. Penzev, Y. Yasuta, and M. Kubota, Phys. Rev. Lett. 101, 065301(2008).
[2] N. Shimizu, Y. Yasuta, and M. Kubota, arXiv:0903.1326., submitted
to Phys. Rev. Lett.
[3] see a review, for example, D. Galli and L. Reatto, J. Phys. Soc. Jpn.
77, (2008) 111010.
[4] E. Kim and M. H. W. Chan, Nature 427 225 (2004).
[5] E. Kim and M. H. W. Chan, Science 305 1941 (2004).
[6] P. W. Anderson, Nat. Phys. 3 160 (2007); Phys. Rev. Lett. 100
215301 (2008).
[7] Y. Wang, L. Li, M. J. Naughton, G. D. Gu, S. Uchida, and N. P.
Ong, Phys. Rev. Lett. 95, 247002 (2005).
Authors
N. Shimizu, A. Penzev, Y. Yasuta, and M. Kubota

Negative Interlayer Magnetoresistanceand Three-Dimensional Dirac Holesin Bulk Graphite
Osada GroupWe have newly found remarkable negative interlayer
magnetoresistance (MR) in bulk graphite, which is considered to originate from the three-dimensional (3D) Diracfermion nature of holes.
So far, many experimental studies have been performedon MR in bulk graphite. However, there has been no clear
report on negative interlayer MR. In graphite, in-plane transverse MR is so huge that the anisotropy between in-plane
resistance and interlayer one inverts under normal magnetic
fields. Therefore, exact interlayer MR measurement is difficult because of the mixing of in-plane MR. In order to limit
this mixing effect, we have prepared Kish graphite samples
with flat surfaces and clear edge sides. Figure 1 shows the
typical trace of interlayer MR observed in well-preparedgraphite under normal magnetic fields. We can see theremarkable negative MR, on which clear Shubnikov-de Haas
oscillations are superposed.
The negative MR observed in graphite is reminiscentof that observed in a layered organic conductor α-(BEDTTTF)2I3 [1]. Recently, we theoretically showed that themultilayer Dirac electron system, where each layer has
two-dimensional (2D) Dirac cone dispersion, shows negativeinterlayer MR [2]. Under magnetic fields, because of its
massless nature, the Dirac electron system can easily reachthe quantum limit where only the n = 0 Landau level (zeromode) always exists at the Dirac point. The increase ofzero-mode degeneracy causes strong negative MR. This

ISSP Activity Report 2008

17

Exciton-Biexciton-Plasma Crossover andOptical Gain in Quantum Wires

References[1] N. Tajima et al., cond-mat 0812.0857.
[2] T. Osada, J. Phys. Soc. Jpn. 77, 084711 (2008).

Akiyama Group

Authors
T. Osada, H. Imamura, K. Uchida, S. Sugawara, and T. Konoike

Roles of electron-hole (e-h) plasmas, biexcitons, andexcitons in formation of optical gain as well as underlying
physics of exciton Mott transition or crossover have attracted
great interests in optical study of semiconductors. We haveinvestigated these problems by developing high-quality
T-shaped GaAs quantum-wire lasers fabricated by cleavededge overgrowth with molecular beam epitaxy [1-4].
Simultaneously measured excitation-power-dependent
photoluminescence (PL) and gain/absorption spectra of a

Intrinsic Properties of SrFe2As2
Single Crystal under Highly
Hydrostatic Pressure Conditions
Uwatoko Group

Fig. 1. Negative interlayer magnetoresistance observed in well-preparedbulk graphite.

Fig. 2. Calculated interlayer magnetoresistance in bulk graphite. Inset
shows the band dispersion along the H-K-H edge of the Brillouin zone.
The broken circle indicate the Dirac-cone of holes.

picture strongly suggested the fact that α-(BEDT-TTF)2I3
has a Dirac-cone dispersion. In the case of bulk graphite,
however, we cannot simply apply our theory, since thenegative MR of graphite appears even in magnetic fields
lower than the quantum limit where the zero-mode conduction is dominant.
Although monolayer graphite (graphene) has 2D Diraccone dispersion, bulk graphite is not a simple multilayer
Dirac electron system because of its alternative stacking. In
its 3D band dispersion, however, there exists the 3D Diraccone dispersion around the H-point in k-space. As a result,H-point holes show Dirac fermion nature whereas K-point
electrons are conventional fermions. So, H-point holes
reach the quantum limit at the much lower magnetic field
than K-point electrons. Therefore, in graphite, negative MR
could be expected in the wide field region corresponding
to the quantum limit of holes. In fact, negative MR accompanied by Shubnikov-de Haas oscillations of electrons has
been successfully reproduced by the calculation of Landau
subband conduction employing the Slonczewski-WeissMcClure band model as indicated in Fig. 2.

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ISSP Activity Report 2008

The recent discoveries of superconductivity (SC) on
iron-pnictide compounds have attracted much attention in
the field of condensed matter physics. The parent compounds
containing FeAs layers exhibit structural and magnetic
phase transitions associated with Fe moments. Chemical
substitution and applying pressure suppress the AF transition, resulting in the appearance of SC. An important clue inregard to the mechanism of SC should be provided by highpressure experiment on a stoichiometric sample since theapplication of pressure does not introduce disorder. However,fundamental problems remain to be solved, as the appearanceof the pressure-induced SC is highly sensitive to pressure
homogeneity. Furthermore, there is no bulk evidence for the
SC transition since most of previous reports are carried out
by resistivity measurement due to the experimental difficultyof high pressure experiment. To resolve these problems, wemeasured electrical resistivity and ac magnetic susceptibilityof SrFe2As2 single crystals under highly hydrostatic pressure
conditions up to 8 GPa.
Figure 1 shows the temperature dependence of the realpart of χac under pressure, relative to that of normal state. At
3.0 GPa, there is no apparent change in χ’. With increasingpressure, a noticeable drop appears close to the temperature
TSC obtained by aforementioned resistivity measurements,
although the SC diamagnetism at 5.0 GPa is around 60 %with a broad transition width, implying the presence of thenormal state portion in the sample. When the pressure is
raised up to 5.5 GPa, the transition becomes sharper, and thenearly perfect shielding was detected. This result indicatesFig. 2. Temperature-pressure phase diagram of SrFe2As2. Filled triangles and circles are determined from resistivity and ac susceptibility,
respectively. The SC transition temperature T SC is defined as a temperature of zero resistance and 50% of the full shielding effect. Brokenlines are guides for the eye.

that the SC transition is a bulk origin. Interestingly, thetransition begins to broaden again at higher pressures, andthus shielding effect becomes abruptly weak at 8.0 GPa. To
show the pressure dependence of the SC volume fraction, weplot the magnitude of 4πχ’ at the lowest temperature studied
here (see the inset of Fig. 1). |4πχ’| remarkably increases
above 5.0 GPa, exhibiting a value (~1) corresponding to thefull shielding effect in a narrow pressure region. Note that
this pressure almost coincides with the PC where the AF andstructural transitions at TS disappear. In Fig. 2, we summarize TS and TSC obtained from our high pressure experiments to construct a temperature-pressure phase diagramof SrFe2As2. As the external pressure increases, TS starts
to decrease steeply above 3.5 GPa, and then seems to be
suppressed to zero in the vicinity of PC ~ 5.0 GPa. At 4.5
GPa, we observed the resistivity anomaly both due to antiferromagnetic and superconducting (zero resistance) transitions,suggesting the coexistence of AF and SC. Indeed, the onset
temperature of the shielding effect approximately corresponds to the zero resistance temperature, but SC volume
fraction is quite small. Since zero resistance due to SC transition can occur with a tiny volume fraction, the inhomogeneity of the pressure distribution leads to the deviation fromthe ideal phase diagram. Therefore, bulk SC only appears
above PC, where TS is fully suppressed. This is consistent
with the rapid sharpening of the SC transition width andremarkable increase of SC volume fraction exceeding PC.
Consequently, we suggest that the SC does not coexist with
the AF/orthorhombic phase. We conjecture that the lattice/magnetic instability in the vicinity of PC plays a crucial rolein the appearance of the SC.

Fig. 1. Absorption (solid curves) and PL (dashed curves) spectra froma single T-wire at 5K for various excitation powers (densities) from (a)nearly zero to (g) 16.6 mW (7.1 × 105 cm-1). All data were measured
for uniform excitation except for the absorption in (a). The vertical lines
indicate the energies of the biexciton (XX) and exciton (X) PL peaks.

Authors
K. Matsubayashi, N. Katayama a , K. Ohgushi, A. Yamada b ,K. Munakata, T. Matsumoto, and Y. UwatokoaUniversity of Virginiab
Saitama University
Fig. 1. Temperature dependence of the real part of ac magnetic susceptibility χac for SrFe2As2 single crystal under pressure, relative to that ofnormal state. Inset shows the pressure dependence of the SC volume
fraction |4πχ’|.

Fig. 2. Absorption spectra (solid curves) and PL (dashed curves) spectraof a 20-wire laser for various e-h densities.

ISSP Activity Report 200819

single-wire laser (Fig. 1) and of a 20-wire laser (Fig. 2) wereobtained. They showed that clear exciton (X) and biexciton(XX) peaks were still visible in the PL when the exciton
peak was bleached from the absorption spectrum, and alsothat optical gain appeared in the absorption spectrum when
the XX biexciton emission overtook the X exciton emission.
This indicated that biexciton-exciton population inversion
led to sharp optical gain before the Mott transition in a 1D
system. As the gain grew and broadened without a peak shift,
the exciton PL was quenched and broad plasma PL evolved
from biexciton PL, indicating a continuous change in gain
mechanism from biexcitons to an unbound e-h plasma. Our
results demonstrated that the Mott transition from excitons
to plasma occurs via biexcitons, and optical gain does notalways signify the Mott transition of excitons. Biexcitons
play key roles in optical spectra in the exciton-plasma crossover or the exciton Mott transition.
References[1] Y. Hayamizu et al., Phys. Rev. Lett. 99, 167403 (2007).
[2] M. Yoshita et al., Phys. Rev. B 74, 165332 (2006).
[3] T. Ihara et al., Phys. Rev. Lett. 99, 126803 (2007).
[4] H. Akiyama et al., Physica E 40, 1726 (2008).
Authors
H. Akiyama, M. Yoshita, Y. Hayamizu, T. Ihara, L. Pfeiffera, K. Westa,
P. Huaib, K. Asanoc, T. Ogawac, and C. Z. Ningda
Bell Laboratories, Alcatel LucentbShanghai Institute of Applied PhysicscOsaka UniversitydArizona State University

Nanoscale Chemical Imaging by Scanning
Tunneling Microscopy Assisted by
Synchrotron Radiation

a topographic image of the sample taken in the constantcurrent mode by the SR-STM. We can see the checkerboard
pattern clearly. Figures 1(c) and (d) represent the photoinduced current images taken simultaneously with the
topographic image at the top of Fe L3 (hν = 706 eV) andNi L3 (hν = 852 eV) edges, respectively by using lock-in
technique [2]. As shown in the figures, the photo-induced
current causes significant contrast that depends on theelement of the stripe structures. That is, the area of Fe is thebrightest and that of Ni is the darkest in Fig.1(c) and (d).
However, the photo-induced current image does not depend
much on the exciting photon energies. The observed almost
unchanged contrast is probably caused by the work-function
difference of the materials. It has been reported that the Fe
has much smaller work-function than that of Ni [3]. Since in
the SR-STM measurement observed photo-induced current
is dominantly due to secondary electrons which are inelastically scattered after excited by incident photons, the intensity of photo-induced current is inversely proportional towork-function and proportional to ionization cross section.
The ionization cross section is nearly the same between Fe
and Ni because the both two elements are neighbor in the
periodic table, and the difference in photo-induced current by
the work-function difference may cause the contrast.
In order to reduce the strong contrast that is not sensitive to the photon energy and to enhance the photon energy
dependence, we have divided the photo-induced current
image taken at the absorption edge-top by that of at theedge-bottom. Figures 1(e) and (f) are the images obtained bythis procedure. In Fig.1(e), the photo-induced current imageat hν = 706 eV (= edge top) has been divided by that taken
at hν = 698 eV (= edge bottom). Now, as shown in Fig. 1(e),the region of Fe has larger intensity than that of Ni. Interestingly, the contrast has been inverted if we divide the images

20

ISSP Activity Report 2008

References[1] T. Matsushima et al., Rev. Sci. Instrum. 75, 2149 (2004).
[2] T. Eguchi et al., Appl. Phys. Lett. 89, 243119 (2006).
[3] D. E. Eastman, Phys. Rev. B 2, 1 (1970).
[4] T. Okuda et al., Phys. Rev. Lett. 102, 105503 (2009).

shifter.
We designed and fabricated a phase shifter prototype
with 0.1-mm-thick permalloy laminations to satisfy therequirements for the phase shifter. The phase shifter prototype consists of three H-type dipole magnets (Magnet A, B
and C) with the yoke-length ratio of 1:2:1. Each magnet has
a yoke divided into two parts at the horizontal symmetricplane and two coils for the upper and lower magnetic poles.
Figure 1(a) shows the fabricated phase shifter prototype.
Each half yoke is made of 0.1-mm-thick permalloy laminations united and insulated by varnish. Figure 1(b) shows apermalloy lamination used for the yokes. All the permalloy
laminations are formed by stamping and annealed at about1100 °C to maximize their magnetic permeability. We
succeeded in reducing the size error of the laminations down
to ±0.1 mm even after annealing. They have a very highmagnetic permeability (about 400000) and very low hysteresis. The three magnets are independently excited by threeidentical power supplies, which are linear amplifier type anddeveloped so that it should have good frequency response
and low ripple and drift less than 100 ppm (10-4) of the
maximum current.
Field measurements were performed using a 3-D fieldmeasurement system with a Hall probe and a 3-D movable

Authors
T. Okuda, T. Eguchi, K. Akiyamaa, A. Harasawa, T. Kinoshitab, Y.
Hasegawa, M. Kawamoric, Y. Haruyamac, and S. MatsuicaTohoku Universityb
SPring-8, JASRIc
University of Hyogo

Development of a Phase Shifter with
Laminated Permalloy Yokes forMulti-Segment Undulators

Synchrotron Radiation Laboratoryand Hasegawa GroupThe microscopy having the spatial resolution of atomic
scale with the function of element specificity is one of
the ultimate microscopy for materials science. To thisend, we have developed scanning tunneling microscopy(STM) system assisted by a core-level electron excitation
by synchrotron radiation light (SR) [1]. Recently, we havereported the possibility of the element-specific imaging by
the SR assisted STM (SR-STM) and a quite high-spatial
resolution (~ few tens nm) has been demonstrated [2]. Here,
we present some more evidences of the capability of theelement-specific STM observation in high-spatial resolution
by the SR-STM.
The experiment was done on the beamline BL-13C
at Photon Factory, KEK. As a test sample, checkerboard
patterned sample consisting of Fe and Ni stripes with the
thickness of about 5 and 10 nm was fabricated on a Au thickfilm formed on Si(100) substrate by an electron beam lithography. The width of the stripes is 1μm each. Because of thesequence of the stripe formations (Fe after Ni), Fe stripes arelocated at the top of the sample surface and covers partially
the Ni stripes as illustrated in Fig. 1(a). The detailed settingof the SR-STM measurement is presented elsewhere [2].
In order to obtain the element specific images, we
performed the STM observation during irradiating thesample surface with the SR light of different photon energies
at around Fe and Ni L absorption edges. Figure 1(b) istaken at the Ni absorption edge-top (hν = 852 eV) by that of
the edge-bottom (hν = 843 eV) as in Fig. 1(f). This contrast
inversion between the Fe and Ni absorption edges indicatesthat one can obtain element specific images by SR-STM by
utilizing the energy tunability of synchrotron radiation.
The spatial resolution of the photo-current image canbe roughly estimated by taking the intensity line profile inFig. 1(c) along the dotted line indicated in Fig. 1(b). Fromthe line profile (not shown here) the resolution of about 30
nm is obtained. Considering the intrinsic edge width of thepatterned sample (~ 20 nm) the real spatial resolution ofphoto-current signal can be estimated as about 10 nm [4].
The obtained value is comparable to or even better than thebest spatial resolution of photoemission electron microscopy
observation.
Although the mechanism of achieving such a high spatial
resolution is still not clear, the local detection of the photoinduced secondary electrons through the surface barrier
lowered by the proximate tip and/or via the tunneling processprobably plays an important role.

Nakamura GroupA phase shifter is used to control the radiation phasebetween two undulator segments by giving a bump orbit
to an electron beam with its magnetic field and can beapplied to novel undulators such as a polarization-controlled
undulator [1,2] and very long undulators in synchrotron
radiation(SR) sources and FELs that have to consist ofundulator segments separated by finite distances. The phaseshifter generally has to make a phase shift of 0 to 2π or more
for all the wavelengths. High stability and reproducibilityof the phase control is required to avoid orbit movements ofthe electron beam and quality degradation of the SR beam.
Furthermore fast control of SR wavelength and polarization
including helicity switching is often needed for the phaseFig. 1. Images of an Fe and Ni checkerboard pattern observed by
SR-STM with (b) constant current mode (topographic image), (c)photo-induced current image at hν = 706 eV and (d) 852 eV. Schematic
illustration is presented in (a). Elemental specific image of Fe ispresented in (e) that is obtained by dividing photo-induced current
image at Fe L absorption edge top (hν = 706 eV) by that taken at edge
bottom (698 eV). (f) the same as (e) but obtained with images at the Ni
L absorption edge (hν = 852 /843 eV) .

Fig. 1. (a) Phase shifter prototype and (b) 0.1-mm-thick permalloy
lamination used for the yokes.

Fig. 2. Field measurement results: (a) measured longitudinal field distribution and phase shift expected from the distribution with the 3-D fieldcalculation result, (b) excitation curve of Magnet B and its ultra-low
hysteresis and (c) frequency response of Magnet A up to 500 Hz.

ISSP Activity Report 200821

stage. Figure 2(a) shows the measured longitudinal field
distribution and the phase shift due to the field distributions
at coil currents of IA = IC = −1170 AT and IB = 1400 AT. The
measurement result is in good agreement with the result of
3-D magnetic field calculation. The maximum phase shift
given by this prototype is about 4π for an assumed maximum
radiation wavelength of λ = 4.959 nm (250 eV). Figure 2(b)shows the excitation curve of Magnet B and its hysteresis
effect. The excitation curve is almost linear and the magnetic
field is not saturated up to the maximum coil current of 1500
AT. Due to the ultra-low hysteresis of the yoke, the hysteresis effect was only 0.25 G at maximum. Figure 2(c) shows
measured frequency response of Magnet A. The gain was
kept constant within 1 dB up to 500 Hz and the phase delay
was less than 1 degree at 10Hz and 10 degrees at 100 Hz.
The phase shifter prototype with laminated permalloy yokes
showed good performance in reproducibility and frequency
response of the magnetic field.
References[1] M. Oshima and A. Kakizaki, Journal of JSSRR 20 No. 6, 383(2007).
[2] T. Tanaka and H. Kitamura, AIP Conference Proceedings 705,
231(2004).
Authors
N. Nakamura, K. Shinoe, I. Ito, T. Shibuya, A. Ishii, H. Kudo, H.
Takaki, H. Tanakaa, H. Kitamuraa, and T. Bizenba
RIKEN SPring-8,b
JASRI/SPring-8

High-Strength Hydrogel with Ideally
Homogeneous Network Structure
Shibayama Group
Hydrogels are defined as 3 dimensional polymer
networks, in which the polymer chain networks are highly
immersed by water and the water content is typically over
90%. By taking advantage of their high water absorption and

Fig. 1. Molecular structures of TAPEG (a) and TNPEG (b).

22

ISSP Activity Report 2008

Fig. 2. Stress-Strain curves of agarose gel (square), acrylamide gel(triangle), and Tetra-PEG gel (circle).

retention properties, hydrogels have been applied to diapers,
contact lenses, drug reservoirs, etc. Despite these practicalapplications, applications of hydrogels to structural materials
are restricted because of their poor mechanical properties,
which result from the micro-inhomogeneities created bycross-linking reactions. Here, we show the new strategy forforming a homogeneous network by decreasing the degree of
freedom of the micro-network structure.
We successfully designed and fabricated a novel homogeneous hydrogel by combining two symmetrical tetrahedronlike macromers (the building blocks) of the same size. The
macromers are tetra-amine terminated PEG (TAPEG) andTetra-NHS-glutarate terminated PEG (TNPEG). Here, NHS
represents for N-hydroxysuccinimide. These are preparedfrom tetra-poly(hydroxylpolyethylene glycol)s which weresynthesized by successive anionic polymerization reaction of
ethylene oxide from sodium alkoxide of pentaerythritol. We
formed the gel by combining two well-defined symmetricaltetrahedron-like macromers of the same size (Figure 1). We
call these gels Tetra-PEG gels. Since each macromer has four
end-linking groups reacting each other, these two building
units must connect alternately without self-binding reaction.
Both TAPEG and TNPEG macromers and the reaction itself
are biocompatible, and the compressive strength of thus
prepared Tetra-PEG gels was in a MPa-range which wasmuch superior to those of agarose gels or acrylamide gelshaving the same network concentrations (Figure 2). It isalready confirmed that the mechanical strength reaches 30
MPa by tuning the molecular weight and concentrations,which is three times as high as that of native articular cartilage (≈ 10MPa).
Small-angle neutron scattering studies on Tetra-PEGs
having various molecular weights revealed that Tetra-PEG
gels form a tetrapod-like structure with extremely low
degrees of defects (Figure 3). This is the reason of theFig. 3. Schematic illustration of a model structure for Tetra-PEG gel.
Red and blue spheres represent TAPEG and TNPEG, respectively.

Fig. 4. (a) Photograph of Tetra-PEG gel (a) before, (b) during, and (c)after compression test.

extraordinarily good mechanical properties of gels.
Hence, Tetra-PEG gel may be valuable in application
for practical use. In the case of biomedical applications,hydrogels are required to meet three criteria at the sametime: high-mechanical properties, biocompatibility and aneasy fabrication process. A Tetra-PEG gel satisfy all threecriteria. First, it has attained high-mechanical properties
using symmetrical tetrahedron-like constitutional polymers
to form an extremely homogeneous structure (Figure 4).
Second, Tetra-PEG gel uses biocompatible tetrahedron-like
PEG with mutually reactive terminal groups; its gel formation reaction does not use or produce any toxic substance.
Third, Tetra-PEG gel can be fabricated within a few minutes
through in situ gelation by simply mixing two macromer
solutions. Because of these combined merits, we strongly
believe that Tetra-PEG gel and its derivatives may be usefulin the biomedical field.

scattering techniques, i.e., neutron diffraction, quasi- andinelastic scattering, spin echo, etc. We have already investigated several imidazolium ILs with relatively shorter alkylchains and found that alkyl-chains are highly disordered
and moving very fast (in ps order) at room temperature. In
this work, we have taken fully-deuterated 1-octyl-3-methyimidazolium chloride (d-C8mimCl) which has relatively
longer alkyl-chains and so may have clear domain structure.
The deuteration was required to avoid strong incoherent
scattering from hydrogen (H) atoms.
Figure 1 shows the neutron diffraction patterns of
d-C8mimCl measured by HERMES (IMR, Tohoku Univ.)
installed at JRR-3, JAEA. The peaks around 0.3, 1.1, and1.4 Å-1 are considered to be due to the correlations of
domains, ions, and alkyl-chains, respectively. As coolingthe sample, the inter-ionic and inter alkyl peaks shifted tothe high-Q side, indicating compact packing of local structure, while the inter-domain peak was grown and shiftedto the low-Q side, indicating clear contrast and expansionof domains. The diffraction pattern was not changed mucharound 190 K because of the freezing phenomenon (glasstransition) around 200 K. This is the first neutron diffraction data demonstrating clear temperature dependence of IL
domains. Figure 2 shows the intermediate scattering functionof d-C8mimCl obtained by a neutron spin-echo (NSE)
instrument installed at National Institute of Standard andTechnology (NIST), USA. There were fast and slow relaxations depending on temperature. These curves were fitted
well with the combination of an exponential and a stretchedexponential functions as shown in Fig. 2. The fast motion
did not change much with temperature while the slow motion
had Arrhenius temperature dependence with the activation

References[1] T. Sakai, T. Matsunaga, Y. Yamamoto, C. Ito, R. Yoshida, S. Suzuki,
N. Sasaki, M. Shibayama, U. Chung, Macromolecules 41, 5739 (2008).
[2] T. Matsunaga, T. Sakai, T. Y. Yamamoto, Y. Akagi, U. Chung, M.
Shibayama, Macromolecules 42, 1344 (2009).
Authors
T . Sakai, T . Matsunaga, Y. Yamamoto, C. Ito, R. Yoshida, S. Suzuki,
N. Sasaki, Y. Akagi, U. Chung, and M. Shibayama

Short- and Intermediate-range Structureand Slow Dynamics ofan Ionic Liquid C8mimCl

Fig. 1. Neutron diffraction patterns of d-C8mimCl ionic liquidmeasured as a function of temperature.

Yamamuro Group
Ionic liquids (ILs) are very interesting materials not
only for various applications but also for basic science ofliquids. Most of ILs have glass transitions at low temperatures and the related temperature dependence of viscosity
is not understood by Stokes-Einstein scenario. This can be
related to hierarchical structure and dynamics of ILs. Most of
cations of ILs have core parts with positive charge and also
have mostly neutral methyl-groups and longer alkyl-chains.
Furthermore, it was recently found that ILs have separatingionic and neutral (alkyl-chain) domains of a nanometer scale.
They may have different time scales of diffusive motions.
The purpose of our study is to clarify the hierarchicalstructure and dynamics of ILs by using various neutron

Fig. 2. Intermediate scattering function of d-C8mimCl ionic liquidmeasured by a neutron spin echo technique as a function of temperature.

ISSP Activity Report 200823

energy of 36 kJ mol-1. The fast and slow motions may berelated to the local motion of alkyl-chain and the diffusionof ions, respectively. The time-scale of domain motion was
found to be slower than 10 ns from the NSE measurement at
Q = 0.28 Å-1. This is also the first data showing the dynamics
of ions and domains of ILs.

Novel Devil’s Staircase Transitions
under Pressure
Y. Ueda Group

Authors
O. Yamamuro, T. Yamada, and M. Nagaoa
aNIST and Indiana University

Single-crystal Neutron-Inelastic-Scattering
Study on BaFe2As2; a Parent Compound
for the Fe-Based Superconductors

Fig. 2. Neutron inelastic scattering spectrum at the antiferromagnetic
zone center Q = (1,0,5) and at T = 5K.

Sato Group
Discovery of the high-Tc superconductivity in Fe-based
compounds in early 2008 generated a tremendous boomin the solid-state-physics community. One of the central
issues for understanding the mechanism of the high-Tc superconductivity in the Fe-based compounds is role of Fe spin
fluctuations. Shortly after the discovery, it was suggested
that the Fe moments originate from itinerant 3d electrons.
Indeed, the parent (non-superconducting) compounds arepoor metals with spin-density-wave (SDW) type antiferromagnetic ordering accompanied by structural transitions
in common. This itinerant magnetism is in striking contrast
to the Mott-insulator-type localized magnetism in parent
compounds of the cuprates, the highest-Tc superconductors
ever found. It is, therefore, of enormous importance to elucidate how the itinerant Fe moments fluctuate both in the SDWphase of the parent compound and in the superconducting
phase of the doped systems. To reveal the spin fluctuation spectrum, we, therefore, performed neutron inelastic
scattering study on the parent compound BaFe2As2 using
single-crystalline sample.
Figure 1 shows the single-crystalline sample, which wasgrown using the vertical Bridgman technique in the present
study. The crystals with total mass of about 0.4 grams wereco-aligned and used in the inelastic neutron scattering study,
performed using the thermal and cold-neutron triple-axis
spectrometers ISSP-GPTAS and HER, installed at JRR-3,
Tokai, Japan.

Fig. 3. Constant-E scans around the antiferromagnetic zone center Q =(1,0,1) in the in-plane and out-of-plane directions.

Figure 2 shows the low-energy spin excitations in theparent compound, measured at the antiferromagnetic zone
center Q = (1,0,5) at the base temperature. We note that weused the low-temperature orthorhombic coordinate in thisreport. Apparently, an inelastic intensity starts to increase
above ℏω = 9.5 meV, and peaks around ℏω = 20 meV. The
Q-dependence of the inelastic intensity is shown in Fig. 3. It
peaks at the antiferromangetic zone center at low energies,
whereas the peak starts to broaden at higher energies.
Detailed analysis shows that the inelastic peak corresponds
to the spin-wave-like excitations with very steep in-planeand relatively weak out-of-plane dispersions. It should
be noted that the ratio of the spin-wave velocities for thein-plane and out-of-plane dispersions is quite large as 0.2,
which is very different from that in the high-Tc cuprates. In
gh-energy regions above 20 meV, we have observed appearance of highly 2D scattering independent of L in addition
to the spin-wave excitations. We suggest that this may beelectron-hole excitation continuum across the SDW gap.
Further studies in higher energies as well as the Co-doped
compounds are in progress.
Reference[1] K. Matan, R. Morinaga, K. Iida, and T. J. Sato, Phys. Rev. B 74
54526 (2009).

Fig. 1. Single crystalline samples of the BaFe2As2 parent compoundgrown by the Bridgman technique. At the bottom part, single grains
were grown. Inset: the crystals removed from the Bridgman ingot.

24

ISSP Activity Report 2008

Authors
T. J. Sato, K. Matan, R. Morinaga, and K. Iida

Among various vanadium bronzes, β-vanadium bronzes,
which have mix-valence vanadium atoms, exhibit wide
variety of attractive properties, such as charge orderings,
metal-insulator transitions, and pressure-induced superconductivities [1]. In addition to these, we discovered novel typeof phase transitions under pressure in the compound with Sr
as a cation, which are strongly related to the symmetry of thecrystal and the one-dimensional nature of the system. The
compound has a monoclinic crystal structure with vanadium
chains along the b-axis, and it is a quasi-one-dimensional
system.
At ambient pressure, the Sr compound takes place acharge ordering transition at low temperature, accompanied
by a formation of three-fold superlattice along the b-axis.
By applying pressure, the charge order state with a tripledunit cell is suppressed, and another charge ordering state
appears, which is well-distinguished in magnetic susceptibility data as shown in Fig. 1. X-ray diffraction study clarified that this new phase has five-fold superlattice. Also in the
charge disorder phase, another phase was found in resistivity
measurements. At least four phases are concentrated in avery narrow pressure and temperature region, suggesting
competition among various states [2].
We conducted detailed structural investigations in thispressure and temperature region, and found many otherphases with different periodicities along the b-axis asshown in Fig. 2. The observed periodicities are five, seven,and eleven, all of which are odd numbers including threeobserved under ambient pressure. All the observed phases
are well-described by symmetry reduction from the crystal
symmetry of the charge disorder phase. Strong couplings ofcharge ordering and crystal symmetry, restricts the periodicity to only odd numbers.
As an origin of the appearance of many phases withdifferent periodicities, we propose a self-doping model
between two types of vanadium chains. The system has
two types of vanadium two-leg ladders. Suppose that all

Fig. 2. Temperature dependence of x-ray diffraction intensity measured
at 0.9 GPa. Superlattice signals indicate devil's staircase-type transitions. At low temperature, signals at 0.2 and 0.8 represent the five-fold
supperlattice. By elevating temperature, the position of the superlatticesignal moves stepwise to 1/7, 1/11, etc.

vanadium atom has the same charge, the occupancy is 1/3,
which leads the periodicity three observed under ambientpressure. By application of pressure and/or change oftemperature, charge could transfer between two types ofladders, and this varies the charge modulation, which generates many periodicities.
References[1] T. Yamauchi and Y. Ueda, Phys. Rev. B 77, 104529 (2008).
[2] T. Yamauchi, H. Ueda, J.-I. Yamaura, and Y. Ueda, Phys. Rev. B 75,
014437 (2007).
Authors
H. Ueda, T. Yamauchi, Y. Ueda, K. Ohwadaa, and H. Sawaba
JAEA, SPring-8bNagoya university

Incommensurate Order andRotational-Symmetry Breaking
in 2D Frustrated Heisenberg Model
Kawashima Group

Fig. 1. Pressure dependence of of magnetic susceptibility. At ambientpressure, susceptibility exhibits Curie-Weiss behavior below the chargeordering temperature, and shows a spin-gap feature at low temperatures.
By applying pressure, additional anomaly is observed in the chargeorder region, indicating appearance of a new charge order state.

The magnetic compound NiGa2S4 is a rare example ofa two-dimensional triangular lattice antiferromagnet andstays in a spin-disordered state at low temperatures. Interesting behaviors of this material were observed by Nakatsuji
group [1,2]. For example, development of incommensurate(IC) spin correlation was confirmed by magnetic neutron
scattering experiment. It is easy to see that the incommensurate magentic ordering can be stabilized at least forzero-temperature by introducing a dominant antiferromagnetic third-nearest-neighbor interaction.
Introduction of such intermediate-range interaction may
cause various effects in addition to the incommensurateordering. To study the consequence in more depth, we investigate a two-dimensional classical Heisenberg model on atriangular lattice with a ferromagnetic nearest-neighbor (NN)interaction J1 (<0) and an antiferromagnetic third-NN inter-

ISSP Activity Report 200825

Development of High-Speed Imaging
System in Pulsed-Magnetic Fields

action J3 (>0). The model Hamiltonian is given by= J1

si · sj
Σ si · sj + J3〈i, Σj〉〈i, j〉NN(1).

If the third-NN interaction is dominant, there are threeequivalent IC ground-state spin-configurations that can betransformed to each other by 120 degree lattice rotations(Fig. 1). Presence of this discrete symmetry makes it possible
to have a finite temperature phase transition without violating
Mermin and Wagner's theorem.
We perform classical Monte Carlo simulation using thestandard heat-bath method [3]. In this calculation, the valueof the interaction ratio J3/J1 is fixed at -3. We find that the
specific heat as a function of the temperature exhibits asingle peak indicating a phase transition. We study the probability distribution of the energy in order to determine theorder of this transition. The distribution exhibits the bimodal
structure and the valley in the middle of the distribution
deepens with increasing lattice size. These evidences clearlysuggest the first-order phase transition.
From ground state properties of the present model, it isnatural to expect the spontaneous breaking of the threefold
lattice-rotational symmetry. Based on this expectation, we
calculate the direction-specified nearest-neighbor correlation
along the three axes separately. To be specific,1
εμ = ——
2LΣ〈i, j〉NN || axis μ

si · sj (μ = 1, 2, 3)(2).

In the symmetry breaking phase, one of the three εμ is
negative, while the others are positive. We therefore sortthe averages of the three quantities in descending order anddefine them as E1, E2, and E3 . As shown in Fig. 2, E1 and E2
increase but E3 decreases below the first-order phase transition temperature. The threefold symmetry is spontaneously
broken and one of the three axes is selected.

We now consider implications of these results on
the experimental observations of NiGa2S4. The important point there was that the IC phase emerges at low
temperatures and the specific heat has a broad doublepeak structure. The present results are consistent with theincommensurability observed in the neutron scatteringexperiments. On the other hand, however, the specificFig. 1. A schematic illustration of spin configuration for the groundstate. The spin configuration along axis 1 is characterized by the wave
number k, and that of axes 2 and 3 by k/2. There is a threefold degeneracy corresponding to 120 degree lattice rotations. The spin configuration on each of the four sublattices (divided by color) is close to, but notexactly, the 120 degree structure.

26

Tokunaga Group

3rd.NN

ISSP Activity Report 2008

Fig. 2. Temperature dependence of the direction-specified bond energy
E1, E2 and E3. The first-order transition temperature is Tc/J3 ~ 0.42.

heat of the present model only exhibits a single divergent
peak. To relate the present results to the experiment, it isnecessary to construct the whole phase diagram and also
take into account other types of interactions that havebeen neglected.
References[1] S. Nakatsuji, Y. Nambu, H. Tonomura, O. Sakai, S. Jonas,
C. Broholm, H. Tsunetsugu, Y. Qiu, and Y. Maeno, Science 309, 1697
(2005).
[2] Y. Nambu, S. Nakatsuji, and Y. Maeno, J. Phys. Soc. Jpn. 75,
043711 (2006).
[3] R. Tamura and N. Kawashima, J. Phys. Soc. Jpn. 77, 103002 (2008).
Authors
R.Tamura and N. Kawashima

Magnetic States of Highly Frustrated Spinel
ZnCr2O4 under Ultra-high Magnetic Fields up to
400 T by Electro-Magnetic Flux Compression
Takeyama and Y. Ueda Groups
ZnCr2O4 is a geometrically frustrated antiferromagnet.
Cr ions form a pyrochlore lattice in this material. The Néel
temperature is 12 K while the Curie-Weiss temperature issuch high as -390 K. This shows that a strong geometricalfrustration prevents the Néel ordering. It is known that thelattice distortion accompanies the Néel ordering. Spin-latticeinteractions act under the strong geometrical frustration inthis material. As the exchange interactions between Cr ionsare very strong, an ultra-high magnetic field is required toinvestigate the magnetization process of this material. We
performed the Faraday rotation measurements under ultrahigh magnetic fields up to 190 T by using a single turn coil[1]. We observed a first order phase transition at 120 T anda linear increase of Faraday rotation angle between 120
T and 135 T and the plateau with 1/2 of the full moment
between 135 T and 160 T. These behaviors coincide with thetheoretical prediction by K. Penc et al[2]. According to their
theory, the region of the linear increase shows a cant 2:1:1
phase. In order to observe the full magnetization process of
this material, a much higher field is required. We conducted
the Faraday rotation measurements in magnetic fields up
to 400 T by an electro-magnetic flux compression (EMFC)

Fig. 1. Magnetization of ZnCr2O4 measured by the Faraday rotation
method under ultra-high magnetic fields generated by the electromagnetic flux compression method. Arrows show the phase transitions
above the plateau phase. The upper left picture shows a primary coil forthe electro-magnetic flux compression. The upper right picture shows ahandmade optical cryostat made of a “stycast” resin.

method. For a precise Faraday rotation measurement in the
EMFC system, we have developed a hand-made opticalcryostat fully made of a “stycast” resin (a picture above thefigure), which enables us to achieve low temperatures witha fairly good vacuum tight. A curve in the figure shows theresult. The left vertical axis shows the Faraday rotation angle(θF). The right vertical axis in the figure shows the magnetization which corresponds to the Faraday rotation angle.
We successfully observed the full magnetization process of
this material at 6 K. The data below 190 T coincides wellwith those measured by using the single turn coil. We foundsome anomalies which show new phase transitions (240 T,
290 T, 350 T, and 390 T) above 190 T. We considered thatthe anomaly at 390 T corresponds to the phase transition to
the Ferro phase which is already explained in the theory by
Penc et al. However, other phase transitions are beyond theprediction of the theory. The values of the magnetization
at the fields of 240 T, 290 T and 350 T are 2/3, 3/4 and 5/6of the full moment, respectively. The details of those phasetransitions are still not clear. However, a possible explanation for these transitions is as follows. We assume that the
1/2 plateau phase has a rhombohedral spin structure (theinset of the figure). This structure shows that Kagome lattices
with up-spins and triangular lattices with down-spins arealternately layered. If the spins in the triangular lattice layers
form 1 up-2 down spin structures or 2 up-1 down structuresin the high field region, new transitions can occur at 2/3
or 5/6 of the full moment. If such a structure is realized, itmeans that dimensional reduction of “magnetic ordering”
takes place from three to two induced by an external strong
magnetic field.

The coupling between spin and lattice degrees of freedom
realizes a variety of magnetic field-induced structural transitions. Recent development of diffraction measurementsin pulsed-fields enabled us to obtain direct evidence of
these phenomena for some materials in high-fields [1,2].
The accuracy, however, remains insufficient to detect faintchanges caused by superstructures. Here, we developed analternative technique to study these transitions: high-speed
polarizing microscope imaging in pulsed-magnetic fields.
Some kinds of structural transitions can be identified throughobservation of creation or annihilation of twin boundaries incrystals. With using a high-speed camera, we can perform
this imaging in high magnetic fields achieved by pulse
magnets. To this end, we introduced a high-speed camera(6000 frames/s for VGA resolution) on a commercial polarizing microscope. Using this microscope system and an
objective lens with long working distance, we obtain realFig. 1. Schematic drawing of the high-speed imaging system combinedwith a pulse magnet. The magnet and sample are cooled by the firstand second stage of a cryocooler, respectively. The sample stage wasmechanically isolated from the vibration of the compressor of thecryocooler and of the magnet. A high-speed camera is mounted on
a commercial polarizing microscope to trace fast changes in images
induced by pulsed-magnetic fields.

References[1] E. Kojima, et al., Phys. Rev. B 77 (2008) 212408.
[2] K. Penc et al., Phys. Rev. Lett. 93 197203 (2004).; Y. Motome, etal., J. Mag. Magn. Mater. 300 57 (2006).
Authors
E. Kojima, A. Miyata , S. Takeyama, H. Ueda, and Y. Ueda

Fig. 2. Polarizing microscope images in the ab-plane of La1/2Sr3/2MnO4
at 200 K at (a) zero field and (b) 28 T. The bright areas represent thedomains of the COO, which disappear by application of a magnetic
field of 28 T along the c-axis.

ISSP Activity Report 200827

Fig. 3. Averaged intensity in a part of the polarizing microscope images
as a function of the applied field. The data at each temperature is offset
for clarity. The observed decrease in intensity corresponds the reduction
in optical anisotropy caused by the COO.

images of samples located on the center of a small pulse
magnet, which can generate magnetic fields up to 37 T
in a duration time of about 5 ms. Using a low-vibration
cryocooler system, the sample and the magnet can be cooled
down to 8 K and 80 K, respectively.
As the first target of the present imaging system, wechose a crystal of La1/2Sr3/2MnO4. This layered manganite
exhibits charge/orbital ordering (COO) below about 220 K.
Since the orbital ordering restricts the direction for hoppingof mobile eg electrons in the Mn3+ ions, optical anisotropy
emerges within the ab-plane in this state. Consequently,
the COO state shows up brightly in polarizing microscope
images at the crossed-Nicols configuration [3]. Figures 2
show polarizing microscope images on the cleaved ab-planeof La1/2Sr3/2MnO4 at 200 K. Corresponding to the fieldinduced melting of the COO, bright domains in zero field(Fig. 2(a)) become dark in 28 T (Fig. 2(b)).
The utilized camera has 12-bit resolution for intensity ineach pixel, so that we can quantitatively analyze the obtained
images in pulsed-fields. In Fig. 3, we show the intensity ina part of the images as a function of applied field at several
temperatures. These clear changes in the optical intensity
manifest the field-induced melting of the COO in contrast
to rather moderate changes in magnetization and magnetoresistance in this temperature region [4]. The present result
provides a novel opportunity for investigation of some kindsof field-induced transitions even in tiny crystals.
References[1] Y. H. Matsuda et al., Physica B 346-347, 519 (2004).
[2] Y. Narumi et al., J. Synchrotron Radiat. 13, 271 (2006).
[3] T. Ogasawara et al., Phys. Rev. B 63, 113105 (2001).
[4] M. Tokunaga et al., Phys. Rev. B 59, 11151 (1999).
Authors
M. Tokunaga, I. Katakura, A. Matsuo, K. Kawaguchi, and K. Kindo

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ISSP Activity Report 2008

ISSP Activity Report 200829