ISSP
Activity Report B
2001
Contents

Pages

Preface1Highlights of Joint Research

2 - 20ISSP Workshop

21 - 23

Cover photo: The cubic anvil apparatus to produce
high pressure up to 10 GPa provides multiple extrme
conditions combined with low temperature and highmagnetic field (top). The apparatus for electromagnetic flux-compression to produce high magnetic
field over 600 tesla (bottom).

ISSP Activity Report B 2001

PREFACE- Activity Report B -ISSP was established in 1957 based on the recommendation of the National Council of Japan topromote the cooperative research throughout the country in condensed matter physics in close contact with other related fields of science such as chemistry and earth science on one hand and to pursue its own scientific research on the other hand. This foreseeing philosophy of the founders together
with the constant efforts afterwards had made ISSP very active in both ways in recent years, especially after the introduction of the large group systems in 1980 and the moving to the new campus of
Kashiwa. So far there were various forms of the publications to make the status of the research activity of ISSP public. New forms are now introduced; the Activity Reports A and B, which show the upto-date status of these scientific activities in separate ways depending on the styles of the pursuing
the research. This Activity Report B is based on the joint research programs conducted by ISSP in
the fiscal year of 2001. In the first half of this Report the highlights are collected, whereas in the latter half all of the publications under this program are listed together with titles of the joint research
programs and ISSP workshop. Those results of the joint research programs making use of the big
facilities in Sychrotron Radiation Laboratory, the Neutron Scattering Laboratory and theSupercomputer Center are reported separately as their own activity reports.
We are happy to receive any comments on these Reports for the possible improvement of theoverall research activity of ISSP.

August 19, 2002

Hidetoshi Fukuyama
Director
Institute for Solid State Physics
The University of Tokyo

ISSP Activity Report B 20011

Highlights of Joint Research
Neutron Scattering Laboratony
In the fiscal year of 2001, the new light source project
was promoted extensively. Synchrotron Radiation
Laboratory (SRL) re-designed the accelerator to be supported by as many users as possible. The accelerator design
review committee has discussed the new design and theapplicability of the light source from both viewpoints ofsynchrotron radiation users and accelerator scientists. The
results of the discussion were applied to the modification of
the design, and now SRL proposes a 1.8 GeV electron storage ring with 12 insertion devices optimised in EUV and SX
region as the future synchrotron light source in Japan.
Nation-wide synchrotron radiation related scientists support
the new light source. The staff members of SRL continue
the research and developments of the light source, beamlines
and monochromators. The SRL is carrying out researchworks of the accelerator physics and developing the accelerator-related technology, many of which will be directly
applied to the new light source.
On the other hand, the SRL has a branch laboratory in
the High Energy Accelerator Research Organization (KEK)at Tsukuba. The branch laboratory maintains an undulator
called Revolver, two beamlines and three experimental stations for angle-resolved photoelectron spectroscopy (BL18A), spin- and angle-resolved photoelectron spectroscopy(19A) and soft X-ray emission spectroscopy (19B). They are
installed in the Photon Factory (PF) and fully opened to outside users. The SRL staffs not only serve the outside users
with technical support and advices, but also carry out theirown research works on advanced solid state spectroscopy aswell as instrumentation. In the fiscal year of 2001, the operation time of the beamlines was more than 5000 hours andthe number of the users was more than 200. The main scientific interests and activities in the SRL at KEK-PF are
directed to the electronic structures of new materials withnew transport and optical properties. The electronic structures of solid surfaces and interfaces are also intensively
studied.
The scientific highlights of the beamlines achieved in2001 are found in the studies of the electronic structures of anovel ferromagnetic semiconductor ZnGeP2:Mn observedby photoelectron spectroscopy, of the time-resolved corelevel photoluminescence from quantum well states of ZnSeand ZnSe/ZnCdSe/ZnSe and of the micro-domain structure
observation in anti-ferromagnetic materials by photoelectron
microscopy utilizing magnetic linear dichroism.

2

ISSP Activity Report B 2001

Since 1961, the ISSP has been playing a central role inneutron scattering activities in Japan not only by performing
its own research programs but also by providing a generaluser program for the university-owned various neutron scattering spectrometers installed at research reactors of JAERI(Tokai). By spending 5 years starting from 1988, the ISSP
constructed total 8.5 highly-sophisticated spectrometers bothin a reactor hall and in a guide hall of the refurbished JRR3M reactor (20MW) with cold source. In addition, Tohoku
University owns 3.5 spectrometers and Kyoto University 1;
therefore, totally 13 spectrometers belong to the university
while JAERI has 9. In order to fully utilize these facilities,
the previous Neutron Scattering Division of ISSP at Tokyo
was promoted to the present Neutron Scattering Laboratory
at Tokai in 1993. Since then, close to 300 proposals are submitted for the ISSP general user program for neutron scattering research each year, and a number of users under thisprogram reaches over 7,000 (person-day/year). Major
research areas are solid state physics (strongly correlated
electron systems, high Tc superconductors, heavy fermion
systems, low dimensional magnetism, high-pressure physicsatc.), fundamental physics and neutron beam optics, polymer, chemistry, biolgy, and materials sciences.
Triple axis spectrometers and a high resolution powder
diffractometer were utilized for a conventional solid statephysics and a variety of research fields on hard-condensed
matter, while in the field of soft-condensed matter physics,
researches were mostly carried out by using the small angle
neutron scattering (SANS-U) and/or neutron spin echo(NSE) facilities. In the fiscal year of 2001, the research topics on the hard-condensed matter science cover stripe orderin high-Tc superconductors, and closely related 2 dimensional systems, charge and orbital ordering in CMR manganites, quadrapolar ordering in rare-earth based intermetalic compounds, spin dynamics of low dimensional dimer systems, etc. The research topics covered structure characterization of polymer blends, micelles, amphiphilic polymers,
block copolymers, liquid crystals, proteins, inorganic gels,dynamics of brush-polymers on surface, slow dynamics ofsurfactants, pressure dependence of dynamics ofamphiphilic membranes, and so on. The details of individual
studies are reported in the NSL - ISSP Activity Report 2002.

Supercomputer Center
Supercomputer System: The main system in the
Supercomputer Center of the institute (SCC-ISSP) consistsof two supercomputers. Hitachi SR8000/60, called SystemA, is a distributed-memory-type parallel supercomputer with
60 nodes and 640GB memory in total. Each node consists of
8 microprocessors which share their memory inside thenode. Each node is optimized for vector-type large-scale calculations and archives 12GFlops peak performance.These
nodes are interconnected by the multidimensional crossbar
network.
On the other hand, SGI Origin 2800/384, called SystemB, is a massively parallel supercomputer and consists ofthree hosts interconnected by high-speed GSN network.
Each host is a distributed-shared-memory-type computerand equipped with 128 MIPS R12000 CPU's and 64GB
memory.
User Program: The system is placed at the service ofgeneral researchers of condensed matter physics through theUser Program. A project(s) can be proposed by any staff inuniversities or public research institutes in Japan. In order torun smoothly the User Program, the Materials Design andCharacterization Laboratory (MDCL) of the ISSP has organized the Steering Committee of the MDCL, the Steering
Committee of the SCC-ISSP, under which theSupercomputer Project Advisory Committee is formed toreview proposals. More than half of the members of eachcommittee are those in other institutions than the ISSP. The
number of projects, the total points applied and approved inthis year are listed on Table 1.
Results: The research projects carried out on the systemcover various fields in the condensed matter physics. They
are roughly classified into the following three:First-Principles Calculation of Materials Properties (39,126)
Strongly Correlated Quantum Systems (51, 275)
Cooperative Phenomena in Complex, Macroscopic
Systems (18,53)
Here the first and second numbers in the parenthesis arethose respectively of the projects and the publications whichwere reported from the project leaders. All the three involve
both methodology of computation and its applications. The
results of all the projects are reported in 'Activity Report
2001' of the SCC-ISSP where the following four invitedarticles are included in this year:"First-Principles Study of CO and NO Absorption at
Transition-Metal Surfaces" by Hideaki Aizawa and Shinji
Tsuneyuki, "Strongly Correlated Electrons Studied by Path
Integral Renormalization Group Method" by Masatoshi
Imada, "New Type of Ordered States in Quantum Spin
Systems and Their Dynamical Properties" by Seiji
Miyashita, " Application of New Monte Carlo Algorithms to
Various Spin Systems" by Yutaka Okabe, Yusuke Tomita
and Chiaki Yamaguchi.

For System-A 1 k point corresponds to charge for CPU time of about 0.43hours by one processing element, while the corresponding figure is 2.32 hours for System-B.

ISSP Activity Report B 20013

Low Temperature Magnetization Studyon the Spin Ice State of the Pyrochlore
OxidesK. Matsuhira and T. Sakakibara
In the pyrochlore oxide Dy2Ti2O7 where spins are residing on the vertices of the corner-shared tetrahedra, ferromagnetic interactions and Ising anisotropy along the local<111> axes lead to a strong frustration. Accordingly, thelocal spin arrangement of two spins pointing outward andtwo spins inwards (so-called ‘two-in two-out’ state) is stabilized in a basic tetrahedron. For every tetrahedron, there arethus six possible combinations of spins under the two-in
two-out rule. The ground state is therefore highly degenerate
and a static disordered state (so-called “spin ice” state) is
formed below 1 K in spite of chemically ordered system [1].
Magnetic field breaks the ground state degeneracy.
Monte Carlo simulations show that the degeneracy breaking
becomes a two-stage process for H//[111] and a magnetization plateau should appear in the intermediate fields [2]. We
have examined the DC magnetization process of Dy2Ti2O7
in magnetic fields applied along the [111] direction, using acapacitive Faraday magnetometer installed in a 3He cryostat.
Figure 1 shows the results. At T = 1.65 K, the magnetization is a gradual function of field with a weak feature ataround 1 T, and saturates at higher fields to the value~5 μB/Dy. This moment value corresponds to the fully saturated one-in three-out (or three-in one-out) state of the Ising
pyrochlore lattice with the local Ising axis pointing along the<111> directions.
On cooling below 1 K, the feature at ~1 T becomes
sharper, and eventually turns into a metamagnetic step at~0.5 K with a preceding magnetization plateau below ~0.9
T. The magnetic moment of the plateau is very close to thevalue 3.33 μB/Dy, expected for the saturated moment along
the [111] direction without destroying the two-in two-out
state. Clearly, the metamagnetic step near 1 T corresponds
to a breaking of the spin ice state by a magnetic field strong
enough to overcome the magnetic interactions, as predictedby the Monte Carlo simulations.
In the previous magnetization measurements done at 1.8
K, only a broad feature had been observed at around 1 T[3].

Fig. 1. Magnetization process of Dy2Ti2O7 in magnetic fields applied along
the [111] direction.At low temperatures below 1 K, a magnetization plateau
develops in the field range of 0.4 ~ 0.9 T where the ice rule 2-in 2-out state ismaintained. The metamagnetic step at around 1 T corresponds to a breakingof the ice rule state. The inset shows the details at around 1 T.

4

ISSP Activity Report B 2001

Our data are the first results in the low temperature regime
where the ice rule configuration is well developed.
Magnetization measurements at still lower temperatures arein progress.
References[1] A.P. Ramirez, A, Hayashi, R.J. Cava, R. Siddharthan and B.S.
Shastry, Nature 399 333 (1999).
[2] M.J. Harris, S.T. Bramwell, P.C.W. Holdsworth and J.D.M.
Champion, Phys. Rev. Lett. 81 4496 (1998).
[3] H. Fukazawa, R.G. Melko, R. Higashinaka, Y. Maeno and M.J.P.
Gingras, Phys. Rev. B 65 054410 (2002).
Authors

K. Matsuhiraa, Z. Hiroi, T. Tayama and T. Sakakibara
aDepartment of Electronics, Faculty of Engineering, Kyushu Instituteof Technology.

Magnetic Circular Dichroism in Ce
L2,3-Absorption Edges of Mixed Valence
Compound CeFe2
I. Harada and K. Kotani
Mixed valence property in Ce compounds is one of themost striking and interesting phenomena in condensed matter physics, caused by the mixing of localized states withstrong correlation and itinerant states. The decomposition of
the total electronic density of states into various contributions of each element and each shell would be a significant
progress in understanding such a mechanism.
The recent experimental development of the X-ray
Absorption Spectroscopy (XAS), I+(ω) + I-(ω), and themagnetic counterpart, the Magnetic Circular Dichroism(MCD), I+(ω) – I-(ω), (see Figure 1), together with thedevelopment of new generation synchrotron radiation facilities with high photon flux enable us to obtain such detailed
information on electronic and magnetic states of selected
atoms and of selected shells.
Let us consider a mixed valence intermetallic compound,
CeFe2, which behaves as a ferromagnetic material but has
anomalously small magnetic moment and very low Curie
temperature compared to other RFe2 (R = rare earth).We
attribute the anomaly to the mixed valence electronic stateof Ce, which can be described by an impurity Andersonmodel[1] where the three electronic configurations, 4f0, 4f1L
and 4f2L2, L being a ligand hole in the spin polarized 3dband of surrounding irons, are considered. In order to treat
the band nature of the Fe 3d states as well as of the Ce 5dstates, we adopt a cluster approach, where the hybridization
between them is taken into account. This effect is the originof the spin polarization of the 5d states and crucial for MCD

Fig. 1. Definition of the intensities, I+(ω) and I-(ω).

Fig. 2. XAS and MCD spectra for CeFe2. The solid curves are the calculated
results while the crosses and the solid circles are the experimental ones [2].

at the Ce L2,3 edges.
Based on this model, we calculate numerically XAS and
MCD spectra, which are compared quantitatively with thoseobserved experimentally [2] in Figure 2. Then, we find thefollowing [3]: (i)XAS shows two peaks which correspond to
the 4f0 and 4f1L states, the 4f2L2 state overlaps with the latter. The intensity ratio of these yields the degree of the mixing of these configurations and the positions depend mainlyon the core hole potential and the Coulomb interaction within the 4f states.(ii)MCD spectrum shows also two peaks but
the peak positions are different from those of XAS. We
attribute the difference as a consequence of the fact that the
spin polarization of the 5d states depends on energy andoccurs only near the Fermi energy. The anomalous magnetic
properties are also naturally explainable with this model.
In conclusion, we show that the calculation based on the
impurity Anderson model combined with a cluster model isappropriate for taking the Coulomb interaction and the
hybridization effect into account and gives a correct interpretation for the MCD spectra at the Ce L2,3 edges of CeFe2,
which gives rise to the detailed information on the characteristic electronic and magnetic states of Ce.

Fig. 1. Conically intersecting bands for the flux state in the 2D Hubbard
model [1]. The conical intersections are located at (±π/2, ±π/2). They are
the sources of a gauge potential Ank. Its time-dependence, which may be produced by an adiabatic deformation of the lattice, is predicted to produce ageometric phase current in our paper [4].

In this situation, electronic motion is far from trivial due
to complexity of the band dispersion curves even in a simple
picture of the band electron; there are numerous crossingsand also sensitively affected with spatial arrangements of
the internal degrees of freedom, which is very likely to becoupled with lattice distortions through vibronic effects.
Even in the cuprate case, the spin band is not simple; on thecontrary, one with non-trivial flux is speculated for a certain
range of parameters [1].
In order to understand the anomalous current in transition-metal compounds from a viewpoint of the band theory,
we have recently reinvestigated the velocity formula for
Bloch electrons. Usually the formula is given by
as is derived in textbooks. In magnetic materials, however, it
has been known that it is not valid; instead, a fictitious magnetic field Bnk has to be included:

References[1] For instance, see a review, A. Kotani et al., Adv. Phys. 37, 37(1988).
[2] C. Giorgetti et al., Phys. Rev. B 48, 12732 (1993).
[3] H. Ogasawara et al., J. Phys. Chemi. Solids. 63, 1487 (2002)
Authors
I. Haradaa, K. Asakuraa, H. Ogasawara, K. Fukuib, and A. Kotani
aOkayama University., bRIKEN/SPring-8

Quantum Transport in Transition Metal
Oxides
H. Koizumi and Y. Takada
Anomalous transport properties in transition metaloxides have been a focus of attention in condensed matterphysics since the discovery of high-Tc superconducting
materials in cuprates. In these compounds, low-lying excitedstates are mostly governed by spin degrees of freedom andthere are indications to show that the ground state must be a
non-Fermi liquid due to strong correlation. Another aspectof the transition-metal oxides, in particular in manganeseand ruthenium oxides, is that they are equipped with orbital
degrees of freedom; thus, the total internal degrees of freedom consist of both spin and orbital degrees.

where Ank is the gauge potential (Berry phase connection)
associated with the Bloch electron[2], giving rise to nontrivial geometric phase effects when the crossings of bands
exist, or when either time-reversal or inversion symmetry is
broken[3].
According to our recent reinvestigation [4], the above
formula has to be further modified; an additional term isfound in the fundamental expression for the electron velocity. By employing a path-integral representation for the propagator of a Bloch electron, the formula is obtained as.

In developing the conventional Fermi-liquid theory, thesecond and third terms (Geometric phase current) in thisvelocity formula are not usually taken into account. Thus afurther work is called for to elucidate the consequence of
these terms, especially in the transition-metal compounds.
Note that the second term, which is produced by the fictitious electric field Enk, explains the adiabatic charge trans-

ISSP Activity Report B 2001

5

port processes, which may be relevant in explaining therecently observed anomalous acoustoelectric effect in thefilm of La0.67Ca0.33MnO3[5].
References[1] I. Affleck and J. B. Marston, Phys. Rev. B 37, 3774 (1988).
[2] J. Zak, Phys. Rev. Lett. 62, 2747 (1989).
[3] M. -C. Chang and Q. Niu, Phys. Rev. Lett. 75, 1348 (1995).
[4] H. Koizumi and Y. Takada, Phys. Rev. B 65, 153104 (2002).
[5] Y. Ilisavskii et al., Phys. Rev. Lett. 87, 146602 (2001).

Authors

H. Koizumia and Y. Takada
aFaculty of Science, Himeji Institute of Technology.

First-principles Computational study ofstable Adsorption sites and Dynamics
for Silver adatom on Ge(001) Surface
A. Ishii and F. Komori
The determination of adsorption site of adatom is very
important to consider the dynamics of adatom on surfaces.
For the growth of novel and functional nano-structure on
surface, the detailed dynamics is significant to recover the
self-assemble mechanism.
Silver is suitable atomic specie for nano-wire because of
the high conductivity. Especially, silver on clean Ge(001)surface is attractive because of the discovery of their superconductivity below 2 K. However, silver on clean Ge(001)does not have 1ML ordered structure; the growth islands ofsilver on Ge(001) has usually up to thirty atoms. The reasonof the lacking of the ordered structure is considered to be thelarge lattice mismatch. Neverthless, the determination of theatomic structure is significant to discuss the superconductivity of this system.
Komori et al. [1,2]observed the sereral adsorption site
for silver adatom on Ge(001) surface using low temperature
STM at temperatures between 65 K to RT. They distinguished the adsorption sites of Ag into several types.
According to our first principle calculations, we obtain thepotential surface for Ag adatom on clean Ge(001) surface.
The contour map of the potential surface is shown in Figure
1. The most stable position for Ag adatom is between the
dimer rows.
Our calculation shows that the two Ag adatoms are
dimerized to be more stable than the individual adsorption.
The calculated STM image of the Ag dimer is shown in
Figure 2. This image is very similar to the STM observation

Fig. 2. Calculated STM image for Ag adatom between the Ge dimer rows on
Ge(001) c(4×~2) surface. The brighter protrusions correspond to the Ag
dimers.

named B1 observed by Naito et. al.[1] As we see in Figure
2, we found that the most of all calculated STM images arevery close to the real atomic configuration. The hopping
motion of adsorbed Ag adatoms was observed using STM
by Naitoh et.al.[2] They distinguish the single Ag adatom
into at least two types, A1 and A2 in which the both sites areplaced between the Ge dimer rows. They found that the Ag
adatom of A1 site cannot move at 80 K but the Ag of A2
site can move at low temperature. According to our calculation, the hopping barrier energy for the Ag adatom of the A2
site along the dimer row direction is 0.3eV, which is very
close to the value determined experimentally, 0.2eV.
The A1 site is considered to be the substitutional site. Ag
adatom of the substitutional site is placed at the backbond of
the Ge dimer. Since the Ag adatom is usually very bright in
STM images, the A1 site would be the Ag of the substitutional site. The calculated hopping barrier energy for the Ag
atom of the substitutional site is 1.1 eV, which is enoughvalue not to move at 80K.
References[1] Y. Naitoh, K. Nakatsuji and F. Komori: Surfaces, to be publishedin Surf. Sci. 513 (2002) 1.
[2] Y. Naitoh, K. Nakatsuji and F. Komori: to be published in J.
Chem. Phys. 117 (2002) 2832.

Authors

Akira Ishiia and Kaori Seinoa
aDepartment of Applied Mathematics and Physics, Tottori University

Cyclotron Resonance of GaN/Al0.13Ga0.87N
and In0.20Ga0.80N/In0.02Ga0.98N QWs
Using Ultra High Magnetic Field
Y. Arakawa and N. Miura

Fig. 1. Calculated potential energy surface for Ag adatom on Ge(001) c(4×2) surface. Blue area corresponds to the stable area for Ag adatom.

6

ISSP Activity Report B 2001

1. Introduction
Gallium nitride and its compounds are the most promising materials for short wavelength light emitting devices andfor high frequency, high power, high temperature operationtransistor. In recent years significant progress has beenachieved in the fabrication of optical and electronic devices
based on these materials. However, the physical propertiesof Ⅲ-Ⅴ nitride materials are not well-known up to now,
especially the effective mass of electron. A few cyclotron
resonance (CR) experiments of AlGaN/GaN heterotructure
were reported at low magnetic field [1] and none of InGaN
QWs have been reported, which is due to extremely low

c-Al2O3/GaN (3μm)/GaN-Al0.13Ga0.87N (30 layers-7.3nm)
or c-Al2O3/GaN (3μm)/ In0.20Ga0.80N- In0.02Ga0.98N (30layers- 6.5nm). We have used the LLO [2] to take off thesapphire substrate and transfer sample onto KRS-5 substratein order to carry out the CR experiment easily. Infrared laserlines at 10.6 μm, 23 μm and 28 μm were used as a radiation source. The transmission signal was detected by using
the extrinsic photoconductivity of a Cu-doped Ge detector
cooled to 4.2 K for a H2O laser, and a HgCdTe photovoltaic
detector cooled to 77 K for a CO2 laser.

Fig. 1. The magnetotransmission spectra of GaN QWs at a wavelength of
28μm.

3. Results and discussion
Figure 1 shows a typical result of the CR experiment of
GaN QWs. There is a CR peak at 101 T. From this value theCR mass is estimated as mCR = 0.264 m0. Impurity cyclotron
resonance (ICR) is also observed at 50 T. In Fig. 2, we plotted the dependence of the CR peak on the CR energy. The
results of GaN are also plotted in the Fig. At near the TO
photon energy, polaron effect becomes larger and the mCR
becomes heavier. Figure 3 displays the magnetotransmissionof InGaN QWs subject to 10.6 μm laser radiation at 300 K.
A very broad CR is observed at 240 T.
4. Conclusion
We have demonstrated the CR experiment of GaN withhigh-field magnetotransmission up to 270 T. A significant
resonant polaron effect was observed near the TO-phonon
energy.
References[1] W.Knap et.al., App.Phys.Lett. 70, 2123 (1997)[2] M.K. Kelly, O. Ambacher, R. Dimitrov, R. Handschuh, and M.
Stutzmann, Phys. Stat. Sol. (A) 159, R3 (1997).

Authors

Fig. 2. Cyclotron energy versus magnetic field.

T. Q. Canha, S. Katoa, K. Hoshinoa, Y. Matsuda, T.Someyaa,b, N.
Miura and Y. Arakawaa,baRcast, University of Tokyo, bISS,University of Tokyo.

Dielectric Enhancement of Exciton
Binding Energy in a Natural
Quantum-Well Material
T. Kondo and N. Miura
Exciton is a quasi particle composed of an electron and ahole which are bound by the Coulomb interaction. Excitons
in low-dimensional semiconductors have been an area ofactive research because of their unique nature and the poten-

Fig. 3. The magnetotransmission spectra of InGaN QWs at a wavelength of
10.6μmelectron mobility and large effective mass of electrons ofGaN. In the present work, we used the laser lift-off (LLO)
technique to remove the sapphire substrate completely andthe single turn coil technique to generate ultrahigh magnetic
field up to 270 T to do the CR experiment of the GaN system.
2. Experiment
The samples for this study were grown by metal organic
chemical vapor deposition (MOCVD) and have structures:

Fig. 1. Energy shift as a function of the magnetic field for the lowest 1s exciton of (C6H13NH3)2PbI4. Circles represent the experimental results and solidlines show the theoretical fitting. Absorption spectrum is shown in the inset.

ISSP Activity Report B 2001

7

tial applications in photonics. It has been predicted that thebinding energy of excitons will be substantially enhanced by
the enforced Coulomb interaction in quantum wells withbarrier layers of dielectric constant εb much lower than thatof well layers εw. The dielectric enhancement due to electric fields penetrating into the low-εbarrier layers, however,has not been verified quantitatively to date.
A two-dimensional crystal (C6H13NH3)2PbI4 is a natural
quantum well system, in which excitons are tightly confinedin the well layer of [PbI6] octahedra sandwiched between
organic barrier layers. We have demonstrated that excitonsin this crystal are highly stable and responsible for extremely large optical nonlinearity which can be applied to a timeto-space conversion of ultra-high bit-rate optical pulses [1].
We have quantitatively verified the exciton binding energyin (C6H13NH3)2PbI4 is dielectrically enhanced, for the firsttime, in this study.
We have performed a two-photon absorption (TPA) [2],magneto-absorption (MA) and detailed reflection spectroscopies.Wannier exciton series up to n = 4 have been identified, and the energy separation of the discrete levels indicates that the excitons in (C6H13NH3)2PbI4 are Wannier ones
with strong two-dimensional characters. Binding energy ofthe lowest 1s exciton is estimated to be 350 meV. MA spectra were measured at 4.2 K in pulsed magnetic fields up to
42 T in the Faraday configuration. Diamagnetic shift clearlyobserved in the MA spectra of the 1s exciton (Figure) yields
an important information on the dielectric environment of
the excitons. The diamagnetic coefficient for a two-dimensional exciton is given by

where EB is the exciton binding energy and εeff is the effective dielectric constant the exciton feels. With the measured
value of c 0 = 2.71 ×10-7 and εB = 350 meV, we obtained
εeff = 3.1. This effective dielectric constant is much lower
than that of the well layer (εw = 6.1) and close to that of thebarrier layer (εb = 2.1). This clearly indicates that the largebinding energy of the 1s exciton in (C6H13NH3)2PbI4 is substantially enhanced by the dielectric effect.

Fig. 1. Room-temperature cyclotron resonance (CR) signals of(In0.2Ga0.8As)8/(AlAs)8 superlattices (SL) measured with a 16.9 μm H2O
laser under a magnetic field at an angle θ to the SL axis. Inset shows θdependence of the normalized resonant magnetic field (closed circles). The
CR positions substantially deviate from a simple cosθ-dependence (green
curve).

than that for a conventional crystal. In the presence of an
applied electric field along the growth direction, an electron
can accelerate and move cyclically around the miniband
before it undergoes a collision. A magnetic field applied
perpendicular to the planes of the SL quantum wells, on theother hand, acts to quantize the in-plane motion into Landaulevels. Thus the electron undergoes Bloch motion along the
SL axis and cyclotron motion in the SL plane.
When a magnetic field applied at an angle θ to the SL
axis, the interaction between Bloch motion and cyclotron
motion occurs resulting in a complicated electronic states.
Recently it has been shown that electrons in a SL under tilted magnetic fields provide an experimentally accessible
non-KAM (Kolmogorov-Arnol'd-Moser) system, which isof great interest due to application in plasma physics, turbulent fluid dynamics, ion traps, and quasicrystals [2].
In the present study, we have performed cyclotron resonance(CR)
measurements
inshortperiodReferences[1] J. Ishi, H. Kunugita, K. Ema, T. Ban and T. Kondo, Appl. Phys.
Lett. 77, 3487 (2000).
[2] K. Tanaka, F. Sano, T. Takahasi, T. Kondo, R. Ito and K. Ema,
Solid State Commun. 122, 249 (2002).

Authors

T. Kondoa, T. Takahashia, K. Tanakaa, K. Uchida and N. MiuraaDepartment of Materials Science, The University of Tokyo

Cyclotron Resonance in Short Period
III-V Semiconductor Superlattices
under Ultra-High Magnetic Fields
N. Mori and N. Miura
The pioneering experiments of Esaki and Tsu [1]demonstrated the potential of the semiconductor superlattice(SL) for investigating electron dynamics in a regime notaccessible in naturally occurring solids. Superlattices can beproduced with SL period many times longer than the lattice
constant of the host crystal. The Brillouin zone, or minizone,of the SL therefore has much smaller range of wavevector

8

ISSP Activity Report B 2001

Fig. 2. Calculated energy levels in (In0.2Ga0.8As)8/(AlAs)8 superlattices (SL)as a function of angle θ between the magnetic field B and the normal to the
SL layers. Yellow rectangle with a red (blue) striped pattern shows the lowest(first excited) Landau level miniband (LLMB) at θ = 0. The energy difference between the lowest LLMB and the first excited LLMB is consistent withthe experimental findings (yellow circles, which correspond to the yellow
curve in the inset of Fig. 1) and differs from a simple cosθ-dependence(green circles).

(InGaAs)n/(AlAs)n superlattices under tilted magnetic fields
to investigate Landau-level-miniband mixing.The SL samples used in the present study are (In0.2Ga0.8As)n/(AlAs)n
SLs grown by molecular beam epitaxy (MBE) on semi-insulating GaAs (001)-oriented substrates. 80 periods of δdoped InGaAs/AlAs SLs are sandwiched with 10 periods
undoped InGaAs/AlAs SLs. Pulsed high magnetic fields up
to 150 T were produced by the single-turn coil technique.
The pulsed magnetic field was applied to the samples at anangle θ between the magnetic field B and the normal to the
SL layers.
Figure 1 shows CR signals of (In0.2Ga0.8As)8/(AlAs)8superlattices, which clearly shows that the CR positions substantially deviate from a simple cosθ-dependence suggesting the Landau-level-miniband mixing. The observed CR
positions are found to agree with the calculated results(Figure 2).
References[1] L. Esaki and R. Tsu, IBM J. Res. Dev., 14, 61 (1970).
[2] T.M. Fromhold, A.A. Krokhin, C.R. Tench, S. Bujkiewicz, P.B.
Wilkinson, F.W. Sheard, and L. Eaves, Phys. Rev. Lett. 87, 046803(2001).

Authors

N. Moria, H. Momose, Y. Tujimoto, M. Itoh, and C. Hamaguchi, N.
Miura
aOsaka University.

Cyclotron Resonance of ConfinedElectrons in Semiconductor Quantum
Well of Near Ten Monolayer Thickness
N. Kotera and N. Miura
Semiconductor
nanometricstructures
calledInGaAs/InAlAs quantum wells (QWs) are widely used inlight-emitting diodes or lasers. The slightly-different two
crystals, InGaAs as a 'well' and InAlAs as a 'barrier', arealternately grown on Indium Phosphide crystal surface.
Conduction electrons in QWs are confined in a squarepotential well (Fig.1), comprised by InGaAs layer. Confined
elecrons in QWs have one or more discrete eigen energies, if
they do not liberated into the barrier by tunneling.
In the recent state-of-the-art technology, well thickness

Fig. 1. Cross-sectional image of nano-scale quantum well (QW) structuresutilizing slightly-different two crystals, red atoms (well region) and whiteones (barrier region). Electro-static potential of square shape is illustrated
where electron wave-function of free electrons is confined in a direction normal to the QW interface. Penetration of wave-function into the barrier was
the problem in QW system so far. Key to solve this difficulty was to raise thebarrier height. Using molecular-beam-epitaxy technique, such a fine structure
was fabricated, where QW thickness was tested in a range from 12 monolayer (ML) to 68 ML. 10 ML thickness corresponds to about 3 nm in theexperimental material system. Monolayer thickness was defined as an averaged distance of constituent atoms like In, Ga, and As.

Fig. 2. Summary of measured results of cyclotron resonance (CR) showing
CR energy vs. magnetic field (Solid marks; experiments, Solid lines; theoretical calculation based on band theory). As quantum well thickness or thickness of InGaAs layer decreases from 68 ML to 12 ML, CR energy decreases
relatively, demonstrating the heavier electron effective mass in thinner QW.
One reason of the heavier mass is the increase of eigen-energy in thinner
QW. It is the evidence of electron confinement in InGaAs layer that theexperiments fit well with a band theory in InGaAs QW under very high magnetic field. Even at 12 ML QW, a simple band theory assuming an infinite
spread of crystals applies though the slight shift appears. This is another evidence of lattice-matching whole through the crystal structure where the averaged lattice-constant is common everywhere in barriers and wells.

can be made as thin as 10 monolayers (MLs). Ten ML thickness of equally mixed crystal of InAs (Indium Arsenide) and
GaAs (Gallium Arsenide) corresponds to 3 nm in length(Fig. 1). A 'band theory' or the 'effective-mass-approximation' describing electronic states in QWs must be checked insuch a thin layer.
In the popular GaAs/GaAlAs QW system, electrons tend
to be free from GaAs because of the lower barrier height of
0.3 eV. Wave functions are penetrating in a direction normal
to the QW interface (Fig.1).
In this experiment, however, high barrier height of 0.52
and 1.6 eV was realized by using InGaAs/InAlAs QW system (specimen names, M5 and M10H) and InGaAs/AlAsSb
QW system (specimen names, D582 and D660), respectively. To generate free electrons in semiconductors impurity
atom, Si, was doped in InAlAs layer (for M5 and M10H
specimens) or InGaAs layer (for D582 and D660 specimens). This QW structure offered a good means to check theband theory in thin-layer semiconductor, where only one
material (InGaAs) controls the electronic states because
highly confined free electrons in QW were realized in thisexperiment. Small 'band-gap energy' of 0.8 eV in InGaAs
crystal also simplified the description of electronic states,
where the (8 x 8) determinant representation can be reduced
to a third-order equation of energy for conduction electrons.
Characteristic 'electron effective-mass' in a direction normal
to the QW interface is given there by a simple algebraic
equation of electron energy.
Pulse cyclotron resonance (CR) up to 100 T for these
specimens was measured near 30 K by using the single-turncoil technique scanning magnetic flux density, B, under various infrared wave-lengths ranging 118.8 to 9.25 micron
generated from carbon-dioxide-gas laser and methanolvapor laser (Fig.2).
If the electron mass is a constant, CR energy changes
linearly on B. If the mass increases with increasing electronenergy, CR energy changes sub-linearly with B (Fig. 2). In

ISSP Activity Report B 2001

9

experiments, as the QW thickness decreases from 68 ML to
12 ML, CR energy tends to decrease (from red mark togreen mark). The inverse of gradient of CR-energy-vs.-B
curve reflects the magnitude of electron mass, which clearly
increased in thinner QW specimen.
Theoretical calculation based on the above simple 'band
theory' (solid lines in Fig. 2) fit quantitatively with experiments (marks in Fig. 2), except the slight shift at the thinnest
12 ML case. A reasonable band parameter, Ep, to fit theexperiments was determined to be 21.2 eV. Wave-function
confinement was thus evidenced by comparing CR experiments with a band theory in a specially tailored QW specimens. Applicability of a simple model of chracteristic electron-effective-mass called 'non-parabolic effective mass'
was first demonstrated experimentally. Anisotopy of masses, mass differences in directions normal and parallel to the
QW interface, will be clarified quantitatively.
References
N.Kotera, K. Shibata, T. Kawano, T. Sakai, T. Ikaida, N. Miura, N.
Georgiev, T. Mozume, International Symposium on Compound
Semiconductors 2001, to be published in IOP Conference Series(Tokyo, Japan, October 1-4, 2001).
Authors
N. Koteraa and N. MiuraaKyushu instilute of technology

Magnetization Plateau in Cs2CuBr4
H. Tanaka and T. Goto
Cs2CuBr4 has an orthorhombic structure with spacegroup Pnma. For isostructural Cs2CuCl4, magnetic properties have been extensively investigated by Coldea et al. [1]using neutron inelastic scattering technique. They demonstrated that Cs2CuCl4 is magnetically described as the quasi2D distorted triangular antiferromagnet in the bc-plane asshown in Fig. 1. The ordering temperature of Cs2CuCl4 is
TN = 0.62 K. In the ordered phase, the magnetic structure isincommensurate along the b-direction due to J1 ≠ J2.
Coldea et al. also showed that the ordered phase vanishes
before reaching the saturation for H || c-axis. The similarmagnetic behavior is also expected in Cs2CuBr4. Therefore,
we have performed the high field magnetization and specificheat measurements in Cs2CuBr4.
From the specific heat measurements, it was found that

Fig. 2. Magnetization curve in Cs2CuBr4 for H || b-axis around H = 15 T
measured at T = 0.4 K. Transitions between helix-uud and uud-fan structures
are expected to occur.

Cs2CuBr4 undergoes magnetic ordering at TN = 1.4K, which
is about twice as large as that of Cs2CuCl4. The magnetization saturates at H ~ 30 T, and the saturation magnetizationis slightly higher than 1μB per Cu2+ ion. This result indicates that the orbital moment is quenched, so that the magnetic moment is approximately given by spin only. Figure 2
shows the magnetization curve around H = 15 T measured at
T = 0.4 K for H || b-axis. A magnetization plateau is clearlyobserved at one third of saturation magnetization Ms. The
plateau was also observed for H || c-axis. One the other
hand, the magnetization curve for H || a-axis is monotonicalup to the saturation.
In the 2D triangular antiferromagnet, the magnetization
plateau can appear at one third of Ms, where the collinear
up-up-down (uud) spin structure is realized with the help of
the quantum fluctuation [2]. Since the magnetic properties
may be described by S = 1/2 quasi-2D distorted triangularantiferromagnet as shown in Fig. 1, we infer that the magnetization plateau in Cs2CuBr4 is attributed to the quantum
fluctuation and the uud spin structure is realized at theplateau.
The transition between the slope and the plateau regions
appears to be of the first order. This implies that the ordering vector Q varies discontinuously at the both edges of theplateau, which suggests an incommensurate-commensuratetransition. The spin structure below and above the plateau
region should be helical and fan structures, respectively.
Reference[1] R. Coldea et al., Phys. Rev. Lett. 86 1335(2001) .
[2] A. V. Chubukov and D. I. Golosov, J. Phys.: Condens. Mater 3 69
(1991) .

Authors

T. Onoa, H. Tanakaa, H. A. Katorib, F. Ishikawa, Y. Mitamura and T.
Goto
aTokyo Institute of Technology, bRIKEN

Magnetization Process in Mixed Magnetic
Chains (CH3)2CHNH3Cu(ClxBrl-x)3
I. Yamada and T. Goto
Fig. 1. Arrangement of Cu2+ ions in the bc-plane and exchange interactions in
Cs2CuCl4 and Cs2CuBr4. Blue circles and open circles denote Cu2+ ion and
Br- ion, respectively.

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ISSP Activity Report B 2001

Recently, we found useful candidate compounds which
consist of the gapless state induced by bond randomness,(abbreviatedasi.e.,(CH3)2CHNH3Cu(ClxBr1-x)3

Fig. 1. The magnetization curves for x = 0.79, 0.73 and 0.60. The spin floptransition appears when H⊥C plane. Each peak position of the dM/dH curveis defined as the spin flop transition field indicated by an arrow.

IPACu(ClxBr1-x)3)[1] obtained by mixing the almost isomorphous
compounds(CH3)2CHNH3CuCl3
and(CH3)2CHNH3CuBr3[2]. As shown in the schematic drawing of the crystal structure of IPACuX3 (X = Cl, Br), which
is given in Fig. 1 in Ref. 3, the neighboring Cu ions both ineach dimer and between the dimers are bibridged by X ions.
From the analysis of the magnetic susceptibility χ(T )data[3], the intradimer exchange interaction Jintra(X, X) andthe interdimer one Jinter(X, X) were determined to be Jintra(Cl, Cl)/k = + 54.1 K, Jinter(Cl, Cl) /k = -23.5 K, Jintra(Br,Br)/k =-61 K and Jinter(Br, Br)/k = -33 K.
When Cl ions in IPACuCl3 are randomly substituted by
Br ions, we expect Jinter(Cl, Br) = Jinter(Br, Cl) < 0 becauseof Jinter(Cl, Cl) < 0 and Jinter(Br, Br) < 0. On the other hand,
Jintra(Cl, Br) = Jintra(Br, Cl) < 0 is obtained from analysis ofthe χ(T) data[4]. To express the degree of the bond randomness, we introduce a parameter P which indicates theprobability of Jintra> 0. Thus, we obtain the relation P =x2[4].
From the χ (T) and magnetic specific heat measurements for 0 < x < 1[1], we confirmed that the antiferromagnetic long-range order occurs at TN ~ 15 K for 0.44 < x <0.87 because of realizing the gapless state induced by bondFig. 2. The variations of the magnetic anisotropy constant K and the spin floptransition field Hsf with x and P=x2, as indicated by closed circles and theopen triangles, respectively. The crosses represent no observation of the spin
flop transition. The solid lines are guides for the eyes.

randomness. To obtain more information about the gaplessstate, we measured the magnetization process up to 41 T at
1.7 K (≪TN) for 0 ＜ x ＜ 1. An external field H wasapplied along the normals of the three orthogonal surfaces ofeach as-grown sample, A, B and C, as named in Ref. 3.
Figure 1 shows the representative M(H) curves over thegapless state. The M(H) curves for H⊥A plane show astraight line, but those for H⊥c plane exhibit a typical spin
flop transition. Therefore, the magnetic easy axis is found tobe perpendicular to the c plane. The variation of the spin
flop transition field Hsf with x as well as P = x2 are shown in
Fig. 2.
The appearance of magnetic long-range order means thatthe expected value of spins is not zero, i.e., ＜Szi ＞ ≠ 0. We
examine how ＜Szi ＞ is affected by changing bond randomness. On the basis of the molecular field theory, Hsf is represented by using χ＝ and χ⊥ as Hsf =,inwhich K is the magnetic anisotropy constant. Since K∝＜Szi ＞＜Szi ＞ in the present compounds, the variation in K
with P suggests how ＜Szi ＞ changes by varying the bondrandomness. Applying the χ(T ) data [1] into χ＝ and χ⊥,
we calculate K at each value of χ (Fig. 2).
As can be seen in Fig. 2, K versus P describes a parabolic curve with the maximum at P ~ 0.5, which indicates that
＜Szi ＞ reaches its maximum at P ~ 0.5, the point at whichbond randomness is the highest.
Reference[1] H. Manaka et al., Phys. Rev. B 63 (2001) 104408.
[2] D. R. Bloomquist and R. D. Willett, J. Am. Chem. Soc. 103 (1981)
2615.
[3] H. Manaka et al., J. Phys. Soc. Jpn. 66 (1997) 564; 66 (1997)
1908.
[4] H. Manaka et al., in preparation.
Authors

H. Manakaa, I. Yamadab, H. Mitamurac, and T. GotoaHigh Energy Accelerator Research Organization, bChiba University
cChiba University

Surface Study of Quan-tum Fluids
K. Kono and H. Ishimoto
Surface phenomena on liquid helium or quantum fluids
provide a unique opportunity to study low-dimensional
physics. Electrons on liquid helium, positive and negative

Fig. 1. Surface profile at a termination point of quantized vortex. Vortex islocated vertically at distance = 0. The profile is according to Harvey andFetter [J. Low Temp. Phys. 11 (1973) 473.]

ISSP Activity Report B 200111

fig. 2. Magneto resistance of the Wigner crystal as a function of the magnetic
field. The rotation of the cryostat smears out the sharp transition due to thedecoupling from the coupled phono ripplon mode.

ions under the free surface of liquid helium, and hydrogenatoms adsorbed on the surface are examples of such uniquelow-dimensional systems. The important advantage instudying these systems is cleanliness and well-defined characteristics. In particular, helium-3 provides the purest surface. Here, we describe recent experimental achievement of
the magneto-conductivity measurement of the Wigner solid
on a rotating superfluid helium-4 surface. Two-dimensional
electrons on a liquid helium surface form the Wigner solid
at sufficiently low temperatures. The formation of the
Wigner solid inhibits the fast diffusion of electrons on thehelium surface and accordingly the electron stays at its lattice site long enough time to produce dimples on the liquid
helium surface. The dimples are produced commensurately
with the Wigner solid and so it is called the dimple
lattice.The dimple lattice results in a prominent influence on
the transport properties of the Wigner solid.Since the motionof the Wigner solid is accompanied with the dimple motion,
electrons become less mobile in the Wigner solid phase. The
dynamical sliding phenomena can be observed by applying
the magnetic field perpendicular to the surface under theconcentric Corbino geometry. When the driving electricfield, which is oriented in the radial direction, is strong
enough, the Wigner solid starts to leave the dimple lattice
and to move in the azimuthal direction. The transition iscooperative because of the strong correlation between electrons. Because the substrate for the Wigner solid is liquid
helium, the system is so homogeneous that it is extremely
suitable to study such a cooperative dynamic transition.
Conversely, however, it is difficult to introduce inhomogeneous pinning of the Wigner solid in this system. In order toovercome this restriction and expand the research possibility
to the inhomogeneous sliding, we develop the idea to utilize
termination points of quantized vortices at the free surface
as pinning centers for the Wigner solid.
Fig. 1 shows the surface profile at the termination pointof the vortex, where the free surface produces a sharp dip ofabout 7 nm deep, with a wide depressed surrounding area.It
may be an interesting problem to consider how 2D electrons
are scattered, trapped, and localized because of such dips, inparticular, in the presence of perpendicular magnetic field.
So far, no theoretical treatment has been carried out on the
problem. Experiment has been carried out using a special
dilution refrigerator equiped with tandem sorption pumps
with which He3 is circulated only in the cryostat. The dilution is mounted on a rotating turn table to produce the quan-

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ISSP Activity Report B 2001

tized vortices in superfluid helium-4. The detailed description of the apparatus is given elsewhere.
Fig. 2 shows the magneto-resistance of the Wigner solidwith and without vortices. Significant influence from thevortices has been observed for the first time.Without vortices the transition to the sliding state from the coupled stateis very abrupt and with hysteresis, whereas under rotation
the transition becomes gradual and without hysteresis. Thisbehavior may suggest the inhomogeneous transition
between two states. Therefore, the role of the dips at termination points of vortices is not so simple to act as pinning
centers. By looking at the overall landscape of the surface
profile, one is aware that the profile has both a very sharp
central dip and long range gradual tail part. The motion ofelectrons under the influence of such an unusual potential
should shed a new light on the physics of 2-dimensional
electrons on the liquid helium surface.
Principal publication and Author

K. KonoAffiliation: RIKEN, Hirosawa 2-1, Wako-shi, 351-0198 Japan

Vortex Nucleation and Texture of
Rotating 3He-A in Narrow Cylinders
T. Mizusaki and M. Kubota
Helium is a rather simple atom, which consists of nucleus made of two protons and either one (3He) or two (4He)
neutrons(s). Although chemical properties as well as hightemperature physical properties are quite the same for 3He
and 4He, drastic difference appears as to the superfluid properties. 4He undergoes into superfluid at Tλ = 2.17 K (at saturated vapor pressure, SVP) whereas 3He becomes superfluid first at about 1 mK at SVP, three decades lower temperature. This is understood as that Quantum statistics of theatoms is essential for superfluid occurrence. For 4He Bose
Einstein Condensation is essential for the occurrence ofsuperfluidity, whereas for Fermion system 3He, BCS mechanism with p-wave Cooper pairs is responsible for superfluidity at Tc << TF = ~1K~ Tλ.
Superfluidity is studied under rotation just as study ofsuperconductivity in magnetic field. Bulk properties ofsuperfluid 3He has been studied under rotation since sometime [1], we have started new activity to study superfluid
3He in restricted geometry [2]. Since the Cooper pair of 3He
has internal freedom with 18 components, there are plentyof possible superfluid states and bulk liquid 3He displays

Fig. 1. NMR absorption line shapes for the 0.2mm sample at T/Tc = 0.75 as afunction of frequency shift from Larmor frequency. The dotted green line
was taken at rest and the solid red line was taken at rotation speed of
6.28 rad/sec.

Authors

R. Ishiguroa, M. Yamashitaa, E.Hayataa, T. Ohmia, Y. Sasakia, T.
Mizusakia, K. Fukudab, O. Ishikawac, T. Takagid, T. Igarashi, T.
Obada, M. Kubota, H. Ishimoto
aDep. of Physics, Graduate School of Science, Kyoto University,bCollege of Medical Technology, Kyoto University. cDep. of Physics,
Osaka City University. dDep. of Applied Physics, Fukui University.

Quantum Critical Phenomena in UNiGa5
R. Settai and Y. Uwatoko

Fig. 2. Main NMR absorption peak height during acceleration (solidcircle)
and deceleration (cross). (a):the sample was cooled to superfluid under rotation at rest. and (b): it was cooled under rotation of + 2rad/sec

three superfluid states, A, A1, and B phases. Quite different
up to 11 types of vortices are expected in the bulk liquid and7 of them have been reported experimentally. There is socalled ‘texture’, which is configuration of arrangement ofinternal freedom, namely of l-vector and d-vector, and determined by external conditions, magnetic field, boundary,flow field, etc. The liquid in a confined geometry is expected to have quite a different configuration.
Superfluid 3He-A in small cylinders has been being studied with the world fastest rotational cryostat at Kubota lab.
ISSP, U-Tokyo. Since 3He has nuclear spin, NMR is one of
the most effective methods to study this superfluid and it isapplied for the present study. Fig.1 shows typical cw-NMR
absorption line shape of superfluid 3He-A phase confined in
0.2mm diameter cylinders at T/Tc = 0.75. The main peak forthe 3He-A phase appears about 4.5 kHz shifted from the normal phase Larmor frequency. Under rotation above a criticalangular velocity, this main peak decreases its height, insteada new peak appears at a lower frequency. The position ofthis new peak is characterized by a parameter R t2, definedby Δf = Rt2 ( f LA)2 / (2f 0), where Δf is the frequency shift
from Larmor frequency f 0 and f LA is temperature dependent
longitudinal frequency in the A phase. So the main peak
position is at R t 2 = 1.0, whereas the new peak position is
Rt2~0.37. The new peak position is very close to the doubly
quantized nonsingular vortex that is observed in bulk 3HeA[1].
Fig. 2 shows the change in NMR absorption signal as afunction of the rotational speed Ω. Since the main peakheight is most sensitive to the change of texture, it is plottedagainst Ω. The difference in (a) and (b) is the difference incooling procedure, either if the system is cooled through Tc
under rotation or not. This is the first observation of single
vortex formation, which shows history dependent configuration. As one can see from Fig. 2 this result could be obtained
only by the speed of rotation in the ISSP rotating cryostat
doubling the former cryostats[1].

Recently, quantum critical phenomena induced by pressure have been investigated intensively in cerium and uranium compounds [1, 2]. As pressure P is applied to some cerium and uranium compounds with the magnetic ordering, theordering temperature decreases and becomes zero at the critical pressure Pc. Around the critical pressure, a heavy fermion state or non-Fermi liquid behavior appears at low temperatures. Moreover, superconductivity appears in some compounds. As a candidate of such compounds, we investigated
the antiferromagnet compound UNiGa5 with TN = 86 K bymeasuring the electrical resistivity down to 2 K under pressures up to 8.5 GPa using a BeCu piston-cylinder cell and a
cubic anvil apparatus.
The electrical resistivity shows a sharp kink at TN = 86.5
K under ambient pressure (Figure 1). TN decreases withincreasing pressure and the kink disappears above Pc = 4.5
GPa. An inset of figure 1 is the P-T phase diagram for
UNiGa5. Figure 2 shows the T 2 -dependence of the electricalresistivity. The Fermi liquid nature is found at 1.5 and 2.5
GPa below about 15 K. From 5.0 to 5.5 GPa, the slopes ofT 2-dependence become steep, indicating an evolution of theheavy fermion state, or the electrical resistivity deviatesfrom the T 2-dependence, reflecting the non-Fermi liquid
nature. With further increasing pressure, the Fermi liquid
nature fully recovers and is satisfied in a wide temperature
range, as seen at 7.0 and 8.5 GPa.
Superconductivity was not found in the present experiment for UNiGa5. However, the superconductivity appears
below about 1 K in uranium compounds in general.
Measurements under the critical pressure at much lower
temperature are needed to confirm the superconductivity ,together with the non-Fermi liquid behavior for UNiGa5.
References[1] N. D. Mathur et al., Nature, 394, 39 (1998).

References[1] O.V. Lounasmaa and E. Thuneberg, Proc. National Acad. Sci.
USA 96 (1999) 7760.
[2] R. Ishiguro, M. Yamashita, T. Igarashi, E. Hayata, O. Ishikawa, Y.
Sasaki, K. Fukuda, M. Kubota, H. Ishimoto, T. Mizusaki, T. Ohmi, T.
Takagi, and R. E. Packard, to be published in LT23 Proceedings.
[3] M. Kubota, T. Obata, R. Ishiguro, M. Yamashita , T. Igarashi, E.
Hayata, O Ishikawa, Y. Sasaki, N. Mikhin, M. Fukuda, V. Kovacik, T.
Mizusaki, to be published in LT23 Proceedings.

Fig. 1. Temperature dependence of the electrical resistivity under pressure in
UNiGa5. A kink in the resistivity corresponds to the Néel temperature. An
inset is the pressure dependence of the Néel temperature. Critical pressure in
UNiGa5 is about 4.5 GPa.

ISSP Activity Report B 2001

13

Fig. 2. T2-dependence of the electrical resistivity under pressure in UNiGa5.
The slope of the T2-dependence is steep around the critical pressure, or theelectrical resistivity deviates from the T2-dependence, reflecting the nonFermi liquid nature.

[2] S. S. Saxena et al., Nature, 406, 587 (2000).

Authors

R. Settaia, M. Nakashimaa , Y. Hagab, Y. Onukia,b and Y. Uwatoko
aGraduate School of Science, Osaka University, bAdvanced Science
Research Center, Japan Atomic Energy Research Institute.

Anomalous Magnetism of a PrCu2Ge2
Single Crystal
T. Shigeoka and T. Uwatoko
The ternary compound PrCu2Ge2, crystallizing in thetetragonal ThCr2Si2-type structure, shows interesting magnetic behaviors; It has an anomalously high Neel temperature and the existence of another magnetic transition at
4.2 K was suggested [1]. In order to elucidate this magnetic
behavior, measurements of magnetic susceptibility and magnetization have been carried out on a PrCu2Ge2 single crystal compound at the Institute of Solid State Physics,
University of Tokyo.
The c-axis susceptibility shows a very peculiar behavior

Fig. 2. Magnetization as a function of applied magnetic field along the main
symmetry axes at 2 K for the PrCu2Ge2 single crystal. The blue and red curveshows an ascending and descending process along the c-axis, respectively.

(Fig. 1); the susceptibility of a virgin state is different fromone after a saturation magnetization process. Below 3.5 K, astrong increase in virgin susceptibility can be observed withdecreasing temperature. After the magnetization is saturated
by an applied field above 1.5 T at low temperature of 2 K,
the susceptibility becomes very small. Then with increasing
temperature, it persists the small value below 3.5 K, increases rapidly around 4 K, reaches the value of the virgin susceptibility at 5 K and at last above 5 K, behaves similarly to
one of the virgin state. This behavior suggests a magnetictransition around 4 K and the ground state may be forced tochange by a saturation process.
The magnetization along the easy c-axis shows a very
peculiar behavior (Fig. 2): an irreversible magnetization
process; in the ascending process, a four-step metamagnetic
process appears. Magnetization increases rapidly around
0.2 T, 0.45 T, 1.15 T and 1.45 T, and reaches the saturation
value of 2.34 μB. Then in the descending process after saturation, it decreases rapidly around 1.3 T, goes across one of
the ascending process and reaches a very small value below
1.2 T. This peculiar irreversible process appears only in thevirgin state and has been never seen yet. The process in thesecond run becomes a one-step metamagnetic one similar to
one of the descending process in the virgin state with small
hysteresis; it becomes a reversible process. The irreversible
process can be observed up to 3.5 K. Above 4 K, the process
becomes a reversible one-step one. Regarding to the fourstep metamagnetic process, the magnetization of eachplateau is 0.09 μB, 0.27 μB, 0.45 μB and 2.34 μB, corresponding to 1/26 Ms, 3/26 Ms, 5/26 Ms and Ms (= the saturation moment), respectively. It is worth noting that the ratios
have a large common denominator (= 26), suggesting a long
period antiferromagnetic structure althoug a simple antiferromagnetic structure reported. And it suggests that thismetamagnetic process occurs by spin-flip transitions.
Though the origin of the irreversible process is unknown
yet, orbital effects, quadrupolar effects may be responsible.
Further study is now in progress.
References

Fig.1. Temperature dependence of magnetic susceptibility along the main
symmetry directions of the tetragonal cell in low temperatures on a PrCu2Ge2
single crystal compound. The red circles and blue triangles show the susceptibility along the c-axis in the virgin state and a state after saturation process,respectively. The green squares show the susceptibility in the basal plane

14

ISSP Activity Report B 2001

[1] E.V. Sampathakumaran, I. Das and R. Vijayaraghavan, Solid State
Commun., 83, 609 (1992).

Authors

T. Shigeokaa, Y. Tanedaa , M. Hedo and Y. Uwatoko
aFaculty of Science, Yamaguchi University.

Modulation in Electrical Conduction ofSingle-Wall Carbon Nanotubes in Zeolite
Single Crystals by Laser Light Irradiation
N. Nagasawa and T. Suemoto
Carbon nanotubes (CNs) formed in micro-channelsaligning along the C-axis of a Zeolite (AFI) single crystal
have unique electrical and optical character. Tang's grouphas reported electrical transport properties of the AFI-CN
crystal [1] as well as its superconductive character at low
temperature [2].According to the analyses of the opticalabsorption [3] and Raman spectra [4], metallic and semiconducting tubes coexist in this system.
We have found that the electrical properties are modulated by the laser light irradiation. The modulation has been
seen not only in the photo-current [5] but also in the bias
current. Figure 1 shows a schematic illustration of ourexperimental set-up to measure the photo irradiation effects
on the bias current.
Linearly polarized laser light at 900 nm was focused
onto the central part of one surface of the crystal. It has been
known that metallic tubes in this system show strong
absorption at 900 nm in E//C configuration, where E and C
are the electric field vector of the laser light and the C‐axisof the crystal or the tube axis, respectively. Before laser
light irradiation, the I‐V characteristics show ohmic nature
as Tang et al. have reported [1]. After strong photo-irradiation with E//C, however, the bias current was dramaticallydecreased and showed slight deviation from the ohmic character in the I‐V characteristics.
Plotted in Fig.2 is the deviation from the ohmic behavior
by subtracting the ohmic part around zero-bias region frommeasured bias current, (a) before and (b) after the laser lightirradiation. The ohmic character is seen in curve (a). On theother hand, the deviation was found to start at V = ± 0.25V
in curve (b). The inset shows the first-order derivative of
curve (b) in terms of V, to show the on-set voltage moreclearly. The character is somewhat semiconducting.
We can speculate the following scenario to understandthe mechanism of the change. The resultant current in thephoto-irradiated sample can be due to remaining un-photomodulated metallic tubes and the photo-modulated semiconducting ones. The on-set energy observed above suggests that the relevant band gap energy would be about

Fig.1. Experimental setup. A single AFI-CN crystal was inserted into a glass
tube, and was pinched by two gold wires from both openings. Bias voltage
was applied to CNs between these two wires.

Fig. 2. Deviation from ohmic behavior of the bias current, (a) before and (b)after the photo-irradiation. Inset: The first-order derivative of curve (b).

0.5 eV. This value is not so unrealistic considering the accuracy of the band gap energies of relevant CNs predictedfrom the LDA calculation.
These results may suggest the potential application ofCNs to novel opto-electronic devices.
References[1] Z.K. Tang, H.D. Sun, J. Wang, J. Chen, and G. Li,Appl. Phys.
Lett., 73 (1998) 2287.
[2] Z.K. Tang et al., Science, 292 (2001) 2462.
[3] Z.M. Li et al., Phys. Rev. Lett., 87 (2001) 12740.
[4] A. Jorio et al., Chem. Phys. Lett., 351 (2002) 27.
[5] Y. Kamada, N. Naka, and N. Nagasawa, Z.M. Li, and Z.K. Tang,
Physica B, in press.

Authors

Y. Kamadaa, N. Nakaa, and N. Nagasawaa, T. Suemoto and S. Saito
aDepartment of Physics, University of Tokyo

Synthesis of Large Single-Crystal
Forsterite using CZ Method
K. Ito and T. Kitazawa
Forsterite is the most abundant phase of the Earth's mantle. Synthesis of high-quality large forsterite single crystals
is essential to understand physical properties of the mantle.
To study anisotropies in seismic velocity, elasticity, anelasticity, viscoelasticity, rheology, electrical conductivity, andthermal property, it is essential to determine the propertiesof mantle minerals along various crystallographic directions.
For systematic anisotropy measurements, it is important to
prepare homogeneous specimens cut for various crystallographic directions from one large single crystal. We reportsynthesis of large high-quality forsterite single crystals foruse in physical property measurements at high pressure andtemperature. We synthesized olivine single crystals by the
CZ method with a pure-iridium crucible. A wide furnace
space of the new CZ apparatus used in this study allows tomount a large Ir-crucible to ~90 mm diameter and ~90 mm
height. The CZ apparatus is equipped with the automatic
diameter control (ADC) system, which automatically maintains uniform crystal diameter. We will show as-grown crystals of forsterite and olivine. Figner 1 shows typical asgrown crystals of forsterite growing along the [100] direction. Careful preparations (weighing and mixing) of starting
reagents enable us to create a high-quality crack-free
forsterite specimen as shown in Fig. 1. In this sample, no
inclusions, voids or defects are observed under stereoscopic

ISSP Activity Report B 2001

15

Fig.1. As-grown forsterite single crystals growing along the [100] direction.
The unit of the measure is cm. The crystal has no cracks, inclusions ordefects. A high-quality crack-free specimen is synthesized by using highpurity MgO and SiO2 reagents.

microscope, except for facet. A thick long crystal can beproduced by using a large volume crucible. Owing to the
new ADC system, the present crystals have uniform diameter, and their cross sections are nearly perfect circle.
Ultimately, we will be capable of producing specimen size
to over 50 mm diameter and to ~250 mm length, by using a
crucible of ~90 mm diameter and ~90 mm height. Large
high-quality single crystals will be successively supplied foruse in property measurements and laboratory experiments.
References

The author thanks the Material Design and Characterization
Laboratory of the Institute for Solid State Physics of the University ofTokyo for the CZ facilities. This work was supported by grants fromthe Earthquake Research Institute of the University of Tokyo (2001-A18).

Authors

K. Itoa, H. Satob, H. Kanazawac, N. Kawamec and T. Kitazawa
aFaculty of Business Administration, Southern Osaka University.
bDepartment of Earth and Space Science, Osaka University.
cDepartment of Environmental Conservation and Development, Kyoto
University.

Non-stoichiometry and Phase Transitionof EuBaCo2O5+x Double Perovskit― A TEM Study――

Fig.1 A high-resolution lattice image of EuBaCo2O5.25 viewed along the[001] direction. The oxygen non-stoichiometry is accommodated by the
microdomain formation. Red lines indicate domain boundaries between thevacancy ordered regions.

RBaCo2O5+x, ranges from 0 to 1 ideally and oxygen vacancies are located at the RO planes. Oxygen vacancy orderingsin the RO planes are reported for the compositions x = 0.5
and 0.43. The formal valence of Co atoms ranges from 3.5
to 2.5. At the specific compositions of x = 0 and 0.5, acharge order transition and a metal-insulator transition arereported, respectively. The oxygen non-stoichiometry has a
crucial role on the physical properties of RBaCo2O5+x.
However, no detailed study has been reported except for
YBaCo2O5+x system [2].
We have investigated EuBaCo2O5+x system containing
the larger R atoms and the results were compared with
YBaCo2O5+x system. The studies were focused on the direct
observation of oxygen vacancy ordering by high-resolution
TEM images and the TEM characterization of the low temperature charge ordered state.
Samples were prepared by the conventional ceramic
method. After calcinations of starting mixtures at 1050°C,
samples were annealed at several temperatures in O2, N2 or
Ar stream to obtain samples with wide range compositions.
The oxygen content was determined by TG-analysis in H2.
For 0.05 ＜ x ＜ 0.25, the orthorhombic phase with the fun~b~~a0, c ~~2a0) wasdamental 112 structure (Pmmm, a ~N. Nakayama and Y. Ueda
The transition metal oxides ABO3 with perovskite-type
structure exhibit a variety of interesting physical properties
such as the colossal negative magnetoresistance ofManganites. The flexibility of perovskite-type structure, theaccomodation of cations with the different ionic charge in A
sites and/or the oxygen deficiency, stabilizes the wide rangeof valence state for transition metals in B sites and leads to
the rich variety of the physical properties. The perovskitetype cobaltate is another candidate of the colossal negative
magnetoresistant materials.
Recently, a series of cobaltate, RBaCo2O5+x (R; rareearth metals), showing fairly high magneto-resistance has
been found [1]. In these compounds, R and Ba atoms occupy
the A-site in an ordered manner along c-axis. The crystal
structure can be described as a layered arrangement of
RO/CoO2/BaO/CoO2. The oxygen deficiency, (1-x) in

16

ISSP Activity Report B 2001

Fig.2. A [010]-zone electron diffraction pattern of the charge ordered state for
EuBaCo2O5.05 below 200 K. The reciprocal lattice reconstructed from several
ED patterns with different zone axes indicates the Pmma symmetry of theordered state, which agrees with the stripe-type ordering model of Co(Ⅱ) andCo(Ⅲ).

obtained. Whereas for 0.25 ＜ x ＜ 0.65, the orthorhombic
phase with doubled a-axis due to the oxygen vacancy ordering was obtained. The 3a0 × 3a0 ordered tetragonal phaseas reported for ABaCo2O5+x with smaller A cations (Y, Hoetc ) was not observed. The oxygen non-stoichiometry isaccommodated by the micro-domain formation (Fig.1).
For the sample with x = 0.05, a charge order phase transition was found. DSC curves show two peaks at 250 and
360 K, corresponding to the charge order and the antiferromagnetic order, respectively. The magnetic susceptibility
shows the anomalies at the transition temperatures.
Electron diffraction patterns at low temperatures indicate
the doubling of a-axis due to the charge order (Fig.2). The
extinction condition agrees with the symmetry of chargeordered phase (Pmma) of Y or Ho systems. Transition temperatures are 20-30 K higher than Y and Ho system as aresult of volume effect.
References[1] A. Maignan, C. Martin, D. Pelloquin, N. Nygyen, and B. Raveau,
J. Solid State Chem. 142 (1999) 247[2] D. Akahoshi and Y. Ueda, J. Solid State Chem. 156 (2001) 355
Authors
N. Nakayamaa, Y. Kubotaa, T. Mizotaa, and Y. Ueda
aFaculty of Engineering Yamaguchi University

A New Macroscopically Degenerate Ground
State in the Spin Ice Compound Dy2Ti2O7
under Magnetic Field
K. Matsuhira, Z. Hiroi and T. Sakakibara
Geometrical frustration can lead to novel phenomena
such as a macroscopic degeneracy in the ground state withno long-range ordering. Recently, a new class of magnetic
state known as “spin ice” (Figure 1) is discovered in the
pyrochlore rare-earth [1].
In the pyrochlore lattice, a ferromagnetic interaction stabilizes the local spin arrangement of two spins pointing outward and two spins inwards (so-called ‘two-in two-out’state) in a basic tetrahedron when there is a strong singlesite anisotropy along the local <111> axes. For every tetra-

Fig. 2. Macroscopic degeneracy in the plateau state (kagomé ice). Here, a, b
and c show the three patterns for the one-in two-out configurations for triangles with another apical spins above, whereas d, e and f are those for two-in
one-out arrangements for the othertriangles with another apical spins below
in the kagomé layer.

hedron, there are six possible combinations of spins under
the two-in two-out rule reflecting the global cubic symmetry. In fact, the typical compound Dy2Ti2O7 show the residual ground state entropy of 1.68 J/mol K, which is numerically in agreement with the Pauling's entropy for water ice[2].
By applying magnetic field along the [111] direction, we
found a new macroscopically degenerate ground state.
Below 1 K, we observed a clear plateau in the magnetization
process. The emergence of the plateau is closely associatedwith a formation of the spin ice state. The magnetic momentof the plateau is very close to the value expected for the satu
rated moment along the [111] direction without destroying
two-in two-out state. From the result of the field dependenceof magnetic entropy at 0.4 K, we found that the plateau stateis disordered with the residual entropy of nearly half thevalue of the zero-field state. The macroscopic degeneracy
comes from a frustration of the spins on the kagomé layers
perpendicular to the magnetic field. The magnetic fieldalong the [111] direction reduces the sixfold degenerate
two-in two-out configurations in the basic tetrahedron to
threefold. In this state, every spin on the triangular layer isoriented along the [111] direction whereas the spins on the
kagomé layer are still frustrated because the two-in two-out
ice rule is maintained in each tetrahedron. The kagomé layer
consists of corner-shared triangles, and under the ice rule,
there are two types of triangles having the spin configurations of either one-in two-out or two-in one-out (Figure 2).
For each type, there are three ways to align the spins. Our
experimental data confirm this new macroscopically degenerate state (‘kagomé ice’ state) in Dy2Ti2O7 under magnetic
fields.
References[1] M.J. Harris et al., Phys. Rev. Lett. 79 (1997) 2554.
[2] A.P. Ramirez et al., Nature 399 (1999) 333.
Authors
K. Matsuhiraa, Z. Hiroi, T. Tayama, S. Takagia and T. Sakakibara.
aKyushu Institute of Technology

Fig. 1. Pyrochlore lattice of the corner-sharing tetrahedra whose vertices areoccupied by Ising spins of Dy ions. The long arrow shows the [111] direction
along which the magnetic field is applied. One of the six “two-in, two-out”
configurations is shown by short arrows. Each spin lies along the axis joining
the vertex and the center of the tetrahedron, due to a strong single-site
anisotropy. Pyrochlore lattice can be viewed as an alternating stacking ofkagomé layers and sparse triangular layers, both of which are perpendicular
to the [111] direction.

Order-Disorder Transition in Cubic
Cd6M and its Low Temperature phase
R. Tamura and Y. Ueda
Recently, a new class of the icosahedral (i) quasicrystal(QC) was found as a stable phase in Cd-Yb and Cd-Ca
alloys[1], which are the first binary stable iQCs to be identi-

ISSP Activity Report B 200117

fied. After the discovery, Takakura et al.[2] pointed out thatcubic Cd6Yb and Cd6Ca crystals are the 1/1 cubic approximants of the Cd-Yb and Cd-Ca QCs. Cd6Yb is reported tohave space group of Im3, which contains 168 atoms in a unit
cell with a = 15.638 Å.The structure can be described as bcc
packing of an icosahedral cluster composed of 66 atoms.
Interestingly, the core of the icosahedral cluster is an atomic
shell of non-icosahedral symmetry, four Cd atoms reside atthe vertices of a small cube with half occupancy forming atetrahedron. In the course of our systematic investigation of
the transport behavior in the binary QCs and their approximants[3], we found the occurrence of an order-disorderphase transition at about 110 K and 100 K in the Cd6Yb andCd6Ca cubic crystals, respectively, as described below[4.]The resistivity of the Cd6Yb and Cd6Ca cubic crystals
show a stepwise change at 110 K and 100 K with a discontinuous change of TCR(temperature coefficient of the resistivity) across the step. No hysteresis is observed in the resistivity change for decreasing and increasing cycles of thetemperature. Results of the specific heat measurements also
exhibit a small peak at about 110 K and 100 K, respectively.
The fact that these temperatures correspond precisely to thetemperatures of the resistivity jump verifies the occurrenceof a phase transition at these temperatures. On the other
hand, no discontinuous shift was evident from powder X-ray
diffraction spectra at temperatures above and below 110 K
for Cd6Yb. However, slight peak-broadening has been
observed at 110 K, which occurs reversibly when the temperature cycle is repeated. This result is taken as indicationthat the phase transition at 110 K accompanies the introduction of some kind of disorder.
As shown in Fig.1, we clearly observe a change of theelectron diffraction pattern below and above 110 K for
Cd6Yb. The figure shows the patterns obtained for the samegrain at 130 K(a) and 14 K(b) for Cd6Yb. Pattern (a) showsa diffraction pattern taken along [013] and is indexed asgiven in the figure, satisfying the extinction rule of the bcc
structure, which is consistent with the reported space group
Im3 for Cd6Yb. Meanwhile, in pattern (b), additional weak
spots appear in a commensurate manner to pattern (a). Thesespots are regarded as the superlattice spots due to the ordering of the structure. We confirmed that the change of thediffraction pattern is reversible for the temperature cycle andthus it indicates that the phase transition is an order-disorderFig.2. Schematic illustration of the B32 ordered arrangement of the central
4Cd tetrahedral. The 4Cd tetrahedra of different orientations are indicated bydifferent colors.

transition. Close examination of diffraction patterns of theordered phase has revealed that the superlattice spots aremostly described as F-type reflections from a superstructureof double unit cell. Considering that the transition is mostprobably due to the orientational ordering of tetrahedral 4Cd
clusters sitting at the center of dodecahedral cages, we are
led to a conjecture that above 110 K (100 K) the central 4Cd
cluster is disordered over two orientations resulting in a bcc
structure (Im3), whereas below 110 K, it is ordered in Ftype superstructure (B32) (c).
Figure 2 illustrates schematically the B32 orderedarrangement of the central tetrahedra. Such B32 ordering isplausible since the point symmetry of the nearest icosahedral cluster arrangement is the same as that of the central
4Cd cluster, which would result in reduction of the strainenergy.
References[1] A.P. Tsai et al., Nature 408 537(2000) .
[2] H. Takakura et al., Philos. Mag. Lett. 81 411(2001) .
[3] R. Tamura et al., Jpn. J. Appl. Phys. 40 L912(2001) .
[4] R. Tamura et al., Jpn. J. Appl. Phys. 41 L5242002) .

Authors

R. Tamuraa, Y. Muraoa, S. Takeuchia, M. Ichihara, M. Isobe and Y.
Ueda
aDepartment of Materials Science and Technology, Science Univesityof TokyoHigh Pressure X-ray Study on Organic
Charge Transfer Salts
H. Sawa and J. Yamaura

Fig.1. Electron diffraction patterns of Cd6Yb taken along [013] at 130 K (a)and at 14 K (b). Electron diffraction patterns expected from the bcc, theordered B2 and the ordered B32 structures (c).

18

ISSP Activity Report B 2001

It is well known for many organic charge transfer salts
that the applying pressure induces a various changes ofphysical properties. It is significant that the crystal structure
determination under pressure to discuss the origin of thehigh-pressure state. We have been developing the X-ray
study under pressure at the X-ray laboratory in the material
design and characterization laboratory.
β '-Pd(dmit)2 anion radical salt is characterized by asolid-crossing columns composed by strongly dimerizedPd(dmit)2 units. This is a unique two-band system associated with two-dimensional HOMO-based band and onedimensional LUMO-based band [1]. Several compounds
show metallic and superconducting states under pressure
which are followed by a non-metallic state under higher
pressure region. X-ray single crystal structure analysis wassured the X-ray diffraction of the powder samples underpressure for DI and DBr salts using a special designed powder type DAC. We found the different pressure dependence
in the lattice parameter (Fig. 2), estimated by the Rietvelt
method, for each salt. This suggests that the electronic state
under pressure is different between them although they have
similar conductive properties under pressure.
References[1] R. Kato, Y-L. Liu, Y. Hosokoshi, S. Aonuma and H. Sawa, Mol.
Cryst. Liq. Cryst. 296, 217 (1997).
[2] R. Kato, H. Kobayashi and A. Kobayashi, J. Am. Chem. Soc. 111,
5224 (1989).

Authors

J. Yamaura and H. Sawaa
aKEK (Japan)
Fig. 1. Fermi surface of the HOMO band under pressure at room and low
temperatues. They are multiple ones due to solid-crossing columns. Lowering
the dimensionality under pressure is related to the non-metallic state in thehigher-pressure region.

Dynamics of an Isolated Twin Boundary
K. Kawabata and H. Takayama

carried out for the high-pressure superconductor
β'-Et2Me2P[Pd(dmit)2]2 (Tc = 4 K at 7 kbar) under pressure
at room temperature and low temperature to clarify thenature of the electronic states. The diffraction data underpressure were collected using an imaging plate type
Weissenberg camera equipped with a diamond anvil cell(DAC). We calculated the band parameters based on theextended Hückel method from the refined crystal structure
under pressure, indicating an increase of bandwidth and adecrease of the dimensionality of a HOMO-based band.
Figure 1 shows the calculated Fermi surface of the HOMO
band, which means the decrease of dimensionality underpressure. The above findings are related to a pressureinduced metallic state and a non-metallic state in the higherpressure region.
(R1,R2-DCNQI)2Cu salts have a 1D column structure of
DCNQI molecules, where the column networks are bridged
by Cu ions. They exhibit various unique physical properties
due to the hybridization between 1D organic DCNQI pπŒband and Cu dxy-orbitals near the Fermi level [2].
Among them, we are interested in the R1 = R2 = I (DI) andR1 = R2 = Br (DBr) salts having the complicated pressuretemperature phase diagram, that is, the ground state changes
successively under pressure as metal(MI) → insulator(II) →metal(MII) → insulator(III) → metal(MIII). In order to identify the origin of the complicated phase diagram, we mea-

In the ISSP collaboration with theory groups(Matukawa, Nemoto, Takayama) and experimental groups(Kawabata, Sambongi), we discussed dynamics of an isolated twin boundary in order to clarify the mechanism of its
mobility. Charge transfer complex (TMTSF)2X has triclinic
crystallographic structure. TMTSF and X denote tetramethyl-tetraselena-fulvalene and an anion such as PF6+,
respectively. Planer TMTSF molecules are stacked alongneedle axis. The twin boundary is formed in a single crystalof (TMTSF)2X when the stress is applied in a suitable manner. The boundary can move easily under a weak shear
stress. It is quite unique that the boundary is visible as a kinkof the needle shape crystal as shown in Fig.1-a.
Then its position can be measured definitely by optical
microscope and a digital recorder in real time. An isolated
boundary was forced to move repeatedly under constantstress within the same area of the crystal. The motion of theboundary was found unusual, as follows. The boundary
moves intermittently; it stops at some positions and moves
again after intermission ranging over tens of seconds evenunder constant external stress. We considered that the intermission occurs at some strong pinning centers like defects
and/or impurities and that many internal degrees of freedom
existing in the present system are essential. We found thatthe mobility of the boundary is affected strongly by theFig. 2. Pressure dependence of the lattice parameter ratio of a/c for DBr andDI salts with the crystal stucture of (R1,R2-DCNQI)2Cu. They show the different dependence of their values in higher-pressure region. The pressure
value is normalized by the critical pressure for each salt.

Fig. 1. (a) A single crystal of (TMTSF)2X before and after twin deformation.
The twin boundary appears as a kink in morphology, which reflects to its
twinned crystal structure. (b) Schematic model for twin boundary motion.
When the boundary moves ahead in lattice spacing, molecules on one side of
the boundary slide along lattice array on the other side. The twin boundary
motion is in a similar situation to dry friction between two commensurate lattices.

ISSP Activity Report B 2001

19

Fig. 2. Temperature dependences of the local velocity for various external
stresses. When the temperature is lowered, the velocity decreases under theexternal stress of 1.2 kPa, while the velocity increases under 0.68 kPa.

waiting time, which corresponds to the time elapsed before
the boundary passes forward through each position after the
backward passage. The velocity becomes lower as the waiting time is longer [1]. We succeeded in proposing a modelthat the imperfections, which created by the backward
motion and recovered for the waiting time, assist the mobility of the boundary.
On the other hand, we focused that the twin boundary
motion is in a similar situation to dry friction of solid on
solid systems. In the case that a unit cell is simple, eachmolecule on one side of the boundary slides over molecule
array of the other side for lattice spacing along the twinboundary plane, when the boundary goes ahead for latticespacing, as shown in Fig.1-b. Then, the twin boundary
motion can be considered as an elementary process of dry
friction between two commensurate lattices without wearing. We found similarity of the motion between the twinboundary and frictional sliding of a paper on paper system,
which was previously reported by Heslot et al [2]. The present system is useful to examine temperature effects on thefriction phenomena, since it is difficult to carry out friction
experiments of solid on solid systems at various temperatures. We found that the obtained temperature dependence iscomplicated, as shown in Fig. 2 [3].
When the temperature is lowered, the velocity decreases
with the stress under 1.2 kPa, while the velocity increases
under 0.68 kPa. Taking into consideration of the temperature dependent threshold stress σc , the obtained results were
replotted as a function of temperature under constant effective stress, which is defined as σ-σc. The temperaturedependence can be represented well as a thermal activation
type. The activation energy was estimated as about 0.1 eV
and almost independent of the effective stress. This implies
that the boundary motion includes some nucleation process.
References
1. M. Mukoujima, K. Kawabata, T. Sambongi, Eur. Phys. J. B7, 365369 (1999).
2. F. Heslot, T. Baumberger, B. Perrin, B. Caroli, and C. Caroli, Phys.
Rev. E49 (1994) 4973.
3. K. Kawabata, Y. Hosokawa, T. Kawauchi and T. Sambongi,
Ferroelectrics, 251, 199-205 (2001).
Authors
K. Kawabataa, H. Takayama, H. Matukawab, K. Nemotoa, T.
Sambongic
aHokkaido University., bOsaka University., cHokkaido Information
University.

20

ISSP Activity Report B 2001

〜 March 2002)
ISSP WorkShop (April 2001〜
1. Chemistry and physics of oxides with 3D geometrical frustration

May 31~June 1, 2001

Geometrical frustration among localized spins is known to cause anomalous ground state degeneracy and fluctuation effects. Recent extensive studies on pyrochlore and spinel oxides have revealed rich variety of anomalous phenomena not only for insulating spin system but for itinerant electron systems. In this workshop, the latest experimental
and theoretical results on these oxides were presented.

2. Chemistry of transition metal oxides: Present status and future of materials search for novel
physics and applications
June 18~20,2001
The remarkable discovery of high-Tc superconductivity and the following enthusiastic research in the last decade
have clearly exemplified how the finding of new materials would give a great impact on the progress of solid statephysics and its application to technology. Now the related topics are spreading over not only superconductivity but also
an unusual metallic behavior which is generally seen near the metal-insulator transition in the strongly correlated electron system. We believe that for the next few decades it will become more important to explore novel physics throughsearching for new materials. Transition-metal oxides are one of the most typical system where the effect of Coulombinteraction plays a critical role on their magnetic and electronic properties. Especially interesting is what is expected
when electrons localized due to the strong Coulomb repulsion start moving by changing the band width or the numberof carriers. We anticipate there an unknown, dramatic phenomenon governed by quantum fluctuations. In this meeting
specialists from various areas of research got together to discuss the general idea on materials research.

3. New developments of charge ordering phenomena

June 27, 2001

Recently charge ordering phenomena have been discovered in many materials, including manganese, cobalt andvanadium oxides and organic charge transfer salts. This one-day workshop was organized to discuss these phenomena,
in particular the behavior under high pressure and magnetic field, as well as the related theoretical models.

4. Superfluidity and quantum vortex in quantum condensed system :Superfluid He and Bose-Einstein condensation ( BEC ) in neutral atomic gases
October18~19,2001
It is not too much to say a BEC in a laser-cooled alkali atomic gases is one of the most active field in condensed
matter physics. The concepts of a condensate, an elementary excitation and a quantum vortex etc. in BEC in gas
strongly reflects those of superfluid He in a conventional low temperature phsics. In Europe and U.S.A., there exists a
close communication among those who work in quantum electronics and low temperature physics, resulting into promoting the reseach in the " quantum coherent system " in both fields. On the other hand, in our country, there was nosuch communication at all, which is an unfortunate situation. Recenkly we started some actions to improve the situation, and the present meeting is also one of such activities. Just before this meeting, this year's Nobel prize in physics
was annonced to be awarded for the realization and basic research in BEC in dilute alkali atomic gases. As a result ofheated atomosphere in related communities, we had about eighty participants and lively discussions among them. We
are very happy if this meeting gives a good chance to cause a rapid progress in the research field of " quantum coherent
system " in our country.

5. High magnetic fields: Physics and technology

November 5 ~ 16, 2001

Recently, high magnetic field science has attracted much attention, and a number of new large facilities for highmagnetic fields have been built in the world. Many novel interesting results have been obtained using the high magnetic fields. It has long been argued in Japan, that a new collaborative scheme is necessary among the research groups
who are working on the physics in high magnetic fields in Japan, in order to stimulate the activity in this research area
and to strengthening cooperation among the groups. It has been agreed that the High Magnetic Forum Japan should beestablished in autumn 2002, and that a seminar should be organized at least once a year to discuss about the latest
results and the future collaboration. This Workshop was organized as the first meeting of this kind. A variety of topics
have been discussed in the two-days workshop. In particular, a focus was put on new techniques for generating highmagnetic fields and for precise data acquisition in the high fields. A new direction of the research was also discussed.

6. Applications of new light sources and future prospect

November 19 ~ 20, 2001

Recently the brightness of synchrotoron radiation (SR) has been improved and long undulators are becoming lightsources with spatial coherence as high as lasers. New coherent light sources such as high-order harmonics at the soft Xray region, free electron lasers at the farinfrared region and terahertz radiation by femtosecond lasers have appeared on
the stage. Furthermore the slicing of femtosecond pulses from SR by lasers is becoming to an application level. On theother hand, in the field of dynamics in structure and phase transition, some interesting demonstrations like timeresolved X-ray diffraction are reported. In this symposium, researchers concerning with the development of lightsources, spectroscopy and theory gathered to discuss about new achievement, present status and future plan on
researches by new light sources and spectroscopic methods at the visible and infrared regions.

ISSP Activity Report B 2001

21

7. Unconventional superconductivity in the heavy-electron compounds CeTIn5
December 7 ~ 8, 2001
The heavy-electron compounds UGe2 and CeTIn5 (T = Co, Ir, Rh) have been attracting considerable interest,
because of their unusual superconducting properties. UGe2 is a ferromagnet at an ambient pressure, and under the pressure of ~ 1 GPa exhibits a superconducting transition that is considered to be coexisting with ferromagnetism. While
CeRhIn5 is an antiferromagnet, CeCoIn5 becomes an ambient pressure superconductor with the highest known transition temperature (2.3 K) for any heavy-electron systems. The CeTIn5 compounds are therefore considered to present anuncommon opportunity to study the interplay of superconductivity and magnetism in heavy-electron systems. Recent
activities on these topics are presented as 27 talks and discussed in this workshop. troscopic methods at the visible andinfrared regions.

8. Physics of Vortex Matter in High temperature Superconductors.

December 10 ~The purpose of this conference is to clarify (1)the static and dynamic phase diagram of the "composite vortex matter", which is realized in highly anisotropic high temperature superconductors in magnetic field, (2) the electronic
structure in the vortex state of unconventional superconductors, and (3) the feasibility of device application of the vortices. In the vortex matter, various kinds of ground states are realized by the interaction, thermal fluctuation, and randomness. Moreover the electronic state of the two dimensional vortices is not well known. In the conference, the above
topics were discussed in 45 talks by 65 participants.

9. Meeting on Cd2Re2O7.

January 30, 2002

Since Cd2Re2O7 was discovered to be the first superconductor among the the pyrochlore oxides at ISSP, active
research has been pursued. In this workshop, the latest results on the superconducting and other physical properties ofthis material were intensively discussed. In particular, the nature of the successive structural transitions and the changesof physical properties caused by them were the target of active discussion that revealed anomalous character of itinerant electron systems on a pyrochlore lattice.

10. Structural fluctuation of Si(100) surface

February 1, 2002

Surface atoms on Si(100) are reconstructed to form a (2x1) dimer row structure. Many experimental and theoreticalstudies have supported the buckled (asymmetric) dimer with c(4x2) phase as a ground state, although some quantum
chemical studies including electron correlation have proposed a symmetric dimer as the most stable state. Recently,
several low temperature STM studies have reported symmetric dimer images. With decreasing the substrate temperature from 80 K to 5 K, the area of symmetric dimer image increases. However, the origin of such symmetric images isa controversial issue (dynamic or static). In this workshop, we have discussed the surface structure of Si(100) at low
temperature. Recent low temperature STM studies have been presented as well as other experimental investigations
including surface diffraction, surface phonon spectroscopy, high resolution core-level photoelectron spectroscopy andtheoretical studies. More than fifty researchers and graduate students participated in this workshop and discussed thiscontroversial issue in detail.

11. Recent developments in quantum Hall systems

February 15~16,2002

The quantum Hall systems, i.e. two-dimensional electron systems in semiconductor heterostructures in strong magnetic fields and low temperatures, are well established as a highly controled experimental stage of the physics of strong
correlation. Recently, there are a few new developments in this field, including, interlayer coherence in double layer
quantum Hall systems, stripe phase (charge density wave phase) in higher order Landau levels, long relaxation phenomena involving nuclear spins, edge states and chiral surface states in multilayer quantum Hall systems, etc. Together
with Professor Z.F.Ezawa, who is a Visiting Professor of ISSP this year, we made a plan for a workshop in which a relatively small number of experimentalists and theorists have a intensive discussions on these and related topics. The
proposal for such a workshop has been granted as a ISSP Workshop, and the Workshop was held in February 15th
(Fri.) and 16th (Sat.) in the Lecture Hall of ISSP. The Workshop turned out to be very successful with about 40
researchers and students participating in active discussions on such subjects as, (1)the many body states of the quantum
Hall systems with extra degrees of freedom, (2)the relation between the stripe phase and externally applied potential
modulation, (3)the mechanism of the breakdown of quantum Hall states, (4)transport phenomena in antidot and multilayer systemsprogress in the research field of " quantum coherent system " in our country.

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ISSP Activity Report B 2001

12. New Synchrotron Light Source in Japan

February 22~23,2002

The symposium attracted over 80 scientists in the field of accelerator, solid state and soft materials science, had 18
invited presentations on the application of undulator radiation, and had two group discussions. The organizers were
Professors S. Sato (Tohoku University), H. Fukuyama (The University of Tokyo, ISSP) and Y. Kimura (KEK, IMSS),
who had been discussing on the future plan of the synchrotron radiation facility in Japan. In opening the symposium,
Dr. N. Kumagai (SPring-8) reported on the discussion at the light source design working group and showed a new 1.61.8 GeV electron storage ring with 10 insertion devices optimised in EUV and SX region. The design and the applicability of the new light source were discussed keenly throughout the two-days symposium from both viewpoints of synchrotron radiation users and accelerator scientists. The results of the symposium were applied to modify the design,and the construction plan of a 1.8 GeV electron storage ring with 12 insertion devices was proposed as the future synchrotron light source in Japan.

13. ISSP Theory forum for the 21st century.

March 7~8, 2002

Discovery of new materials and development of nano-structure and soft-materials have made condensed matter theory even more exciting and challenging. At the same time, however, one tends to focus on specialized problems andcommunications among researches working on different disciplines are becoming increasingly difficult. This workshop
is organized to provide opportunities for young condensed matter theorists to discuss new concepts and methods common to many sub-fields of condensed matter physics across different disciplines. The program included 30 minutes
review talks intended to serve for non-specialists and poster sessions from general contributors, as well as a few keynote speech from experimentalists that provided new theoretical problems. The workshop was very successful withactive discussions among more than 90 participants.

14. Proposal of the monochromator and the characterization of the optical elements for the VUVSX high brilliant light source beamlines
March 19, 2002
The high brilliant light source project around the extreme ultraviolet(EUV) and the soft-X ray (SX) region is now
discussed and designed as a national project. The members of the working group for the accelerator, the beamlines andthe research field are selected from many facilities, institutes and universities in Japan. In order to proceed the designof the monochromator suitable for the high brilliant light source with highperformance, i.e., high energy resolution,high photon flux, small spot sizeetc, the R&D’s to develop the optical elements such as multilayer mirrors, zone plate,grating and so on are very important. In this workshop, the present status of the characterization of optical elements are
discussed. The proposal and the demand to achieve the high performance beamlines are also discussed.

ISSP Activity Report B 2001

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