The Institute for Solid State Physics
The University of Tokyo

Highlights
ofJoint Research

Activity Report2005Highlights of Joint Research

Synchrotron Radiation Laboratory
The Synchrotron Radiation Laboratory (SRL) was established in 1975 as a research group dedicating to study solidstate physics using synchrotron radiation. In 1989, the SRL
started to hold the Tsukuba branch, a branch laboratory in
the Photon Factory (PF), High Energy Accelerator Research
Organization (KEK). SRL maintains an undulator called
Revolver, two beamlines and three experimental stations;
BL-18A for angle-resolved photoemission spectroscopy andtwo beamlines, BL-19A and BL-19B for spin-resolved
photoelectron spectroscopy and soft X-ray emission
spectroscopy experiments using undulator radiation. They
are fully opened to outside users for experiments using highbrilliant synchrotron radiation from the undulator. The
operation time of these beamlines are about 5000 hours andthe number of users is more than 200 a year.
The present SRL consists of the accelerator physics
group and the solid state spectroscopy group. The membersof the accelerator group have been carrying out researchworks on the accelerator physics and developing various new
accelerator related technology in collaboration with other SR
facilities. The spectroscopy group has been not only serving
users at the Tsukuba branch with technical supports andadvices, but also carrying out their own research works on
advanced solid state spectroscopy.
In 2005, the operation of the PF ring was quitted from
April to the end of September to improve its characteristics
as a light source. Several straight sections were elongated, sothat many insertion devises could be installed into thestorage ring. The long term shut down of the PF ring reduced
the scientific activities using synchrotron radiation at the
Tsukuba branch. Instead, some of the staff members of SRL
have made experiments at other SR facilities and someothers have dedicated to improve experimental apparatuses
at the beamlines, which results some new achievements. One
is a new angle-resolved photoemission spectrometer with ahigh resolution analyser (PHOIBOS 150) and a equipment
for sample preparation. The new system was commissioned
already and will be installed at BL-19A in 2006. The other isa new inverse photoemission spectrometer for measuring
unoccupied electronic states of solids and solid surfaces.
They have observed an energy dispersion of unoccupied surface
states of LaB6(100) and showed that the low work functionof the LaB6(100) are originated from the charge redistribution
at the surface.
Since 1980s, SRL has been promoting the "Super-SOR"
project for constructing a new synchrotron radiation facility
with a third generation light source dedicated to the sciencesin vacuum ultraviolet and soft X-ray (VSX) regions.
However, during last year, the president office of the university found out that the promotion of the project is very
difficult or almost impossible for financial reasons. The
budget for the new facility project is too big to be supported
by a single university in Japan even after the reformation of

30

ISSP Activity Report 2005

the national universities. The decision of the president office
was made the staff members of SRL to face to a great
difficulty, and they started to find out a new direction of thesolid state spectroscopy in VSX region.
After many discussions, SRL has inclined to construct
beamlines using undulator radiation in other SR facilities
instead of constructing the facility with a light source and topromote advanced materials sciences using high brillianceand small emittance of SR which have been considered in the
Super-SOR project. They are those such as microscopy andtime-resolved experiments which will only be possible using
third generation light source and will promise us a considerable progress in the studies of nano-particles and real timeobservation of magnetic domains and of chemical reaction at
catalytic surfaces, etc. The details of the future plan is notdefinite yet, however, we are planning to follow up the desireof the user community and to construct competitive beamlines for new scientific opportunities in VSX region using
high brilliance SR.
The accelerator group studied feasibility of an energy-upof the Super-SOR light source for request of the university
and found that the energy could be increased up to 2.4 GeV
only with small modification and cost-up. They successfully
designed the light source based on a 2.4 GeV storage ringand its injector (2.4 GeV booster synchrotron and 200 MeV
linac), which satisfied the requirements of both materialscience and biotechnology, though the budget was still too
big for the president office of the university to start the
project in near future. They also developed the advanced
accelerator component and subsystem such as the highresolution beam profile monitor with zone plates, theone-cell magnet girder, the Java/CORBA control system,
copper-plating insertion-device vacuum chamber, highresolution beam position monitor and stabilization systemand so on. Furthermore they started to collaborate with
JAEA and KEK to develop a new accelerator system, anenergy recovering linac (ERL), and to construct an ERL
prototype in the KEK site in near future, which will be used
as one of the best light sources in VSX region.

Neutron Science Laboratory
The Neutron Science Laboratory (NSL) has been playing
a central role in neutron scattering activities in Japan since
1961 by performing its own research programs as well as
providing a strong general user program for the universityowned various neutron scattering spectrometers installed at
the JRR-3 operated by Japan Atomic Energy Agency inTokai. In 2003, the Neutron Scattering Laboratory was
reorganized as the Neutron Science Laboratory to further
promote the neutron science with use of the instruments in
JRR-3. Under the general user program supported by NSL,
14 university-group-owned spectrometers in the JRR-3

Supercomputer Center

reactor (20MW) are available for a wide scope of researches
on material science, and proposals close to 300 are submitted
each year, and the number of visiting users under thisprogram reaches over 6000 person-day/year.
Triple axis spectrometers and a high resolution powder
diffractometer are utilized for a conventional solid statephysics and a variety of research fields on hard-condensed
matter, while in the field of soft-condensed matter science,researches are mostly carried out by using the small angle
neutron scattering (SANS-U) and/or neutron spin echo
(iNSE) instruments. The upgraded time-of-flight (TOF)
inelastic scattering spectrometer is now opened to the user
proposals.
Major research topics on the hard-condensed matterscience cover stripe order in high-Tc superconductors, andclosely related 2 dimensional systems, charge and orbital
ordering in CMR manganites, quadrapolar ordering inrare-earth based intermetallic compounds, spin dynamics oflow dimensional dimmer systems, etc. On the other hand, theresearch topics on the soft-condensed matter science cover
structural characterization of polymer blends, micelles,
amphiphilic polymers, block copolymers, liquid crystals,
proteins, inorganic gels, dynamics of brush-polymers on
surface, slow dynamics of surfactants, pressure dependenceof dynamics of amphiphilic membranes, and so on. In
addition, there are a variety of activities on fundamental
physics, neutron beam optics, developments of neutron
scattering techniques.
The NSL also operates the U.S.-Japan co-operative
program on neutron scattering, providing further research
opportunities to material scientists who utilize the neutron
scattering technique for their research interests.
The details of individual studies and research hightlights
in JFY2004 are reported in the NSL-ISSP Activity Report
vol. 12.

The supercomputer system in the Supercomputer Centerof the Institute (SCC-ISSP) is placed at the service of general
researchers of condensed matter physics through the User
Program conducted by the Materials Design andCharacterization Laboratory (MDCL). One of its aims is toselectively promote and support huge computations.
In March 2005, the SCC-ISSP renewed its main system
which has two super-computers. Hitachi SR11000/48, called
System-A, consists of 48 high performance nodes composedof tightly-coupled microprocessors. A node can be utilized as
an uniprocessor computer by automatic parallelization of its
Fortran compiler. System-A has 2.8 TB memory andachieves 5.8 TFlops peak performance in total. On the other
hand, SGI Altix 3700/1280, called System-B, is a looselycoupled parallel supercomputer consisting of 19 nodes interconnected by a gigabit Ethernet network. Each node is a distributed-shared-memory-type computer consisting of 64 Intel
Itanium 2 CPU's interconnected by a rather high performance network and have 64 GB memory. System-B achieves
7.7 TFlops total throughput performance.
A project can be proposed by any staff in universities or
public research institutes in Japan. The projects proposed are
judged by the Steering Committee of the SCC-ISSP, underwhich the Supercomputer Project Advisory Committee isformed to review proposals. In fiscal year 2005 totally 179
projects were approved. The total points applied andapproved are listed on Table 1 below.
The research projects are roughly classified into thefollowing three (the number of projects approved):
First-Principles Calculation of Materials Properties (72)Strongly Correlated Quantum Systems (67)
Cooperative Phenomena in Complex, Macroscopic
Systems (40)
All the three involve both methodology of computationand its applications. The results of the projects are reported
in 'Activity Report 2005' of the SCC-ISSP. In the report thefollowing four invited articles are included:"Electronic Structure Calculations for Strongly
Correlated Electron Systems", by T. Fujiwara et al.,"A New Approach to First-Principles Calculation ofCharged Surfaces", by M. Otani and O. Sugino,"Numerical Study of Low Dimensional Quantum Spin
Systems with Spatial Structures", by K. Hida,"Particle-Dynamics Approach to Non-Equilibrium
Phenomena", by F. Ogushi et al..

Table 1: Research projects approved in 2005========================================================================Class Max.Point Application# of Proj.
Total points
AppliedApproved
Sys-A
Sys-B
Sys-A
Sys-B
-------------------------------------------------------------------------------------------------------------------------A
100k
any time
2
100K
100K
100K
100K
B
2M
twice a year
60
86.9M
27.55M
86.6M
27.55M
C
20M
twice a year
105
1210M
603.9M
962.8M
597.9MDnone
any time
11
82M
179M
179M
147M
S
>20
twice a year
1
30M
5M
30M
5M
========================================================================The maximum points allotted to the project of each class are the sum of the points for the two systems; 1 K point of
System-A corresponds to charge for 0.37 hours × node, while the corresponding figure is 0.22 hours × 64CPU for System-B.

ISSP Activity Report 200531

Synthesis of 3d Transition-Metal
Nitrides under High Pressure in
Supercritical Nitrogen Fluid
M. Hasegawa, K. Niwa, and T. Yagi
Synthesis of metal nitrides are much more difficult,compared to metal oxides, because they usually decompose
at high temperatures. As a result, only limited studies havebeen so far made to clarify their physical properties, although
they are expected to be quite interesting. Here we reportsynthesis of various metal nitrides using a new technique; adirect reaction between transition metal and supercritical
nitrogen fluid under high pressure.
Diamond anvil cell (DAC) provides us powerful means
to achieve very wide range of high pressures. By combining
it with laser heating system (LASER-DAC), high temperature conditions above several 1000 K can also be achieved
simultaneously, because the laser beam can permeate
through the diamond anvil and is absorbed by the sample in
the DAC. In other words, LASER-DAC can be a powerfultool to synthesize new materials under very wide P-T
conditions and many geoscientists have applied thistechnique to study the Earth's deep interior. The DAC has
another big advantage over large-volume high-pressure
apparatus that gaseous materials can be compressed easily.
By heating solidified gas under pressure, a supercritical fluid
can be formed, which is highly reactive with various
materials. Direct reactions using such supercritical fluid
provides possibility to synthesize new materials which weredifficult to make by other means.
We have reported crystal growth of GaN, which is one of
the most famous nitrides because of its capability for a blue
laser diode, in a supercritical nitrogen fluid at about 10 GPa
using the LASER-DAC system [1]. High quality colorlesstransparent fine crystals of GaN in the wurtzite-type structure
were grown from molten gallium and fluid nitrogen. Using
the same experimental technique, we have also succeeded tosynthesize Co2N [2]. These results indicate that the LASERDAC system opens us a new way of synthesis and crystal
growth of variety of nitrides. The 3d-transition metal nitrides
are attractive materials from the view point of materials
science, because they exhibit variety of unique physicalproperties such as superconductivity, antiferromagnetism,
ferromagnetism and so on, depending on the combination ofelements. It is difficult to synthesize these nitrides, particularly for the late 3d-transition metals, by a simple direct
nitriding reaction between nitrogen and metal at ambientpressure. The reported synthesis processes for these nitrides
vary depending on the transition metal and it is not easy to
study their properties systematically, such as their phasestability, crystal structures, and intrinsic defect of nitrogen.

(a)

(b)

(c)

Fig. 1. Photographs of the sample in DAC before, during, and after(from left to right) the laser heating at 10 GPa. The sample was heated
at about 1800 K.

32

ISSP Activity Report 2005

Fig. 2 Temperature dependence of magnetization of synthesized VN.
The magnetization was measured by SQUID using one tiny sample. A
superconducting transition is observed at about 9 K.

In the present study the syntheses of the 3d-transition metal
nitrides have been tried by only one kind of reaction, i.e. a
simple direct nitriding reaction between 3d-transition metaland supercritical nitrogen fluid at high pressures using the
LASER-DAC system.
Small piece of transition metal (about 150 µm × 100 µm
× 20 µm) was placed in the sample chamber of DAC together
with the ruby pressure marker. The chamber was then filledwith nitrogen gas of about 150 MPa and then closed. By
increasing the applied load of the DAC, the sample chamber
was squeezed and the pressure was increased to a desired
value. Nitrogen in the sample chamber was solidified below
2 GPa. A multi mode YAG laser (100 W CW) was employedfor heating and the laser beam was split into two by a half
mirror to irradiate the sample from both sides of DAC to heat
the sample homogeneously. The sample during heating wasobserved from both sides using two CCD cameras andrecorded by the DVD recorder. Figure 1 is an example of thephotographs of the sample before (a), during (b) and after (c)the laser heating at 10 GPa. The sample was heated at about
1800 K. The solid nitrogen was melted during the laser
heating and this supercritical fluid has reacted with thetransition metals directly. After the heating, the pressure wasreleased and the reacted sample was recovered to ambientcondition and was examined by X-ray diffraction. X-ray
measurements were made at room temperature by a labbased micro-diffractometer (Cr Kα) using a position
sensitive proportional counter. All the 3d-transition metal
nitrides, except Cu nitride, such as TiN, VN, CrN, Mn3N2,
Fe2N, Co2N and Ni3N were synthesized easily at thesame condition described above [3]. This indicates thathigh-pressure and high-temperature synthesis in a supercritical nitrogen fluid using diamond anvil cell and YAG laser
heating is highly useful to synthesize metal nitrides.
The stoichiometry and the crystal structure changed
systematically with the atomic number. The Ti, V and Cr
form 1 to 1 compounds, TiN, VN and CrN, with NaCl-type
structure. The Mn nitride is an IrU2C2-type nitrogen-rich
non-stoichiometric Mn3N2. The Fe and Co nitrides are the
PbO2-type Fe2N and CFe2-type Co2N, respectively, while
the Ni nitride is the O3Re-type Ni3N. No Cu nitride wasformed in the above condition. These results indicate that theratio of nitrogen to metal, N/M, of the nitride decreases from
1 to 0 with the sequence from the early transition metal to thelate transition metal. This means that in the 3d-transitionmetals, nitriding reaction becomes more difficult withincreasing the atomic number. The systematic change of theN/M ratio and crystal structure of the 3d-transition metal
nitrides can be interpreted on the basis of the electron
arrangement of the 3d-transition metal, which is relevant to
its coordination number. Besides, the superconducting
transition of the NaCl-type nitrides (TiN, VN) obtained by
the present study were measured by the SQUID measurement of the magnetization [3]. Figure 2 is an example ofsuch measurement and although the size of the sample
synthesize by this technique is very small, it is clear that
we can still measure these physical properties. It can beconcluded from these results that the LASER-DAC is a
powerful technique to synthesize novel metal-nitrides andstudy their property systematically. Taking this advantage,
we are now making exploratory study of the synthesis of new
nitrides using this technique.
References[1] M. Hasegawa and T. Yagi, J. Crystal Growth 217, 349 (2000).
[2] M. Hasegawa and T. Yagi, Solid State Commun. 135, 294 (2005).
[3] M. Hasegawa and T. Yagi, J. Alloys and Compnd. 403, 131 (2005).
Authors
M. Hasegawaa , K. Niwa, and T. Yagi
aInstitute for Materials Research, Tohoku University

Compensation of the Effective Fieldin the Field-Induced Superconductor
κ-(BETS)2FeBr4
S. Fujiyama and M. Takigawa
The organic charge-transfer-salts containing magnetic
ions have been attracting strong interest since field-induced
superconductivity (FISC) was discovered in λ-(BETS)2FeCl4[1]. At zero magnetic field, this material shows simultaneous
antiferromagnetic (AF) order of localized moments(spin 5/2) on Fe3+ ions and metal-insulator transition of
π-conduction electrons on BETS molecules. A magnetic
field destabilizes both the antiferromagnetic order and theinsulating state. The FISC appears at high fields between
18 and 41 T [2]. This was suggested to be due to theJaccrino-Peter (JP) mechanism [3]: if there is antiferromagnetic coupling between Fe spins and π-electrons, superconductivity is expected when the external field compensates theexchange field on π-electrons from Fe spins, minimizing the

Fig. 1. (a) Crystal structure of κ-(BETS)2FeBr4 (b)Structure of BETS
molecules.

Fig. 2. 77Se NMR spectra at T=1.5 K for the field along the b-axis(a) and the c-axis (b). The horizontal axes show the frequency shift
from the resonance in diamagnetic materials.

spin-Zeeman pair breaking effect.
The field-induced superconductivity has been observed
also in the κ-type polymorph κ-(BETS)2FeBr4 for a much
lower range of the field between 10 and 15 T [4] below 300
mK. Here we report the frequency shift of the 77Se NMR
in κ-(BETS)2FeBr4 to measure the spin polarization of theπ-electrons. Our results demonstrate that the polarization of
π-electrons at low temperatures vanishes for the field near 12
T, which is the central field for FISC, hence support the JP
mechanism.
The crystal structure of κ-(BETS)2FeBr4 consists ofalternating stacks of the conducting BETS layers andthe insulating FeBr4 magnetic layers (Fig. 1(a)). The four
inequivalent Se sites in a molecule (Fig. 1(b)) are resolvedin the NMR spectra for a single crystal at T=1.5 K (Fig. 2).
The frequency shift δν has two contribution,δν = γNAπi,αmπ + γN(Adipi,α + Bi)md: the first term represents
the contribution from the π-spin polarization mπ through theanisotropic hyperfine coupling Aπi,α while the second term isdue to the Fe magnetization md through the direct dipolar
coupling Adipi,α and the transferred hyperfine coupling Bi.
(γN is the nuclear gyromagnetic ratio.) Since Adipi,α canbe calculated and md is completely saturated to 5 µB for
the fields of our measurements, the dipolar field can be
subtracted from the measured shift. The remaining shiftδν’= δν − γNAdipi,α md = γNAπi,αmπ + γNBimd is plotted inFig. 3 against the external field for various resonance lines.

Fig. 3. Field dependence of the frequency shift of various resonancelines for H // b and H // c.

ISSP Activity Report 2005

33

The linear relation between δν’ and Hext indicates validity of the mean field expression mπ = χπ(Hext + Jmd) where
Jmd is the exchange field from Fe spins. If the compensationof the effective field occurs, mπ should vanish and δν’ should
become isotropic at the compensating field. Thus the valueof the compensation field is known from the crossing of lines
in Fig. 3 for H // b and H // c corresponding to the same sites.
The plot in Fig. 3 indeed shows that such crossing occurs in
the field range 10 – 12 T, supporting the JP mechanism. We
also measured temperature dependence of the shift at aconstant field. The analysis of the data lead to similar valuesof the exchange coupling between the Fe moments and the
π-electrons. The consistent results from the two independent
set of data provide convincing evidence for the JP
mechanism of FISC in κ-(BETS)2FeBr4.
References[1] S. Uji et al., Nature 410, 908 (2001).
[2] L. Balicas et al., Phys. Rev. Lett. 87, 067002 (2001).
[3] V. Jaccarino and M. Peter, Phys. Rev. Lett. 7, 290 (1962).
[4] T. Konoike et al., Phys. Rev. B 70, 094515 (2004).
[5] S. Fujiyama et al., Phys. Rev. Lett. 96, 217001 (2006).
Authors
S. Fujiyamaa,b, M. Takigawa, J. Kikuchic, H-B. Chud,b, H. Fujiwarad,b,and H. Kobayashid,baDepartment of Applied Physics, University of Tokyo
bCrest-JST
cDepartment of Physics, Meiji University
dInstitute for Molecular Science

Thermoelectric Power of a Quantum
Dot in a Coherent Region
T. Nakanishi and T. Kato
A thermoelectric power (TEP) in mesoscopic systems has
been of an important issue as another probe for electrontransport properties. TEP in quantum-dot systems have beenstudied theoretically by the Landauer formula and masterequations. Effects of electron-electron interaction have also
been discussed by examining a deviation from the Mott
formula. In these studies, however, coherency of thetransmission through the quantum dot has not been fully
considered. In recent experiments, the phase change by π
between adjacent conductance peaks due to the zerotransmission has been observed in an AB ring with aquantum dot [1]. It remains an important question to consider
how TEP behaves around the zero transmission between
conductance peaks.
In this study, we have considered the enhancement of
TEP through the quantum dot in a coherent region [2]. It has
been shown that a pronounced structure appears in abias-voltage dependence of TEP due to the transmission
zero. This indicates that TEP can be a good probe of thephase change between conductance peaks.
Within the Landauer formula, we have first derived thegeneralized Mott formula for TEP. The Sommerfeld
expansion up to first order of the temperature T gives the(ordinal) Mott formula S=-Tπ2/(3e)d(logG)/dµ, where G is aconductance. We here consider the system of the quantum
dot with several quantum levels coupled to the leads.
Assumed time reversal symmetry, the wave function in thequantum dot is chosen real. By neglecting the Coulombinteraction, one can obtain the transmission coefficientthrough the dot as a function of the bias voltage. By
substituting this transmission coefficient to the Mott formula,

34

ISSP Activity Report 2005

Fig. 1. Thermoelectric power (TEP) through the quantum dot with twolevels at ε1=-50Γ and ε2=50Γ. As the temperature decreases, the biasdependence of TEP changes from a sawtooth-like shape to one withcharacteristic peaks and dips at both the quantum levels (ε=ε1, ε2) andzero transmission point (ε=0) between the levels.

the bias-dependence of TEP is calculated as shown in Fig.1.
Here, the quantum levels in the dot are located at ε1=-50Γ
and ε2=50Γ, with Γ a level broadening. The significant
enhancement of TEP can be seen around at the transmission
zero point ε=0. The height of the peak and dip of the TEParound ε=0 approaches πkB/(31/2e)~1.81kB/e at low temperatures. The prominent structure at ε=0 shows clearly thetransmission zero, while it is hard to see it in the tail of the
conductance.
Figure 2 shows TEP at low temperatures in the case of
five conductance peaks at εn/Γ =100(n-3) for n=1~5, where
the energy levels are indicated by arrows in the figure. The
signs of the coupling amplitude between each quantum level
and two leads include the phase information of the wave
function inside the dot. Here, the coupling between left andright leads is taken as symmetric for n=1~4, while as

Fig. 2. Thermoelectric power (TEP) at the low temperature kBT=0.2Γ.
The arrows indicates the energy levels in the quantum dots. Between
the quantum levels corresponding to the conductance peaks, characteristic structures with peaks and dips are observed. This structure may
disappear as seen in the region between ε4=100Γ and ε5=200Γ. The
presence of the peak and dip structure depends on the sign of the coupling between the dot and leads indicating the phase information of the
wave function in the dot.

asymmetric for n=5. As shown in Fig. 2, TEP shows a sharp
structure at transmission zero between the arrows for thein-phase case; three structures with peaks and dips are seen
between the levels εn for n=1~4. The transmission zero,
however, does not exist between ε4 and ε5, for the out-ofphase case. In the latter case, the transmission zero is outsideof the peak; it corresponds to the sharp structure at ε~230Γ.
Thus, the phase information can be indirectly obtained bymeasuring the bias-dependence of TEP at low temperatures.
In the present study, coherency between the dot and leads
is essential. In real experiments, however, decoherence
effects due to the environment may significantly affectthe bias dependence of TEP. We expect that the present
study will be naturally connected to the previous study
based on incoherent processes like sequential tunneling andinelastic cotunneling [3] by introducing decoherence/dephasing mechanism.
References[1] A. Yacoby, M. Heiblum, D. Mahalu, and H. Shtrikman, Phys. Rev.
Lett. 74, 4047 (1995).
[2] C. W. J. Beenakker and A. A. M. Staring, Phys. Rev. B 46, 9667
(1992).
[3] M. Turek and K. A. Matveev, Phys. Rev. B 66 115332 (2002).
Authors
T. Nakanishia and T. KatoaNational Institute of Advanced Industrial Science and Technology

THz Emission from (Ga, Mn)AsJ. B. Héroux, M. Kuwata-Gonokami,
and S. Katsumoto
The quest for a THz light source with a high efficiency is
an important issue in optics. It has been known that the THz
radiation from non-magnetic semiconductors after excitation
with a short light pulse is largely enhanced by applying an
external magnetic field. The origin of the emission is theacceleration of excited carriers by built-in surface electricfield or by diffusion. The external magnetic field bends theorbit of the carriers through Lorentz force, which improves
the radiation coupling out of the sample. However, theapplication of a strong external field requires a large bulky
system outside the source, which cannot be handled easily.
In this work we observe that the “build-in” magnetic field ina ferromagnetic semiconductor (FMS) produces a similareffect on the THz radiation. The bending of charged carrier
orbit with no external magnetic field in FMSs is often called
anomalous Hall effect (AHE), the origin of which is now
subject to renewed interest [1].
We have observed strong THz radiation from an FMS(Ga,Mn)As just after irradiation with light pulses [2]. The
Mn content of the sample was 6% and the Currie temperature
TC was 110 K. The excitation pulse width was 130 fs with acentral wavelength of 400 nm. Figure 1(a) shows typicalresults of a measurement of THz emission collected at
normal incidence at 4.3 K, where the sample magnetization
M is controlled by an external magnetic field B applied
parallel to the sample surface. The polarization of the THz
wave is linear and always perpendicular to the magnetization
M in this configuration. A typical frequency-domain spectrumof the wave is shown in Fig. 1(b). Figure 1(c) shows curvesof the peak THz intensity as a function of the B field strength

Fig. 1. (a) Typical experimental spectra of THz emission from GaMnAs
in the time domain for a varying external B field. (b) Frequency domain
spectrum. (c) THz hysteresis curves at varying temperatures. (4.2 K
corresponds to the top curve in the right part of the graph.) Inset shows
the axis definition and experimental configuration.

at varying temperatures. Clear temperature-dependent hysteresis curves are seen, and the signal could not be detected
above TC so that the emission is directly related to theexistence of a ferromagnetic phase in the sample.
We have explained the results adopting a simple model
based on a Drude-Lorentz equation of motion, in which the
AHE is phenomenologically introduced. The observed strong
enhancement of THz emission by ferromagnetism demonstrates the potential of diluted magnetic semiconductors for
THz light source applications, and at the same time sheds
light on the nature of AHE.
References[1] T. Jungwirth, Q. Niu, and A. H. MacDonald, Phys. Rev. Lett. 88,
207208 (2002).
[2] J. B. Héroux, Y. Ino, Y. Hashimoto, S. Katsumoto, and M. KuwataGonokami, Appl. Phys. Lett. 88, 221110 (2006).
AuthorsJ. B. Hérouxa, Y. Inoa, Y. Hashimoto, S. Katsumoto, and M. KuwataGonokamia
aDepartment of Applied Physics, University of Tokyo

Electronic States of Clean Ge(001) Surface
A. Ishii and F. Komori
The dangling bond states of the clean Ge(001) surface
plays important roles in various dynamical phenomena at thesurface, such as chemical reaction and electronic excitation.
Recently, hot carrier dynamics is discussed on the basis of a
topological defect motions induced by its local tunnelinginjection from the STM tip into the surface states [1].
However, the previous theoretical and experimental
studies on the electronic structure were insufficient for the
discussion on such hot carrier dynamics. Thus, we haveinvestigated it using the band calculation based on thefirst-principles method, high-resolution ARPES and standing
wave observations by STM.
The atomic structure of this surface is c(4×2) in the
ground state as schematically shown in Fig. 1(a). The
calculated surface band structure for this superstructure isshown in the upper panel of Fig. 2. The dangling bond empty
π* band is in the bulk band gap while the dangling bondfilled π band is a resonance with the bulk states near Γ.
The valence band structure was measured by ARPES. We

ISSP Activity Report 2005

35

80 K in differential conductance images of STM. All theseexperimental results agree well with the band calculation.
References[1] Y. Takagi, Y. Yoshimoto, K. Nakatsuji, and F. Komori, J. Phys.
Soc. Jpn. 74, 3143 (2005).
Authors
K. Nakatsuji, Y. Takagi, H. Kusuharaa, A. Ishiia, and F. Komori
aDep. Appl. Math. & Phys., Tottori University

Superconductivity of Nanometer-size Pb
Islands Studied by Low-temperature
Scanning Tunneling Microscopy
H. Sakata and Y. Hasegawa

Fig. 1. (a) a schematic model of the c(4×2) surface structure. (b) The
surface Brillouin zones for the c(4×2) (thick lines) and p(2×1) (thinlines) surfaces. (c,d) band dispersion around Γ observed at 130 K forthe clean (c) and H-adsorbed (d) Ge(001) surfaces by ARPES.

found that the top band at Γ remains after adsorption ofhydrogen atoms on the surface while the surface states clearly disappear as shown in Figs. 1(c, d). At temperatures higher than 470 K, the bottom of the π* band in the bulk band
gap is thermally filled, and almost flat dispersion is detectedby ARPES along the dimer-axis direction [Γ-J] as in thelower panel of Fig. 2. A large dispersion of the π* band in
the direction perpendicular to the dimer-axis [Γ-J'] isobtained from the analysis of the standing waves observed at

Fig. 2. Upper; Surface band structure for the clean Ge(001)-c(4×2)
surface. The axes are defined for the 2×1 surface shown in Fig. 1(b).
Blue areas indicate the bulk bands projected to the surface. Lower;band dispersion observed at 680 K for the clean Ge(001) surface by
ARPES. Solid blue circles and green triangles indicate the surface andbulk states detected at 130 K. Open blue circles and squares above EF
are the states in the Γ-J' direction determined from the standing wave
observation for the c(4×2) (circles) and p(2×2) (squares) surfaces.

36

ISSP Activity Report 2005

When the size of superconducting materials becomes
smaller than the coherence length of superconductivity, theirsuperconducting properties are modified from the bulk. So
far, various studies have been reported on superconductivityof nanometer-size particles. Detailed analysis has not,however, been successful since precise characterizationof the size, shape and thickness of the particles has beendifficult, and measuring the spatial variation of superconductivity within a single particle has been impossible. Using low
temperature scanning tunneling microscopy (STM) we
studied superconductivity of nanometer-size island structuresby measuring tunneling spectra on the islands while characterizing the geometry of the islands from STM images [1].
Since all experiments including sample preparation were
performed in situ under ultrahigh vacuum (UHV) conditions,
unwanted effects of contamination or oxide layers can beeliminated.
As a sample, we chose Pb islands grown on Si(111)
surface. Lead is one of the elemental superconductors withrelatively high superconducting transition temperature(Tc = 7.2 K). We formed clean and well-characterized Pb
islands on the semiconductor substrate and studied superconductivity in situ by taking tunneling spectra showing thesuperconducting gap to investigate how the superconductivity varies locally and how it depends on size and thickness of
the Pb islands. For the measurements it is necessary to takethe tunneling spectra far below Tc of the material. We used

Fig. 1. STM image of Pb islands formed on Si(111) substrate. The size
is 800 nm × 800 nm. The white line in the image corresponds to 87 nm,
coherence length of bulk Pb. The inserts show local thickness of Pb on
Island A. The thickness of Island B is 9 ML.

Spin Flow Dynamics in
Magnetically-Polarized Superfluid 3He
H. Kojima and H. Ishimoto

Fig. 2. Differential tunneling conductance spectra taken at 3.9 K on
island A, whose diameter is larger than the coherence length, and island
B, whose diameter is almost equal to the coherence length, are displayed.

low-temperature STM cooled by liquid 3He, whose lowest
temperature at the sample position is 1.2 K.
Figure 1 is an STM image of 4 ML Pb-deposited Si
substrate showing several island structures on the surface.
The islands are single crystal having a (111) plate on the top
as the step height on the islands is multiple of the step heightof Pb(111) (0.285 nm). From the height profile, the local
thickness of the islands is measured, as is written with asmall number in the inset drawings of Fig. 1. For convenience we named some Pb islands A, and B, as is marked in
the STM image. The size of the islands is 58000 nm2 for A,
5000 nm2 for B. The coherence length of the bulk Pb superconductor is 87 nm.
First, we measured tunneling spectra showing the superconducting gap on various points in island A at 1.2 K. Fromthe spectra it is found that all spectra taken on the island
including the ones taken around the peripheral of the island
have the same shape. A recent study reports thickness
dependent superconductivity of Pb thin film on the same
substrate. The same spectra independent of the thickness
indicate that the superconductivity does not change within
the nano-size Pb island, probably because the spatial variation of superconductivity is limited by the coherence length.
We then investigated its island dependence at various
temperatures. Tunneling spectra measured on the twoislands, A and B at 3.9 K are shown in Fig. 2. It is foundfrom the figure that the shape of the tunneling spectra
depends on the measured islands and that the dependence
is enhanced at the higher temperature. The gap measured
on the nano-size islands is larger than the bulk Pb value(1.35 meV). The smaller island shows the larger gap and thegap of the small island (B) increases with the temperature.
These peculiar properties are due to the superconducting
fluctuation, we speculate. For the superconducting materials
whose dimension is small the thermal fluctuation issignificant and the effect is enhanced as the size becomes
smaller and the measured temperature is higher. According
to the theory which handles superconductivity of small
particles, the gap is enhanced for smaller islands and at
higher temperature because of the fluctuation, which isconsistent with our results.

When liquid 3He is cooled in a magnetic field to low
enough temperatures, the normal liquid first makes a transition into superfluid A1 phase at Tc1 and then into superfluid
A2 phase at a lower temperature Tc2. The most important
aspect of the A1 phase is that its superfluid component is
totally spin-polarized along the magnetic field. It is a kind offerromagnetic superfluid. A mass superflow is then simultaneously a spin flow. The phase transition into the A1 phaseis accompanied by the spontaneous breaking of relative
spin-gauge symmetry. Interesting hydrodynamic effects
occur in the A1 phase such as a unique magnetic fountainpressure effect which was predicted by Liu [1] andestablished by Kojima [2]. Nevertheless earlier experimentson this interesting magnetically driven superflow haveuncovered puzzling problems [3]. To clarify these, themagnetic fountain detector should be placed in the sample
cell containing only A1 phase liquid without interfaces withany other superfluid and normal phase. To realize suchconditions, the ISSP nuclear refrigerator has been employedowing to its large experimental space and high magnetic
field capability up to 15 T (originally in the frame of
JSPS-NSF joint scientific program). A newly designed
magnetic fountain pressure cell was constructed in which alarge reservoir is connected to a small detector chamber
through a super leak made of several channels 10 µm wide
(Fig. 1). When magnetic field gradient is applied acrossthe superleak, a magnetic fountain pressure is induced byspin-polarized superfluid flow. From the observed decay of
the fountain pressure, spin relaxation time was determined
between 1 and 29 bar and 0.2 and 8 T. The observed
relaxation time τ is shown in Fig.2 as a function of reduced
temperature (Tc1 - T)/(Tc1 - Tc2). It varies from less than
1 sec near Tc2 to about 80 sec near Tc1. Surprisingly τ tends
to vanish towards Tc2 at all magnetic fields. To our knowledge, there is no theory which predicts such increase inrelaxation rate over a relatively large temperature range near
Tc2. A preliminary interpretation is given in terms of intrinsic
spin relaxation based on Leggett and Takagi theory [4] andthe predicted small but increasing presence of minority spinpair condensate in A1 phase.

References[1] T. Nishio, M. Ono, T. Eguchi, H. Sakata, and Y. Hasegawa, Appl.
Phys. Lett. 88, 113115 (2006).
Authors
T. Nishio, M. Ono, T. Eguchi, H. Sakataa, and Y. Hasegawa
aTokyo University of ScienceFig. 1. Magnetic fountain pressure cell

ISSP Activity Report 2005

37

Fig. 2. Nominal spin relaxation time as a function of reduced temperature at various magnetic fields.
References[1] M. Liu, Phys. Rev. Lett. 43,1740 (1979).
[2] R. Ruel and H. Kojima, Phys. Rev. B 28,6582 (1983).
[3] S. T. Lu, Q. Jiang, and H. Kojima, Phys. Rev. Lett. 62, 1639 (1989).
[4] A. J. Leggett and S. Takagi, Annals of Physics 106, 79 (1977).
Authors
A. Yamaguchi, S. Kobayashi, H. Ishimoto, and H. Kojimaa
aSerin Physics laboratory, Rutgers University

Multistep Magnetization of Triangular
Lattice Antiferromagnet CuFe1-xAlxO2
in High Magnetic Fields

to noncollinear, from noncollinear to collinear, and fromcollinear to collinear structure, depending on the strength of
H. By the previous studies the spin structures have been
determined in each field-induced phase as follows. In thefirst field-induced phase, spins establish a complicatedincommensurate structure, which is called a five-sublatticelike (5-sub-like) phase tentatively. In the second, spins on thec-plane establish the commensurate five-sublattice (5-sub)structure parallel to the c-axis. In the third, the commensurate three-sublattice (3-sub) structure is established. Above
32 T we found a linear increment of M(H), indicating anoncollinear spin structure. We call this region a fourthfield-induced (4th) phase.
Multistep magnetization processes similar to CuFeO2
were observed for the samples with x≤0.025. However
magnetic plateaus originating from a collinear spin structure
becomes less pronounced when x is increased. For the
samples with x=0.035 and 0.050 where the OPD phase isestablished in H=0, no field-induced phases was observed.
Here we focus on the effect of Al3+ impurities in eachfield-induced phase.
The 4-sub phase. This phase appears in the samples with
x<0.015, which is consistent with the fact xc~0.014. In thesamples with x=0.010 and 0.012, however, when H isdecreased from high fields, M(H) decreases with the same
slope as in the 5-sub-like phase. It implies that the 5-sub-likespin structure is kept down to H=0 once it is established byapplying high magnetic fields.
The 5-sub-like phase. The slope of M(H) in this phase
does not depend on x up to 0.025. Moreover, it has been
confirmed that the LT phase of the sample with x=0.020
continues up to around 10 T by the neutron diffraction
experiments [2]. Then we deduce that the 5-sub-like phasewill be identified with the LT phase.

H. Aruga Katori, S. Mitsuda, and K. Kindo
Delafossite-type compound CuFeO2 with layered structure of triangular lattices has been extensively investigatedas one of the model materials of a triangular lattice antiferromagnet with geometrical spin frustration. Although theorbital singlet Fe3+ (L=0, S=5/2) magnetic ions have
Heisenberg spin characters, CuFeO2 shows exotic magnetic
orderings with quasi-Ising character.
Recently, the effect of nonmagnetic Al3+ impurities on
the magnetic orderings of CuFeO2 has been investigatedusing single crystals CuFe1-xAlxO2. The Al3+ concentration x
versus temperature phase diagram is shown in the inset of
Fig. 1 [1]. The Al3+ impurities cause the dramatic changeof the ground state of CuFeO2 by disturbing the delicate
balance of the competing exchange interactions. The
commensurate four-sublattice (4-sub) phase with quasi-Isingcharacter is vanished by the substitution of the criticalconcentration xc~0.014. The low-temperature (LT) phaseof the samples with 0.015≤x<0.035 is a complex incommensurate spin state. The samples with x≥0.035 undergo theoblique partial disordered (OPD) phase at low temperatures.
In order to clarify the magnetic properties of the samples in
the 4-sub, LT and OPD phases, we measured the magnetization of single crystals of high quality CuFe1-xAlxO2 in highmagnetic fields.
Figure 1 shows the magnetization curves M(H) of thesamples with various x. The sample with no impurity(x=0.000) shows multistep phase transitions. We foundvarious changes of spin structures of CuFeO2, from collinear

38

ISSP Activity Report 2005

Fig. 1. Magnetization curves of single-crystal CuFe1-xAlxO2 withvarious x measured at 1.6 K. The magnetic field was applied parallel tothe c-axis. The inset shows the Al3+ concentration versus temperaturephase diagram.

The 5-sub phase. This phase is strongly affected by
impurities. The collinear spin structure seems to be destroyed
by a small amount of Al3+ impurity. By increasing x, thenoncollinear spin structure becomes to be constructed, which
was observed in the region called a field-induced (FI) phasein the sample with x=0.020 [2].
The 3-sub phase. This phase is observed for the samples
with x≤0.025. The collinear 3-sub spin structure is stronger
against the Al3+ impurities than other collinear 4-sub and5-sub spin structures.
The 4th phase. The slope of M(H) does not change up to
x=0.025, indicating that this noncollinear spin structure isstable.
References[1] N. Terada, S. Mitsuda, T. Fujii, K. Soejima, I. Doi, H. Aruga Katori,
and Y. Noda, J. Phys. Soc. Jpn. 74, 2604 (2005).
[2] N. Terada, S. Mitsuda, K. Prokes, O. Suzuki, H. Kitazawa, and
H. Aruga Katori, Phys. Rev. B 70, 174412 (2004).
Authors
H. Aruga Katoria, H. Mitamura, S. Kanetsukib, N. Teradab,
S. Mitsudab, K. Kindo, and S. Takeyama
aRIKENbDepartment of Physics, Tokyo University of Science

Pressure-Induced Spin Density Wave
from Local-Moment
Antiferromagnetism in Heavy Fermion
Ce1-yLay(Ru0.85Rh0.15)2Si2
S. Murayama, T. Nakano, and Y. Uwatoko
Typical heavy-fermion compound CeRu2Si2 showsdifferent types of antiferromagnetic ordering by substitutionof each Ce-, Ru- and Si-site by its particular second element.
Ce(Ru1-xRhx)2Si2 shows an incommensurate spin density
wave (SDW) transition with partial gap on the Fermi surface
along the c-axis due to the Fermi surface nesting in theheavy-fermion band [1, 2]. The gap opening is clearlyobserved from a gap-like jump in resistivity only along thec-axis [2]. Ce1-yLayRu2Si2 shows an antiferromagnetic (AF)ordering with different wave vectors and rather large
magnetic moment [3]. Recently, we have measured thesusceptibility and resistivity of Ce1-yLay(Ru0.85Rh0.15)2Si2 invarious La concentrations y [4, 5]. We found that the Kondo
temperature TK decreases rapidly and that the gap-like jumpin resistivity disappears with increasing y as shown in
Fig. 1(a). These results suggest that the La substitution
introduces decrease of the c-f hybridization due to thenegative chemical pressure and that the SDW changes toa local-moment-type AF (LAF) phase without any gap
opening. In order to make clear the magnetic phase of thisCe1-yLay(Ru0.85Rh0.15)2Si2 system, we have done detailed
resistivity measurements under hydrostatic pressure.
Fig. 1(b) shows the temperature dependence of theresistivity of Ce1-yLay(Ru0.85Rh0.15)2Si2, y = 0.25 along thec-axis under hydrostatic pressure. We observe a downwardbend in resistivity at around the Néel temperature TN for
P = 0, which is usually observed at the normal AF transition.
With increasing pressure, the bending at TN becomes small
and the gap-like jump occurs at P ≥ 0.43 GPa. With further
increasing P, the jump disappears at P ≥ 1.16 GPa witha large T2 term observed in the low temperature limit. We
conclude from these results that an SDW with partial gap on

Fig. 1. Temperature dependence of the resistivity of
Ce1-yLay(Ru0.85Rh0.15)2Si2 along the c-axis. (a) shows the variationon the La concentration y and (b) shows the variation underhydrostatic pressure for y = 0.25.

the Fermi surface of the heavy fermion band is induced bypressure from the LAF phase of Ce1-yLay(Ru0.85Rh0.15)2Si2,
y = 0.25. This is just the opposite direction to the case ofchemical La substitution. For P ≥ 1.16 GPa, the SDW gap
disappears and typical Fermi-liquid (FL) state remains with alarge effective mass. The present system can be one of thegood candidates for the Doniach picture concerning thecompeting Kondo effect and RKKY interaction.
Figure 2 shows the magnetic phase diagram as a functionof the La concentration y, pressure, and temperature determined from the susceptibility and resistivity. It is shown that
the La substitution is always destructive for the SDW or FL
phases. On the other hand, hydrostatic pressure plays a constructive role for the SDW or FL. This conflicting tendency
can be well explained by change of the c-f hybridization due
to the negative pressure by the La substitution and the positive hydrostatic pressure.

Fig. 2. Three-dimensional magnetic phase diagram ofCe1-yLay(Ru0.85Rh0.15)2Si2 as a function of the La concentration y,pressure P, and temperature T determined from the susceptibility
and resistivity.

ISSP Activity Report 2005

39

Unfortunately, the mechanism of the Fermi surface
nesting in the heavy-fermion band for the present system isstill not clear, because there is no direct observation of the
Fermi surface or band calculation except for LaRu2Si2 andCeRu2Si2. The nesting condition may be very delicate
depending on the electron number and band state for
4f- and 4d-electrons. We need exact band investigation
for Ce1-yLay(Ru0.85Rh0.15)2Si2 to study further. Anotherinteresting point to solve is the cross-over between the
LAF and SDW. Is it a quantum phase transition or
coexistence of the two phases? The present resistivity of
Ce1-yLay(Ru0.85Rh0.15)2Si2, y = 0.25 shows clear change
between the downward bend and gap-like jump in resistivity
by both the La substitution and hydrostatic pressure, and no
definite evidence of the coexistence. This system may bealso one of the good examples for the quantum phase transition under pressure.
References[1] S. Kawarazaki, M. Sato, H. Kadowaki, Y. Yamamoto, and
Y. Miyako, J. Phys. Soc. Jpn. 66, 2473 (1997).
[2] S. Murayama, C. Sekine, A. Yokoyanagi, K. Hoshi, and Y. Onuki,
Phys. Rev. B 56, 11092 (1997).
[3] S. Quezel, P. Burlet, J. L. Jacoud, L. P. Regnault, J. Rossat-Mignod,
C. Vettier, P. Lejay, and J. Flouquet, J. Magn. Magn. Mater. 76-77, 403(1988).
[4] T. Nakano, H. Yamashiro, K. Fujita, and S. Murayama, J. Phys.
Soc. Jpn. 74, 1602 (2005).
[5] A. Isoda, S. Shimada, Y. Kanda, S. Murayama, H. Takano,
T. Nakano, M. Hedo, and Y. Uwatoko, Physica B, 359-361, 169(2005).
Authors
S. Murayamaa, T. Nakanob,c, S. Shimadaa, T. Horiia, M. Hedo, andY. Uwatoko
aMuroran Institute of Technology
bWaseda University
cJapan Science and Technology Agency/CREST

Two-Peak Structure of Superconductivity
in a Doped-Type Organic Superconductor
H. Taniguchi and Y. Uwatoko
The most well-known ambient-pressure organic superconductors, κ-(BEDT-TTF)2X (X=Cu(NCS)2 and Cu[N(CN)2]Bretc.), can be regarded as a bandwidth-controlled superconductor at fixed band filling [1]. This is because, in thesesystems, the superconductivity is realized by pressurizing
the Mott insulator. In this case, with increasing pressure,the electrons localized at dimers of BEDT-TTF experience
the first-order insulator-metal or insulator-superconductortransition [2] and show Fermi liquid behavior [3] just as they
start to delocalize.
The (BEDT-TTF)4Hg2.89Br8 are also known to crystallize
into the κ-type structure [4]. However, there is a peculiar
structural characteristic not seen in other κ-type salts. Thissalt has incommensurate structure between lattice periodicity
of mercury chains and that of donor molecules [4]. Due tothis incommensurability of mercury ions, the electron fillingis possibly shifted from the usual half-filled state of 2:1 salts.
Therefore, kind of hole-doped state may be realized. Indeed
this salt shows metallic temperature variation and superconductivity at 4.3K [4] even at ambient pressure in spite ofhighly enhanced U/W value (U/W=1.79). So we believe thatthe metallic and superconducting states of this Hg2.89Br8
salt are caused by the effect of “natural doping”. In thissense, the Hg2.89Br8 salt offers the possibility of making a

40

ISSP Activity Report 2005

Fig. 1. Temperature-pressure phase diagram (upper panel) and pressure
dependence of the power in the normal state resistivity (lower panel) of
κ-(BEDT-TTF)4Hg2.89Br8. The resistivity data were obtained by usinga piston-cylinder-type pressure cell (red symbols) and the cubic anvilpress in Uwatoko laboratory (blue symbols).

connection between other κ-type organics and the cuprates.
In order to clarify the difference in physical properties
between doped and nondoped organic superconductors, wemeasured resistivity of κ-(BEDT-TTF)4Hg2.89Br8 as functions of temperature and pressure.
The electrical resistivity measurements were performedby using a piston-cylinder-type pressure cell and the cubic
anvil press (Uwatoko laboratory). The obtained phase diagrams
are displayed in Fig. 1. One of the most remarkable feature ispressure-insensitive nature of Tc. Pressure derivative of Tc at
the intermediate-range, for example, is – 0.45 K/kbar, of
which absolute value is far smaller than approximately |-2|
K/kbar of other κ-type materials’. This feature may suggestthat the present superconductor is not a bandwidth-controlled
type superconductor but a doped-type one.
More importantly, there is a two-peak structure in the Tc
diagram. Such behavior has never been observed in otherorganic system and quite unusual. We found that the lowtemperature part of the resistivity data in the metallic state
were well fitted by R=R0+T n just before superconducting
transition. The obtained n is shown in the lower panel. As
observed in the figure, value of the power at the lowest
pressure is approximately unity. This power is not seen inother organic systems and is reminiscent of anomalous
metallic state of underdoped cuprate or rare earth conpounds
near quantum critical point. With increasing pressure, thepower gradually increases toward almost two and becomesalmost stable around two over the higher-pressure superconducting phase. This behavior may reflect that the system
undergoes a crossover or a transition to the Fermi liquidstate. By applying pressure of 42.5 kbar, the superconductivity disappears within the measured temperature-range.
In summary, we observed quite unusual Tc-behavior in
the doped-type organic superconductor. In addition, we
found that transport properties showed strong violation fromones of other nondoped organic superconductors. We believethat the present organic salt is laid on the unique situation
with the unique values of U/W and doping rate. Therefore,

the observation of two-peak structure of Tc and a crossoveror a transition between the two superconducting phases are
an important finding for the profound physics of the highly
correlated electron system.
References[1] K. Kanoda, Hyperfine Interact. 104, 235 (1997).
[2] S. Lefebvre, P. Wzietek, S. brown, C. Bourbonnais, D. Jerome, C.
Meziere, M. Fourmigue, and P. Batail, Phys. Rev. Lett. 85, 5420 (2000)[3] T. Ishiguro, K. Yamaji, and G. Saito, Organic Superconductors(Springer-Verlag, Berlin) 2nd Ed.
[4] R. N. Lyubovskaya, E. I. Zhilyabva, S. I. Pesotskii, R. B.
Lyubovskii, L. O. Atovmyan, O. A. D’yachenko, and T. G. Takhirov
Pis’ma Zh. Eksp. Teor. Fiz. 46, No. 4, 149 (1987).
Authors
H. Tanigcuhia, N. Nagaia, T. Okuhataa, K. Satoha, M. Hedo, and,Y. Uwatoko
aSaitama University

Change of the Electronic States under
Pressure in Ferromagnet CePtAl
M. Nakashima and Y. Uwatoko
Cerium and uranium compounds form heavy fermions at
low temperatures, which is a consequence of competition
between the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction and the Kondo effect. High pressure is useful intuning the electronic states in these compounds. Whenpressure is applied to the magnetically ordered cerium anduranium compounds, the ordering temperature Tmag decreases
and becomes zero at a critical pressure P = Pc. CePtAlcrystallizes in the orthorhombic TiNiSi-type structure. There

Fig. 2. Pressure dependences of T1, ρ0 and A in CePtAl.

Fig. 1. Temperature dependence of electrical resistivity at several pressures in CePtAl. Arrows indicate the magnetic ordering temperature.

are three successive magnetic transitions at T1=5.9 K, T2=4.3 K
and T3=2.2 K [1,2]. The present experiments were performed
using a cubic -anvil cell at pressures up to 8.0 GPa in thetemperature range from 2 to 300 K.
Figure 1 shows the temperature dependence of electricalresistivity at several pressures. At ambient pressure, theresistivity has a broad hump centered at approximately
150 K and decreases steeply below the ordering temperature
T1=5.9 K. The broad hump is due to a combined effect of the
Kondo effect and the crystalline electric field effect. With
increasing pressure, T1 increases slightly and the broad humpis shifted to lower temperature and another broad humpappears below 20 K. This hump seems to be enhanced withincreasing pressure. The former broad hump and the latterhump are merged into a shoulder-like hump at pressureshigher than 6.0 GPa. The temperature at which the merged
hump is centered increases with increasing pressure. Thisbehavior above 6.0 GPa is often observed in a heavy fermion
compound such as CeAl3, indicating a valence fluctuatingstate. The ordering temperature T1 increases slightly withincreasing pressure. Interestingly, the ordering state disappears abruptly above 6 GPa, indicating a first-order like
phase transition of an electronic state from a heavy fermion
state to the valence fluctuating state. Figure 2 shows thepressure dependences of T1, the residual resistivity ρ0 and thecoefficient A of the T 2-dependence in electrical resistivity.
Both ρ0 and A increase as functions of pressure up to 5 GPa.

ISSP Activity Report 200541

A=2.4µΩcm/K2 at 5 GPa is larger than the value of the heavy
fermion superconductor UPt3. A characteristic feature in
CePtAl is that marked changes in ρ0 and A are found in avery narrow pressure region from 5.0 to 6.0 GPa.
Correspondingly, the ordering temperature does not decreasesmoothly as a function of pressure, but becomes zero abruptly.
These results suggest that a first-order phase transition isrealized in CePtAl.
References[1] A. Dönni, H. Kitazawa, P. Fischer, J. Tang, M. Kohgi, Y. Endo,and Y. Morii, J. Phys.: Condens. Matter 7, 1663 (1995).
[2] H. Kitazawa, S. Nimori, J. Tang, F. Iga, A. Dönni, T. Matsumoto,and G. Kido, Physica B 237-238, 212 (1997).
[3] H. Kitazawa, A. Dönni, L. Keller, J. Tang, F. Fauth, and G. Kido,
J. Solid State Chem. 140, 233 (1998).
[4] K. Miyake and H. Maehashi, J. Phys. Soc. Jpn. 71, 1007 (2002).
Authors
T. Ueda, M. Nakashima, M. Hedo, Y. Uwatoko, H. Nakashima,
A. Thamizhavel, T. D. Matsuda, Y. Haga, R. Settai, and Y. Onuki.

Ultra-Large-Scale Electronic Structure
Theory and Nano-Structure Process
T. Hoshi and T. Fujiwara
We have developed a set of theories and program codes
in ultra-large-scale electronic structure calculations, particularly, for process of nano structures, structures in nano-meteror ten-nano-meter scales [1-5]. In the test calculations of
Fig.1, the computational cost of our methods shows the‘order-N’ or linear-scaling property with the system size (N),up to ten-million atoms. The common theoretical foundationof our methods is the quantum-mechanical calculation of the
one-body density matrix or the Green's function, instead ofone-electron eigenstates. Practical calculations were carriedout using Hamiltonians in the Slater-Koster form.
As an application, 10-nm-scale molecular dynamics
simulation was carried out for surface formation process insilicon cleavage [1, 3, 5]. As shown in Fig.2, we obtained not
only the elementary formation process of the (experimentallyobserved) (111)-(2×1) surface but also step formation andbending of cleavage path. These results are consistent toexperiments and capture the dynamical aspect of cleavage,
which is beyond the traditional understanding with surface

Fig. 2. Surface formation process in silicon cleavage; [3] (a)-(c):Bending of cleavage path into the experimentally observed ((111) or(110)) planes. The present snapshots describe a process on the order of
10 ps. (d)-(e): Two successive snapshots with the time interval of 0.6
ps. A pair of five- and seven-member rings is the unit structure of theexperimentally observed (111)-(2 × 1) surface, called Pandey structure.
A step structure appears with a six-member ring. Bond sites in the initial (crystalline) structure are indicated by letters, such as ‘A’ and ‘B’.

energy [3]. Moreover, the local density of states, calculated
from the Green's function [4], shows characteristic surface
states at the step-edge region, which reproduce bias-dependent
STM image [5]. The step structures are experimentallyobserved but their explicit atomic structures have notbeen settled. In conclusion, our results propose a promising
candidate of the step structure and should be confirmed
experimentally.
In general, the variety of structure and function in nanosystems stems from the competition of bulk and non-bulk
parts whose electronic states are essentially different. The
present electronic-structure theory reproduces this competition,
as in the above application [3], and enables us systematicinvestigations of nano-structure processes among different
materials and phenomena. Other applications are now
proceeding.
References[1] T. Hoshi and T. Fujiwara, J. Phys. Soc. Jpn. 72, 2429 (2003).
[2] R. Takayama, T. Hoshi, and T. Fujiwara, J. Phys. Soc. Jpn. 73,
1519 (2004).
[3] T. Hoshi, Y. Iguchi, and T. Fujiwara, Phys. Rev. B 72, 075323 (2005).
[4] R. Takayama, T. Hoshi, T. Sogabe, S.-L. Zhang, and T. Fujiwara,
Phys. Rev. B 73, 165108 (2006).
[5] T. Hoshi, R. Takayama, Y. Iguchi, and T. Fujiwara, Physica B
376-377, 975 (2006).

Fig. 1. The ultra-large-scale calculation with up to 11,315,021 atoms;[1,3,5] The computational time of our methods is plotted for fcc Cu andliquid C with 64 CPUs and for bulk Si with single CPU and 512 CPUs.
The time of the conventional eigenstate calculation is also plotted forfcc Cu with single CPU. The computations were carried out using Intelor SGI CPUs.

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ISSP Activity Report 2005

Authors
T. Hoshia and T. Fujiwaraa,
aUniversity of Tokyo.

Oxygen Non-Stoichiometry
in the Mixed Valence
RBaCo4O7+δδ (R: Dy-Lu, Y) Systems
N. Nakayama and Y. Ueda
Mixed valence cobalt oxides, RBaCo4O7+δ (R: Dy-Lu,
Y; hexagonal P63mc or trigonal P31c), show interesting
successive structural and magnetic phase transitions [1, 2].
Barium and oxygen atoms compose a 4H(abac)-type closed
packed BaO7 sublattice. Cobalt and R atoms occupy thetetrahedral and the octahedral sites, respectively. CoO4
tetrahedra link their corners to form a 3-dimensional
network, in which 2-dimensional Co kagomé and triangularlattices are stacked alternately. The magnetic frustration andthe charge ordering may feature in the structural andmagnetic phase transitions, although the details have notbeen clarified yet.
Another interesting property is the oxygen nonstoichiometery with the highly excess amount. For example,
LuBaCo4O7, when annealed in 1 atm of oxygen stream at
300 ºC, topochemically absorbs oxygen up to the composition of LuBaCo4O7.8. The reversible oxygen absorption anddesorption occur at around 350 ºC. The crystal lattice showsan anisotropic shrinkage on oxygen absorption; the a-axis
shows 0.4% contraction and the c-axis is almost unchanged.
The sample with the excess oxygen does not show any
structural phase transitions and the magnetic susceptibility
suggests somewhat glassy magnetic ordering.
The excess oxygen atoms chemically order at room
temperature. Figure 1 shows an electron diffraction pattern
and a HREM lattice image of LuBaCo4O7.8. The structuralmodulation is clearly seen along the <110> direction of
the fundamental P63mc lattice with a period of 1.88 nm:
six times of the (110) lattice spacing. The Yb and Tm
compounds also show the similar oxygen over-stoichiometry
and the chemical ordering. The Er, Ho, Dy, and Y
compounds with the larger R atoms form the more overstoichiometric phases with the composition around
RBaCo4O8.3. However, on oxygen absorption, the crystal
lattice shrinks along the c-axis and expands in the a-plane asshown in Fig. 2. The period of the chemical ordering also
turns to the ten times of the (110) lattice spacing. Because Ba
and O atoms are well assumed to form the closed packed
BaO7 lattice, there is no room to accept the excess oxygen
composition. This means that the excess O atoms occupy theinterstitial sites in the closed packed lattice. However, theFig. 2. Oxygen compositions and lattice parameters of over-stoichiometric RBaCo4O7+δ (R: Dy-Lu, Y). Lattice parameters of nearlystoichiometric RBaCo4O7 are also shown (black marks). Notethe anisotropic lattice shrinkage on oxygen absorption and also thedependence on the ionic size of R.

details of the ordered structure are unknown. The neutron
diffraction study is in progress.
References[1] M. Valldor, Solid State Sci. 6, 251 (2004).
[2] N. Nakayama, T. Mizota, Y. Ueda, A. N. Sokolov, and A. N. Vasiliev,
J. Magn. Magn. Mater. 300, 98 (2006)
Authors
N. Nakayamaa, T. Nakajima, and Y. Ueda
aYamaguchi University

Preparation and Magnetic Properties
on A0.33Sr0.67CoO3-δδ (A = Y, Gd)
Y. F. Zhang, M. Izumi, and Y. Kiuchi
Recently, extensive studies have been carried out on
the cobalt oxide due to its unique magnetic behaviors andvarious spin state [1-2], LS (low spin) state, IS (intermediatespin) state and HS (high spin) state. By the theoretical calculation in the perovskite LaCoO3 [2], the eg orbital ordering
exists in the IS state configuration. The cobaltite RBaCo2O5-d
can have Co-O octahedron, square pyramid or both
structures depending on the amount of oxygen [1, 2]. The
unusual paramagnetic (PM) reentrant phenomenon on
the GdBaCo2O5-d compound that occurs via a couple ofspin-state transition has been reported [1]. In our work,
we choose the nonmagnetic Y ion and magnetic Gd ion
to prepare A0.33Sr0.67CoO3-d (A = Y, Gd) samples by theconventional solid state reaction technique to study structuraland magnetic properties.
The homogeneity and stoichiometry of the samples wereanalyzed by the electron-probe microanalysis (EPMA) and
inductively coupled plasma (ICP) spectroscopy which confirmed that the compositions were Y : Sr : Co = 0.323 : 0.672: 1 and Gd : Sr : Co = 0.345 : 0.699 : 1. The oxygen content
Table 1. The lattice parameters, the average Co-O bond length,
deduced from the Rietveld refinement of XRD data at room temperature for A0.33Sr0.67CoO3-d (A=Y, Gd) samples (space group=I4/mmm).
Here iodometric titration data shown include the standard deviation.
SampleFig. 1. An electron diffraction pattern and a lattice image ofover-stoichiometric LuBaCo4O7.8. The excess oxygen atoms form achemical superlattice at room temperature with a period of 1.88 nm
along the <110> direction of hexagonal fundamental lattice with P63mc
symmetry (a = 0.6235nm, c = 1.0220nm). The electron beam incidence
is parallel to the c-axis.

a (A)

c (A)

Co1-O

Co2-O

A:Sr:

3-d

Co ratio
Y0.33Sr0.67

7.616415.2747 1.9064

1.9810

CoO3-d

2.614(3) 2.917

0.672 :1

Gd0.33Sr0.67 7.6272
CoO3-d

0.323:

Co
valence15.3082 1.8676

1.99720.345:

2.824(0) 3.215

0.699:1

ISSP Activity Report 2005

43

References[1] W. S. Kim, E. O. Chi, H. S. Choi, N. H. Hur, S. -J. Oh, and
H. -C. Ri, Solid State Commun. 116, 609 (2000).
[2] M. A. Korotin, S. Yu. Ezhov, I. V. Solovyev, and V. I. Anisimov,Phys. Rev. B 54, 5309 (1996).
[3] M. Karppinen, M. Matvejeff, K. Salomäki, and H. Yamauchi,
J. Mater. Chem. 12, 1761 (2002).
[4] S. Ya. Istomin, J. Grins, G.Svensson, O. A. Drozhzhin, andE. V. Antipov, Chem. Mat. 15, 4012 (2003).
Authors
Y. F. Zhanga, S. Sasakia, O. Yanagisawab, M. Izumia, and Y. Kiuchi
aTokyo University of Marine Science and Technology
bYuge National College of Maritime Technology

Fig. 1. (a) Field magnetization vs temperature curve ofY0.33Sr0.67CoO2.614 at 0.01 T. (b) Field magnetization vs temperature
curve of Gd0.33Sr0.67CoO2.824 at 0.01 T.

was determined through iodometric titration [3]. The
Rietveld profile analysis of the XRD pattern reveals the twocompounds have a common perovskite-based structure(Table 1). Two Co ions form layered alternating order of
CoO4 tetrahedron and CoO6 octahedron [4]. The averagecharge valence in Y0.33Sr0.67CoO2.614 is 2.917, implying that
8% of cobalt ions are in Co2+ and the rest of them are in
Co3+. However, in Gd0.33Sr0.67CoO2.824 the averaged chargeof Co ions is 3.215, implying that about 8% of cobalt ions
are in Co4+ and the rest of them are in Co3+. The different
oxygen contents between two compounds would lead to different magnetic properties.
Figure 1 (a) shows the magnetization versus temperature
data of Y0.33Sr0.67CoO2.614 sample under an applied field of
0.01 T. There is a DC magnetization jump (TJ) at about 180
K during the magnetic field cooling, and subsequently themagnetization normally returns to the high temperature, indicating a kind of magnetic memory effect. The magnetization
decrease is correlated to the AFM appearance in IS spinstate. The phase transition from FM to AFM at TJ is not simply interpreted as the spin-state transition; we should consider the orbital ordering of eg electron suggested by Korotin et.
al [2]. The IS state configuration is eg1. Due to the strong
Jahn-Teller nature of this configuration, this state may be asource of the ordering of orbital. It supports the fact that the
spin structure in AFM state may take a FM ordering in-plane
but AFM ordering interplane. With temperature increase,
there is higher Curie temperature at about 304 K.
The complicated magnetic behavior of Gd-doped sample
is shown at Fig. 1(b), where magnetization jump does notappear. It is PM behavior above 139 K, shows a FM transition near 139 K (TC), and develops like a ferromagnet down
to about 100 K. Below 100 K, the magnetization increases
like a typical PM phase, which is similar to the phenomena
in GdBaCo2O5+d [1]. We suggested that reentrant PMtransition at about 100 K is mainly due to the spin statetransition from IS to LS state of Co3+ ions.
A comparative study of both Y-doped and Gd-doped
cobaltites with different populations of cobalt ions as well as
spin states may give a new kind of phase transition underregulated oxygen contents.

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ISSP Activity Report 2005

ISSP Workshop

1. Rattling and superconductivity

May 9, 2005

Z. Hiroi
The Workshop was held on May 9, 2005 to discuss on the rattling phenomena and its relation to the occurrence ofsuperconductivity in several classes of compounds like filled-skutterudite, clathrate and pyrochlore compounds. About 50
people attended, and 14 lectures were presented. The rattling phenomena have recently attracted many interests because it isbelieved to suppress the thermal conductivity and thus give rise to a large thermoelectric figure of merit in application study.
However, the physical meaning of this effect is still under debate and to be clarified in various materials. Particularly, theinfluence of the rattling motion to conducting carries in the surrounding cage is an interesting topic, which was discussed
extensively in this Workshop.
http://www.issp.u-tokyo.ac.jp/contents/seminar/short/tanki20050509.html

2. Evolution of high magnetic field research on condensed matter utilizing micro-probes
June 13-14, 2005

M. Hagiwara
In this meeting, we discussed recent results of research on condensed matter physics in high magnetic fields, such as
strongly correlated electron systems, quantum spin systems, semiconductors, molecular conductors and so on. In particular, we
discussed a future plan and evolution of neutron and x-ray scattering experiments in high magnetic fields as central topics ofthis meeting.
http://www.issp.u-tokyo.ac.jp/labs/extreme/himag-forum/page006.html

3. Neutron scattering under high pressures

July 22, 2005

K. Hirota, M. Nishi, Y. Uwatoko, H. Amitsuka, and H. Yoshizawa
It is more than 15 years since the JRR-3 research reactor, a center of neutron science in Japan, was built in Tokai. Manyefforts have been being given to renovate and sophisticate the instruments recently. As to triple-axis spectrometers, the ISSP
Neutron Science Laboratory has refurbished them with a completely new control system, and introduced two sets of newliquid-He-free 1K cryostat to expand the temperature range easily accessible for our users. For a couple of years, we also havebeen developing new high pressure cells which have higher neutron transmittance and are easy to use. Since this year, we havestarted constructing neutron focusing devicec using supermirrors to obtain a focused and intense neuron beam required for highpressure experiments on triple-axis spectrometers. Motivated by these rapid technological advancements, we had a one-day
workshop to discuss “neutron scattering under high pressures”, particularly focusing upon possibilities for studying d and felectron systems as well as relaxor ferroelectrics, and new types of high pressure equipments applicable to these issues.
To our welcome surprise. more than 50 participants have attended to this meeting despite the condition that the workshop
was held in Tokai and that only a month was given for preparation. 16 lectures were given by invited speakers: In the beginning, sophistication of neutron spectrometers, development of neutron focusing devices and new high pressure cells for neutron
scattering were discussed. Then, various recent experimental research works were presented, many of which utilize neutrons
from reactors combined with complementary techniques such as synchrotron radiations, ?SR and pulse neutrons. These talks
were followed by experimental challenges for higher pressures with lower temperatures. In the end, a series of lectures were
given for several materials, which are expected to show novel anomalies under high pressures. We were all overwhelmed byenthusiastic talks and following discussions. We have realized that high pressure experiments are frontiers for not only neutron
scattering but also all kinds of beam line sciences: This consensus will be a driving force for constructing a new scientific field
through collaborations among the participants.
http://www.issp.u-tokyo.ac.jp/contents/seminar/short/tanki20050722.html

ISSP Activity Report 200545

4. Hydrogen in solids

Nov. 24-25, 2005

K. Fukutani, K. Aoki, S. Ikeda, M. Kubota, H. Sugimoto, S. Tsuneyuki, and J. Yoshinobu
Hydrogen in solids dramatically affects the electronic, structural and mechanical properties of solids as realized in hydrogen
embrittlement, H-storage, and H-induced metal-insulator transition. Due to the light mass and nuclear spin, hydrogen frequently reveals particular quantum effects such as tunneling, zero-point vibration, extended features of the nuclear wavefunction.
The research area related hydrogen interacting with solids encompasses metals, semiconductors, dielectrics, surfaces andinterfaces, solid hydrogen, high-pressure physics, low-temperature physics and so on. Reflecting the increasingly sophisticatedrange of experimental and theoretical techniques, much progress has been made in the understanding of hydrogen transport,
reactivity, and correlations with electronic and structural properties. Exotic beams including positron, muon, and anti-hydrogen,are broadening the field as an interdisciplinary research area.
This workshop highlighted the topics of each area in aimed at promoting mutual communications between different communities. A total of 180 scientific delegates attended the workshop from all over Japan and 20 invited talks pointing to current
status and directions of the research area were presented. One-minute presentations by the poster presenters stimulated extended and excited discussions.
http://bilbo.phys.s.u-tokyo.ac.jp/hydrogen/5. Theory of electronic structure calculations and application for new nano-scale electronics
December 26-27, 2005

K. Kusakabe
Intending to find new functionality in characteristics of materials at around the phase transition, much attention is paid forthe correlated electron systems. For this purpose, development in the first-principles electronic structure calculations isrequired. This is because, for the design of materials based on the precise quantum mechanical analysis, only the ab-initio
method is able to give the materials solution. Proper treatment of non-local correlation effects due to both short-ranged
and long-ranged correlations in electronic systems, large scale simulation in both length and time scales, and otherrecognized problems are now gradually solved. Response to the outer electro-magnetic fields is rather nicely described in thefirst-principles calculations, so that the full-quantum-mechanical device simulation is now available. This ISSP workshop wasthus planned to discuss recent rapid progress of the electronic structure calculation and to review achievement in theoreticaltechniques. The program contained 34 aural presentations and 16 posters and over 100 participants were attended and over 100
participants were took part in active discussion. In addition, a special panel discussion was held. The panel discussion wasentitled ‘prospects and expectations of the peta-flops machines in the condensed matter physics’.
http://www.issp.u-tokyo.ac.jp/public/ISSP_WS_20051226/

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ISSP Activity Report 2005