Highlights of Joint Research

Synchrotron Radiation Laboratory

Neutron Science Laboratory

Synchrotron Radiation Laboratory (SRL) has been continuing the research and developments of the 3rd generationsynchrotron light source project, Super-SOR project of the
University of Tokyo, which is to be constructed in the
Kashiwa campus. The accelerator group of SRL is carryingout research works of the accelerator physics and developing the accelerator-related technology, many of which will
be directly applied to the new light source. The staff members in the spectroscopy group are developing newexperiemtnal techniques using undulator radiation. They
also maintain the branch laboratory, Tsukuba branch, at
KEK-Photon Factory with an undulator called Revolver,
two beamlines and three experimental stations for angleresolved photoelectron spectroscopy (ARPES), spin- andangle-resolved photoelectron spectroscopy (SARPES) andsoft X-ray emission spectroscopy (XES).
The accelerator group has made a progress in the designof the Super-SOR light source. In the lattice design of thestorage ring, two kinds of optics with a large dynamic aperture were prepared. The injector including a booster synchrotron and a pre-injector linac was designed so that it
could be tuned to be suitable for top-up injection as well as
normal injection in the single- and multi-bunch operationsof the storage ring. The designs of magnet and vacuum systems were almost completed and the other ring components
were also designed and developed. A new orbit correction
method, eigenvector method with constraints, that we had
proposed as an orbit correction method suitable for orbit
feedback systems was experimentally studied by utilizing
the PF-AR ring in order to confirm its performance.
The scientific highlights achieved at the three beamlinesof the Tsukuba branch laboratory, are as follows. Electronic
structure of the Cu-O/Ag(110)(2 # 2)p2mg surface wasinvestigated by Dr Sekiba, et al. at BL18A (Phys. Rev. B
67, 035411 (2003)). They have discussed a scaling law
describing the Slater-Koster (SK) parameters between theoxygen and d-electron metals from angle-resolved photoemission spectra. At BL-19B, Prof. Aita’s group has investigated the electronic structure of CuIr2S4 (Phys. Rev. B 68,
195124 (2003)). They have discussed the effect of the lattice
distortion to the charge-ordering by means of photoemission
spectra and the cluster model calculations. As the joint project with Prof. Hasegawa’s group, the synchrotron radiation
light illuminating scanning tunneling microscope (SR-STM)
project is undergoing at BL-19A. The Si 2p absorption spectrum from the small area of the Si surface was obtained by a
STM tip, the details of which are described in this issue.

Science 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 staring 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, total 13 spectrometers belong to the university
while JAERI owns 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 the number of visiting
users under this program reaches over 7000 (personday/year). Major research areas are solid state physics(strongly correlated electron systems, high Tc superconductors, heavy fermion systems, low dimensional magnetism,high-pressure physics etc.), fundamental physics and neutron beam optics, polymer, chemistry, biology, 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) instruments. In the fiscal year of 2002, the research
topics on the hard-condensed matter science covered stripe
order in high-Tc superconductors, and closely related 2
dimensional systems, charge and orbital ordering in CMR

26

ISSP Activity Report 2003

manganites, quadrapolar ordering in rare-earth based intermetallic compounds, spin dynamics of low dimensional
dimmer systems, etc. On the other hand, the research topics
on the soft-condensed matter science covered structure
characterization of polymer blends, micelles, amphiphilicpolymers block copolymers, liquid crystals, proteins, inorganic gels, dynamics of brush-polymers on surface, slow
dynamics of surfactants, pressure dependence of dynamics
of amphiphilic membranes, and so on. The details of individual studies are reported in the NSL-ISSP Activity
Report.

The research projects are roughly classified into the following three (the number of projects approved):
First-Principles Calculation of Materials Properties (50)
Strongly Correlated Quantum Systems (62)
Cooperative Phenomena in Complex, Macroscopic
Systems (31)
All the three involve both methodology of computationand its applications. The results of the projects are reported
in 'Activity Report 2003' of the SCC-ISSP. In the report thefollowing four invited articles are included in this year:

Supercomputer Center
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 which
are hardly carried out on other computers such as work-station clusters in individual laboratories.
The SCC-ISSP main system consists of two supercomputers. Hitachi SR8000/60, called System-A, is a distributed-memory-type parallel supercomputer with 60 nodes,
640GB memory in total and 12x60GFlops peak performance. On the other hand, SGI Origin 2800/384, called
System-B, is a massively parallel supercomputer with
192GB memory in total and 0.8x384GFlops peak performance.
A project(s) 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,
under which the Supercomputer Project Advisory
Committee is formed to review proposals. In fiscal year
2003 totally 143 projects were approved. The total points
applied and approved are listed on Table. 1 below.

"First-Principles Study on N/Cu(001)" by Y. Yoshimotoand S. Tsuneyuki,"Ab-Initio Quantum Dynamics Calculation of Hydrogen
Interaction with Surfaces ---Exploiting the Inherent
Orientation
Dependenceof
Hydrogen-Surface
Reactions---" by W. A. Di nu o and H. Kasai,"Mott Transition in Multi-Orbital Electron Systems" by
N. Kawakami,"Randomness Effects on Quantum Phase Transitions of
Bond-Alternating Haldane Chain" by S. Todo, T.
Arakawa and H. Takayama.
For communicating information not only on the SCC-ISSP
but on general research activities in the field of`Computational Material Physics', the home page
http://www.issp.u-tokyo.ac.jp/public/cmp
and the mailing list `cmp-ml' are open to community of the
field.

Table 1: Research projects approved in 2003==================================================================================
Class

Max.Point

Application# of Proj.

Total points
AppliedApproved
Sys-A
Sys-B
Sys-A
Sys-B
-----------------------------------------------------------------------------------------------------------------------------------------A
100k
any time
4
210K
190K
210K
190K
B
2M
twice a year
44
58.7M
23.55M
58.5M
23.55M
C
20M
twice a year90
996.9M
508.6M
815.2M
481.0MDnone
any time
5150.0M
70.0M
71.0M
70.0M
==================================================================================For System-A 1 K point corresponds to charge for CPU time of about 0.43 hours by one processing
element, while the corresponding figure is 2.32 hours for System-B. For one of the D class projects, a
special queue which exclusively uses 2 nodes of system-A was allowed to perform extremely long-timejobs necessary.

ISSP Activity Report 2003

27

Anomalous Behaviour Close to theMetamagnetic Quantum Critical Pointin Sr3Ru2O7
R.S. Perry, Y. Maeno, and T. Sakakibara
Quantum criticality continues to generate much interest
within the international condensed matter community.
Conventional phase transitions occur when a systemchanges state, for example as the temperature of water islowered it freezes to create ice. However, it is also possible
to change the state of a system using a non-thermal variable,
for example pressure, magnetic field or chemical concentration. When such a phase transition occurs at zero temperature then quantum fluctuations, as opposed to thermal orclassical fluctuations, dominate the properties of the material. In this region of the phase diagram novel behaviour, forexample unconventional superconductivity [1], has been
observed.
Up to now, both pressure and chemical concentration
have almost exclusively been used to tune materials to quantum criticality. Recently magnetic-field-induced (MFI)
quantum phase transitions have become the subject of attention due to the ease with which they can be accessed experimentally and the opportunity to study the critical behaviour
low disorder environments. A relatively unusual MFI phasetransition occurs in metallic metamagnetic systems. These
are materials close to a ferromagnetic instability that undergo a MFI first order phase transition into a higher polarizedstate at a critical field BM. The phase diagram naturally
involves a classical critical point terminating a line of firstorder phase transitions, and quantum critical behaviour canoccur if the critical point sits at zero temperature. This may
seem to be a fairly unlikely scenario, but just such a situation is realized in the bilayered perovskite ruthenate
Sr3Ru2O7 [2].
Sr3Ru2O7 is a strongly correlated (electronic specificheat, c = 110 mJ/Ru mol K2), d band (4d4), Fermi liquid
metal exhibiting a metamagnetic transition with BM=7.9 T(for the field perpendicular to the ruthenium oxygen planes).
Previously, it was shown that the Fermi liquid state is suppressed as a function of magnetic field and non-Fermi liquidbehaviour extending to high temperatures was measured in
the vicinity of the quantum critical point (see figure 1) [2].

Fig. 1. (left) The bilayered perovskite structure of Sr3Ru2O7. The
large solid circles are Strontium atoms and the small solid circles are
Ruthenium atoms surrounded by oxygen atoms. (right) Plot of theresistivity t temperature exponent a (defined as t = t res + A. T a )as a function of field and temperature. a =2 (blue) implies a Fermi
liquid power law. The system can be tuned from Fermi liquid at low
and high fields to non-Fermi liquid close to the critical field (7.9 T).

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ISSP Activity Report 2003

Fig. 2. Recent low temperature magnetization measurements madeon ultra pure single crystals of Sr3Ru2O7. (main plot) The static susceptibility at 0.91 K, 0.74 K, 0.54 K, 0.36 K, 0.18 K and 0.07 K(from blue to green). Three metamagnetic transitions are clearly
resolved. (left inset) The magnetization versus magnetic field at 70
mK. (right inset) The magnetization as a function of temperatureinside the critical region (7.9 T).

In ultra clean samples (impurity resistivity ~ 0.5 nX cm)
even more anomalous behaviour in the resistivity at low
temperatures has been observed around BM [3]. In order toprobe the thermodynamic properties of the system we havemade the first low temperature magnetization measurements
on ultra clean samples [4]. The samples were grown by
floating zone technique at Kyoto University.
The metamagnetic signature, i.e. the step in the magnetization, is clearly observed as shown in the left inset in Fig.
2. Within an itinerant model, this step originates from achange in the topography of the spin polarized Fermi surfaces. As such, the critical fluctuations can be considered to
be fluctuations of the Fermi surface itself. Around BM multiple metamagnetic transitions have been resolved (see main
plot in Fig. 2). The transitions at 7.8 T and 8.0 T are firstorder (from dynamic susceptibility measurements) and coincide with the anomalous peak in the magnetoresitivity [3].
The observation of multiple metamagnetic transitions in an
itinerant system is fairly unique in itself but what is particularly interesting is the temperature dependence of the peaks
at 7.8 T and 8.0 T. 2 Tpeak/ 2 BM <0 and >0 for the 7.8 T and
8.0 T transitions, respectively. Within a mean field theoryof itinerant electron magnetism involving, for example,
proximity of the Fermi surface to multiple peaks in the density of states, BM would be expected to increase with temperature. More intriguing still is the temperature dependence of the magnetization for magnetic fields between 7.8 T
and 8.0 T (see right inset to Fig. 2). Far for BM, Pauli
behaviour was observed. At BM = 7.9 T, the magnetization
increases smoothly as a function of temperature, as one
would expect for the approach to a critical point. However,at ~1 K a distinct change of slope is observed. This feature
coincides with a minimum observed in the resistivity as afunction of temperature. It should be noted that this anomalous, non-Fermi liquid behaviour is only observed between
these two first order phase transitions.
The work so far raises several interesting questions.
Why does the behaviour of the magnetization deviate frommean field theory? Is this a consequence of proximity to aquantum critical point? What causes the cut-off in thedivergence of the fluctuations very close to BM and for temperatures less than 1 K? We are currently working on
resolving these issues and are planning measurements,
including uniaxial pressure work, in the future.
References[1] N. D. Mathur et al., Nature 394, 6688 (1998).
[2] S. A. Grigera et al., Science 294, 5541 (2001).
[3] R. S. Perry et al., Phys. Rev. Lett. 92, 166602 (2004).
[4] R. S. Perry, T. Tayama, T. Sakakibara and Y. Maeno, in preparation.
Authors
R. S. Perrya,b, T. Tayama, K. Kitagawaa, T. Sakakibara, K. Ishidab,
and Y. Maenoa,ba Department of Physics, Kyoto University.
b Kyoto University International Innovation Center

Electron Correlation and MolecularVibration in the Strongly Correlated
Organic Conductors
T. Sasaki and H. Tajima
A series of the l -type BEDT-TTF organic superconductors has attracted considerable attention from the point ofview of the strongly correlated Q2D electron system. The
unconventional metallic, antiferromagnetic insulator andsuperconducting phases appear next to one another in thephase diagram. The transitions among these phases are controlled by the applied pressure, which must change the conduction bandwidth W with respect to the effective Coulomb
repulsion U on a dimer. Optical conductivity measurements
are powerful tool for investigating such correlated electron
system. We focus on the change of a specific molecularvibration mode v3(ag) of l -(BEDT-TTF)2Cu[N(CN)2]X(X=Br and Cl). The symmetric v3(ag) mode becomes visible
in the IR spectra due to the large electron-molecular vibration (EMV) coupling. Thus the mode is sensitive to the electronic state on the BEDT-TTF dimer.
Figure 1 shows the optical conductivity of X=Br and Cl
salts. The peaks at 1320-1330 for E//c and 1250-1270 cm-1
for E//a shift to higher frequency below about 30-40 K in
X=Br, and to lower frequency below 50-60K in X=Cl . The
former temperature corresponds to T* and the latter Tins.
These relatively broad peaks have been assigned to thev3(ag) mode. In concurrence with such frequency shifts, theshape of the conductivity peaks change at T* and Tins. The
peak becomes sharper and larger below Tins in X=Cl, while
the further broadening seems to be observed below T* in
X=Br.
The peak frequency ~IR of the v3(ag) mode is shown in
the inset of Figure 2. The peak frequencies in both salts andboth polarization directions increase monotonically withdecreasing temperature from room temperature. This
monotonic change of ~IR is in accordance with the changeof the same mode of the Raman shift frequency ~R . Thenthe monotonic change of ~IR and ~R represents the hardening of the molecular viblation itself by cooling. Below about
Tins =50-60K, however, the peak frequency starts to shift tolower frequency in both polarization directions of X=Cl.
Then a small downward jump appears at TN = 28K. To thecontrary, the same v3(ag) mode of superconducting X=Br
shifts to the higher frequency abruptly below T* = 38 K in
both the polarization directions. The characteristic temperature dependence of the v3(ag) mode in the IR optical conduc-Fig. 1. The conductivity spectra of l -(BEDT-TTF)2Cu[N(CN)2]Brand l -(BEDT-TTF)2Cu[N(CN)2]Cl. The peaks at 1320-1330 for E//cand 1250-1270 cm-1 for E//a are assigned to the EMV coupled v3(ag)mode.

Fig. 2. Temperature dependence of the IR conductivity peak frequency ( ~IR ) of the v3(ag) mode normalized by the same mode of theRaman shift ( ~R ) in l -(BEDT-TTF)2Cu[N(CN)2]Br andl -(BEDT-TTF)2Cu[N(CN)2]Cl. Insets show the temperature dependence of ~IR and ~R .

ISSP Activity Report 200329

tivity can be attributed to the change of the electronic states
through the EMV coupling. The main panel of Fig. 2 shows
the temperature dependence of the relative frequency shiftof the v3(ag) mode in the IR spectra, which is normalized by~R shown in the inset.
The IR frequency ~IR is expected to be smaller than thebare phonon frequency ~0 ( . ~R ) when the transfer integral tdimer for hopping between molecules within the dimeris larger than ~0 ( . ~R ). These plots support further thatthe anomalous frequency shifts of the v3(ag) mode at T* in
X=Br, and Tins in X=Cl are caused by changing not themolecular vibration itself but the electronic states on the
dimers through the EMV coupling.
In view of such change of ~IR of the v3(ag) mode, let us
then consider the relation to the electronic states. In the l type BEDT-TTF system, the effective half filling bands are
expected and results in the Mott insulator due to the strong
dimerization. This situation is realized below Tins in X=Cl.
Large negative frequency shift from the bare phonon frequency observed below Tins corresponds to the enhancementof the EMV coupling. The large EMV coupling on the
dimer system implies that the carriers tend to localize at the
dimer and then the conductivity becomes small. The larger
and sharper peak shape of the v3(ag) mode also demonstrates
the localization of a carrier (one hole) at each dimer. Thenthe system has became the Mott insulating state before the
AF static order appears at TN. On the other hand, the positive frequency shift below T* indicates that the carriers on
the dimer show the itinerant behaviour in the superconducting salt.
Reference[1] T. Sasaki et al.: Phys. Rev. B 69, 064508 (2004).

the phenomenon has been elucidated by means of low-temperature X-ray crystallography and calorimetry; the unit cellat room temperature contains two crystallographically independent biferrocenium cations, one of which undergoes
valence localization at around room temperature, whereas
the other one below 120 K. The former transition is a thermal process, which originates from an asymmetrical effectof the crystal environment around the cation, while the latter

Fig. 1. Packing diagram of (1',1'''-isopropyl-1,1''-biferrocene)[Ni(mnt)2] in the low-temperature phase (90 K), together with thestructural formula. The acceptors constitute a uniform column atroom temperature, while dimerization occurs in the low temperaturephase, as seen in the Figure.

Authors
T. Sasakia, I. Itoa, N. Yoneyamaa, N. Kobayashia, N. Hanasaki,
H. Tajima, T. Itob, and Y. Iwasaa.
aInstitute for Materials Research, Tohoku University
bJAIST Hokuriku

Biferrocenium Charge-Transfer Salts
with M(mnt)2. Successive Valence
Transitions Coupling with 1-D
Magnetic Instability of the Anion
T. Mochida and H. MoriElectron transfer phenomena in biological systems aswell as in molecular materials have attracted considerable
attention for years. We have focused on the electron transfer
processes in biferrocenium salts as an approach to novel
electronic functions [1, 2]. To introduce conducting and/or
magnetic functions into biferrocenium charge-transfer complexes, we chose M(mnt)2 (mnt = maleonitrilodithionate) as
the counter anion, and prepared (1',1'''-R2-1,1''-biferrocene)[M(mnt)2] (1a: R = isopropyl; 2a: R = dineopentyl)and (1',1'''-R2-1,1''-biferrocene)2[Co(mnt)2]2 (1b: R = isopropyl; 2b: R = dineopentyl). Complexes 1a and 1b showed
segregated-stack crystal structures, while molecular arrangements in 2a and 2b were discrete. All the complexes containmixed-valent biferrocenium monocations.
Here we focus on the unusual successive valence transitions found in the crystal of 1a. The 57Fe M op ssbauer spectra
indicates the presence of valence ordering processes of thecations at around 250 K and 120 K (Fig. 1). The origin for

30

ISSP Activity Report 2003

Fig. 2. 57Fe M op ssbauer spectra of complex 1a, suggesting the presence of successive valence transitions. A doublet signal is seen above
298 K, which is characteristic of a valence-detrapped state of thecation. Below 230K, the spectra show three doublets, which indicatesthe coexistence of valence-detrapped and valence-trapped species. At
temperatures below 100 K, the spectra show two doublets, being
characteristic of a valence-trapped state.

transition is coupled with a phase transition that accompanies a cell-doubling. The transition is associated with a spinPeierls-like dimerization of the acceptor (Fig. 2), which isalso manifested in its temperature dependent magnetic susceptibility. Thus, the 1-D instability of the Ni(mnt)2 spins
was found to trigger the charge localization of the cation
through symmetry-lowering of the crystal environment. On
the other hand, complexes 1b, 2a, and 2b exhibited trappedvalence states at room temperature. The strong tendency of
charge localization in these salts was shown to originate
from local electrostatic interactions.
References[1] T. Mochida, S. Yamazaki, S. Suzuki, S. Shimizu, and H. Mori,
Bull. Chem. Soc. Jpn., 76, 2321-2328 (2003).
[2] T. Mochida, Mol. Cryst. Liq. Cryst., 343, 205 (2000).
Authors
T. Mochidaa ,K. Takazawaa, H. Matsuia, M. Takahashia, M. Takedaa,
M. Satoa, Y. Nishioa, K. Kajitaa, and H. MoriaToho University

New Mixed Valence Cobalt Oxides,
ABaCo4O7 (A: Ca, Dy-Lu)
N. Nakayama and Y. Ueda
The mixed valence cobalt oxides, such as NaxCoO2 or
YBaCo2O5+x, are known to show interesting magnetic andelectrical properties. We have synthesized new mixed
valence cobalt oxides ABaCo4O7 (A: Ca, Dy-Lu) iso-structural to YBaCo4O7 recently reported. Ba and O atoms forma close packed structure with 4H (abac) stacking, in which A
and Co atoms occupy the octahedral and tetrahedral sites,
respectively. The CoO4 tetrahedra form a non-centrosymmetric lattice (P63mn) by the corner sharing, where the Co
kagom el lattice and the ordered ACo triangular lattice arestacked alternately. The Lu and Ca compounds are particularly interesting because A-site atoms are nonmagnetic. The
formula valences of Co in LuBaCo4O7 and CaBaCo4O7 are
2.25 and 2.5, respectively. The crystal symmetry of
CaBaCo4O7 is lowered to orthorhombic Cmc21, suggesting
the charge ordered state of Co(II) and Co(III). No evidenceof the charge ordering for LuBaCo4O7 was detected at room
temperature. However, LuBaCo4O7 shows a phase transitionindicating the charge ordering at low temperature.
LuBaCo4O7 shows an antiferromagnetic ordering below
50 K, whereas CaBaCo4O7 shows a ferromagnetic ordering
below 60 K. The fundamental magnetic interaction in the Co
lattice is antiferromagnetic. Above the magnetic ordering

Fig. 1. High resolution lattice images of LuBaCo4O7 (a=6.264 Å,
c=10.227 Å; P63mc, Z=2). The non-centrosymmetric structure isclearly resolved when viewed along [10.0].

Fig. 2. The temperature dependence of lattice parameters and theelectrical resistivity for LuBaCo4O7. The phase transition at 160 K
must be originated from the charge ordering in the Co sublattice. A
tentative ordered structure of Co(II) and Co(III) in the low temperature orthorhombic Pmc21 phase is also shown.

temperature, LuBaCo4O7 shows a first order phase transitionat around 160 K, which can be clearly detected by electricalresistivity, heat capacity, x-ray diffraction and electron diffraction. X-ray and electron diffraction patterns indicate thatthe crystal symmetry lowers to orthorhombic Pmc21. The
increase of the electrical resistivity in the low temperaturephase also suggests that the phase transition is originatedfrom the charge ordering. The magnetic susceptibility also
shows a small decrease of paramagnetic effective moments(from 1.75 to 0.99 n B) below 160 K indicating a change inthe spin state. Similar phase transitions were observed for
YbBaCo4O7 and TmBaCo4O7 at 175 K and 230 K, respectively. The investigations on the charge ordered structures
and the oxygen non-stoichiometry are now in progress.
Authors
N. Nakayamaa, T. Mizotaa, A. N. Vasil'evb, M. Isobe, and Y. Uedaa Faculty of Engineering, Yamaguchi University
b Low Temperature Physics and Superconductivity Department,
Moscow State University

Quantum-Number Projected
Path-Integral Renormalization Group
MethodM. Imada
Symmetry plays a crucial role in understanding quantum
many-body systems. For example, the Hubbard model preserves total spin, total momentum, and some geometricalsymmetries on a lattice, by which we can classify low-energy eigenstates and can elucidate the nature of low-energy
phenomena like excitation spectra and spectroscopic properties.
For strongly correlated electron systems, numericalapproaches are an indispensable tool for their studies andvarious methods such as quantum Monte Carlo methods andDensity Matrix Renormalization Group method have beenpresented, while except exact diagonalization method whose
tractable system size is severely limited, these methods cannot fully take an advantage of symmetry and excitation
spectra have not been well explored.
Recently we have developed a quantum-number projection technique [1], by which advantage of symmetry can be
fully extracted. For instance, we can handle wave functions
with definite and exact symmetries and lowest energy state

ISSP Activity Report 2003

31

Fig. 1. We demonstrate the efficiency of the QP-PIRG [1] in the caseof the 2D half-filled Hubbard model with 6 by 6 square lattice andthe periodic boundary condition. The parameters are at t=1, t'=0 and
U=4. The energy is obtained by the extrapolation of the energy to thezero energy variance [2]. The extrapolations of the ground state (S=0,k=(0,0)) and first excited state (S=1,k=( r , r )) energies are shown.
Although exact ground state energy is not available, the ground-stateenergy of Monte Carlo calculation is shown by open diamond at zerovariance with its statistical error bar. The ground state energy by the
QP-PIRG is within the error-bar of the Monte Carlo calculation.

with different symmetries can be treated in the same footingof ground state. Moreover we have presented its implementation [1] to the path-integral renormalization group (PIRG)
method [2] and succeeded in extremely enlarging the feasibility of the PIRG.
The quantum-number projection method can pick up acomponent with required symmetries from symmetry broken wave functions. In the Hubbard-type models, symmetries such as spin and momentum have a significant role in
the low-energy states, while explicit construction of symmetry imposed wave function is quite complicated for a large
number of electrons. However, the quantum-number projection technique enables us to easily handle symmetryimposed wave function in a compact and numericallytractable form by integral or summation [1]. For instance,
for spin symmetry, we can consider spin projection, which isrepresented by one-dimensional integral over Euler's angle
in spin space. The present spin projection is a counterpart ofangular momentum projection, which is often used innuclear many-body problem. The momentum projection is
also simply given from the superposition of spatially translated basis functions. Geometrical symmetry on a latticesuch as the inversion and rotation symmetries is also considered in the similar way. By this method we can exactly treat
spin, momentum and other quantum numbers to investigate
the Hubbard-type models.
Moreover this method is well harmonized with the PIRG
[2]. In this method, the wave function is expressed by a linear combination of L basis states which are deliberately
optimized by the auxiliary-field Quantum Monte Carlo technique [2], while the symmetries are not retained in eachbasis state in general because the numerically manageable
number of the basis states, L, is limited. Therefore by applying the quantum-number projection onto each basis state, we
can implement it into the PIRG. Consequently we can exactly treat the symmetry and extract the state with specified

32

ISSP Activity Report 2003

quantum numbers by the PIRG. Concerning its procedurefor quantum-number projection and PIRG process, we haveproposed two ways of implementing quantum-number projection into the PIRG [1]. One way is to carry out quantumnumber projection afterwards for the already obtained PIRG
wave function (PIRG+QP). Other is to carry out the PIRG
by using quantum-number projected basis states (QP-PIRG).
By both methods, the ground state can be efficiently extracted by specifying the quantum number with higher accuracy
than the PIRG without projection. Comparing these two
methods, the latter method can give more accurate eigenenergy than the former does.
By this extension of the PIRG, we have made following
essential progresses: (1) Precision of the numerical calculation is substantially improved. (2) The quantum number ofthe ground state is exactly determined. (3) The extended
PIRG by the quantum-number projection can handle excitedstates and their spectroscopic properties in addition to theground state.
As a numerical demonstration, we investigate the standard Hubbard model on two-dimensional square lattice. We
can obtain an excited state with S=1 and k=( r , r ) in addition to the ground state with S=0 and k=(0,0) for the 6x6
half-filled lattice with U/t=4 as shown in Fig. 1. The groundstate energy by the QP-PIRG is very close to one by theMonte Carlo method and the QP-PIRG can obtain the excited energy in addition to the ground state energy. We also
test the QP-PIRG in the case of t-t’ Hubbard model withlarge geometrical frustration effect and with severe minus
sign problems in term of Monte Carlo methods. We consider
the half-filled system on 4 by 4 lattice with U/t=5.7 andt'=0.5 and confirmed that the total energy difference
between the present result and exact one is very small and isless than 0.01 [1].
Recently by using the PIRG method, the non-magnetic
insulator (NMI) phase has been found near the Mott transition for relatively large t' [4]. As the present extended PIRG
can explicitly handle quantum numbers, it will be useful toclarify its nature. It is also worth noting that the present
quantum-number projection technique and the PIRG method
work well irrespective of the details of the considered system. Therefore the present approach will work for other systems in addition to Hubbard-type models.
References[1] T. Mizusaki and M.Imada, Phys. Rev. B69, 125110 (2004).
[2] M. Imada and T. Kashima, J. Phys. Soc. Jpn. 69, 2723 (2000);
T. Kashima and M. Imada, J. Phys. Soc. Jpn. 70, 2287 (2001).
[3] M. Imada, T. Mizusaki, S. Watanabe, cond-mat/0307022.
[4] T. Kashima and M. Imada, J. Phys. Soc. Jpn., 70, 3052 (2001); H.
Morita, S. Watanabe and M. Imada, J. Phys. Soc. Jpn., 71, 2109
(2002),[5] Y. Noda and M. Imada, Phys. Rev. Lett., 89, 176803 (2002).
Authors
T. Mizusakia and M. Imadaa Institute of Natural Sciences, Senshu University“Half Metallicity” of a Diluted
Magnetic Semiconductor (Ga, Mn)As
E. Kojima, S. Katsumoto,and M. Kuwata-Gonokami
Since the first synthesis of a III-V based diluted magnet-

Development of Milli-Kelvin Scanning
Tunneling Microscope
Y. Hasegawa M. Kubota, and A. Sakai

Fig. 1. Temporal evolution of the extracted magnetization component. The data are normalized at the maximum point. A positive
direction shows demagnetization. A solid line shows a result obtainedusing the three temperature model considering thermal diffusion.

ic semiconductor (Ga,Mn)As [1], it has attracted attentions
due to its high (110K) Curie temperature, high controllability in structure and good connectivity to sophisticated GaAsbased superstructures. The origin of the ferromagnetism insuch a diluted spin system has been the most important
problem in this system from both the physics and the applications because the Curie temperature is still not highenough to be applied to so called spintronics devices, working at room temperatures. There have appeared many theoretical proposals and interestingly many of them predicts
half-metallic band structure [2], i.e., at the Fermi surface
only single spin subband exists and hence spin injection
with the efficiency of 100% is, in principle, possible [3].
Though no sound experimental evidence has been given tothis attractive hypothesis.
In order to investigate this problem we performed timeresolved magneto-optical Kerr effect (TR-MOKE) measurement [4,5], in which the spin and carrier dynamics can be
simultaneously probed. The “three temperature modeling”
of the transient properties of reflectivity etc. can lead to theobservation of spin-charge thermal isolation, which is strong
evidence of the half metallicity.
Though TR-MOKE is a powerful technique, always aproblem is the extraction of the pure magnetization component from the MOKE signal. In order to overcome the difficulty, we developed a two-color probe method, with which
time-dependent magnetization after an initial strong excitation is successfully extracted as shown in Fig.1.
In Fig.1, after rapid change in the magnetization, very
slow relaxation is observed. Such very slow oscillation can
only be explained by assuming that the density of states forone spin direction is much smaller than that of the otherdirection, i.e., this observation constitutes direct evidence ofhalf metallicity in (Ga,Mn)As.

An ultralow temperature scanning tunneling microscope(STM) was developed by installing an STM head in a dilution refrigerator. Atomic images and tunneling spectra
showing a superconducting gap were taken on a cleaved surface of 2H-NbSe2 with the sample cooled down to 90mK.
The STM was constructed by attaching a home-madeSTM head on a mixing chamber of a commercial dilution
refrigerator. The head is compact with ca. 30 x 30 x 50 mm3
in size, so that it can be fitted into an internal vacuum chamber (IVC) containing cooled parts of the refrigerator. A linear motion motor developed by Pan et al. is used for coarse
approaching. A sample holder can be transferred between
the STM head and a UHV preparation chamber where one
can do basic surface preparation of samples, such as annealing, deposition and so on.
In order to test its performance, we carried out STM andtunneling spectroscopic studies on a cleaved surface of
2H-NbSe2. The sample shows charge-density-wave (CDW)
and superconducting transitions at 33 K and 7.2 K, respectively.
Figure 1 shows an STM image taken at the sample tem-

Fig. 1. STM image of the cleaved surface of 2H-NbSe2 taken at asample temperature of 90 mK. The size of the observed area is 4.6
nm # 6.2 nm.

References[1] H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto,and Y. Iye, Appl. Phys. Lett. 69, 363 (1996).
[2] e. g., H. Akai, Phys. Rev. Lett. 81, 3002 (1998).
[3] M. Tanaka and Y. Higo, Phys. Rev. Lett. 87, 026602 (2001).
[4] T. Kise, T. Ogasawara, M. Ashida, Y. Tomioka, Y. Tokura and M.
Kuwata-Gonokami, Phys. Rev. Lett. 85, 1986 (2000).
[5] E. Kojima, R. Shimano, Y. Hashimoto, S. Katsumoto and M.
Kuwata-Gonokami, Phys. Rev. B 68, 193203 (2003).
Authors
E. Kojima, S. Katsumoto, and M. Kuwata-Gonokamia
aDepartment of Applied Physics, The University of Tokyo
Fig. 2. dI/dV spectrum taken on the NbSe2 surface at 90 mK.

ISSP Activity Report 200333

perature of 90 mK. Atomic arrangement of the surface witha unit distance of 0.35 nm can be clearly observed. A slight
modulation in height with a periodicity of 3 times unit cell,marked with green circles in Fig. 1, is due to CDW.
In a tunneling conductance (dI/dV) spectrum, known torepresent the electronic density of states (DOS) of the sample surface, a gap sandwiched by two peaks, which is characteristic for quasiparticle DOS of superconductor, isobserved (Fig. 2). The shape of the spectrum can be fitted
well with the superconducting DOS formulated by BCS theory.
This apparatus enables us to study local electronic structure of various materials at < 100mK in nanoscale spatial
resolution.
Reference[1] E. Ishikawa, R. Shoda, N. Matsuura, M. Ono, T. Eguchi, A. Sakai,
M. Kubota and Y. Hasegawa, e-Journal of Surf. Sci. and Nanotech.,
2, 151 (2004) .
Authors
Y. Hasegawa, T. Eguchi, M. Kubota, and A. Sakaia
a International Innovation Center, Kyoto University

Thermal Stability of SrTiO3 Surfaces
T. Ohnishi, M. Lippmaa, and Y. Matsumoto
Strontium titanate, SrTiO3, is a perovskite-type oxidethat is widely used as a substrate material for growing oxidethin films. In the field of oxide electronics, this material isalso attracting attention due to its potential use as a semiconductor. Stoichiometric non-doped SrTiO3 is an insulator
with a band gap of 3.2 eV. The Fermi level, however, isclose to the bottom of the conduction band and even slight
doping will induce metallic conductivity and even low-temperature superconductivity.
Our main interest is in studying the properties of nanostructures or thin layers deposited on SrTiO3 surfaces. In
order to understand the transport and magnetic properties ofsuch structures we need to know the atomic arrangement
and the chemical composition of the interface layers. Thistask is greatly simplified if we can use an atomically wellordered starting surface for our experiments.
Commercially available single-crystal SrTiO3 substrates
are typically polished and possibly wetetched to obtain a
step-and-terrace surface morphology. Although the selectivity of the etching process ensures that the surface is perfectly terminated by the TiO2 atomic layer, it does not produce athermally stable surface. A recrystallization process of thesurface starts at temperatures as low as 250 cC . The usualapproach is to anneal the surface at close to 1000 cC until

Fig. 1. Scanning tunneling microscope images of ex situ re-etched
SrTiO3 (001) surfaces after cleaning by heating at 300 oC (a) and
exposure to ultraviolet light (b). Image sizes are 250 nm x 250 nm.
All step edges in the images have a height of a single unit cell.

34

ISSP Activity Report 2003

Fig. 2. Sheet resistance of 2 unit cell-thick LaTiO3 films grown on athermally stable Sr-free SrTiO3 (001) surface (red) and on a commercial SrTiO3 substrate (blue). The sheet resistance of the film grown
on a Sr-free surface shows a large directional anisotropy. Resistancemeasured along surface step edges is lower than the value measured
perpendicular to the step edges.

surface migration restores a regular step-and-terrace morphology. The drawback of this technique is that the annealed
surface contains a small amount of segregated strontium, theprecise amount of which is difficult to control. The segregated Sr at the interface works as an unintentional dopant in theinterface layer.
We have developed a secondary etching procedure thatcan be performed after original high-temperature annealing.
The etching time is very short and is intended only toremove selectively Sr from the SrTiO3 surface. Suitableetchants are buffered HF acid or even warm water. We haveshown that such surfaces are stable at up to 700 cC , whichmeans that many heterointerface studies can now be performed on nearly ideal Ti-terminated surfaces. Heating asubstrate above 300 cC after etching (Fig. 1a) is an effective
way of removing contaminants from the crystal surface.
Scanning tunneling microscope images show a nearly perfect surface structure after additional exposure to UV light(Fig. 1b). The terraces are atomically flat, all step edges
have a height of one unit cell, and both the surface structure
and the terminating layer remain stable up to 700 cC .
The importance of controlling the chemical compositionof the substrate surface was studied by measuring the transport properties of 2 unit cell thick LaTiO3+ d layers grown
on SrTiO3 surfaces. The films were grown at 700 cC . A nonannealed substrate would typically have a significant
amount of segregated Sr present on the surface at this temperature. As expected, the transport behavior (Fig. 2, blueline) of the samples grown on non-annealed as-supplied substrates is similar to La1-xSrxTiO3+ d . The films grown on anannealed and re-etched surface show a large drop in sheet
resistance as a function of temperature and also a large
directional anisotropy (Fig. 2, red line). This behavior isconsistent with the presence of doped SrTiO3 at the interface, either due to charge transfer from LaTiO3 or due to thepresence of oxygen vacancies. The directional anisotropy ofresistance also shows that there is a large difference in conductivity along and across the surface steps.This work has
clearly shown that careful control of the surface structure
and composition of oxide single-crystal is extremely important for the design and characterization of heteroepitaxial

interfaces and nanostructures.
Authors
M. Lippmaa, T. Ohnishi, Y. Matsumotoa, T. Ohsawaaa Frontier Collaborative Research Center, Tokyo Institute of Technology

Itinerant Electron Metamagnetism andMagnetocaloric Effect
H. Yamada and T. Goto
Large magnetocaloric effects have recently been
observed in 3d transition metal compounds MnFe(P,As) andLa(Fe,Si)13. These compounds exhibit a first-order transitionof magnetization at the Curie temperature TC. Above TC, ametamagnetic transition (MT) takes place from the paramagnetic state to the ferromagnetic one under magnetic
fields. Such a MT has been observed in many Co-based
compounds [1]. These peculiar magnetic properties are well
explained by the theory of itinerant-electron metamagnetism(IEM), where the effect of spin fluctuations is taken into
account on the phenomenological Ginzburg-Landau theory[1]. In this report, magnetocaloric effects are discussed,
based on the theory of IEM.
The equation of state for the magnetization M and magnetic field B is written by
B=a(T)M+b(T)M3+c(T)M5
where a(T), b(T), and c(T) are Landau coefficients renormalized by spin fluctuations asa(T) = a(0) + (5/3)b(0) p (T)2 + (35/9)c(0) p (T)4b(T) = b(0) + (14/3)c(0) p (T)2
c(T) = c(0)Fig. 2. Isothermal magnetic entropy changes - D Sm/S0 at
B/B0 = 0.1 # 10-2 Curves (1)-(10) are those for a(0)c(0)/b(0)2 = 0.1780.187 at an interval of 0.01.

where p (T)2 is the mean square amplitude of spin fluctuations, which is well-known to increase as T2 with increasing
temperature T at low T. Then, the reduced temperature
T/Tmax is given by p (T) / p (Tmax), where Tmax is defined
below.
The susceptibility | (T) is given by 1/a(T). It iseasily found when a(0)>0, b(0)<0, and c(0)>0 that
| (T) shows a maximum in its T-dependence atp (Tmax)2= (3/14)|b(0)|/c(0). Tmax is the susceptibility maximum temperature. It is also found that, when a(0)>0,
b(0)<0, c(0)>0, and 3/16<a(0)c(0)/b(0)2<9/20, the groundstate is paramagnetic and the MT takes place from M=0 to a
finite M at a certain magnetic field. In this case, the criticalfield BC is shown to increase as T2 at low T, which isobserved as a common property for itinerant electron metamagnets. When 5/28<a(0)c(0)/b(0)2<3/16, the ground stateis ferromagnetic and M(T) shows a first-order transition at
TC. The transition becomes second-order when
5/28>a(0)c(0)/b(0)2. The magnetic phase diagram is shownin [1].
The magnetization curves M(T,B) can be calculated fromthe equation of state (1). Figure 1(a) shows the calculated
M(T,B) on the plane of T and B for a(0)c(0)/b(0)2=0.185. At
B=0, a clear first-order transition can be seen at T/Tmax=0.4.
Above TC, the field-induced MT takes place at the criticalfield BC. At the same time, the susceptibility maximum canbe seen at high temperatures. This maximum grows up withincreasing B, and connects smoothly with BC of MT.
By the Maxwell relation the magnetic entropy change
D Sm between B=0 and finite B is written byB

DSm (T, B) = # (2M (T, B')/ 2T)B' dB'.
0

Fig. 1. Calculated magnetization curve (a) and isothermal magnetic
entropy change (b). M0 and B0 are those at the critical end point of
MT (a(0)c(0)/b(0)2=9/20). T/Tmax= p (T)/ p (Tmax). S0=B0M0/Tmax.

The calculated result of the isothermal magnetic entropychange is shown in Fig. 1(b). A large magnetic entropychange is seen at TC, associated with the first-order transition of the magnetization. More details about the magnetic
entropy change based on the theory of IEM are given in [2].
In Fig. 2, the calculated results of the magnetic entropychanges - D Sm/S0 at B/B0 = 0.1x10-2 for a(0)c(0) /b(0)2 =0.178-0.187 are shown . Negative and large entropy changes
are obtained along the critical field BC, while positiveand small changes are obtained along Bmax where( 2 M(T,B)/ 2 T)B is positive. The trend of - D Sm/S0 as a function of T (Fig. 2) is a general one in itinerant electron metamagnets. More details of numerical calculations are given in[2].

ISSP Activity Report 2003

35

References[1] T. Goto, K. Fukamichi and H. Yamada, Physica B 300, 167(2001).
[2] H. Yamada and T. Goto, Phys. Rev. B 68, 184417 (2003).
Authors
H. Yamadaa and T. GotoaShinshu University

Possible One-dimensional 3He Fermi
Fluid Formed in Nanometer Pores of
FSM-16
N.Wada and H.Ishimoto
Bulk 3He liquid is known as a typical three-dimensional,
3D, Fermi fluid. Two-dimensional 3He Fermi fluids havebeen produced on flat solid surfaces. Our purpose is to realize a new 1D 3He Fermi fluid using a substrate with 1D
nanometer pores. In the 1D pores of FSM-16 with diameter
2.8 nm, 4He atoms form homogeneous layers on the porewalls up to the coverage of two atomic layers. Pre-platingwith 4He at the coverage of 1.47 layers, 3He atoms areadsorbed on the 1D pore walls whose diameter ( 2b ) is estimated to be about 1.8 nm. In this 4He pre-plated nanopore,the 3He atom adsorbed on the wall by a van der Waals
potential has discrete energy levels in the cross section. Theenergy gap D / kB between the ground state and the firstexcited state is estimated to be about 0.34 K. Therefore if
the Fermi energy kBTF is sufficiently smaller than the gap
energy D and kBT << ( D -kBTF ), one can realize 1D 3He
Fermi degeneracy.
The heat capacity of the adsorbed 3He was measured
down to 5mK for small 3He coverages n3 that satisfy the 1D
condition. The results are shown in the upper part of Fig.1,
where the two lowest coverages ( marked with solid andopen circles ) satisfy the 1D condition around the lowest
temperature. Below about 50mK, all of the data approach to
the T - linear dependence, which clearly indicates the Fermi
degeneracy of the 3He adatoms. The molar heat capacity C
at the low coverage shows a maximum of about 1.5R
( R : gas constant ) around 0.1-0.15 K, and approaches to R
corresponding to the 2D classical gas above about 0.4K.
The heat capacity calculated for 1 D non-interacting gas isshown in the bottom of Fig.1 as a function of n3 where TF ismarked with an arrow. The calculation at the low coverages
reproduces the maximum around 0.1K as an essential character of the crossover from the low temperature 1D state to
the high temperature 2D state. The maximum disappears
when kBTF approaches to the gap energy D at the higher
coverages.
In conclusion, we have succeeded in realizing a 1D
Fermi fluid in 3He nanotube adsorbed on the 4He pre-plated
pores about 1.8 nm in diameter. At the lowest coverage it isa truely Fermi degenerated 1D system, because the motionin the cross section is in the ground state and there remains
only one motional degree of freedom along the 1D pore.

The Low Temperature Phase
Boundary of Pd-Hx System
H. Araki, S. Harada, and M. Kubota

Fig. 1. (Upper): Molar heat capacities of 3He adsorbed on the 4He
pre-plated 1D pores as a function of the 3He coverage together withthe mean inter-particle distance along the 1D pore. (Bottom):Calculated heat capacities of non-interacting gas in the 1D pore 2b=1.82nm in diameter.

36

ISSP Activity Report 2003

Particular interest has been focused on the peculiar
physical properties of Pd-H(D)x systems at low temperature:
the superconducting properties discovered in 1972 by
Skoskiewicz[1] and the anomalous behavior of several physical properties of the hydride phase PdHx near 55 K. The latter is called the 55 K anomaly and was originally pointed
out by Aston in 1957[2]. As to the specific heat measurements of Pd-H(D)x systems, there have been many reports.
A lot of them, however, have been carried out with regard to
the superconductivity of high hydrogen concentration
regions at low temperature T < 10 K[3]. Other researchinterest was concentrated in the phenomena of the 55 K
anomaly for x < 0.75[2]. The 55 K anomaly has been interpreted in terms of an order-disorder transition of the hydrogen sublattice. In the Pd-Hx systems, two ordered structures
have been shown as I41/amd and I4/m on the basis of neutron experiments for Pd-D systems[4]. The phase diagram of
the Pd-Hx system around 55K is expected to be rather complicated because of the long-range-ordering of hydrogen in
Pd with a long relaxation time. Our present specific heat
results, however, suggest a different phase diagram ofhydrogen in palladium around this temperature.
We report systematic measurements of the specific heatof PdHx over a wide temperature range using a Pd specimen
with higher purity than was used in our previous work[5].

Because of the large diffusion coefficient, when the density of the H/D is increased in Pd, we may expect somemacroscopic quantum phenomena because of overlapping of
the wave function below some temperature. It should differ
depending on the Fermi and/or Bose statistics of the hydrogen atoms. And the phase diagram may be modified whenthe quantum nature becomes stronger as is seen in Helium
isotopes phase diagrams. In order to further clarify thisphase boundary, specific heat measurements are being prepared for the PdDx system in order to check if the phaseboundary has to do with quantum statistics, and further
investigation will be required.

Fig. 1. Phase diagram for PdHx. Open circles mean the Tp in thiswork, the solid lines are the new phase boundary: open squares andbroken curve are the transition temperature of superconductivity.

With this specimen we could reproduce the concentration
dependent superconducting transition for PdHx with x > 0.8.
This is an important point because it can be difficult todetermine the hydrogen concentration in situ, especially forhigher concentration specimens. We describe a new phase
diagram of hydrogen in palladium obtained using these careful procedures[6].
The specific heat for PdHx (x = 0 and 0.1777 to 0.8753)
was measured between 0.5 K and 150 K by the quasi-adiabatic heat pulse method. The PdH0.8753 and PdH0.8514 specimens exhibited superconducting transitions at 2.0 K and 1.5
K, respectively. From these transition temperatures, we canconfirm that the hydrogen concentration remained constantduring measurements. The electronic specific heat coefficient and the Debye temperature for pure Pd and PdHx were
evaluated by measured data from 3 K to 7 K. These values
are in good agreement with the literature. Although the electronic state of PdHx changes with x, the Debye temperatureof PdHx is hardly different from that of pure Pd. Because of
the large mass difference of palladium and hydrogen, it is
supposed that the hydrogen in the Pd lattice is vibrating
independently. Therefore, it is assumed that the specificheat of PdHx can be expressed as the sum of contributions
due to the lattice specific heat of Pd, the electronic specificheat of PdHx, and the excess contribution caused by hydrogenation of the specimen. Two characteristic features arerecognized in the excess specific heat converted into Cexcess
per mole of hydrogen. One of them is the existence of abroad hump of specific heat which starts to appear from 10
K upwards independent of x. This hump can be interpreted
as the change of the elastic properties of the matrix latticecaused by hydrogenation of Pd.
The other is the existence of mirror- m shape’s specificheat. The sharp peak which indicates a phase boundary withtransition temperature Tp = 55 K to 85 K depending linearlyon the hydrogen concentration from x = 0.6572 to 0.8753.
We do not observe anomalies at specific x values as would
be expected for the specific ordered structures.
In this study, we have reported here the specific heatmeasurements of PdHx over the wide temperature range
0.5 K < T < 100 K in the high hydrogen concentration
region where the superconductivity transition was observed.
The obtained phase diagram of hydrogen in Pd shown in the
figure indicates new features which are not consistent withan order-disorder phase transition between two orderedstructures, but are consistent with other measurements.

References[1] T. Skoskiewicz, Phys. Status Solidi a11, K123 (1972).
[2] D. M. Nace and G. J. Aston, J. Am. Chem. Soc. 79, 3627 (1957).
[3] C. A. Mackliet et al., J. Phys. Chem. Solid 37, 379 (1976).
[4] O. Blaschko, J. Less-Common Met. 100, 307 (1984).
[5] H. Araki et al., Physica B 284-8, 1255 (2000).
[6] H. Araki et al., J. Low Temp. Phys. 134, 1145 (2004).
Authors
H. Arakia, M. Nakamurab, S. Haradab, and M. Kubota
aNagaoka National College of Technology.
bFac. Eng. Niigata Univ.

Pinning of Vortices of Rotating B-likePhase in Superfluid 3He Confined in
98 % Aerogel
O. Ishikawa and M. Kubota
The vortex in fluids is an interesting phenomenon, suchas in turbulence in many fluids. Especially, when the fluid isquantum liquid, the vortex has a circulation quantum and isoften called a vortex line. When you rotate a sample cell, avortex cluster composed of many straight vortices is formedin the middle part of the cell and creates the superfluid
velocity field of rigid body rotation in the cluster region.
The cluster of vortices itself rotates with the sample cell.
Stopping rotation makes a vortex cluster unstable and novortex remains.
Such a vortex motion in bulk liquid is drasticallychanged by existing Aerogel, which is made of tangling silica strands of a few nm in diameter with a large porosity
more than 90%. The diameter of strand is much shorter thanthe coherence length of superfluid 3He, a few tens of nm,
but the average distance between strands is comparable withthe coherence length. So the coherence of superfluidity is
not completely destroyed and Aerogel plays as impurity.
Recently the first rotating experiment on superfluid 3He in
Aerogel was performed in the ultra low temperature group at
ISSP [1].
In Fig. 1, NMR absorption curves are shown at several
rotating speeds. The reason why the NMR signal changed
by rotation was that the large counter-flow between vn andvs changed the relative orientation of order parameter vector
to the magnetic field. The large rotating speed made theincrease of absorption in higher frequency region and thedecrease in lower frequency region simultaneously in Fig.
1(a). But at a speed in deceleration from the maximum
angular velocity, NMR spectra was almost the same as that
before rotation and in further deceleration the same change
for the large rotating speed appeared even at rest in Fig.

ISSP Activity Report 2003

37

introduced by Glaberson and Donnely [3] is applicable toexplain how the vortex density redistributes.
References[1] M. Ymashita, J. Low Temp. Phys. 134, 749 (200).
[2] C. P. Bean, Rev. Mod. Phys. 36, 31 (1964).
[3] W.I.Glaberson and R.J.Donnely, Phys.Rev. 141, 208 (1960).
Authors
M. Yamashita, A. Matsubara, R. Ishiguro, Y. Sasaki, Y. Kataoka,
M. Kubota, O. Ishikawa, Y. M.Bunkov, T. Ohmi, T. Takagi, andT. Mizusaki

Tunnel Magnetoresistance of Co-Al-O
Granular Films under High Pressure
S. Kaji, G. Oomi, and Y. Uwatoko

1(b). This means existing of a large counter-flow at rest.
Only the superfluid velocity should remain in aerogel
because the normal fluid velocity at rest should be zero in
the whole cell.
From the NMR spectrum we can obtain the intensity ofcounter-flow. which is shown as a function of X in Fig. 2.
When the angular velocity once exceeded X c, the hysteresis
was observed. Such a hysteresis curve is observed in thehard type II superconductor as a result of vortex pinning [2].
Keeping the superfluid velocity finite while cryostat stays at
rest is due to the pinning of vortices in Aerogel. The mechanism of pinning of vortex in superfluid 3He by Aerogel is
not so clear that we can only mention the inhomogeneity of
Aerogel density may be responsible for the pinning center.
We think that the instability mechanism for vortex motion

In granular films consisting of ferromagnetic metal Co
granules of nanometer size (2-3 nm) in an insulating matrix,
which includes the randomly oriented magnetic moments,
the conduction of electrons is well known to be dominatedby the tunneling between the granules. Since the electron
tunneling in these materials depends on the relative orientation of magnetic moments between ferromagnetic granules(spin-dependent tunneling), it gives rise to the large tunnel
magnetoresistance(TMR) of the order of 10 %. Such films
have a large number of granules with a broad distribution ofsize. The distribution and the Coulomb blockade have beenshown to play an important role in TMR. Furthermore, higher-order tunnel conduction has been pointed out to show asignificant effect on the magnitude of TMR in the low temperature[1,2].
The TMR in insulating Co-Al-O granular films has beenstudied under high pressure up to 3 GPa[3]. It is found thatthe pressure enhances the TMR at 4.2 K and its effectdepends on temperature. The results provide clear evidence
that the successive onset of higher-order processes in tunneling conduction at low temperature below ca. 50 K, which isaffected by strong Coulomb blockade, is enhanced at highpressure. In order to clarify the effect of pressure on the
TMR above 3 GPa more precisely, we made an attempt tomeasure the magnetoresistance(MR) of Co-Al-O filns up to
8 GPa in the wide range of temperature.
Figure 1 shows the MR ratio at 4.2 K defined as
MR = ( t (H) - t max )/ t max, where t max is the maximum
resistivity around zero field. The maximum in t (H) curve at
4.2 K is observed around the coercive force Hc. The MR
ratio increases from 13.8 % to 14.5 % as pressure increases

Fig. 2. Rotation dependence of the counterflow intensity at T = 0:67
Tc. X c = 2:6 rad/s is the critical velocity of the vortex penetration
into the aerogel.

Fig. 1. MR curves measured at 4.2 K under high pressure. The vertical axis indicates the values of D t / t rmax, D t = t (H) – t max.

Fig. 1. Rotation dependence of NMR spectra at T = 0:67 Tc in acceleration (a) and in deceleration (b).

38

ISSP Activity Report 2003

Development of the High Field ESR
System under Pressure
T. Sakurai and Y. Uwatoko

Fig. 2. Pressure dependence of the MR ratio at 4.2 K, 77 K and R.T..

below 6 GPa, but the MR ratio decreases above 6 GPa. The
Hc decreases at the rate, dHc / dP = - 4.3 x 10-3 T / GPa.
Pressure dependence of MR at R.T., 77 K and 4.2 K isshown in Fig. 2. It is found that the behavior of MR againstpressure is different depending on the temperature. The MR
in high temperature range, R.T. and 77 K, decreases monotonously with pressure approximately in the linear fashionand the behaviors are nearly the same. The decreasing rate
1/MRT (∂(MRT) / ∂P) at T = 77 K and R.T. are - 7.1 x 10-2
GPa-1 and - 5.4 x 10-2 GPa-1, respectively. On the other
hand, the pressure coefficient of MR at 4.2 K changes its
sign from positive to negative with increasing pressure, i.e.,
it shows a broad peak centered around 5 GPa. This result
indicates that the higher order tunneling effect is suppressed
above 5 GPa because it becomes significant at low temperatures below 50 K. The magnitude of MR is approximated by
the following equation, MR ≈ (n* + 1) v 2, where n* is related to a number of Co granules on a conduction path and v
is the spin polarization. In the previous study[3], it has been
revealed that n* increases with pressure by the enhancementof higher order tunneling at high pressure. But above 3 GPa
the effect of pressure dependence of v , which decreases
with pressure, may not be neglected anymore. This is theorigin of the broad peak in the pressure dependence of MR
at 4.2 K. On the other hand, the TMR at 77 K and room temperature decreased as pressure increased. This result is well
explained by considering that the pressure change in v
becomes predominant at r.t. or 77 K since the higher ordereffect is suppressed or the contribution from n* is relatively
small.
Thus the large difference in the pressure dependence of
TMR between at 4.2 K and at R.T. or 77 K is consistently
explained by using the pressure dependence of n* and v .

We have developed a new system which enables ESR
measurements under pressure [1, 2]. Although there aresome types of X-band ESR system under pressure, our system has advantages that the high magnetic field up to 16 T
obtained by the pulse technique and wide frequency range
from 70 GHz to at least 460 GHz are available. In addition
to these advantages, a characteristic pressure cell for our
ESR system is used and a unique pressure-calibration
method has been established as described below.
A clamped-type pressure cell is used for the high field
ESR system. The inner parts such as pistons are made fromsapphire because millimeter and submillimeter electromagnetic wave must be transmitted. The cylinder is made from
CuBe alloy with the inner and outer diameter of 3 mm and 6mm, respectively. The pressure can reach 3.5 kbar and it islimited by the ratio of the inner and the outer diameters. The
outer diameter is determined by the bore of the solenoid
magnet. On the other hand, the inner diameter favors above
3 mm because the signal intensity directly depends on theamount of the sample. We are now examining a new cylinder made from NiCrAl alloy with almost the same design
and try to achieve the higher pressure.
Next, the pressure-calibration method for ESR measurements will be described. Figure 2 shows typical ESR spectraof ruby under ambient pressure and under pressure. Ruby
includes a small amount of Cr3+ ion. Cr3+ ion has the trigonal symmetry and the spin is S = 3/2. The spin-Hamiltonianof ruby is described as g n BSH + DSz2, where g is theg-factor, H is the magnetic field and D is the single ion
anisotropy. When the magnetic field is applied parallel tothe c-axis, which corresponds to the z-axis in the spinHamiltonian, the resonance fields are obtained to be
Ha = (hv + 2D)/g n B, Hb = hv/g n B and Hc = (hv - 2D)/g n B.
As shown in Fig. 1, three absorption lines with almost thesame intervals are observed. The resonance field Ha shifts tothe lower field side and the resonance field Hc shifts to thehigher field side by applying the pressure, while the resonance field Hb does not shift. These results show that theabsolute value of D increases under pressure. We obtain the

Reference[1] S. Takahashi and S. Maekawa, Phys. Rev. Lett., 80, (1998) 1758.
[2] S. Mitani, S. Takahashi, K. Takanashi, K. Yakushiji, S. Maekawa
and H. Fujimori: Phys. Rev. Lett. , 81, (1998) 2799.
[3] S. Kaji, G. Oomi, S. Mitani, S. Takahashi, K. Takanashi, S.
Maekawa, Phys. Rev. B, 68, (2003), 054429.
Authors
S. Kajia, G. Oomia, S. Mitanib, S. Takahashib, K. Takanashib,
S. Maekawab, M. Hedo, and Y. Uwatoko
aDept. of Phys., Kyushu University
bInstitute for Material Research, Tohoku University

Fig. 1. Typical ESR spectra of ruby at 1 bar (black) and at 2.21 kbar(red). The horizontal axis is normalized by the resonance position of
the DPPH, where DPPH is the magnetic field marker. Three signals
Ha, Hb and Hc due to ruby are observed and Ha and Hc shifts underpressure while Hb does not change.

ISSP Activity Report 2003

39

Time-Resolved Auger Decay in CsBr
by Using High Harmonics
T. Shimizu, M. Itoh, and S. Watanabe

Fig. 2. Frequency-field diagram of CsCuCl3 at 6 K for H//c. The discontinuous change of ESR mode indicates the magnetic phase transition. The transition field shifts to the higher field side under pressure.

first differential coefficient as -4.3 # 10-2 GHz/kbar at 4.2 K
below 3 kbar and this value can be used to calibrate thepressure by setting ruby with a sample at the same time.
Finally, we demonstrate an example of high field ESR
measurement under pressure. Figure 2 shows the frequencyfield diagram of S = 1/2 quantum spin system CsCuCl3
which is a well-known triangular antiferromagnet [2, 3].
Below the antiferromagnetic transition temperature TN =10.5 K, the spins form 120˚ structure in the c-plane [4]. This
compound shows a small jump in the magnetization process
when the magnetic field is applied parallel to the c-axis. It isshown by the theoretical studies that this jump in the magnetization process corresponds to the magnetic phase transition
from the “umbrella-type” spin configuration to the“collinear-type” spin configuration induced by the quantum
fluctuation [5]. The discontinuous change of ESR mode in
Fig. 2 reflects the magnetic phase transition and the transition field Hc is found to shift to the higher field side by
applying the pressure. We analyzed this change of ESR
mode within the mean field approximation. As a result it isconcluded that one of the key parameters is the easy-plane
type anisotropy of this system and we found this anisotropy
energy increases under pressure. Theoretically, theanisotropy is considered to compete with the quantum fluctuation and to stabilize the “umbrella-type” spin configuration [5]. These facts support the experimental results that theincrease of the transition field Hc under pressure is related tothe increase of the easy-plane type anisotropy.

A vacant state in an inner-shell relaxes with time constants ranging from a few attoseconds to a few femtoseconds, depending on the binding energy. The inner-shellholes created vanish through an ultrafast rearrangement of
the electronic system, leaving two holes in the outer levels.
This process, known as an Auger process, is a universal phenomenon in atoms, molecules, and condensed matter. The
lifetime of core holes has been estimated from the full widthat half maximum of the photoelectron spectra. However,spectral measurements can provide only indirect informationon the Auger decay process. Real time measurements are
greatly needed to study Auger dynamics. The time-resolved
spectroscopy of core holes needs high temporal resolution
along with high photon energy to excite the inner shell. Highharmonics (HH) of a femtosecond laser have a pulse width
down to sub-fs in the extreme-ultraviolet and soft x-ray
regions. Therefore, HH are eminently suitable for timeresolved spectroscopy following core hole creation.
In condensed matter, interatomic Auger transitions are
possible among neighboring atoms. Such interatomic Auger
processes could be largely affected by the environment ofcreated holes, such as lattice deformation or vibration (i.e.,
temperature or injected carrier density), and are a challenging subject in solid-state physics. CsBr is of special interestfor the study of the Auger process, because it has two competing relaxation channels of holes created in the Cs+ 5p
outermost-core band, radiative decay and nonradiative
Auger decay. At low temperatures, Auger decay is energetically forbidden because the energy difference EVC between
the top of the outermost core band and the top of the valenceband is smaller than the band gap energy Eg. However, theUrbach exciton tail states can provide suitable final states
for Auger decay products, causing the rate to increase withtemperature. In the present experiment, we measured thetemperature-dependent decay time of the luminescence arising from the radiative decay of Cs+ 5p core holes, and determined the Auger decay probability at the high-temperature
limit. Figure 1 shows the temperature dependence of the
decay time of the luminescence. The interatomic Auger
decay time was, for the first time, estimated to be 2.4+3.8/1.5 fs in condensed matter, in good agreement with a theoretical calculation of alkali halides. The present work veri-

References[1] H. Ohta, et al., J. Phys.: Condens. Matter 14 (2002) 10637.
[2] T. Sakruai, et al., J. Phys. Soc. Jpn. 72 (2003) Suppl. B 156.
[3] T. Sakruai, et al., Physica B 346-347 (2004) 221.
[4] K. Adachi, et al., J. Phys. Soc. Jpn. 49 (1980) 545.
[5] T. Nikuni and H. Shiba, J. Phys. Soc. Jpn. 62 (1993) 3268.
Authors
T. Sakuraia, H. Ohtab, and Y. Uwatoko
aCenter for Supports to Research and Education Activities, Kobe
University.
bMolecular Photosience Research Center, Kobe University.

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ISSP Activity Report 2003

Fig. 1: Temperature dependence of the decay time of the luminescence in CsBr. Closed triangles correspond to the data obtained by
SR and closed circles to the present data. Solid line is the best fit of atheoretical model to the experimental result.

fied that HH enable time-resolved spectroscopy with hightemporal resolution that was not accessible before by synchrotron radiation (SR).
Reference[1] T. Shimizu, T. Sekikawa, T. Kanai, S. Watanabe, and M. Itoh,Phys. Rev. Lett. 91, 017401 (2003).
Authors
T. Shimizu, T. Sekikawa, T. Kanai, S. Watanabe, and M. Itoha
Department of Electrical and Electronic Engineering, Shinshu
UniversityaUnoccupied Electronic Structure ofLightly-Doped SrTiO3 by
Resonant-Inverse Photoemission
Spectroscopy
T. Higuchi and S. Shin
Recently, there has been a renewed interest in the electronic structure of SrTiO3 doped with charge carrier. Whena small number of electrons chemically doped into SrTiO3,
e.g. by La3+ substitution for Sr2+ site or by Nb5+ substitution
for Ti4+ site, the doped electrons enter the bottom of theempty Ti 3d band. In the limit of lightly doping concentration, there will be no electron correlation between the doped
electrons since the probability of electron-electron scattering

Fig. 1. IPES spectra of SrTi0.98Nb0.02O3 near the Ti 3p " 3d absorption edge measured by various kinetic energy of the incident electron(Ek). The abscissa is the energy above Fermi level (EF=0 eV).
Arrow shown in each spectrum indicates the normal fluorescence.

is vanishingly small. The photoemission spectra have two
features in the band gap region, which are generally
believed to be a coherent part at the Fermi level (EF) and an
incoherent part at ~1.5 eV that is attributed to a remnant of
the lower Hubbard band. The band structure calculations
that assume a rigid-band filling cannot reproduced the peakat ~1.5 eV, though the bandwidth of O 2p valence bandshows a good agreement with the photoemission spectra.
The origin of the ~1.5 eV peak has been studied by several
theoretical calculations. It is proposed that the ~1.5 eV peakis contributed to be a polaronic feature or a surface structurethat is created due to the degree of correlation and disorderin the surface. A similar feature is also expected in the conduction band side.
In this paper, we present resonant inverse-photoemission
spectroscopy (RIPES) spectra of lightly Nb-doped SrTiO3(SrTi0.98Nb0.02O3). The RIPES is a powerful technique toinvestigate the unoccupied electronic structure in the surface
state. Although the technique and new physics obtained
from RIPES spectra were investigated in rare-earth metalcompounds by Kanai et al.[1], the RIPES spectra of lightly
3d transition metal compounds has not been reported thus
far.
Figure 1 shows the excitation energy dependence of the
RIPES spectra of SrTi0.98Nb0.02O3 in the incident electronenergy (EK) region of Ti 3p core level. The abscissa represents the energy above the Fermi level (EF) that was calibrated by the Fermi edge of Au. The intensity of the feature
D at about ~7.3 eV becomes strong at lower EK. This isconsidered to be due to the electron energy dependence of
the ionization cross section, indicating the existence of theSr 4d band. The broad bands indicated by arrow are the normal Ti 3p fluorescence that is caused by the Ti 3p core holecreated by the electron excitation. In the energy region from0 to 5 eV, two prominent features A and B are found at ~1.7

Fig. 2. (a) On-resonance (red circle) and off-resonance (blue circle)
spectra measured at Ek= 47 eV and 45 eV, respectively. Difference
spectrum (green line) subtracted from on- and off-resonance spectrum is also shown. (b) O 1s XAS spectrum.

ISSP Activity Report 2003

41

and ~3.9 eV, respectively. The intensities of the features A
and B increase with decreasing EK and decrease at EK=45
eV. This indicates the resonance effect of the RIPES spectra
at EK=47 eV.
Figure 2(a) shows the comparison of on- andoff-resonance spectra of the conduction band measured at
EK= 47 and 45 eV. One can find that the Ti 3d components
in both the features A and B are resonantly enhanced by theTi 3p " 3d excitation in the on-resonance spectrum, though
the resonance effect of the feature B is very weak in comparison with that of the feature A. The difference spectrum
from on-resonance to off-resonance spectra is also shown.
The difference spectrum corresponds to the Ti 3d partial
density-of-state (DOS) in the conduction band. The O 1s Xray absorption spectrum of SrTi0.98Nb0.02O3 is also shownin Fig.2 (b). The energy position of Sr 4d state of the O 1s
X-ray absorption spectroscopy (XAS) spectrum matcheswith the broad band at ~7.3 eV (feature D) in the RIPES
spectra. On the other hand, two features at ~1.7 eV and ~3.9
eV of O 1s XAS spectrum reflect the t2g- and eg-subbands of
the Ti 3d states. It is striking that the energy positions of the
t2g and eg subbands of O 1s XAS spectrum are good agreement with the features A and B of the RIPES spectra, as
shown in two vertical dashed lines. These positions of thefeatures A and B mismatch with those of the coherent andincoherent parts, which are expected from the disordered
Hubbard model [2].
In the difference spectrum of Fig.2 (b), the feature C at
~6.1 eV, which show the Ti 3p " 3d resonant effect in Fig.2,
does not match with the O 1s XAS spectrum. The existenceof the feature C might be attributed to the surface-induced
structure, because the O 1s XAS spectrum is the bulk sensitive. In recent years, the band structure in the conduction
band region of the lightly-doped SrTiO3 has been calculatedby Sarma et al [2]. They used the linearized muffin-tin
orbital method within the atomic-sphere approximation.
From the second order perturbation applied to the DOS
obtained from LMTO supercell calculation, a correlationinduced satellite appears outside the one-electron bandwidth
in the unoccupied states. The energy position of the correlation satellite structure matches with that of the feature C.
This fact indicates that the correlation effect exists in thesurface state of lighly-doped SrTiO3 [3].

Electronic Structure and its
Contribution to Large ThermoelectricPower in Layered Cobalt Oxides
T. Takeuchi and S. Shin
Layered cobalt oxides characterized by two-dimensionally spanned CoO2 triangular-lattice consisting of edgeshared CoO2 octahedrons have attracted a great deal ofinterests because of their possession of unusual electricalproperties[1-3]. Large thermoelectric power simultaneously
observed with metallic electrical conduction is especially
important from the technological point of view because thelarge thermoelectric power (S) and the large electrical conductivity ( v ) together with a low thermal conductivity ( l )are of necessities for the development of a practical thermoelectric material that has been regarded as one of the key
technologies of energy-saving and environmental protection.
In the present study, electronic structure in the c -phase
Na0.6CoO2, Bi2Pb2Co2O9, and Ca3Co4O9 is investigated indetail using high-resolution ultraviolet photoemission spectroscopy (UPS) and Co 2p-3d resonant photoemission spectroscopy (RPES). The electron transport properties of bothelectrical conductivity and thermoelectric power for theselayered cobalt oxides and their sample dependence are discussed in terms of the measured electronic structure near the
Fermi level EF, and the specific electronic structure responsible for the metallic electrical conduction and the large
thermoelectric power is experimentally investigated. The
temperature dependence of the thermoelectric power S(T)
was calculated from the measured electronic structure within
the context of the Boltzmann transport equation. We demonstrate that the large thermoelectric power in the layered
cobalt oxides can be well accounted for in terms of the
Boltzmann-type metallic conduction in a narrow band specific to the two-dimensional CoO2 layers.
Figure 1 shows the Co 2p-3d on-resonant and off-resonant photoemission spectra accumulated at hv = 778 eV and
771 eV, respectively. The off-resonant spectra of all thesecompounds exhibit an intense peak at ~1.0 eV below EF .
We consider that the electronic states causing this intense
peak are closely related with their possession both of theReferences[1] K. Kanai and S. Shin, J. Electron Spectrosc. Relat. Phenomn. 117118 (2001) 383.
[2] D. Sarma, S. R. Barman, H. Kajueter and G. Kotliar, Europhys.
Lett. 36 (1996) 307.
[3] T. Higuchi, S. Nozawa, T. Tsukamoto, R. Eguchi, H. Ishii, Y.
Tezuka, S. Yamguchi, K. Kanai, and S. Shin, Phys. Rev. B 66 (2002)
153105.
Authors
T. Higuchi,a T. Tsukamoto,a K. Kanai,b Y. Tezuka,c R. Eguchi, andS. Shina Department of Applied Physics, Tokyo University of Scienceb Department of Chemistry, Nagoya Universityc Department of Electronic and Information System Engineering,
Hirosaki University

Fig. 1. (a) Co 2p-3d on-resonant spectra of Ca3Co4O9 (CCO),
Bi2Sr2Co2O9 (BSCO) and c -Na0.6CoO2 ( c -NCO) measured with anincident energy of hv = 778 eV at 20K. (b) Off-resonant XPS spectraof CCO, c -NCO, and BSCO measured at 20K. All spectra are characterized by an intense peak centered at ~ 1 eV. The Fermi level islocated near the high-energy edge of this peak.

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ISSP Activity Report 2003

Strain Field under the SiO2/Si(001)Interface Revealed by thePhase-sensitive X-ray Diffraction
TechniqueW. Yashiro, K. Miki, and T. Takahashi

Fig. 2. (a) UPS spectra at 300K (+) and those divided by the FermiDirac distribution function (UPS/FD spectra; with thin lines) are
shown for Bi2Sr2Co2O9 (BSCO). The density of states above EF isrevealed in the UPS/FD spectra. The UPS/FD spectra were used as
N(E) for the evaluation of thermoelectric power. The deduced thermoelectric power are shown in (b) with the experimentally observed
data. A positive temperature coefficient and large positive value ofthe thermoelectric power of the Bi2Sr2Co2O9 was reproduced.

large thermoelectric power and metallic conduction, because
EF is located near the high-energy end of this peak.
However, this peak turned out to be less obvious in the onresonant spectra, though the intensity itself was increased
more than five times larger due to the Co 2p-3d resonance.
This means, thereby, that the band(s) lying in the energy
range from EF to 2.0 eV below EF consists both of the O 2p
and Co 3d components, and is dominated more by O 2p
component than by the Co 3d component.
The density of states at EF is finite but negligibly small
at room temperature because EF is located near the highenergy edge of this narrow band. Since thermoelectric
power is inversely proportional to the density of states at EF,
one may naturally expect that this characteristic electronic
structure would be responsible for the large thermoelectric
power of these layered cobalt oxides. We calculated thermoelectric power S(T) using the Boltzmann transport equation
with the electronic structure near EF determined by the highresolution UPS measurement. The calculated S(T) shows
fairly good consistency with the measured value both in its
magnitude and the temperature dependence. We conclude,
therefore, that the large thermoelectric power observed inthese layered cobalt oxides can be explained within theframework of the Boltzmann-type electrical conduction
mechanism and the electrons in the narrow band with its
band edge near EF. Detailed results were reported elsewhere[4].

Strain near an interface affects its electronic structure,
but a full understanding of such strains is still lacking evenin the case of SiO2/Si, which has been applied to electronic
devices since 1960s. We have developed a new technique,the phase-sensitive X-ray diffraction (PSXD) technique,which is a powerful way to characterize strain fields near
crystal surfaces, and applied it to a Si(001) wafer whose surface is covered with a thermal oxide layer [1]. It was
revealed that there is a small strain field distributing overmesoscopic-range depth (up to several hundred of nm)
under the SiO2/Si interface and having a static fluctuation inlateral direction.
The PSXD technique is an application of a phenomenon,
modulation of the intensity of the crystal-truncation-rod(CTR) scattering under an excitation of a Bragg reflection[1,2], which is an interaction between a two-dimensionallydiffracted wave (CTR scattering) and a three-dimensionallydiffracted (Bragg reflection) wave. An example of the phenomenon in the case of the Si(001) wafer is shown in Fig. 1,
where intensity of the 50 rod CTR scattering is modulatedby the excitation of the 004 Bragg reflection. It was shownthat the modulation profile can be characterized by two
parameters: the phase shift, which represents the dip or peak
position of the modulation profile, and the visibility. The
solid (red) line in Fig. 1 is the best-fit curve calculated,
where the phase shift and visibility are fixed at-2 r # (0.117 ! 0.001) and 0.521 ! 0.002. Both the experimentally obtained values were different from those of an
ideal perfect crystal (0 and 0.711, respectively), which isshown by the dotted line in the figure.
Figure 2 shows an illustration of the strain field under
the SiO2/Si(001) interface which can explain the experimen-

References[1] K. Takeda et al., Nature 422, 53 (2003).
[2] T. Valla et al., Nature 417, 627 (2002).
[3] I. Terasaki et al., Phys. Rev. B 56, R12685 (1997).
[4] T. Takeuchi et al., Phys. Rev. B 69, 125410 (2004).
Authors
T. Takeuchia, T. Kondob, U. Mizutanib, S. Tsuda, T. Kiss, T. Yokoya,
and S. Shina Ecotopia Science Institute, Nagoya University.
b Department of Crystalline Materials Science, Nagoya University.

Fig. 1. Intensity of the CTR scattering is modulated by the excitationof a Bragg reflection [1,2]. An example is demonstrated in the case ofa Si(001) wafer covered with a thermal oxide layer. The experiment
was carried out at BL09XU in SPring-8. The solid and open circles
represent the experimentally obtained intensities corresponding to the
50 rod CTR scattering and 004 Bragg reflection, whose scales (inarbitrary units) are on the right and left axes, respectively. The horizontal axis is the deviation in the incident angle from the 004 Bragg
angle. The dotted line is the intensity calculated for an ideal perfect
crystal. The solid and broken lines are the best-fit curves to the experimentally obtained data.

ISSP Activity Report 2003

43

faces.We have found that rough as-grown (110) surface
turns to an atomically smooth surface if we perform growthinterrupt in-situ annealing at 600 cC for 10 min [1]. Thisannealing method enables us to realize quantum wires andwells with unprecedentedly high quality.
The purpose of this joint study is to understand formation mechanisms of the atomically flat (110) GaAs surface.
For this purpose, we formed annealed (110) GaAs surface
with fractional monolayer (ML) coverage, and performed
AFM measurements on characteristic step-edges shapes ofisolated pits and islands [1], and also performed statisticalanalysis on the size and shape distribution of 1-ML-deep

Fig. 2. Illustration of the strain field under the SiO2/Si(001) interface
which can explain the experimentally obtained modulation profile.
The open circles represent to the cites of atoms or unit cells in bulk
crystal, and the solid circles represent the position of them in thestrained layer near the interface. The arrangement of the experiment
was set to be particularly sensitive to strain fields distributing mesoscopic-range depth. The phase shift of modulation profile can be simply interpreted into the sum of displacements of atomic planes under
the interface projected onto the direction perpendicular to the 004
plane. The phase shift in Fig. 1 shows that there is a total displacement of -0.16 Å in the direction perpendicular to the crystal surface.
On the other hand the experimentally obtained visibility indicates thatthe total displacement has static fluctuation in the direction parallel tothe interface. The visibility was interpreted into a static fluctuation ofat least ! 0.13 Å in the lateral direction.

tally obtained modulation profile. The phase shift can be
simply interpreted into the sum of displacements of atomic
planes under the interface projected onto the direction perpendicular to the 004 plane (-0.16 Å in the figure). On theother hand the visibility indicates that the total displacement
has a static fluctuation of at least ! 0.13 Å in the direction
parallel to the interface. The features revealed by the PSXD
technique are expected to provide a new window to understand the oxidation mechanism of Si surface.
References[1] W. Yashiro, K. Sumitani, T. Takahashi, Y. Yoda, and K. Miki,
Surf. Sci. 550 93 (2004).
[2] T. Takahashi and S. Nakatani, Surf. Sci. 326 347 (1995); W.
Yashiro, K. Sumitani, Y. Yoda, and T. Takahashi, Jpn. J. Appl. Phys.
42 6658 (2003), and references therein.

Fig1 . (a) Top and side views of the atomic arrangement model of a
1-ML-deep pit observed on the annealed (110) surface. This modelhas the Ga-terminated three-bond step-edge (A-B), the As-terminatedthree-bond step-edge (C-D), and the two-bond step-edges (A-D and
B-C). (b) Atomic step kinetics of the surface evolution during
annealing. Proposed schematic drawings of the evolution for theisland and pit structures from an assumed square initial shape.

Authors
W. Yashiroa,b, K. Sumitani, Y. Yodac, K. Mikia,b, and T. Takahashia) Nanomaterials Laboratory (NML), National Research Institute forMaterials Science (NIMS)b) Nanotechnology Research Institute (NRI), National Institute ofAdvanced Industrial Science and Technology (AIST)c) Japan Synchrotoron Radiation Research Institute (JASRI)

Atomically Smooth GaAs (110) Surface
Fabricated by Growth-interrupt
Annealing and Cleaved-edge
Overgrowth
Ji-Won Oh, A. Ishii, and H. Akiyama
MBE growth of GaAs on a cleaved (110) surface isbecoming of increasing importance as a fabrication method
for novel semiconductor nanostructures. The overgrowth ona cleaved (110) surface requires, however, a low substratetemperature range (470 - 510cC) and a very high As4-vapor
overpressure for GaAs, and overgrown film has rough sur-

44

ISSP Activity Report 2003

Fig. 2. Contour map of migration barrier energy for (a) Ga adatomand (b) As adatom on a GaAs (110) surface with the surface atomic
configuration. The dotted and solid circles correspond to As and Ga,
respectively.

pits [2].
Figure 1 shows a model for the atomic step-edges of the
1-ML-deep pit, which is based on the stability of atomic
sites. Our model, namely Ga atom migration occurs fromtwo-bond sites to three-bond sites having higher stability,
well explained the driving force to form an atomicallysmooth surface and characteristic atomic step edges [2].
We also performed the first-principles calculation on the
migration barrier energy for Ga and As adatoms on a GaAs
(110) surface [3]. The results show far smaller barrier energies than those on a (100) GaAs surface, which enable
adatoms to migrate in a very long length and explain the formation of a very wide atomically smooth ((110) surface asstated above.
Figure 2 (a) and (b) show the calculated potential surfaces for the Ga and As adatoms on the GaAs (110) surface.
Migration barrier energy surface of Ga adatoms is 1-D,
while that of As adatoms is 2-D. For both Ga and As
adatoms, the stable near As of the topmost layer are very
stable, while those near Ga are unstable. This fact explains
the asymmetric shapes of 1 ML deep pits shown in Fig. 1(a).
Reference[1]M. Yoshita et al., Jpn. J. Appl. Phys. 40, 252L (2001); Appl. Phys.
Lett. 81, 49 (2002); J. Cryst. Growth, 251 62 (2003).
[2]J-W. Oh et al., Appl. Phys. Lett. 82, 1709 (2003).
[3]A. Ishii et al., Appl. Phys. Lett. 83, 4187 (2003).
Authors
Ji-Won Oha, A. Ishiib, M. Yoshita, H.Akiyama, L. Pfeifferc, and
K. WestcaJSPS foreign visiting researcher
bDepartment of Applied Mathematics and Physics, Tottori University
cBell Laboratories, Lucent Technologies

ISSP Activity Report 2003

45

ISSP Workshop

1. Superfluidity under rotation 2003 (SUR2003)

May 13, 2003

M. Kubota, H. Ishimoto
Superfluidity is being studied under rotation as often
compared with the study of superconductors in various
magnetic field. Actually superfluid study under rotation
has a unique opportunity to study interplay between rotation and magnetic field for the p-type Fermion superfluid,
liquid 3He. Another unique superfluid consists of submonolayer 4He film condensed on controlled porous
media presents a Bose superfluid whose properties can be
modified by the pore size and the film thickness over quite
a wide range. One of its unique features is the possibility
to reach X c2, which is practically impossible for the caseof the bulk He superfluids. The present ISSP Mini Work
Shop is actually a collection of lectures by the pioneering
workers and People started the activity using newly completed ISSP rotating Cryostats, Professors W.F. Vinen(Birmingham), Erkki Thuneberg (Oulu), Matti Krusius(Helsinki), Takao Mizusaki (Kyoto), Minoru Kubota(Tokyo), and John D. Reppy (Ithaca). This public ISSP Mini Work
Shop(May 13-14, 2003) was followed up by continued International work shop(May 14-17), which was held by Chuzenji
Lake. The information can be found at the following url:
http://www.issp.u-tokyo.ac.jp/contents/seminar/workshop/20030513-17t.html

2. Accelerators for the Super-SOR projectAugust 22, 2003

N. Nakamura
The Super-SOR light source is a VUV and soft X-ray
third-generation synchrotron radiation source and to be
operated for nationwide and worldwide users. The
University of Tokyo proposed to construct it in Kashiwa
Campus. The light source consists of a 1.8-GeV storage
ring, booster synchrotron and pre-injector linac and top-up
injection is planned to overcome a relatively short beam
lifetime. The accelerators including their buildings andradiation safety system have been designed intensively formore than two years. In this workshop, the latest accelerator design and the present status of R&D on the accelerator components and subsystems were presented. The
workshop was successfully completed with active discussions among the nationwide accelerator scientists and SR
users of more than 40.
http://www.issp.u-tokyo.ac.jp/labs/sor/vsx/community/2003.htm

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ISSP Activity Report 2003

3. New findings and views on creation and destruction of coherence of lights withshorter wavelengths
November 14-15, 2003
T. Miyahara, S.Shin, and T. Suemoto
Production of coherent lights with short wavelengths has come to a new stage since plasma sources, synchrotron radiation,
free electron lasers, together with conventional lasers have made a remarkable progress in the time structure, the stability, thereproducibility, as well as the spatial coherence. One of the important problems left is how to maintain the obtained coherence
against various processes to cause decoherence. This problem is not easy to solve especially when the mechanism to produce
the coherence could also plays a role of decoherence. In this workshop various mechanisms to create or destroy coherence
were discussed with emphasis on advantages and disadvantages of various techniques to produce coherent lights. It is also
confirmed that precise control of the phases of wavepackets on the material system could provide us in the future with a new
method to suppress the decoherence of lights.
http://www.issp.u-tokyo.ac.jp/contents/seminar/short/20031114ws.html

4. Material researches using high-brilliance light sources

November 28-29, 2003

N. Ueno and T. Kinoshita
Recent hot results and new research possibilities using high-brilliance synchrotron radiation in materials science and technology were presented by actively working young researchers. The workshop attracted various scientists from universities,
national institutes and private companies. The workshop covered wide variety of studies on nanostructures, magnetic thinfilms, superconducting materials, organic-device related materials, bio-related materials, surface chemistry, and molecular andatomic physics. Potential use of rather small-energy x-rays near SK edge in structural analyses of biomolecules was presented. In addition to these, actual applications of synchrotron radiation to industrial fields were introduced. Very exciting andvaluable discussions were made throughout the workshop. Discussions demonstrated steady increase in scientific potential ofspectromicroscopy including photoelectron emission microscopy using high-brilliance light sources.
http://www.issp.u-tokyo.ac.jp/labs/vsx/community/2003keikaku

5. Novel condensed-matter properties created by frustration

December 1-3, 2003

H. Kawamura and M. Imada
Frustration is ubiquitous in nature. It describes the situation where several optimization conditions compete with eachother and are incompatible. Under frustration, the system tends to be unstable and fragile, often being subject to strong fluctuation effects. This could give rise to many interesting novel phenomena not encountered in standard unfrustrated systems.
Last year, we held an ISSP workshop "Frustrated magnetism and novel properties". Following the success of this workshop, we held this year the second one in the series, i.e., "Novel condensed-matter properties created by frustration", which
covered even wider areas, including, not only frustrated magnetism, but also electron transport phenomena, metal-insulatortransitions, various charge and orbital phenomena closely related with frustration. Mentioning some of the specific topics
hotly discussed in the workshop, magnetic ordering of kagome, spinel and pyrochlore magnets including spin and kagome ice,quantum spin ordering of geometrically frustrated magnets, chirality and its effect on transport properties such as anomalous
Hall effect, nature of the ordering of spin glasses, geometrical frustration and strongly correlated electron systems including
absorbed helium-three film and organic materials, superconductivity in the novel triangular-lattice oxide NaxCoO2yH2O, etc.
http://thmat8.ess.sci.osaka-u.ac.jp/Meeting2003/

6. Frontier of surface spectroscopy and its application to nano-science

December 4-5,2003

M. Kawai and J. Yoshinobu
Recent development of surface spectroscopy in time-domain and spatial-domain enables us to investigate atoms and molecules on surfaces in detail. For instance, the dynamics including the energy tranfer and dissipation between vibrational modes
and bond-fomation and bond-scission at surfaces can be observed in ps/fs time-scale using non-linear spectroscopy withpulsed laser. In the latter case, single molecular spectroscopy and manipulation on surfaces can be achieved using a local
probe methods (STM, AFM, etc.). These state-of-the-art tools play an important role in nano-science and nano-technology but
also gives us an opportunity to extract new concepts in physics and chemistry. Therefore the collaboration between physicist
and chemists, and theoreticians and experimental researchers becomes more important. In this workshop, we presented anddiscussed our recent results and idea including surface vibrational spectroscopy, local-probe methods, photoelectron spectroscopy using SR and laser.
http://sas.k.u-tokyo.ac.jp/ads/

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7. Superfluidity by means of vacancy excitations

December 20-21, 2003

M. Kubota
Supersolid state, which maintains the lattice order and at the same timesuperfluid properties, has been discussed since 1970 for the quantum solid
4He. Although various experimental search for such a state lasted for morethan 30 years without any success. The general situation has been changed byrecent reports by Goodkind (California) on bulk solid He and by Moses Chan
(Pensilvernia) on solid He formed in porous media, especially by the latter.
Namely their torsional oscillator results could derive critical velocity information. Theoretical study by Galli and Reatto predicted BEC and off diagonal
long range order(ODLRO) on much more quantitative ground. ISSP mini
work shop was organized to review the above situation and to present otherrelated systems which may come into the same or similar category situation. Systems include solid He mixture near hcp- bcc
phase boundary, He films condensed on glass surfaces, Hydrogen in metalhydrides. There will be new developments expectedin the field. December 20-21, 2003. The information about this miniworkshop is available at the following url:
http://www.nagaoka-ct.ac.jp/mb/lab/araki/issp-tanki2003-12/index.html

8. Frontiers in research of quantum condensed systems

January 7-9, 2004

K. Shirahama, H. Ishimoto, and M. Kubota
Condensed helium is unique in condensed matter physics: it shows a large variety of quantum phenomena at low temperatures. Since similar phenomena have been observed in other systems such as alkali Bose gases and heavy fermion superconductors, liquid and solid helium draw renewed interests as counterpart and model systems. Moreover, recent contributions
from Japanese groups have been of great importance in the research fields. We organized this three-day workshop to serve asa forum for discussing recent outcome, common problems and concepts in helium and various research fields. We have madefruitful discussions on the following topics: anisotropic superfluids and superconductors, low dimensional 3He and quantum
spin systems, helium in restricted geometries, surface and interface phenomena, quantized vortices and Bose condensation. A
series of workshops are planned including the ninth ISSP International Symposium (ISSP-9) on Quantum Condensed System,
held at 16 - 19 November, 2004 (http://www.issp.u-tokyo.ac.jp/issp-9/).
http://www.phys.keio.ac.jp/labs/sirahama/2004_ISSP-tanki/ sirahama-jp.html

9. High pressure material seminar21:
Pressure induced quantum critical phase transitions

February 5-6, 2004

Y. Uwatoko
In recent years, many researchers do investigate a study to high pressure science. The seminar is focused on the high pressure science and technology. To cover the entire diversity in methodology, experimental techniques, application, and theory ofhigh pressure science and technology, the scientific program is organized in lectures and poster sessions. In addition to the scientific program, the progress and development of new pressure techniques are an important part of the workshop. Special
emphasis are put on the needs of the high pressure user community, future developments and new perspectives.
http://www.issp.u-tokyo.ac.jp/labs/extreme/uwatoko/tanki_top.html

10. Recent progress in ultrafast laser spectroscopy

February 16-17, 2004

T. Suemoto
Ultrafast laser spectroscopy has become a matured technology owing to the titanium sapphire lasers with a very high performance and stability developed in the 1990s.
In addition to the lasers in the near infrared and visible regions, coherent light sources in the vacuum ultraviolet and theterahertz frequency regions are being developed in recent days. This workshop focused on these new light sources, spectroscopic methods utilizing the properties of these advanced lasers, and their applications to material science. It was also intended to overview the activities of this field in Japan before the 16th International Conference on Ultrafast Phenomena, which
will be held at Niigata in July. Twenty-three oral presentations involved topics on non-linear phenomena, photoelectron spectroscopy, wave-packet dynamics, photo-induced phase transitions, terahertz radiation and so on. Dynamical studies on a wide
range of materials, i.e., semiconductors, nano-structures, strongly correlated systems, organic crystals and biomaterials werecovered in the workshop.
http://www.issp.u-tokyo.ac.jp/contents/seminar/short/tanki20040216.html

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ISSP Activity Report 2003