The Institute for Solid State Physics
The University of Tokyo

Activity Report
2005

Activity Report
2005
Preface1

Research Highlights

2 - 29Highlights of Joint Research

30 - 44

ISSP Workshop

45 - 46

Proposal for Joint Research

47 - 84

Publications

85 - 122

ISSP Activity Report 2005

Preface

The ISSP was established in 1957 based on the recommendation of the National Council of Japan to set up a new institutefor condensed matter physics. The idea was to establish a topclass research institute for experimental and theoretical studies
of condensed matter and to promote cooperative research in
the field throughout the country. Soon after the inauguration,
the name of ISSP became recognized world-wide in thecommunity of condensed matter physics.
Since then there have been two major changes in the historyof the institute. The first was the reorganization of the institutein the mid-eighties. The system of divisions consisting of several groups was introduced andthe project of the "Physics in extreme conditions", which includes Ultra-High Magnetic Fields,
Laser Physics, Surface Physics, Ultra-Low Temperatures, and Very High Pressures, was started. The
second major change was the relocation of the ISSP to the new Kashiwa campus which wascompleted in 2000.
To make the status of the research activities in the ISSP public, new forms were introduced in 2001:
Activity report A for research mostly by the staff of the ISSP and Activity report B for joint research
conducted by the ISSP. This copy is the Activity Report for the fiscal year 2005 in the style of single
volume for all the research activities at the ISSP. The system of national universities in Japan
has made a profound change from April in 2004 by the so called "houjinka", which means thatnational universities, once directly belonged to the ministry of education, have become independentadministrative entities. In the new scheme importance of the Activity Report has been increased.
We are happy to receive any comments on the Report for the possible improvement of the research
activities of the ISSP.

June 21, 2006

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

ISSP Activity Report 2005

1

Research Highlights

Pressure Effect on Superconductivity
and Antiferroquadrupolar Orderin PrOs4Sb12
Sakakibara Group
Heavy-fermion (HF) behavior occurs in a variety ofCe-, U-, or Yb-based compounds, but less is known for the
Pr-based compounds where doubly occupied f electrons aremuch more localized. Thus, Pr-based filled skutterudites
have attracted a considerable amount of attention inrecent years because they offer unique opportunities for
exploring the HF behavior. Unlike the Ce-, U-, or Yb-based
compounds, most of Pr-based filled skutterudites have anonmagnetic ground state in the crystalline electric field(CF). Hence, its HF behavior has often been discussed interms of the interaction between the electric quadrupolar
moments of Pr3+ and conduction electrons.
Here we report on our recent magnetization study on the
HF superconductor PrOs4Sb12 (Tc1=1.85 K and Tc2=1.75 K)
under pressure. This compound exhibits an antiferroquadrupolar (AFQ) transition in high fields above 45 kOe,in addition to superconductivity. The occurrence of the
field-induced AFQ transition can be understood in termsof a level-crossing of a Γ1 singlet ground state and a Γ4
triplet excited state located at ∆=8 K. Recent inelastic
neutron-scattering experiments have revealed that low-energy
excitations are mainly due to nonmagnetic quadrupolarinteractions associated with the AFQ order. It is also suggested
that occurrence of HF superconductivity in this compound isclosely related to the quadrupolar excitons, although it is still
unclear how the quadrupolar excitons affect the supercon-

ducting state. Recently, it is theoretically shown that Tc
strongly depends on the magnitude of the energy splitting∆ in a multipole-exciton-mediated superconductivity scenario. Hence, pressure experiment becomes very important,because pressure dependence of Tc and ∆ enables to clarify
the relationship between superconductivity and thequadrupolar excitons.
Figure 1 shows the magnetic phase diagram of
PrOs4Sb12, as deduced from the present magnetization
measurements. The blue and red lines represent the data at
P=0 and 1.45 GPa, respectively. For the superconducting(sc) phase, only the phase boundary for Tc2 is shown. The
phase diagram clearly indicates that the sc region decreases
with an increase in pressure. On the other hand, the AFQ
ordered phase moves to lower fields as the pressure increases, while its area remains almost the same. The crosses indicate the temperature T* at which the M(T) curve becomesmaximum, and the dashed lines are fits to the T* data. The
T*(H) line at P=0 approaches a point where TQ is the largest.
Similar behavior is observed in the T*(H) line at 1.45 GPa.
Analysis of our experimental data on the CF model reveals
that the CF energy splitting ∆ between the Γ1 and Γ4 states
decreases with an increase in pressure, whereas the two sc
transition temperatures decrease significantly. Therefore, theobservations raise a question about the scenario for superconductivity mediated by the quadrupolar excitons alone.
Inelastic neutron scattering experiments have revealed
that quadrupolar excitons dominate low-energy excitationsin PrOs4Sb12, indicating that quadrupolar interactions must
be relevant in the sc state. The present phase diagram
demonstrates that the sc region decreases with increasingpressure, although the AFQ phase approaches the sc phase.
Such behavior can be expected to occur when the quadrupolarinteractions associated with the AFQ order suppress
superconductivity. Thus, our data suggests that the sc statecompetes with the AFQ ordered state.
References[1] T. Tayama, T. Sakakibara, H. Sugawara, and H. Sato, J. Phys. Soc.
Jpn. 75, 043707 (2006).
Authors
T. Tayama, T. Sakakibara, H. Sugawaraa, and H. SatobaThe University of TokushimabTokyo Metropolitan University

Fig. 1. H-T phase diagram of PrOs4Sb12 deduced from magnetization
measurements. The blue and red lines are obtained from the data at 0
and 1.45 GPa, respectively. For the two superconducting transitions,only the phase boundary for Tc2 is shown. The crosses represent thetemperature where the T-dependence of the magnetization exhibits amaximum. The dashed lines are a guide to the eye. SC: superconducting state and AFQ: antiferroquadrupolar ordered state.

2

ISSP Activity Report 2005

A Voltage-Induced Transition of
Hemin in BIODE(Biomolecular Light-Emitting Diode)
Tajima and Akiyama Groups
A thin-film organic light-emitting diode (OLED) was
fabricated by Vincett et al. for the first time [1]. This device

is of interest not only for the practical use but also for thebasic research. By fabricating this device, we can measureelectro-luminescence (EL) spectra of insulating organic
compounds, and can study their electrical and opticalproperties simultaneously. We recently succeeded in
obtaining the EL spectra of cytochrome c [2]. This is the firstreport on the fabrication of the biomolecular light-emitting
diode ("BIODE"). Subsequently, we fabricated BIODE
devices using other porphyrin-containing biomolecules(myoglobin [3], chlorophyll a [4]) and reported the EL
spectra of the compounds. Interestingly, heme proteins(heme = iron porphyrin) do not or hardly exhibit photoluminescence (PL), although chlorophyll a, which does notcontain iron, exhibits PL. In this respect, the mechanism of
EL in heme proteins is of interest not only for bioscience but
also for physics [5].
Hemin is a small molecule containing an ironprotoporphyrin IX unit (see the inset of Fig. 1a). This
compound does not exhibit PL. Figure 1a and 1b show thecurrent and EL intensity simultaneously measured for ahemin BIODE [6]. The two characteristics are plotted as afunction of the biased voltages applied to the electrodes.
Interestingly, both characteristics took a peak above V=4 V.
Similar behavior was observed in other samples, suggestinga voltage-induced transition. Hereinafter, we call the state
before the transition as the low-voltage (LV) state and thestate after the transition as the high-voltage (HV) state,
respectively. The voltage-induced transition was irreversible.
In order to examine the origin of the transition, we
measured the EL spectra in both LV and HV state. Figure 2
shows the EL spectra [6]. The EL spectrum of hemin in the
LV state is composed of several narrow bands, while thespectrum in the HV state exhibits broad peaks. The spectralfeatures in the HV state are very similar to those ofcytochrome c and myoglobin.
The spectral changes associated with the applied voltages
remind us of the transition from a low-spin state into ahigh-spin state in the compounds containing iron porphyrins.
The native form of cytochrome c is a low-spin compound ofiron. Both the high-spin and low-spin states are known inmyoglobin. Hemin is a high-spin five-coordinate compoundof iron. However, the spin state of these compounds is very

Fig. 2. The EL and absorption spectra of hemin. The EL spectrum inthe low-voltage state (red curve) is attributable to hemin in thehigh-spin state, while that in the high-voltage state (blue curve) is
associated with hemin in the low-spin state [6].

subtle and easily changes when the substitution or additionof ligands occurs. Thus in order to ascertain the spin state of
the compounds used for the device fabrication, we measured
the magnetic susceptibility, Raman spectra of the compounds[6]. After the measurements, we concluded that the broadfeature of the electroluminescence is characteristic for iron
porphyrin in the low-spin state, and that hemin in the LV
state is in the high-spin state while hemin in the HV state isin the low-spin state.
References[1] P. S. Vincett, W. A. Barlow, R. A. Hann, and G. G. Roberts, Thin
Solid Films 94, 171 (1982).
[2] H. Tajima, S. Ikeda, M. Matsuda, N. Hanasaki, J. Oh, and
H. Akiyama, Solid Sates Commun. 126, 579 (2003).
[3] H. Tajima, S. Ikeda, K. Shimatani, M. Matsuda, Y. Ando, J. Oh,
and H. Akiyama, Synth. Met. 153, 29 (2005).
[4] K. Shimatani, H. Tajima, T. Komino, S. Ikeda, M. Matsuda,
Y. Ando, and H. Akiyama, Chem. Lett. 34, 948 (2005).
[5] S. Ikeda, H. Tajima, M. Matsuda, Y. Ando, and H Akiyama, Bull.
Chem. Soc. Jpn. 78, 1608 (2005).
[6] H. Tajima, K. Shimatani, T. Komino, M. Matsuda, S. Ikeda,
Y. Ando, and H. Akiyama, Bull. Chem. Soc. Jpn. 79, 549 (2006).
Authors
H. Tajima, K. Shimatani, T. Komino, M. Matsuda, S. Ikeda, Y. Ando,and H. Akiyama

Competition between"Superconducting" State and"Checkerboard-type Charge-ordered"State of New Organic Superconductors;
β -(DMBEDT-TTF)2X (X = PF6, AsF6)
Mori and Uwatoko Groups

Fig. 1. (a) Current-voltage, and (b) EL intensity-voltage characteristics
of an ITO/hemin/Al junction. The inset shows the molecular structureof hemin. The broad peak around 4.5 V is due to the voltage-induced
transition [6].

So far, the organic superconductors, whose superconducting state is competitive with magnetic order like
antiferromagnetic state or SDW state, have been intensively
studied for κ-type ET salts and Bechgaard TMTSF salts.
However, an organic superconductor, whose superconducting(SC) state is competing with a charge-ordered (CO) state isvery rare [1]. Therefore, its superconducting mechanism has
not been clarified experimentally yet, and only theoretical
proposal submits the competitive superconducting symmetry
between singlet and triplet states [2].

ISSP Activity Report 2005

3

Fig. 1. Checkerboard-type charge-ordered state for β-(meso-DMBEDTTTF)2PF6. The ~+0.7 charge rich (r; red) and the ~+0.3 charge poor(p; blue) molecules are arranged as a fashion of “rrpp” in the column.

On the way to apply the chemical negative pressure
to ET salts by the introduction of the steric hindrance partto ET molecule, Mori group has discovered the newpressured-induced organic superconductors of β-(mesoDMBEDT-TTF)2X (X = PF6 [3] and AsF6). At an ambientpressure, the metal-insulator (MI) transition occurs at 90 K
for the PF6 and AsF6 salts. The magnetic susceptibility,
however, follows the Heisenberg model with J = 105 K
even in the metallic region without the anomaly at the MI
transition at 90 K. In order to clarify the insulating state
below 90 K, the crystal structure analysis by utilizing
synchrotron X-ray radiation has been performed at 11.5 K.
As a result, the checkerboard-type CO state was exhibited asshown in Fig.1 [4]. The ~+0.7 charge rich (r) and the ~+0.3
charge poor (p) molecules are arranged in the manner of“rrpp”. This checkerboard-type CO pattern could not be
explained only by the intermolecular Coulomb repulsion (V),but also by the electron-phonon interaction and moleculardegrees of freedom.
By applying pressure to the CO state, the MI transition
temperatures are lowered and the superconducting transitions
were observed at the onset temperature of resistance drop
Tc = 4.3 K under 4 kbar for the PF6 salt and Tc = 4.3 K under
3.8 kbar for the AsF6 salt. The characteristic features are theresistivity upturn observed just above the resistance dropfrom 1.9 to 9.0 kbar. By applying magnetic field under
3.8 kbar, the temperature of resistivity drop (Tc) is lowered
and the resistivity upturn is extended to lower temperatures
owing to the suppression of superconductivity. Moreover,the magnetic-field-induced insulating states emerged below
13 K, indicating not only that the SC state is competitive
with the CO state, but also that the SC state is deeply related

Fig. 3. Electronic phase diagram of β-(meso-DMBEDT-TTF)2AsF6,
indicating the competition between superconducting and chargeordered states.

to the CO state as shown in Fig. 2. Under 10.5 to 15 kbar,
the metallic behavior is observed down to 0.5 K and thetemperature dependence follows T n with an increase ofn from 1.3 (9 kbar) to 1.7 (15 kbar). It implies that theelectronic state changes from non Fermi liquid at theboundary between the superconducting and the metallic
region to the Fermi liquid in the metallic region under highpressure. The elucidated electronic phase diagram as shownin Fig. 3 demonstrates that the CO state covers the SC statein the large pressurized region.
These salts β-(meso-DMBEDT-TTF)2X (X = PF6 [3] andAsF6) which show the superconductivity under the moderatepressure are suitable and promising materials to clarify
the nature of CO suppressed superconductivity and thecompetition between SC and CO states.
References[1] H. Mori, J. Phys. Soc. Jpn. 75, 51003 (2006).
[2] Y. Tanaka, Y. Yanase, and M. Ogata, J. Phys. Soc. Jpn. 73, 2053
(2004).
[3] S. Kimura, T. Maejima, H. Suzuki, R. Chiba, H. Mori,
T. Kawamoto, T. Mori, H. Moriyama, Y. Nishio, and K. Kajita, Chem.
Commun. 2454 (2004).
[4] S. Kimura, H. Suzuki, T. Maejima, H. Mori, J. Yamaura,
T. Kakiuchi, H. Sawa, and H. Moriyama, J. Am. Chem. Soc. 128, 1456
(2006).
Authors
H. Mori, S. Kimura, R. Chibaa, H. Suzuki, J. Yamaura, M. Koeda,
M. Hedo, Y. Uwatoko, T. Kawamotob, T. Morib, Y. Nishioa, and
K. Kajitaa
aToho University
bTIT

Control of the Spin State of a Light
Atom by Changing the Density ofSurrounding Conduction Electrons
Takada Group

Fig. 2. Superconducting state and the magnetic-field-induced insulatingstate for β-(meso-DMBEDT-TTF)2AsF6.

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ISSP Activity Report 2005

Interests in spintronics are on the rise from both scientificand technological points of view. Since devices in spintronics involve active control and manipulation of spin degrees
of freedom in solid-state systems, it is absolutely necessary
to have a deeper understanding of fundamental interactions
between electron spins and its solid-state environments. Inview of this situation, we are interested in a composite system of an atom immersed into the otherwise homogeneousFig. 1. Calculated total spin of the pseudoatom versus the EG density
parameter rs. Solid curves are the fittings of the data with the universal(rs-rsc)0.5 relation.

electron gas (EG).
In an isolated atom, the ground state obeys the Hund’s
multiplicity rule that requires the highest spin configurationcompatible with the Pauli’s exclusion principle. Physicallythis rule is interpreted as the consequence of an effectively
larger nuclear charge in a higher spin configuration due
essentially to the exchange effect.
Similarly in a uniform EG, the same effect favors spinpolarization, bringing about the spontaneous spin-symmetry
breaking or the spin-density-wave state which was proven to
be the ground state at arbitrary electron densities within the
Hartree-Fock (exchange only) approximation. The correlation effect, however, acts in the opposite direction and thiseffect is so strong in an EG as to lead eventually to the paramagnetic ground state for the majority of metals.
We have studied the ground state of the composite system (or pseudoatom) in the spin-density functional theory [1]and found several intriguing features such as (i) sharp transition from a spin-neutral state to a spin-polarized one with thedecrease of the electron density of the EG, if the immersed
atom is either B, C, N, or O; (ii) smooth evolution from thespin-polarized state of the pseudoatom to that of the negative
ion of the corresponding isolated atom with the further
decrease of the density; (iii) formation of the spherical combined spin density/charge density wave, which slowly decays
with the distance from the immersed-atom site; and (iv) significant shrinkage of the size of the spin-polarized
pseudoatom as compared with its spin-neutral counterpart.
In order to elaborate the first two points, we show the calculated total spin S of the ground-state pseudoatom as a function of rs (the conventional parameter characterizing the electron density of the enveloping EG) in Fig. 1, which clearly
indicates a spontaneous magnetization of the pseudoatom forrs larger than some threshold value rsc. The net electronic
spin S depends on both rs and the atomic number of the atom
Z. This finds itself in contrast with the result for the net electronic charge around the atom, which is uniquely determinedby Z due to the full screening of a charge in the EG.
The steep fall in S near the critical point in Fig.1 issuggestive of a “phase transition” of the second order. In
fact, S is found to exhibit a universal behavior in accordancewith “the mean-field theory” or in proportion to (rs-rsc)0.5
for rs near rsc.
Generally, the obtained S is not a multiple of 1/2. This
fundamental difference between an isolated atom and thispseudoatom is brought about by the contribution of an infi-

nite number of delocalized electrons in the latter, implying acomplicated many-body nature in this atom-EG composite
system.
The trend in S at low densities (large rs) has a clear qualitative interpretation: Because of the positive electron affinity(EA) of the B, C, and O isolated atoms (0.010, 0.046, and0.054 a.u., respectively), each atom immersed into the EG in
the limit of zero density should be reduced into a negativeion (NI) of the corresponding atom. According to the Hund’srule, the population of the 2p orbitals is with 2 electrons withspin up (3P), 3 electrons with spin up (4S), and 3 electrons
with spin up and 2 electrons with spin down (2P) for B-, C -,and O- ions, respectively, corresponding to the total spin of
1, 3/2, and 1/2, respectively, which is clearly satisfied in Fig.
1 at large rs. On the other hand, the NI of the N atom is
unstable although long living (EA=- 0.003 a.u.), and the slow
growth of S of this pseudoatom between 1 and 3/2 at large rs
can be understood as the competition between the NI 3P andatomic 4S states.
Our model might be too primitive to discuss actual systems, but because of the sharpness of the transition in Fig. 1,
we may suggest a novel method of spin manipulation bychanging the carrier density in a system across the threshold.
For example, we may think of an impurity atom in an appropriate semiconductor host, where the carrier density in thesemiconductor will be altered by either applying the gate
voltage in the FET (field-effect transistor) structure or opticalpumping.
References[1] V. U. Nazarov, C. S. Kim, and Y. Takada, Phys. Rev. B 72, 233205(2005).
Authors
V. U. Nazarova, C. S. Kima, and Y. Takada
aChonnam National University

Ab Initio Simulation of Biased
Surfaces/Interfaces
Sugino Group
Applying bias voltage is an effective means to modify
surface properties as has been exemplified by recent
scanning tunneling microscopy (STM) and electrochemical
experiments. The bias of up to several eV can be applied to
the region between the surface and the STM tip, typicallyless than 10 Å thick, inducing large electric field and surface
charges. Even larger field and charge are induced when thesurface is faced with the water solution because of theelectric double layer effect. By precisely measuring the

Fig. 1. A snap shot of the first-principles molecular dynamics
simulation for the biased platinum-water interface done at the room
temperature. The effective screening medium (ESM) with infinite ε isplaced in the region painted with blue. Surface charge of about 1/150[C/surface atom] is induced because of the applied bias.

ISSP Activity Report 2005

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6

ISSP Activity Report 2005

Authors
M. Otani, Y. Yoshimoto, and O. Sugino

Resistively-Detected NMR
in Quantum Hall Systems
Iye and Katsumoto Groups
Quantum Hall (QH) systems, i.e., two-dimensional (2D)
carrier systems in semiconductor heterostructures under
the condition of high magnetic field and low temperature,
exhibit a wide variety of many body ground states dependingon the Landau level (LL) filling factor ν, including integer
and fractional quantum Hall liquids, Wigner solids, stripe
and bubble phases, and composite fermion liquids. In recent
years, phenomena associated with the spin degree of freedom
have attracted much attention. They include, spin phaseBxx

where the first two terms are kinetic and exchangecorrelation energy for the electrons and the last two terms areelectrostatic energy for the electrons, nuclei, and electrolyteions (ions in the water solution). This is different fromthe conventional total-energy functional in that the local
permittivity ε appears here to represent the effective
screening medium (ESM). Importantly, the equations derived
from the total-energy functional, i.e., the Poisson and the
Kohn-Sham equations, usually have a structure in our target
systems which can be solved without introducing theperiodic boundary condition (PBC) in the surface normal
direction. In this case, contrary to the conventional scheme,
Green's function technique needs to be introduced for the
Poisson equation but the computational demand required
additionally is almost negligible.
The effective screening medium (ESM) approach thus
enables us to handle surfaces even when biased and charged

References[1] M. Otani and O. Sugino, Phys. Rev. B 73, 115407 (2006).

xx

electronic and geometric response to this extreme condition,much experimental effort has been made to reveal undiscovered properties of the surface.
Introducing bias voltage to the first-principles simulation
is a rather difficult problem because the simulation
techniques for the surfaces have been developed mainly
assuming the periodic boundary condition (PBC), which isincompatible with the biased surfaces. The induced charge isalso incompatible because the electrostatic energy diverges
unless the screening effect of the STM tip or the water
solution is properly taken into account. To overcome these
problems without increasing computational demand of thesimulation, we developed a new approach to the biased
surfaces/interfaces, called effective screening medium(ESM).
In this approach, the system is represented by the surface
slab model that is in contact with a continuum represented
by the local permittivity, ε, and by the distribution ofclassical ions in the solution, i.e., the electrolyte ions. For
example, the STM tip is modeled by using infinite ε andthe water solution is modeled by using ε=78.4 and the
Poisson-Boltzmann (PB) distribution of the classical ions.
Importantly, owing to the screening effect of the continuum,
charged surface can be handled without the electrostatic
energy divergence, and also, the vacuum level can be
unambiguously determined to define the bias voltage inaccordance with the experimental conditions.
The total-energy functional is then2f (r)K 8 teB + E xc8 teB - # drdV (r) + # dr ttot (r) V (r) ,8r

xx

Fig. 2. Potential energy averaged over the surface parallel directions.

up, when the counter electrodes, e.g., the STM tip or thewater solution, can be modeled by the continuum. We havesuccessfully applied the ESM to organic molecules on thesilicon surface, carbon nanotubes on the backgate, and theplatinum-water interfaces with help of collaborators. Figure 1
is a snapshot for the first-principles molecular dynamics
simulation for the biased platinum-water interface. The Pt
surface layers and water molecules are represented within
the generalized gradient approximation (GGA) for thedensity-functional theory (DFT) and the effective screening
medium (ESM) with infinite ε is placed beyond the water.
The potential profile, shown in Fig. 2, shows that thepotential gradient has a finite and almost constant value
because of the dielectric screening of the water but is zero in
the substrate because of induced surface charge and strong
metallic screening of the platinum. With intensive use of the
ISSP supercomputers and the Earth Simulator (ES), thesimulation is now performed to elucidate the electrolysis ofwater, which is the most fundamental electrochemical
reaction and the microscopic detail has been unknown yet.

Fig. 1. (a) Magnetoresistance of high mobility 2DEG sample showing
hysteretic behavior at around ν=2/3 associated with the spin transitionof the quantum Hall system. Resistively detected nuclear magnetic
resonance signals for the 69Ga and and 75As nuclei at ν=2/3 are shownin (b) and (c).

Authors
K. Kodera, H. Takado, A. Endo, Y. Hashimoto, S. Katsumoto, andY. IyeQuantum Dot with Zero Electron
Still Causes the Fano EffectKatsumoto Group

Fig. 2. The inset shows an anomalous “dispersive” lineshape of
RDNMR in the vicinity of ν=1. The main panel is a color-scale plot
showing the evolution of the anomalous RDNMR lineshape as afunction of ν.

transition of fractional quantum Hall states, spin polarizationof composite fermions, skyrmion excitations in quantum
Hall ferromagnets, and interaction between electron andnuclear spins. A dramatic effect involving nuclear spins
has been observed in 2D electron system (2DES) at ν=2/3,
where the effect manifests itself as a hysteretic behavior ofmagnetoresistance with a very long relaxation time scale[1].
The ground state can be spin-polarized or spin-unpolarized
depending on the magnetic field range where this state
occurs. When the two spin states are nearly degenerate,domains of different spin polarization states may be formed.
When the energy difference between the different electron
spin states is comparable to the nuclear Zeeman energy, theelectron-nuclear spin flip process via hyperfine coupling isenergetically allowed.
Figure 1-(a) shows the magnetoresistance trace of a highmobility 2DEG sample, which shows hysteretic behavior at
ν=2/3, when it occurs at relatively low magnetic fields(B<8T). Shown in Figs. 1-(b) and (c) are the resistively
detected nuclear magnetic resonance (RDNMR) signals the69Ga and and 75As nuclei in this regime.
With appropriate tuning, RDNMR signals can bedetected over a rather wide range of filling factors[2], though
the signal intensity varies significantly. Another QH regime
where RDNMR signal can be conspicuously detected is thevicinity of ν=1 where skyrmions (spin textures) constitute
low-lying excitations. In this regime, anomalous “dispersive”
RDNMR lineshape, whose origin is not completely
understood, is observed. The dispersive lineshape is foundto evolve into an ordinary one as we move further away from
ν=1. The temperature dependence of the nuclear spin-lattice
relaxation rate 1/T1 in the latter regime is found to be
Korringa-like, while that in the range of ν where anomalouslineshape is observed is larger in magnitude and increases
with decreasing temperature. The implication of theseanomalous behaviors for the skyrmion physics is currently
explored.
References[1] S. Kronmueller et al., Phys. Rev. Lett. 81, 2526 (1998); ibid. 82,
4070 (1999).
[2] K. Hashimoto et al. Phys. Rev. Lett. 88, 176601 (2002); W. Desrat
et al., Phys. Rev. Lett. 88, 256807 (2002).

A quantum dot (QD) is often defined as a "droplet" ofelectrons. Here we would like to show, however, thata potential dip with no electron also works as a QD in
interference circuits. A quantum wire with a side-coupled
QD is a simplest circuit to cause quantum interference, thatis, the interference between the direct path through the wire
and the path with reflection to the dot. Because squeezing of
the dot by surrounding depletion layer pushes it to the wire,
the number of electrons can be reduced to zero without
reducing the coupling strength between them. This featureenables us to explore the interference effect around resonance points, i.e., the Fano effect in QDs with few electrons[1].
The inset of Fig.1 shows a micrograph of the sample. We
measured the conductance of the wire as a function of thegate voltage Vw, which suppresses the width of the wire andthe gate voltage Vg, which controls both the dot size andthe electrostatic potential of the dot. As is well known, theconductance of the wire is approximately quantized versus
Vg and the interference effect appears as small deviation fromsuch stepwise variation. Figure 1 displays the variation ofwire conductance from the stepwise baseline in a colored
plot as a function of Vw and Vg.
In the transition regions between the conductance steps,
the response to Vg is a mixture of the interference effect andthe effect of changes in the electrostatic potential andinevitably has large amplitudes. The regions with small
amplitudes indicated by Qm's are on the quantized plateaus
and pure interference effect is observed. There verticallyelongated blue-red stripes are observed, which indicate theFig. 1. Color plot of variation in conductance of a quantum wire with aside-coupled quantum dot. Quantized stepwise conductance variation issubtracted as the baseline. Qm is the height of "base" conductance stepand n is the number of electrons in the dot counted by starting at n =0
region.

ISSP Activity Report 2005

7

Fano effect around the Coulomb peaks of the dot. The
distances between them in Vg axis thus reflect the "additionenergy" of an electron to the dot.
We see no such vertical structure in the region captioned
as "n=0", where no electron exists in the dot. This is
supported by several facts, e.g., disappearance of strong
signal in the plateau-plateau transition regions. Now we cancount the number of the electrons in the dot starting from thezero-electron state and then find that the spacing between the
Coulomb peaks is apparently wider at "n=2" and "n=6".
They correspond to closed shells of a two-dimensional
isotropic harmonic potential, that is, N(N+1) (N=1,2). Thismeans the electrons are confined with a comparatively
circular potential.
Then why we still see blue-red structures (not vertical but
slanted to Vg axis) in the region n=0? Part of electron
wavefunction goes even into the empty dot and causes someresonance and interference. Hence an empty QD can cause
the Fano effect.
References[1] K. Kobayashi, H. Aikawa, S. Katsumoto, and Y. Iye, Phys. Rev.
Lett. 88, 256806 (2002).
Authors
T. Otsuka, Y. Iye, and S. Katsumoto

Switching Magnetization of
Nano-Scale Ferromagnetic Particle
by Non-Local Spin Injection
Otani Group
Spin-dependent transport properties in ferromagnetic/
nonmagnetic hybrid structures have drawn much attention
owing to the novel idea to utilize the spin angular momentum
for future spintronic devices. Instead of the conventional
inductive recording method, the spin angular momentum

Fig. 1. (a) Schematic illustration of the non-local spin injection using
lateral spin-valve geometry. Only spin current is injected into the small
ferromagnetic particle. (b) Scanning electron microscope image of thefabricated lateral spin valve. The device consists of a large Py injector
30 nm in thickness, a Cu cross 100 nm in width and 80 nm inthickness, and a Py nano-scale particle, 50 nm in width, 180 nm inlength and 6 nm in thickness.

8

ISSP Activity Report 2005

Fig. 2. (a) Field dependence of the non-local spin signal. The resistancechanges at low and high fields correspond to the relative magnetic
switching of the Py injector and particle, from parallel to anti-parallelstates and vice versa. (b) Non-local spin valve signal after the pulsed
current injection as a function of the current amplitude with corresponding magnetization configurations.

transfer of conduction electrons is now employed to switch
the magnetization. The switching mechanism due to the spintransfer can be understood by considering the spin torquewhich is proportional to the injected spin current. So far,most of the current studies related to the spin torque are
performed using vertical multilayered nanopillars in which
two magnetic layers are separated by a nonmagnetic metal
layer. In such structures, the charge current always flows
together with the spin current, thereby undesirable Joule heat
and Oersted field are generated.
Our recent experiments revealed that the spin currents
induced by the non-local spin injection are effectively
absorbed into an additionally connected ferromagnet becauseof the short spin diffusion length. There is therefore a highpossibility that the spin current without a charge flow is
selectively injected into a ferromagnetic small particle andswitch the magnetization of the small ferromagnetic particle.
To test this idea, we fabricated a lateral spin valve including
a nano-scale Permalloy (Py) particle as shown in Figs. 1 (a)and (b).
To confirm that the spin current from the Py injector
is injected into the Py particle, the non-local spin valve
measurements are performed. As in Fig. 2 (a), the fielddependence shows a clear spin-valve signal with themagnitude of 0.18 mΩ, assuring that the spin current reaches
the Py particle.
Then, we examine the effect of the non-local spin
injection into the Py particle with using the same probe configuration. Before performing the non-local spin injection,
the magnetization configuration is set in the anti-parallel
configuration by controlling the external magnetic field. The
non-local spin injection is performed by applying large
pulsed currents up to 15 mA in the absence of magnetic field.
As shown in Fig. 2(b), when the magnitude of the pulsed
current is increased positively in the anti-parallel state, nosignal change is observed up to 15 mA. On the other hand,for the negative scan, the abrupt signal change is observed

at -14 mA. The change in resistance at -14 mA is 0.18 mΩ,
corresponding to that of the transition from anti-parallel toparallel states. This means that the magnetization of the Py
particle is switched only by the spin current induced by the
non-local spin injection. The responsible spin current for
switching is estimated from the experiment to be about
200µA, which is reasonable compared with the values
obtained for conventional pillar structures.
References[1] T. Kimura, Y. Otani, and J. Hamrle, Phys. Rev. B 72, 014461(2005).
[2] T. Kimura, Y. Otani, and J. Hamrle, Phys. Rev. Lett. 96, 037201(2006).
Authors
T. Kimura and Y. Otani

Remote Atomic Manipulation of Dimer
Buckling through Carrier Propagationin the Surface States
Komori GroupElectronic excitation induced by tunneling electrons inscanning tunneling microscopy (STM) often moves theatoms under the tip. On the clean Ge(001) surface, it locallyinverts the buckling orientation of the Ge dimers without
cutting the Ge bond below 80 K [1]. Moreover, we canreversibly control the orientation to change the area of local
c(4×2) or p(2×2) structures by selecting the sample bias
voltage Vb in the STM operation. The two different local
structures coexist on the same surface when Vb is between- 0.6 V and 0.7 V. In the other range of Vb, one of the two
structures covers the whole surface when it is scanned by the
STM tip. The mechanism of the transformation is explainedby a coherent climbing of the vibrational state of the dimerby electronic excitation [2].

Fig. 2. STM images showing one-dimensional motion of the topological defect on the Ge(001) surface by pulse current injection. Each timeafter the injection of electrons from the STM tip at the point of thegreen circle, the defect moves in the dimer-row direction. The center of
its random walk is 8 nm apart from the point of the electron injection
by the bias voltage pulse to 0.8 V. The center position separates further
with increasing the bias voltage during the electron injection. When we
decrease the bias voltage below 0.7 V, the defect comes just under the
STM tip.

The electron tunneling from or to the STM tip remotely
creates and moves a metastable topological defect betweenc(4×2) and p(2×2) shown in Fig. 1. Hot electrons or holes
created in the Ge surface states by the tunneling propagate
along the surface, and induce the defect motion in the dimerrow direction through the electron-lattice interaction, as
demonstrated in Fig. 2. The direction of its motion depends
on the electric field due to STM bias through the electric
dipole of the Ge dimer. We have measured the rate of thedefect motion induced by tunneling electrons as a function of
the distance and the direction between the defect and thetunneling point, and the bias voltage during the pulse. The
threshold distance of the motion rapidly increases withincreasing the absolute value of the bias voltage for the bothpolarities. It is much longer in the dimer row direction thanin the dimer direction for the transformation by electron
injection. On the other hand, the distance is almost isotropic
for the transformation by hole injection. The difference
is attributed to that in the lifetime of the excited carriersin the surface electronic states on the basis of the bandcalculation [3].
References[1] Y. Takagi, Y. Yoshimoto, K. Nakatsuji, and F. Komori, J. Phys.
Soc. Jpn. 72, 2425 (2003).
[2] H. Kawai and O. Narikiyo, J. Phys. Soc. Jpn. 73, 2362 (2004) .
[3] K. Nakatsuji, Y. Takagi, F. Komori, H. Kusuhara, and A. Ishii,
Phys. Rev. B 72, 241308(R) (2005).
Authors
Y. Takagi, K. Nakatsuji, Y. Yoshimoto, and F. Komori

A Compact UHV System for Fabricationand in situ Analysis of Electron Beam
Deposited Structures using a Focused
Low Energy Electron BeamYoshinobu Group
Fig. 1. (a) Schematic model of the topological defect between c(4×2)
and p(2×2) areas on the Ge(001) clean surface. (b) STM images of thedefect for two sample bias voltages. The image area is 6 nm × 6 nm. (c)Simulated STM images including the defect(red circle) on the basis of
the first-principles calculation.

A UHV system has been developed in order to fabricate
and in situ analyze micro- and nanostructures. The system
includes a low energy electron gun which provides a minimum
spot size of ~25 nm in a diameter using electrostatic lenses, acylindrical mirror analyzer for Auger electron spectroscopy

ISSP Activity Report 2005

9

Fig. 1. Top view of the present UHV system including a focusedelectron beam source, a CMA, a QMS, an MCP-LEED and a µ-metalshield.
Fig. 3. AES spectra of the Fe M2,3VV and SiL2,3VV regions for(a) clean surface (b) after electron-induced deposition of Fe(CO)5,
at the position of electron beam irradiation (c) at the position of
non-irradiation. The temperature of the Si(100) substrate was 80 K.

Fig. 2. The process diagram of electron-induced deposition of Fe(CO)5.
(a) Fe(CO)5 molecules are physisorbed on the Si(100) surface below
80 K. (b) The Fe(CO)5 multilayer is irradiated by an electron beam
area-selectively. (c) Intact Fe(CO)5 molecules are desorbed by heating
up to 150 K. (d) An iron artificial structure is formed at the electron
beam irradiated position.

(AES) and electron energy loss spectroscopy (EELS), a
low energy electron diffraction (LEED) instrument, and ascintillation counter for scanning electron microscopy (SEM)
as schematically shown in Fig. 1 [1]. Using the electron gun,we can analyze electronic states of microscopic structure. In
addition, we also fabricate submicron structures artificiallyby means of scanning a focused low-energy electron beam.
In order to demonstrate the performance, we fabricatedan iron microstructure by electron beam induced
deposition of iron pentacarbonyl (Fe(CO)5), which wascharacterized by in situ AES, EELS and LEED analysis.
The details of the process are as follows: (a) an Si(100) cleansurface is prepared; (b) gaseous Fe(CO)5 molecules aredeposited on the Si(100) clean surface at 80 K; (c) theFe(CO)5 multilayer on the Si(100) clean surface is irradiatedby a focused electron beam by controlling the beam positionand scanning speed; (d) the Si substrate is heated to 150 K
in order to desorb intact Fe(CO)5 molecules which havenot been irradiated by an electron beam. The diagram of thisprocess is summarized in Fig. 2.
Figure 3 shows AES spectra of Fe M2,3VV and SiL2,3VV
for (a) clean surface (b) after electron-induced deposition ofFe(CO)5, at the position of electron beam irradiation (c) at
the position of non-irradiation. These spectra were taken
after desorption of the remaining intact Fe(CO)5 by heating.
At the electron-irradiated position, the Si L2,3VV peak at
92 eV is decreased in intensity, and an Fe M2,3VV peak
appears at 47 eV (Fig. 3(b)). On the other hand, at the
non-irradiated position, only an Si L2,3VV peak is observed

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ISSP Activity Report 2005

after the desorption of intact Fe(CO)5 (Fig. 3(c)). Thus, theFe exists only at the electron beam irradiated position. In
addition, the newly formed structure is metallic Fe judging
from the Auger peak position. Note that metallic Fe andFe(CO)5 can be clearly distinguished in their energies; theAuger peak is observed at 37 eV for the Fe(CO)5 molecules
and at 47 eV for metallic Fe, respectively.
Figure 4 shows an example of a fabricated submicron
iron structure. The size (including film thickness) and theposition of microstructures are controllable with deposited
molecules and electron beam irradiation. The present low
energy electron beam system with electrostatic lenses is very
useful for electron spectroscopy of artificial microstructureson surfaces.
References[1] Y. Kakefuda, Y. Yamashita, K. Mukai, and J. Yoshinobu, Rev. Sci.
Inst. 77, 053702 (2006).

Fig. 4. (a) An SEM image after Fe deposition by an electron beamirradiation at 80 K followed by heating to 150 K. The area of this image
is 3×3 µm2. The total dose of electrons is 2×10-14 C on the Fe(CO)5
multilayer (600 L). (b) An SEM line profile. The full width at half maximum of this iron structure is 190 nm.

Authors
Y. Kakefuda, Y. Yamashita, K. Mukai, and J. Yoshinobu

Real-Space Observation of Screened
Potential by Scanning Tunneling
Spectroscopy
Hasegawa GroupThe electrostatic potential around a single charge invacuum is described with the Coulomb potential. If thecharge is situated in a metallic electron system, the Coulomb
potential is modified by electrons in the system. The
modification of the potential, called screening, is one of thefundamental phenomena in the condensed matter physics.
Using scanning tunneling microscopy / spectroscopy (STM/S)
we developed a method for measuring electrostatic potential
in high spatial and energy resolutions, and performed areal-space observation of the potential around external
charges screened by two-dimensional surface electron gas(2DEG). In the real-space potential mapping, characteristic
decay and oscillation in the potential, so-called, the Friedel
oscillation, were clearly visible around the charges [1].
As a sample having a two-dimensional surface electron
system, we used the Si(111)- 3 × 3 -Ag surface. The
electron standing wave patterns [2] are observed intunneling conductance (dI/dV) images and an energy
dispersion curve obtained from the images exhibits an
isotropic and parabolic shape, indicative of free-electron likebehaviors of the electrons in the states. It has been known
that when the surface potential is changed, for instance, byadditional Ag adsorption on the surface, an energy level ofthe surface states also shifts accordingly. Therefore, by

measuring the energy level of the surface states locally byusing STS, one can obtain a mapping of the potential on thesurface.
The obtained potential profiles around Ag adsorbates andstep edges, where the Ag adsorbates are accumulated, werefitted well with theoretically predicted potential profiles, as
shown in Fig. 1. In the area marked with a rectangle on the
STM image of Fig. 1, the tunneling spectra were measured
from the terrace to the step edge, and the obtained spectra areplotted in a color-coded manner in the middle of Fig. 1. In
the spectra a peak around -0.85eV is due to one of theelectronic states of the surface, called S2/S3 states. The
binding energy of the S2/S3 state is found to shift to thehigher binding energy as the measured position approaches
to the step edge. The amount of the shift as a function of a
distance from the step edge is plotted in the lower panel ofFig. 1. This is the electrostatic potential profile around thestep edge of the surface, we thought.
In order to confirm our speculation, we calculated an
expected potential profile on the surface. Assuming the stepedge has positive charges, which is due to the adsorbed Ag
adatoms on the step edge, the potential around the step edge
should behave like a Coulomb potential. Since the surface
has 2DEG, the Coulomb potential is modified or "screened"by the 2DEG electrons. The screened potential can becalculated from a dielectric function of 2DEG, which wasderived from the Lindhard theory. The calculated screened
potential is fitted to the observed profile in the lower panel ofFig. 1, showing good agreements.
A wiggling feature in the screened potential is observedin the figure and called Friedel oscillation. The oscillation
should have a period of the half Fermi wavelength of
the 2DEG since it arises from a sharp cutoff in the 2DEG
dielectric function at the corresponding wave number. From
careful comparison, we found that the oscillation period issame as that of the electron standing wave at the Fermi
energy, which is also a half Fermi wavelength. These results
confirm that the observation of the screened potential and theFriedel oscillation from the STS.
References[1] M. Ono, Y. Nishigata, T. Nishio, T. Eguchi, and Y. Hasegawa,Phys. Rev. Lett. 96, 016801 (2006).
[2] Y. Hasegawa and Ph. Avouris, Phys. Rev. Lett. 71, 1071 (1993)
Authors
M. Ono, Y. Nishigata, T. Nishio, T. Eguchi, and Y. Hasegawa

Insulator-Metal-Superconductor
Transitions Induced byElectric Field EffectLippmaa Group

Fig. 1. (upper): an STM image (40×40nm2) on the Si(111)- 3 × 3 -Ag
surface. (middle): A series of tunneling spectra taken in the rectangulararea are shown in a color-coded manner. The -0.85eV peak shifts tohigher binding energy as approaching to the step edge. (bottom): thepeak energy shift as a function of distance from the step edge. It can be
fitted well by the screened potential.

The physical properties of many oxides depend in asensitive way on the concentration of charge carriers. The
phase diagrams of cuprate superconductors and colossal
magnetoresistance manganites have been studied inparticular detail. Our work is aimed at controlling the carrier
density in thin oxide films by field effect, rather thanby chemical doping. Field-effect devices can be used toinduce sufficient effective doping levels in thin layers tocause phase changes, such as metal-insulator or normalsuperconducting transitions. Some of the device structures

ISSP Activity Report 2005

11

that we are using are shown in Fig. 1. The biggest challenge
is to grow very thin epitaxial layers and heterointerfaces ofoxides with sufficient quality for observing doping-related
phase changes. So far, we have focused on structures basedon undoped SrTiO3 and Nb-doped SrTiO3.
An oxide field-effect transistor (FET) is illustrated inFig. 1(a). A SrTiO3 substrate was used as the active channelmaterial and a wide-gap insulator, CaHfO3, was used as agate insulator. The channel-insulator interface was grown
epitaxially to reduce trapping [1]. SrTiO3 is suitable forobserving phase changes caused by field-effect doping,because the normally insulating material can become
metallic and even superconducting at very low carrier
densities of about 1020 cm-3. Indeed, by applying a gate bias
to a SrTiO3 FET, we can observe an increase of channel
current with reducing temperature, as shown in Fig. 2(a). The
transition into a metallic state occurs at a gate bias of 1.3 V.
Below this bias, the device is in an insulating state, with thechannel current dropping at lower temperatures.
A different route to field-effect doping is illustrated
in Fig. 1(b). In this case the gate electrode is replacedwith a ferroelectric thin film Pb(Zr,Ti)O3, which can bepoled locally with the conducting tip of an atomic-force
microscope (AFM) [2]. By choosing the polarity of thepoling voltage, it is possible to either accumulate or deplete
carriers in the thin Nb-doped SrTiO3 layer. Doped SrTiO3
becomes superconducting below about 300 mK. The transition temperature is a function of the number of carriers,
which we modulate by poling the ferroelectric cover layer.
The device is patterned as a usual Hall bar, allowing varioustransport measurements to be made. Only the region shownin red in Fig. 1(b) is poled by AFM.
The effect of poling the Pb(Zr,Ti)O3 layer in opposite
directions on the superconducting transition temperature ofa thin Nb:SrTiO3 layer can be seen in the bottom panel ofFig. 2. A clear shift to higher temperature can be seen in the
P- state, which corresponds to an increase of carrier density.
If the temperature is fixed at about 260 mK, local switchingof superconductivity is possible.

Fig. 2. (Top) The channel current of a SrTiO3 FET as a function of gatebias and temperature. The channel becomes metallic above a gatebias of 1.3 V. (Bottom) The shift of the superconducting transition
temperature of a Nb:SrTiO3 layer, caused by accumulating (P- state) or
depleting (P+ state) carriers from the film.
References[1] K. Shibuya, T. Ohnishi, T. Uozumi, T. Sato, M. Lippmaa,
M. Kawasaki, K. Nakajima, T. Chikyow, and H. Koinuma, Appl. Phys.
Lett. 88, 212116 (2006).
[2] K. S. Takahashi, M. Gabay, D. Jaccard, K. Shibuya, T. Ohnishi,
M. Lippmaa, and J.-M. Triscone, Nature 441, 195 (2006).
Authors
M. Lippmaa, K. Shibuya, T. Ohnishi, K. S. Takahashia, and J.-M. Trisconea
aUniversity of Geneva, Switzerland

Magnetization Measurementsof Two Dimensional Solid 3He
on a Layer 4He Pre-Plated Graphite
Ishimoto Group
3He film adsorbed on graphite surface is one of the mostFig. 1. (a) The device structure of an epitaxial oxide FET. A transmission electron microscope image shows details of the channel interface.
(b) A device where the carrier density in a thin superconducting
Nb-doped SrTiO3 layer is modulated by changing the poling directionof a ferroelectric Pb(Zr,Ti)O3 cover layer.

12

ISSP Activity Report 2005

ideal two-dimensional (2D) Fermi system which can realize
not only a Fermi liquid but also a quantum solid with atriangular lattice only by changing the adsorption density.
The transition between them is recently considered to be asort of Mott transition [1]. The solid forms a quantum spin
magnet with nuclear spin S = 1/2 on a triangular lattice.
Because of the hard-core potential between 3He atoms, thehigher order multiple spin exchange (MSE) processes as wellas two-particle exchange play important roles. The exchangeof even number of particles is anti-ferromagnetic (AFM),
while that of an odd ones is ferromagnetic (FM). The
competition between them, in addition to the geometricalfrustration inherent to a triangular lattice structure, makes thesolid 3He film strongly frustrated.

Development of Implosive Magnet Coils
for Mega-gauss Magnetic Field Generation
Takeyama Group

Fig. 1. Schematic drawing of double gradient coil Faraday-typemagnetometer.

Recent theoretical studies in the exact diagonalization
method for this finite size system show that none of longrange ordered AFM phases survive even at T = 0 K due toquantum fluctuations. The exchange parameters so far
obtained in the AFM region ( so called 4/7 phase ) predicts
a quantum spin liquid (QSL) ground state with a finite
excitation gap [2], and a magnetization plateau at around
5 to 6 T. Nevertheless the heat capacity and susceptibility
measurements seem to indicate a gapless spin liquid groundstate [3,4]. To solve this discrepancy, the magnetization
curve in high magnetic fields has been tried.
A Faraday-type magnetometer has been developed tomeasure small nuclear magnetization of monolayer 3He films
adsorbed on Grafoil at temperatures well below 1 mK in highmagnetic fields up to 10 T. The magnetic force is measured
capacitively by monitoring the displacement of a wiresuspended copper plate, on which are diffusion-bonded
many copper foils interleaved by Grafoils. To eliminate alarge background signal from Grafoil and copper, a double
gradient coil system is employed to produce opposite field
gradients in two regions equidistant from the field center.
The upper region contains Grafoil and the lower region
contains non-exfoliated graphite sheets as shown in Fig.1.
The paramagnetic first layer and ferromagnetic second
layer show the expected magnetization curve, proving thatmost of the background signal is well cancelled out. While
the results of AFM 4/7 phase on both 3He and 4He mono
layer pre-plated graphite show no saturated magnetizationeven at 1 mK and 10 T, suggesting the existence ofmagnetization plateau. Further development to clarify thewhole magnetization behavior is now under way.
References[1] A. Casey, H. Patel, J. Ny'ki, B. P. Cowan, and J. Saunders, Phys.
Rev. Lett. 81,115301 (2003).
[2] G. Misguich, B. Bernu, C. Lhuillier, and C. Waldtmann, Phys. Rev.
Lett. 81,1098 (1998).
[3] K. Ishida, M. Morishita, K. Yawata, and H. Fukuyama, Phys. Rev.
Lett. 79, 3451 (1997).
[4] R. Masutomi, Y. Karaki, and H. Ishimoto, Phys. Rev. Lett. 92,
025301 (2004).
Authors
A. Yamaguchi, H. Nema, Y. Tanaka, and H. Ishimoto

The magnetic field over 100 Tesla can be attained only
by a destructive pulsed magnet. There are three kindsof destructive pulsed magnets for the generation of such
an ultrahigh magnetic field (the single turn coil [1], theexplosive-driven flux compression [2], the electro-magnetic
flux compression (EMFC) [3-5]). Although one can generate
ultrahigh magnetic field with the inner sample space
intact by the single turn coil method, the maximum field islimited to about 300T for a reasonable bore radius. The
explosive-driven flux compression or the electro-magnetic
flux compression can generate much higher field than the
single turn coil in spite of complete destruction of thesubstances in the sample space after the field generation.
Explosive-driven flux compression can generate the highest
magnetic field. However, there are limitations in the application to measurements for solid-state physics because thenecessary use of up to 200 kilograms of explosives results intotal destruction of all equipment placed within a few metersfrom the magnet. Electro-magnetic flux compression ismore suitable for measurements of solid-state physics than
explosive-driven flux compression because the destruction islimited to a relatively small space, and can be placed in aindoor space.
The EMFC was originally developed by E. Cnare [3].
The EMFC system at ISSP has been developed since theearly 1970s. This system is now acknowledged as generating
the world-highest field (622 T) as an indoor system [5]. It
has been served to various measurements of solid statephysics in ultrahigh magnetic fields. Recently, a group at
Loughborough University developed the EMFC system byusing a fast condenser bank system which was normally used
for the single turn coil method. They reported the success ofgenerating over 300T [6].
The EMFC system consists of a primary coil and a liner
ring (a liner). We have recently conducted successful
improvement for the primary coil system to generate higher
field with less energy injection and with more simplified coilpreparation processes. Steel has been adopted as a material
for the primary coils for a long time at ISSP. Two technicalproblems are inherent in the old type primary coil. One is a
structural problem. The old coil consists of three separated
parts (one of a coil part and two of upper and lower
electrodes). A small gap near the junction is unavoidable
because of the inadequate geometrical contact due to the coilstructure (see the top picture at the left side of Fig.1). Thisgap can bring an undesirable electric discharging during the
primary current injection. We have found that such a spark
discharging is an important factor for the reduction of
the energy transfer efficiency and for emission of electromagnetic noise. Another is the problem on the material itself.
The old coil is made of steel which has a high electricalresistance and accordingly renders a deep high-frequency
current skin depth. A high electrical resistance reduces
efficiency of the electro-magnetic energy transfer. The deepskin depth serves as increase of the effective feed-gap, whichis unfavorable for the liner symmetrical implosion.
To solve these problems, we changed the structure andthe material of the primary coil. A new coil system has abended cupper sheet inside a steel coil (see the top picture at
the right side of Fig.1). In the new coil system, the cupper

ISSP Activity Report 2005

13

The effect of feed-gap in a new coil system was reduced
as we expected. The middle of Fig. 1 shows the high speed
photograph of the liner implosion, where the effect of afeed-gap was pronounced in an old coil, whereas the effectwas much reduced in a new coil. The cylindrical symmetryof the liner implosion was comparable even with that byadopting “the feed-gap compensator”, which has recently
attained a great success to record the highest magnetic field
up to 622 T [6].
In the high field generation by the flux compression, themost important factor is a velocity of the liner implosion.
There is a definite correlation between the velocity of theliner implosion and the maximum field [6]. The velocity of
inner radius of the liner was estimated from the high speed
photograph of Fig.1. In the new coil, the estimated velocity
was 2.4 km/s which was the fastest record ever obtained at
ISSP. The rising time of the field was also the shortest,
which supports the fastest implosion of a liner.
In our recent experiments by using a newly designed coil,a pick up coil for the field detection was unfortunately
destroyed by a luminous jet before the field reached the
maximum. We are convinced, however, that the new system
is promising for generation of higher fields than thoserecorded ever before by the EMFC techniques.
References[1] N. Miura et al., Physica B 294-295, 562 (2001)[2] C. M. Fowler, W.B. Garn, and R. S. Caird, J. Appl. Phys. 31, 588(1960)[3] E.C. Cnare, J. Appl. Phys. 37, 3812 (1966)[4] F. Herlach et al., IEEE Trans. Nucl. Sci. NS-18, 809 (1971)[5] M. H. Matsuda et al., Rev. Sci. Inst. 73, 4288 (2002)[6] B. M. Novac, I. R. Smith, D.F. Rankin, and M. Hubbard, J. Phys.
D: Appl. Phys. 37, 3041(2004)
Authors
E. Kojima and S. Takeyama

Interlayer Coupling and Magnetotransportin Layered Conductors
Osada Group
Fig. 1. A comparison of the new and old coil used for the EMFC at
ISSP. From top to the bottom; the primary coil structure, the high speed
photograph of the liner implosion, the primary current and the magnetic
field.

sheet beers the main primary current flow. The shape ofcupper sheet is specially designed to improve the electriccontact with an electrode of the current collector plate fromthe condenser bank. Cupper has 1/9 times smaller resistanceand 1/3 times shallower skin depth than steel. The role of thesteel coil in the present coil system is different from that in
the conventional old type coils. The steel coil in the new coil
is designed for a provision of mass to avoid a deformation ofcupper sheet during an acceleration of the liner.
We performed the EMFC experiments by using the newcoil system. The bottom of Fig.1 shows a comparison of themagnetic field and the primary current between a new coiland an old coil. The new coil system could generate up to
475 T by a 2.3 MJ energy injection (the maximum primary
current 3.2MA). This is a remarkable progress from theold system, in which 350 T was obtained with 4 MJ (themaximum primary current 4.2 MA). It is evident that theelectro-magnetic energy transfer efficiency has been
improved a great deal by employing a new coil system.

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ISSP Activity Report 2005

Low-dimensional layered conductors, which include
organic conductors, layered oxides, intercalation compounds,
artificial superlattices, etc., show rich magnetotransportfeatures as a function of magnetic field orientations. The
most remarkable examples are angle-dependent oscillations
or resonances, such as AMRO (Yamaji oscillations) in
quasi-2D conductors or Lebed resonances in quasi-1D
conductors. Usually, the background of interlayer resistance,
on which angle-dependent oscillations or resonances
superpose, takes a local maximum when the field is parallel
to layers, since the Lorentz force works most effectively.
Sometimes, a sharp peak structure superposes at the parallel
field orientation (peak effect). These phenomena have been
believed to be bulk effects originating from electron orbital
motion on the Fermi surfaces. On the other hand, somelayered conductors show anomalous background taking a
local minimum at the parallel orientation. The origin of this"background inversion" has been an open question.
Does the angle-dependent interlayer magnetotransport
in layered conductors truly originate from bulk effects?
What causes the background inversion? To answer thesequestions, we have performed experimental tests using
GaAs/AlxGa1-xAs artificial multilayer systems withbackground inversion occurs only in the "dirty" samples.
This is the first experiment which has related the backgroundinversion to interlayer coupling. We have also found that
AMRO survives even in the "dirty" samples, while the peakeffect vanishes.
In the next test, we have reduced the number of layer todistinguish local effects from bulk effects. The bulk effects
originating from electron orbital motion should be strongly
suppressed in a few layer samples. As shown in Fig. 2, wehave found that the background inversion occurs even in
the "dirty" systems with a few layers, and that AMRO still
survives even in a few layers ("clean" and "dirty"), but thatthe peak effect vanishes in a few layers.
From these results, we can conclude that AMRO
originates not from the electron orbital motion on the Fermi
surfaces but from the local interlayer tunneling between two
neighboring layers. On the other hands, the peak effect is abulk effect due to electron orbital motion. We can also
conclude that the background inversion is a local phenomenon in the "dirty system", which might originate fromin-plane diffusion.
Authors
T. Osada, M. Kobayakawa, K. Kuraguchi, and E. Ohmichi

Charge-Transfer Phase Transition ofIron Mixed-Valence Complex(n-C3H7)4N[FeIIFeIII(dto)3]
Fig. 1. Interlayer resistance of GaAs/AlxGa1-xAs multilayers fordifferent field orientations. The angle-dependent features dramaticallychange depending on the ratio of interlayer tunneling rate and in-plane
scattering rate.

controlled structures and parameters.
In the first test, we have changed the strength ofinterlayer coupling in GaAs/AlxGa1-xAs superlattices tosimulate "clean" and "dirty" layered conductors. In the"dirty" samples, in-plane scattering occurs more often thaninterlayer tunneling due to small interlayer coupling, and theinterlayer mean free path is much shorter than the interlayer
spacing. As shown in Fig. 1, we have found that theFig. 2. Interlayer resistance of a GaAs/AlxGa1-xAs bilayer system fordifferent field orientations. We can conclude that the features observed
also in a bilayer originate from local mechanism.

Uwatoko GroupThe iron mixed-valence complex,(n-C3H7)4N[FeIIFeIII(dto)3](dto = dithiooxalato C2O2S2) shows a charge-transfer (CT)phase transition at TCT = 122.4 K. In the vicinity of TCT,
the spin state changes from FeII (S = 2) − FeIII (S = 1/2)
(high-temperature phase: HTP) to FeII (S = 0) − FeIII (S = 5/2)
(low-temperature phase: LTP) accompanied by a chargetransfer between FeII and FeIII. This complex also undergoes
a ferromagnetic transition at 7 K in the LTP. The ferromagnetic interaction takes place over a long distance involving
FeIII (S = 5/2) - S2C2O2 - FeII (S = 0) - O2C2S2 - FeIII (S = 5/2).
The ferromagnetic ordering is presumably induced by the
charge-transfer interaction between FeII and FeIII [1].
The most remarkable point of CT transition in(n-C3H7)4N[FeIIFeIII(dto)3] is, that the result of heat capacity
measurements enable us to speculate that the spin-entropy
drives the CT phase transition [2]. Therefore, the cooperative
effect between a metal ion and lattice strain is extremely
small, compared to typical spin crossover complex [3].
We chased the structural changing around CT phasetransition by high-resolution powder x-ray diffraction (PXD)
at Spring-8 BL02B2. At CT phase transition, abrupt
constriction was observed in 2-D honeycomb network layer[FeIIFeIII(dto)3]- (ab plane). For c axis, monotonous constriction was observed between 20 K and 300 K. As a result,small volume change was observed at CT transition.
Fundamentally it is considered that abrupt constriction of abplane induces the electron-transfer between iron atoms. From
the result of Rietveld analysis, a volume constriction of[FeS6] octahedron and a volume expansion of [FeO6] in[FeIIFeIII(dto)3] were observed at 120 K, it is revealed thatthese two conflicting vibration entropy cancel out each other,as a result, the cancellation appears as origin of the small
excess entropy beyond the contribution from change in the

ISSP Activity Report 200515

pressure, we revealed that the organ of CT phase transition isa small volume change and spin-entropy difference between
HTP and LTP. We have also succeeded in direct observationof electron-transfer between iron atoms at CT transition.
References[1] N. Kojima, W. Aoki, M. Itoi, Y. Ono, M. Seto, Y. Kobayashi, and
Y. Maeda, Solid State Commun. 120, 165 (2001)[2] T.Nakamoto, Y. Miyazaki, M. Itoi, Y. Ono, N. Kojima, M. Sorai,
Angew. Chem. Int. Ed. 40, 4716 (2001)[3] M. Sorai and S. Seki, J. Phys. Chem. Solids. 35, 555 (1974) .
[4] Y. Kobayashi, M. Itoi, N. Kojima, and K. Asai, J. Phys. Soc. Jpn.
71, 3016 (2002)
Authors
M. Itoi, C. Lin, M. Koeda, M. Hedo, Y. Uwatoko, N. Kidaa,
N. Kojimaa, Y. Kobayashib, K. Asaib, K. Katoc, K. Osakac, Y. Ohishic,
M. Takatac
aGraduate School of Arts and Sciences, The University of TokyobDepartment of Applied Physics and Chemistry, University ofElectro-Communications
cSpring-8/JASRI

Investigation of YbInCu4
at Pressures to 7 GPa
Uwatoko Group

Fig. 1. Crystal structure of (n-C3H7)4N[FeIIFeIII(dto)3] with [002]
section (left) and projection of [110] (right).(a) MEM charge density of[0 0 2] section at 180 K, (b) 120 K, (c) 90 K. Counter lines are drawn
between 0.0 and 1.0 e/Å3 with 0.001 e/Å3 intervals. The total chargesof Fe1 and Fe2 are estimated as 24.51 e and 25.23 e (at 180 K), 24.70 e
and 24.69 e (120K) and 24.38 e and 23.26 e (at 90 K), respectively.

spin multiplicity.
The electron density distributions around CT transition
have been succeeded in carrying out by the maximum
entropy method (MEM) combined with Rietveld method.
Figure 1 shows the MEM charge density of [0 0 2] section at(a) 180 K, (b) 120 K, (c) 90 K. Counter lines are drawn
between 0.0 and 1.0 e/Å3 with 0.001 e/Å3 intervals. The
strong covalent bonding of sulfur atoms clearly appears withwidely distributed electrons, compared to the density aroundoxygen bonding. The total charges of Fe1 and Fe2 areestimated as 24.51 e and 25.23 e (at 180 K), respectively. At
120 K, the charge distribution on dto molecules expands inwhole honeycomb ring, at this point, the charge of Fe1 andFe2 take almost same values of 24.70 e and 24.69 e. At 90 K,
the honeycomb ring shrank, the charge on two Fe1 and Fe2
became 24.38 e and 23.26 e (at 90 K). We can clearly
describe the electron transfer between FeII and FeIII at CT
phase transition by x-ray structure analysis.
(n-C3H7)4N[FeIIFeIII(dto)3] is very sensitive for externalpressure. The TCT increases linearly with applied pressure,
the TCT close to room temperature under 1 GPa [4]. We also
investigated the structural change of (n-C3H7)4N[FeIIFeIII(dto)3]
under high pressure using diamond anvil cell (DAC) at
Spring-8 BL10XU. The tendency of structural change isdeferent from that of ambient pressure. Under high pressure,on cooling an abrupt constriction of ab plane was observed at
CT transition. However, on heating, we observed abrupt
expansion of c axis. No abrupt expansion of ab plane wasobserved on heating. As a result, small cell volume change
was observed at the CT transition under pressure.
In summary, by high-resolution PXD and PXD under

16

ISSP Activity Report 2005

It is well-known that YbInCu4 shows a first orderisostructural valence transition at Tv ~ 40 K [1], above which
Yb is in a magnetic state of Yb3+, while below which it isnon-magnetic Yb2+. In addition, it is found that the valencetransition is suppressed with applying pressure anddisappears with further pressurization. In previous work, wefound an anomalous decrease of the resistivity at T ~ 1.4 K
under several pressures, which led us to speculate theoccurrence of a superconducting state under pressure [2].
On the other hand, Mito et al. [3] recently found that aferromagnetic state is induced together with the disappearance of the valence transition upon pressurization. Therefore,much attention has been paid to how the electrical systembehaves under pressures after the valence transition
disappears. To elucidate a pressure-induced phase at thelowest temperatures, electrical resistivity measurements havebeen performed under higher pressures.
We have newly developed a novel diamond anvil cellFig. 1. Temperature dependence of the electrical resistivities of
YbInCu4 at low temperatures and at representative pressures. Arrows
indicate the temperature where there is a ‘knee’ in ρ(T). The inset
shows the resistivity under 2.9 GPa.

detail around 4 GPa, may enable us to better understand theessential physical properties of YbInCu4.
References[1] I. Felner et al., Phys. Rev. B 33, 617 (1986).
[2] M. Hedo et al., Acta Phys. Pol. B 34, 1193 (2003).
[3] T. Mito et al., Phys. Rev. B 67, 224409 (2003).
[4] D. Jaccard et al., Physica B 259-262, 1 (1999).
[5] T. Park et al., Phys. Rev. Lett. 96, 046405 (2006).
Authors
N. Kurita, M. Kano, M. Hedo, Y. Uwatoko, J. L. Sarraoa,
J. D. Thompsona, and S. W. Tozerb
aLos Alamos National LaboratorybNational High Magnetic Field Laboratory

Selective Observation of the Nuclear
Wave-packet Dynamics in the Excited State
Fig.2. The temperature-pressure phase diagram for YbInCu4. Cross
marks and open circles indicate the valence transition temperature Tv
and some transition temperature TM, respectively. TM is almost constantat 2.4 K for high pressures.

designed to perform high pressure electrical transportmeasurements based on the concept of a turnbuckle. Thispressure apparatus, with a 1:1 mixture of Fluorinert FC70
and FC77 as the pressure-transmitting medium, was used inthis work.
Electrical resistivity measurements were performed
under 2.9 < P < 7.0 GPa and for 300 < T< 0.3 K. Figure 1
shows the temperature dependence of the electricalresistivity of YbInCu4 at low temperature for P = 3.3, 4.0,
4.6, 5.5 6.0, 7.0 GPa. As shown in the inset in Fig. 1, theresistivity at P = 2.9 GPa exhibits a hysteresis near thevalence transition temperature Tv ~ 6.2 K. For P ≥ 3.3 GPa,the valence transition disappears, with no detectable hysteresis at low temperatures. As is recognizable from Fig. 1, theresistivity at low temperature is suppressed by increasingpressure from 3.3 GPa to 5.5 GPa and enhanced with
applying higher pressure. We tentatively fit the data obtainedin the previous and this work at low temperature toρ = ρ0+ AT 2. Both the residual resistivity ρ0 and the resistivity coefficient A as a function of pressure show broad
peaks around 4 GPa as expected in ref. [2]. This suggeststhat YbInCu4 around 4 GPa may be in the vicinity of aquantum critical point, given the report of similar variationsof A and ρ0 in the pressure-induced superconductor
CeCu2Ge2 [4]; although, P-induced changes in carrier
density and ground-state degeneracy could provide analternative interpretation [5].
In this work, slight decreases of the resistivities, indicatedby arrows in Fig. 1, are observed at TM ~ 2.4 K that is
independent of pressures from 3.3 GPa up to 7.0 GPa. These
decreases were also found by T. Mito et al who concluded
that TM signaled a paramagnetic to pressure-induced
ferromagnetic state [3]. A temperature-pressure phase
diagram up to 7.0 GPa is shown in Fig. 2. The diagram isdifferent from that of Mito et al. in terms of the pressure
dependence of TV and TM. Moreover there is no explanation
for why TM hardly changes with applying pressure.
In conclusion, electrical resistivity measurementsof YbInCu4 by a four-wire method was performed under
2.9 < P < 7.0 GPa down to 0.3 K. We have clarified that
YbInCu4 around 4 GPa may be near a quantum critical point
and that an anomalous transition appears at TM ~ 2.4 K
above 3.3 GPa where the valence transition has vanished.
Various kinds of measurements, including resistivity in more

Suemoto GroupThe dynamics of atoms or molecules in condensed
matters is an important issue for understanding the chemical
processes, defect formation and phase transitions, and also
for controlling the photo-induced phase transitions [1]. The
local motions of atoms are suitably described in terms of a
nuclear wave-packet (WP) propagating on the adiabatic
potential-energy surfaces (APEs).
Up to now, the WP dynamics has been studied mostwidely by means of transient absorption or reflection
spectroscopy using femtosecond laser pulses. However, it
has been difficult to separate the effects of absorption tohigher electronic states and induced emission to the groundstate. In addition, the discrimination between the dynamics
in the ground state and the excited state has also beendifficult. In contrast, the femtosecond luminescence
spectroscopy allows us to solve these problems.
Here, we demonstrate a definitive difference between the
ground- and excited-state dynamics by comparing ourluminescence experiment [2] with the pump-probe ones by
Nisoli et al. [3] at the F center in KBr, which is a prototype
of an optical center with strong electron-lattice interactions.
Figure 1 shows the time evolution of the luminescenceFig. 1. Time evolution of the luminescence intensity at 1.10 eV andits continuous wavelet transform. (a) The oscillating component in the
0-4 ps delay range. The inset shows the original wave form. (b)Continuous wavelet transform of (a). The color scale indicates thesignal amplitude, red being the maximum.

ISSP Activity Report 200517

Development of Highly Advanced SoftX-Ray Lasers and its Applications
Kuroda Group

Fig. 2 The red and blue upward arrows connecting two APESs show
the Franck-Condon transitions in our luminescence measurement andthe pump-probe ones by Nisoli et al., respectively. In the cross-section
drawings of the lattice around the F center, the green and yellow circles
represent halogen and alkali ions, respectively. The solid arrows
represent the direction of the coherent movements of neighboring ionsjust after photoexcitation.

intensity observed at 1.10 eV (a) and its wavelet analysis
showing the time-dependent Fourier power spectrum (b).
The oscillation reflects the WP motion on the APES. In
contrast to the transient absorption measurements, in which
they observed a pronounced peak at 3.3 THz [3], we
observed two peaks at 2.6 and 4.8 THz as seen in Fig. 1(b).
The situation is explained in Fig. 2. An optical pulse
excitation creates the WP as a coherent superposition of
the vibrational wave-functions on the APESs. As thepump-probe measurements described in [3] were performed
under off-resonant excitation (blue case in Fig. 2), theoptical excitation initiated the swinging of the negative WP
along the ground-state APES. Since the probe pulse energy isclose to the pump pulse one, the probe signal reflects
mainly this phenomenon (lower illustration in Fig. 2). The
wave-function of the ground state is compact and interacts
strongly with a local breathing mode, i.e., A1g mode at 3.3
THz. On the other hand, the luminescence measurement was
performed under resonant excitation from the bottom of the
APES (red case in Fig. 2), and the detected photon energy of
1.10 eV is far from the absorption band. Thus, the result
reflects solely the WP dynamics in the excited state (upper
illustration in Fig. 2). In the excited state, the electronic
wave-function spreads out, so that the electron preferably
couples to the bulk phonon modes LA and LO, which peak at
2.6 and 4.8 THz, respectively. Our result clearly shows that
the WP in the ground state and that in the excited state havesignificantly different character and that the luminescencemethod has an advantage in studying the WP dynamics in the
excited state.

Since the time when first ruby laser was demonstrated,
future development of x-ray laser remains as one of a dreamof laser physicists and almost all researchers engaged in laser
applications involving biomedical applications and basic
medicine. We have developed the new 18.9 nm Ni-Mo x-ray
laser based on the transient-collisional-excitation (TCE)
scheme extended to the Ni-like ion, which has extraordinary
small divergence and also operates with only 150 mJ pumplaser energy [1,2]. These are over ten times good divergenceand 100 times smaller energy compared to other results.
This method uses a sub-picosecond or femtosecond,
high-intensity laser pulse to generate a short-lived, high gain
laser medium of only 2 mm length within preformed plasma.
The experimental results reveal an additional favorable
characteristic of longitudinal-pumped TCE x-ray laser,i.e., ultra-small divergence of less than one milliradian(0.3 mrad). These unique features will have a large impact
on x-ray laser application. An example is the precise
measurement of time-resolved emission spectroscopy in live
tissue, which is tremendously difficult even with advanced
synchrotron radiation sources, and we hope to renew ourunderstandings of new fields such as nanotechnology,
nanobiology, and laser medicine. Efforts to develop shorter
wavelength around “water window” region (2.2 nm-4.4 nm)
based on Ni-W 4.3 nm laser will be continued.
We show the intensity trace of a typical on-axis soft-xray spectrum between 15 and 25 nm observed using TCE
experimental setup in Fig.1. The background originatingfrom the noise of the detector was subtracted to obtain
the actual spectral intensity trace. The pump laser energy
in the long and short pulse is only 30 mJ and 120 mJ. The
localization of the Ni-like ion abundant occurred at theregion of the longitudinal beam. Simulations show that the
1.5×1011 W cm-2 intensity pre-pulse is only capable ofFig. 1. Intensity trace of the on-axis soft-X-ray spectrum between 15
and 25 nm. The background was subtracted from the trace to obtain theactual intensity profile.

References[1] K. Nasu et al., in Photoinduced Phase Transitions, edited by
K. Nasu (World Scientific Publishing, Singapore, 2004).
[2] T. Koyama, Y. Takahashi, M. Nakajima, and T. Suemoto, Phys.
Rev. B 73, 161102(R) (2006).
[3] M. Nisoli et al., Phys. Rev. Lett. 77, 3463 (1996).
Authors
T. Koyama, Y. Takahashi, M. Nakajima, and T. Suemoto

18

ISSP Activity Report 2005

Fig. 2. Spectrum image of the on-axis emission for plasma length of (a)2.0 mm and (b) 0.5 mm. The sensitivity of the photocathode camera
was increased by about eight times for (b).

Fig. 3. Harmonic output from B plasma as a function of the wavelengthof generated XUV radiation in the cases of (1) orthogonal and (2)longitudinal pump schemes.

producing pre-plasma with an average ionization of 8, muchless than 14 required for Ni-like molybdenum ions.
However, by introducing a 300 ps duration, 1×10-5 contrast
ratio pedestal to the main pulse, the temperature and density
is increased to 90 eV and 1×1020 cm-3. As a result, narrow
Ni-like molybdenum abundant region is produced in the
pre-plasma, which can generate high gain with the irradiationof the high intensity main pump.
We are also devoting extensive studies on higher
harmonic generation from solid surface plasma, which is also
recognized as an alternative way to pursue coherent lightsource in soft X-ray regions. The availability of compact,
high-intensity sub-picosecond lasers with chirped pulse
amplification (CPA) has opened the new field of study for
laser-matter interactions with solid targets. At the high laser
intensities above 1016 Wcm-2, the electron temperature
is more than one keV and the main laser absorption isrelated to collisionless absorption. Collisionless absorption
mechanisms include vacuum heating, J × B heating, sheath
inverse Bremstrahlung, and the anomalous skin effect. These
results are important demonstrations of the large harmonic
blue shift that can provide a tool to realize a tunable coherent
subpicosecond light source. We observed an interesting blue
shift of generated higher harmonics without broadening due
to self phase modulation. These shifts are brought about
by collisionless absorption process, which shows an onsetof new interaction process predicted for a mechanism
responsible for ultra short and intense, laser-matterinteraction [3].
The high-order harmonic generation (HHG) from theinteraction of ultrashort laser pulses with gas jets and solid
targets has proven to be an effective source of coherent
short-wavelength radiation. The HHG in visible and extreme
ultraviolet (XUV) ranges has been investigated using thelaser plasma formed by an optical breakdown in gases and at
the surface of solid targets. The method for generating
harmonics from solid target atoms apart from solid surface isbased on the preparation of a gas-like ablation plume via
laser-surface interaction with further propagation of shortlaser pulse through this medium. The application of various
ions provided important information on their role for the
HHG. However, the maximum observed order of harmonics
reported in those studies was limited to the 27th one due
to some concurred effects in high-excited plasma. The
peculiarity of the harmonic generation from the surface
plasma with comparing to the conventional gas-jet technique
includes its capability of generating the plasma with highdensity, long length, and variable conditions of plasma
excitation. This technique also gives new degrees of freedomthat can be used to optimize the HHG. The use of any
elements in the periodic table that can be formed as solid
targets may disclose many interesting peculiarities for thegeneration of coherent XUV radiation. The optimization ofplasma conditions can lead to further growth of HHG
efficiency. In particular, the low-excited plasma could
possess some attractive properties in comparison withhigh-excited ones (i.e., less reabsorption of generated harmonics, less influence of free electrons on phase-matchingand self-defocusing, etc.) that can increase both theconversion efficiency and cutoff energy of harmonics. We
demonstrated the efficient generation of coherent XUVradiation after the propagation of femtosecond pulse throughthe low-excited plasma produced on the surface of different
solid targets. We showed the high harmonics up to the 65thorder (λ=12.6 nm) using B target in Fig.3 and the conversion
efficiencies at the plateau region up to 8×10-6 wereachieved [4].
References[1] T. Ozaki, H. Kuroda et al., Phys.Rev.Lett. 89, 252902 (2002).
[2] H. Kuroda, M. Suzuki, R. A. Ganeev, M. Baba et al., Proc. of 9th
International Conference on X-ray lasers, I2802 (2004).
[3] A. Ishizawa, T. Ozaki, R. A. Ganeev, and H. Kuroda, Phys. Rev. E.
66, 026414 (2002).
[4] R. Ganeev, M. Suzuki, M. Baba, H. Kuroda, T. Ozaki, Opt. Lett.
30, 768 (2005).
Authors
H. Kuroda, M. Baba, M. Suzuki, R. A. Ganeev, and T. Ozakia
aInstitut National de la Recherche Scientifique, Canada

Low-Threshold Single-ModeCurrent-Injection Lasing in T-shaped
GaAs Quantum Wire Lasers
Akiyama Group
Quantum-wire lasers are expected to exhibit superior
performance to conventional semiconductor lasers due totheir one-dimensional electronic states. However, such
performances have not yet been realized most likely becauseof difficulty in preparation of homogenous quantum-wirestructures. Recently we fabricated GaAs T-shaped quantum
wires (T-wires) with high interface homogeneity bycleaved-edge overgrowth with molecular beam epitaxy, andreported lasing by optical excitation and basic one-dimensional optical properties [1-3]. The first current-injection
T-wire laser was reported in 1994 [4], where multimodelasing occurred with 0.4 mA-0.8 mA thresholds at 4 K, butthere has been no report since then. In this year, we demonstrated low-threshold current-injection single-mode lasing
at 30-70 K based on a new simple current-injection scheme.
At 30 K, threshold current was 0.27 mA and differential
quantum efficiency was 17%.
In our laser sample, 20-period GaAs quantum wires(T-wires) are formed at T-shaped intersections of 20-layersof 14-nm-thick stem wells and a 6-nm-thick arm well. The
T-wires are embedded in the core of the T-shaped opticalwaveguide surrounded by the cladding layers. Since
Si-doped (C-doped) multiple quantum wells (MQWs) areformed as an n-type (p-type) injection layer on the bottom(top) of the stem slab waveguide, both electrons and holes
are injected into the arm well and then flow into the T-wires.
We call this new injection scheme as arm-arm injection. A
laser bar with 500 µm cavity length was cut from the wafer

ISSP Activity Report 2005

19

probably due to carrier freezing in C-doped and/or Si-doped
GaAs MQW layers.
Micro-photoluminescence study on the same sample has
demonstrated very high uniformity of quantum wires, towhich the low lasing threshold is firstly ascribed. We studied
carrier-injection behavior in this structure by luminescenceimaging, and found that significant portion of carriers
overpass the wire region without recombination. Thissuggests that improvement in the doping design should
further improve threshold current and quantum efficiency ofthe present laser.
References[1] Y. Hayamizu, et al., Appl. Phys. Lett. 81, 4937 (2002).
[2] H. Akiyama, et al., J. Phys. Condensed Matter 16, S3549 (2004).
[3] Y. Takahashi, et al., Appl. Phys. Lett. 86, 243101 (2005).
[4] W. Wegscheider, et al., Appl. Phys. Lett. 65, 2510 (1994).
Authors
Shu-man Liu, M. Yoshita, M. Okano, T. Ihara, H. Itoh, H. Akiyama,
L. Pfeiffera, K. Westa, and K. Baldwina
aBell Laboratories, Lucent Technologies, USA
Fig. 1. High-resolution emission spectra of the 500 µm-long quantumwire laser at four input currents.

by cleavage, and the cavity facets were coated with 50-nmthick (front facet) and 200-nm-thick gold films.
Emission spectra of the laser at 40 K for four injectedcurrents are shown in Fig.1. As the current injection
increases, the emission band narrows showing red-shifts, andfinally reaches stable single-mode lasing. Single-mode lasing
is similarly observed at all cryostat temperatures between
30 K and 70 K. Figure 2 (a) shows the corresponding output
power vs. input current. The threshold current and differential
quantum efficiency vs. temperature are depicted in Fig.2 (b).
At 30 K, we observe the lowest threshold current of 0.27 mA
and the highest differential quantum efficiency of 17%. As
the temperature is increased, we observe higher thresholdcurrent and lower differential quantum efficiency. At lower
temperatures than 30 K, we do not observe single mode
lasing. The current vs. voltage (IV) characteristic at temperatures below 30 K shows rather high series resistance,

Fig. 2. (a) Light output vs. input current for a quantum wire laser. (b)Threshold current and differential quantum efficiency vs. temperature.

20

ISSP Activity Report 2005

Developments of FZP Beam Profile Monitor
Nakamura GroupThe FZP (Fresnel zone plate) beam profile monitor
constructed at the KEK-ATF damping ring is a real-time andhigh-resolution monitor using an X-ray imaging optics basedon two FZPs. In the monitor beamline, the synchrotron
radiation from the electron beam is monochromatized by acrystal monochromator and the transverse beam image istwenty-times magnified by the two zone plates (CZP and
MZP) and detected on the X-ray CCD camera. This monitor
can take real-time images of the electron beam and measure
the beam profiles and sizes with a high spatial resolution ofless than 1 µm. Clear electron-beam images with the verticalbeam sizes less than 10 µm were successfully observed withthis monitor [1].
Recently the FZP beam profile monitor was greatly
improved [2]. The new monochromator was made in order tosuppress the angle drift of the crystal. In the new monochromator, the stepping motor was thermally isolated from themain part of the monochromator by ceramic insulators andthermally stabilized by copper lines cooled by the water. The
FZP holders were newly designed and fabricated so that
the FZPs could be inserted in and removed from the optical
path in the vacuum when necessary. A precise beam-based

Fig. 1. Block diagram of the synchronization system with an ultra-fast
shutter. This system enables the FZP beam profile monitor to have theminimum time resolution of 0.35 ms and to measure the beam profiles
in synchronization with beam injection.

Hydrophobic Solvation of Block
Copolymer Aqueous Solutions:
Pressure and Temperature Dependence
Shibayama Group

Fig. 2. Two-dimensional image profiles of an electron calculated fromthe Fresnel-Kirchhoff diffraction integral with the CZP tilt of (a) 0mrad and (b) 15 mrad in the vertical direction.

alignment scheme of the optical elements could beestablished by utilizing the new FZP holders. The CCD
background due to transmitted X-rays through the two FZPs
was much reduced by an X-ray pinhole mask installedaround the focus point of the CZP.
We added a system for synchronization with injection tothis monitor and succeeded in obtaining the damping timewith the damping wiggler on and off by measuring the beam
size as function of the time after injection and confirming theeffect of the damping wiggler on the damping time.
Furthermore time resolution of the monitor was improved
from 20 ms to 0.35 ms by installing the ultra-fast mechanicalshutter around the focusing point of the CZP [3]. As a result,the influence of fast beam position fluctuation on the beamprofile measurement was removed. Figure 1 shows the block
diagram of the synchronization system with the ultra-fast
shutter.
We developed a computer code to calculate the spatial
resolution of the monitor numerically from the FresnelKirchhoff diffraction integral, because the Fraunhofer
approximation is invalid for the FZPs used in the monitor[4]. The spatial resolution corresponds to the image profile
width of an electron moving on the orbit. The effects of the
FZP alignment errors on the spatial resolution were also
evaluated by using the developed computer code. Figure 2
shows the image profiles calculated with the CZP tilt of 0
and 15 mrad. The spatial resolution was calculated to be
0.5 – 0.6 µm in both vertical and horizontal directions
without the FZP alignment error. The tolerable range of thealignment error for the resolution of less than 1 µm was
obtained to be less than 0.6 deg. for the CZP tilt and less than
4 deg. for the MZP tilt. Since it is easy to keep the alignment
error within the tolerable range, the FZP beam profile
monitor has a sub-micron resolution.

Hydrophobic interaction has been one of the mostintriguing problems in physical chemistry, theoreticalchemistry, and in biology. The concept of "hydrophobicbond" proposed by Kauzmann [1] is now recognized as the"hydrophobic interaction" and as one of the fundamental
molecular interactions ruling the structures and functions ofproteins, nucleic acids and so on. However, it is moreconvenient to use the term "hydrophobic solvation" when
one discusses miscibility and phase behavior of aqueous
systems of organic compounds, including synthetic- andbio-polymers. Block copolymers undergo microphaseseparation transition (MiST) due to connectivity between
unlike polymer chains, e.g., A and B block chains, which ischaracterized by a molecular-order transition. On the other
hand, in the case of polymer blends and polymer solutions, aphase separation means a macrophase separation transition(MaST) because the characteristic size of phases can be
infinite. Here, we report the first observation of pressure (P)and temperature (T)-induced micro- and macro-phaseseparation transitions in a block copolymer solution, anddiscuss the role of hydrophobic solvation in the P-T plane.
This system can be regarded as a prototype of protein
folding.
Poly(2- (2-ethoxy) ethoxyethyl vinyl ether)-block-poly(2-methoxyethyl vinyl ether) diblock copolymer (pEOEOVEpMOVE) with a narrow molecular weight distribution(Mn = 5.7 × 104, Mw/Mn = 1.28) was synthesized by living
cationic polymerization, where Mn and Mw are the number
and weight average molecular weights, respectively [2]. The
degrees of polymerization of the individual block chains, i.e.,

References[1] K. Iida et al., Nucl. Instr. and Meth. A 506, 41 (2003).
[2] N. Nakamura et al., Proc. of the 8th EPAC, Lucerne, 2350 (2004).
[3] H. Sakai et al., Proc. of the 2nd Annual Meeting of Particle
Acclerator Society of Japan, 57 (2005).
[4] T. Muto et al., Proc. of the 2nd Annual Meeting of Particle
Acclerator Society of Japan, 21P049 (2005).
Authors

N. Nakamura, H. Sakai, T. Mutoa, and H. Hayanoa
aHigh Energy Accelerator Research Organization (KEK)

Fig. 1. SANS intensity functions, I(q)'s, of pEOEOVE-pMOVE solutions at (a) various temperatures and (b) various pressures. The solidlines in Fig. 1b denote the fit with OZ functions. The inset of Fig.1bshows the variation of the correlation length with P.

ISSP Activity Report 200521

References[1] W Kauzmann, Adv. Protein Chem. 14, 1 (1959).
[2] S. Sugihara, S. Kanaoka, and S. Aoshima, Macromolecules 38,
1919 (2005).
[3] I. Nasimova, T. Karino, S. Okabe, M. Nagao, and M. Shibayama,
Macromolecules 37, 8721 (2004).
[4] N. Osaka and M. Shibayama, Phys. Rev. Lett. 96, 048303 (2006).
Authors
N. Osaka and M. Shibayama

Charge Disproportionation in
β-Na0.33V2O5 Revealed by
Neutron Magnetic Diffraction
Hirota, Yoshizawa, and Y. Ueda Groups
Fig. 2. Phase diagram for polyA-polyB-copolymer aqueous solutions.
The solid and dashed curves denote the demixing temperatures of A, TA(MiST) and of B, TB (MaST). P0 divides the strongly hydrophobic-solvated (SHS) and the weakly hydrophobic-solvated (WHS) regions. The
arrows indicate repulsive interactions among A, B, and S (solvent).

pEOEOVE and pMOVE, were 200 and 400, respectively. A
15.0 wt% polymer solution was prepared by dissolving
pEOEOVE-pMOVE in deuterated water (D2O). Pressuredependent small-angle neutron scattering (SANS) measurements were conducted at SANS-U, a small-angle neutron
scattering apparatus, The University of Tokyo, Tokai, Japan.
The incident wavelength was 7.0 Å and the wavelength
distribution was 10 %.
At ambient pressure, the solution underwent a two-steptransition at 40 and 65 ºC, both of which were convexupward functions of P having a maximum around P0 ≈ 150
MPa. Fig. 1 shows SANS intensity curves, I(q)'s, ofpEOEOVE-pMOVE solution obtained (a) at various
temperatures by fixing P (= 0.1 MPa), and (b) at variouspressures by fixing T (= 28 ºC). Here, q is the momentum
transfer. Above 35 ºC, I(q) drastically increased with increasing T, and an MiST with a bcc packing of pEOEOVE
domains took place at 40 ºC. The structure grew gradually byfurther increasing T to 45 and 50 ºC. It is noteworthy that areentrant MiST took place by increasing P at 45 ºC (notshown). In contrast to the temperature variation, I(q) changed
rather moderately with increasing P as shown in Fig. 1b andwas well represented by an Ornstein-Zernike (OZ) function(solid lines). The inset shows that the correlation length, ξ,diverged at Psp ≈ 350 MPa indicating an occurrence of MaST
with divergence. The experimental findings indicate thefollowing. The pEOEOVE-pMOVE aqueous solution
undergoes two types of phase transitions in P-T plane. A
temperature increase at the ambient pressure resulted in
MiST with a bcc structure. Pressurizing at low temperatures
resulted in a MaST with a divergence of the correlation
length and susceptibility. More interestingly, pressurizing athigh temperatures above the MiST temperature ofpEOEOVE solution exhibited a reentrant phase transition froma microphase to a new phase by way of dissolution process. It
is clear from this work that the pressure effect on thehydrophobic solvation is different from temperature effect.
Since a selective solvation was observed only in the low-P
region, the solvation effect was found to be exclusively important at low pressures where a negative volume change by mixing is observed [3, 4]. This may be why the hydrophobic solvation plays an important role in the diversity of biological
functions, such as molecular recognition and replication.

22

ISSP Activity Report 2005

A family of the β-Vanadium bronze, β-A0.33V2O5 (A =Li+, Na+, Ag+, Ca2+, Sr2+, Pb2+) is a quasi-one dimensional
conductor as expected from their crystal structure, which has
three characteristic V chains: The V1 sites form an infinite
zigzag chain of the edge-shared VO6 octahedra, the V2 sites
form a two-leg ladder of the corner-shared VO6 octahedra,and the V3 sites form a zigzag chain of the edge-shared VO5pyramids. Among the family, β-Na0.33V2O5 has attracted
most attention because this is the first vanadium oxide toshow superconductivity, which was discovered by Yamauchi
et al. [1] under high pressures. Even more interestingly, thesuperconducting (SC) phase is the adjacent to the insulating(I) phase in the PT (pressure-temperature) phase diagram.
We thus pay much attention for the nature of the I phasecompeting the SC phase, and the mechanism of the MI(metal-insulator) transition (TMI ~ 130 K at ambientpressure) of β-Na0.33V2O5 because we believe that theessential interaction can be revealed through the study of the
MI transition.
Recent studies suggested the charge ordering (CO) was a
possible origin of the MI transition. This picture, however, isvery controversial: An x-ray diffraction study revealed
the MI transition accompanied by a 6b lattice modulation
implying a carrier localization in the V2 ladder with a periodof 6b along the b axis [2], while an ESR study concluded thatcarriers are primarily located on the V1 sites [3]. An NMR
study, on the other hand, attributed a splitting of the Knight
shift at TMI to the charge localization in the V1 and V2 sites[4]. In the present research [5], we have employed theFig. 1. Crystal structure of β-Na0.33V2O5 (Monoclinic C2/m). V2O5
framework formed by (V1)O6, (V2)O6 and (V3)O6 octahedra.

[4] M. Itoh, N. Akimoto, H. Yamada, M. Isobe, and Y. Ueda, J. Phys.
Chem. Solids 62, 351 (2001).
[5] S. Nagai, M. Nishi, K. Kakurai, Y. Oohara, H. Yoshizawa,
H. Kimura, Y. Noda, B. Grenier, T. Yamauchi, J. Yamaura, M. Isobe,
Y. Ueda, and K. Hirota, J. Phys. Soc. Jpn. 74, 1297 (2005).
Authors
S. Nagai, M. Nishi, K. Kakuraia, Y. Oohara, H. Yoshizawa,
H. Kimurab , Y. Nodab , B. Grenierc, T. Yamauchi, J. Yamaura,
M. Isobe, Y. Ueda, and K. Hirota
aJapan Atomic Energy Agency
bIMRAM, Tohoku University
cCEA-Grenoble, France

Structures and Low-energy Excitationsof Amorphous Clathrate Hydrates
Yamamuro Group

Fig. 2. Result of the least square fitting. (a) A plot of |F|2cal (solid
circle) vs. |F|2obs (open circle). Nine parameters (six are amplitudes in
the a’ and c directions, three are phases) are optimized in the fitting.
(b) A schematic illustration of the arrangement of magnetic moments.

neutron magnetic diffraction. β-Na0.33V2O5 undergoes anantiferromagnetic (AF) transition at TN ~ 24 K, which yields
useful information of the charge density distribution because
the spin density directly corresponds to the charge density;
S = 1/2 and 0 correspond to V4+ and V5+ states, respectively.
Neutron diffraction experiments were performed on asingle crystal of β-Na0.33V2O5, grown by the Czochralski
method, using the four-circle diffractometer FONDER andthe triple-axis spectrometer GPTAS installed by ISSP to the
JRR-3 reactor in JAEA. We have found that the magnetic
Bragg reflections appear at (h, k ± 1/6, 0) with h + k = odd
below TN, which indicates that the amplitude of the magnetic
moments at the V sites in the all three types of V-O chain are
modulated with a period of 3b along b. We have also
confirmed that the lattice modulations occur at ~ 244 K andat ~ 131 K with a period of 2b and 6b along b, respectively.
Through a systematic calculation of the magnetic structure models under the constraint of the magnetic symmetry
P21/a, we have concluded that the 131 K transition should be
attributed to a charge disproportionation in the V 3d orbitalwith a period of 3b, not 6b as previously believed. This picture is also supported by the 3b periodicity in the amplitudeof magnetic moments. Therefore, the 6b lattice modulation
developing below ~131 K is attributed to an instability inthe V 3d band to form the 3b charge modulation on thepreexistent 2b lattice modulation of Na below ~244 K.
References[1] T. Yamauchi, Y. Ueda, and N. Mori, Phys. Rev. Lett. 89, 057002(2002).
[2] J. Yamamura, M. Isobe, H. Yamada, T. Yamauchi, and Y. Ueda,
J. Phys. Chem. Solids 63, 957 (2002).
[3] M. Heinrich, H.-A. Krug von Nidda, R.M. Eremina, A. Loidl,
Ch. Helbig, G. Obermeier, and S. Horn, Phys. Rev. Lett. 93, 116402(2004).

Clathrate hydrates are well-known inclusion compounds
which accommodate various guest molecules in theircage-like structures formed by water molecules. There are
two major structural types: type I (space group: Pm3n) andtype II (Fd3m) depending on the van der Waals diameter ofguest molecule; molecules larger than 0.55 nm and smaller
than 0.40 nm prefer type II. The ideal compositions of type I
and II hydrates are M·5.75H2O and M·17H2O, respectively.
Recently, natural methane hydrate deposited under seabed isremarked as future energy resource and various applications
using clathrate hydrates (e.g., molecular sieves, heat
reservoir, etc.) are studied actively. From the basic scientificpoint of view, however, the clathrate hydrates have been
important materials as exotic disordered crystals (both guest
and water orientations are disordered) and model systems ofhydrophobic hydration for a long time.
Recently, we found that amorphous clathrate hydrates are
realized by vapor-deposition technique and they exhibit
interesting low-energy phonon excitations [1,2]. We chose
Ar (0.38, II), CD4 (0.41, I), Xe (0.44, I) and SF4 (0.58, II) as
guest molecules to investigate their size dependence; thenumbers in the parentheses denote average van der Waals
diameter in nm and structural type. The amorphous samples
were prepared in a novel cryostat designed for in-situ
neutron scattering measurements [3,4]. The mixed gas of theguest and water vapor was deposited slowly on a substrate at
ca. 10 K. The mass of the deposited sample was ca. 0.5 cm3

Fig. 1. Radial distribution functions calculated from the neutron diffraction data of vapor-deposited amorphous Xe·5.75D2O, Xe·17D2O,
CD4·5.75D2O and D2O. The tick marks denote the atomic distances forthe three adjacent D2O molecules in the figure.

ISSP Activity Report 200523

E/T-scaling Behavior of Spin Fluctuationsin the p-Zn-Mg-Ho Quasicrystal
Sato Group

Fig. 2. Dynamic structure factors of vapor-deposited amorphousXe·5.75H2O, Xe·17H2O, CD4·5.75H2O, SF6·17H2O and H2O.

corresponding to a thickness of 0.13 mm. Their neutron
diffraction and inelastic neutron scattering data were measured by HIT and LAM-D spectrometers (both at KEK),
respectively. The data of as-deposited sample were collected
first and then the samples annealed at 120 K and 150 K were
measured to investigate the effects of structural relaxationand crystallization.
Figure 1 shows the radial distribution functions of thethree vapor-deposited hydrates and D2O. The all of intramolecular and intermolecular atomic correlations were reproduced well. The RDF’s of the amorphous Xe and Ar
hydrates are considerably larger than that of amorphous D2O
around 4 Å corresponding the center-to-wall distance of the
12-hedral hydrate cage; the RDF of CD4 hydrate is larger
than others because a CD4 molecule consists of five atoms.
The present result indicates that local cage-like structure is
kept even in the amorphous solids.
Figure 2 shows the incoherent inelastic neutron scattering
data. The peak of amorphous H2O around 7 meV is due tothe acoustic mode and the intensity below 7 meV is sensitive
for annealing condition. Therefore it is believed that excess
density of states, sometimes called boson peak, exists below
7 meV for amorphous H2O. The intensity below 7 meV wasclearly reduced by adding guest molecules, depending on theconcentration and size of the guests. This may be due to thefact that the hydrogen-bond formation is promoted anddisorder, defects and distortion of amorphous structure,
which might be related to the origin of the boson peak, werereduced by adding guest molecules with large hydrophobic
hydration effect. It is also of interest that excess excitation
appeared below 3 meV for amorphous hydrates. This may berelated to the cooperative motion of the cage and guest
molecules but further details of the origin are unknown. We
are now planning the experiments on the amorphous hydrateof H2 (or D2) which is the smallest guest molecule.

Quasicrystals have distinct spatial symmetry characterized by highly-ordered but non-periodic atomic arrangement.
This special atomic structure, called quasiperiodic structure,
has hidden translational symmetry in higher dimensional
space, and thus differs both from the periodic crystals andrandom glasses.
Since the discovery of the quasicrystals in 1984 [1],extensive efforts have been made to understand theirstructure and physical properties. Ordering and dynamics ofmagnetic moments (spins) in the quasiperiodic structures
have been one of fundamental topics to date. Theoretically,
a number of interesting phenomena are expected, such as
non-collinearly ordered ground state with hierarchicalspin-fluctuation distribution [2], nevertheless, earlier experimental studies only detected rather normal behavior; most of
the magnetic quasicrystals exhibit typical spin-glass-likebehavior in macroscopic susceptibility, suggesting random
freezing of spins. Using the neutron scattering technique, wehave revealed several non-trivial features in the spin orderingand dynamics in magnetic quasicrystals, such as significant
short-range order with localized collective modes in the
Zn-Mg-Tb quasicrystal [3]. Here, we will give a briefsummary of our recent discovery --temperature independent
spin excitation spectrum--, which is, we believe, the mostprominent feature ever found in the magnetic quasicrystals.
Figure 1 shows the magnetic excitation spectra at several
temperatures from 1.4 K to 200 K, observed using a powder
sample of the p-type Zn-Mg-Ho quasicrystals. The experiment was carried out using LAM-40 inverted-geometry
time-of-flight spectrometer installed at KEK. One may
immediately notice that the scattering intensity for theneutron-energy-loss side ( '~ > 0) is mostly temperature
independent. This surprising independence can never be an
experimental fault, since the energy-gain side ( '~ < 0)shows usual temperature dependence. Neutron scatteringcross-section for spin systems may be given in terms ofdynamic susceptibility as:
S (Q, '~, T ) " [1 + n('~, T)] Im | (Q, '~, T ),and thus, the temperature-independence requires a special
function form in Im | (Q, '~, T ) , which compensates theusual Bose-temperature factor term [1 + n('~, T)]. Afterseveral trials, we found that the spectrum in 0.2 < '~ < 1.5
meV can be well described by the following function in

References[1] O. Yamamuro, T. Matsuo, I. Tsukushi, and N. Onoda-Yamamuro,Can. J. Phys. 81, 107 (2003).
[2] O. Yamamuro, Y. Inamura, and N. Onoda-Yamamuro, Physica B,(2006) in press.
[3] O. Yamamuro, Y. Madokoro, H. Yamasaki, T. Matsuo, I. Tsukushi,
and K. Takeda, J. Chem. Phys. 115, 9808 (2001).
[4] O. Yamamuro, T. Matsuo, N. Onoda-Yamamuro, K. Takeda,
H. Munemura, S. Tanaka, and M. Misawa, Europhys. Lett. 63, 368(2003).
Authors
O. Yamamuro, T. Kikuchi, Y. Inamuraa, and N. Onoda-Yamamurob

aJapan Atomic Energy Agency
bTokyo Denki University

24

ISSP Activity Report 2005

Fig. 1. Inelastic excitation spectra in the p-type Zn-Mg-Ho quasicrystal
at several temperatures ranging from 1.5 K to 200 K. Scattering angle
was fixed to 24.3 deg.

Fig. 2. Imaginary part of the dynamic susceptibility (with weak
prefactor T1/3) scaled by '~/kBT . Solid line shows the scaling
function with α = 1.00(2). See text for details.

entire temperature range:
u /kB T ),Im | (Q, '~, T ) " ('~)-1 3 tanh (a'~which has similar '~ - and T-dependence as the inverse of
the Bose temperature factor. This form of the dynamic
susceptibility further suggests a scaling law:
u /kB T ) (kB T)1 3 " ('~/kB T)-1 3 tanh (a'~u /kB T ).
Im | (Q, '~This is a typical E/T-scaling function, where the susceptibility
is scaled only by E/T, except for the weak prefactor T1/3.
This E/T-scaling is frequently observed in the non-Fermi-liquid(NFL) compounds, such as UCu4Pd [4]. Several mechanisms have been proposed to understand this E/T-scaling in
the NFL compounds, such as the local quantum criticality or
the disordered quantum Griffiths singularity. At the present
moment we are not sure what mechanism is responsible for
the E/T-scaling in the magnetic quasicrystal, however, we
believe there must be interesting common background in thetwo seemingly different systems.

easy axis. This feature is more clearly seen in M-H curves asa spin-flop transition. By taking isothermal magnetization
curves at various temperatures, we obtain a H-T phase
diagram associated with spin-flop transition (Fig. 1(g)).
Compared with conventional uniaxial antiferromagnets, thepeculiar feature of the phase diagram is that the spin-flop
field (HSF) shows a steep decrease near TN and the first-orderline of the spin-flop transition meets the H = 0 line at a
certain temperature T’ (~ 83.5 K). If we assume that the spineasy axis varies from the b- to a-axis upon the spin-floptransition without a change of the pattern of spin arrangements(Ci-C'i ; i = a, b, and c), the observed weak ferromagnetism
can be well understood; the irreducible representation related
to Cb-C'b type spin arrangements has neither Fi nor F’i typecomponent, whereas the irreducible representation responsible for Ca-C'a type spin arrangements includes both Fc andF’c type component (ferromagnetism along the c-axis).
The system of A = Ge exhibits essentially similar temperature dependence of M/H to A = Si case (Fig. 1(d)-1(f)).
However, there are several differences. A careful examinationof M-H curve at 86 K tells us that spontaneous weakferromagnetism does not appear near TN, but M/H along thec-axis shows merely an enhancement under small magnetic
field near TN. Another difference is that M/H along the a-axis
shows an enhancement at low temperatures, which is notdiscernible in A = Si. These two characteristics are also
understood through a phase diagram (Fig. 1(h)). In marked
contrast to A = Si, the first-order line does not meet the H = 0

References[1] D. Shechtman et al., Phys. Rev. Lett. 53, 1951 (1984).
[2] S. Wessel et al., Phys. Rev. Lett. 90, 177205 (2003).
[3] T. J. Sato, Acta Crystallogr. 61, 39 (2005); T. J. Sato et al., Phys.
Rev. B 73, 054417 (2006).
[4] M. C. Aronson et al., Phys. Rev. Lett. 75, 725 (1995).
Authors
T. J. Sato and A. P. Tsaia
aIMRAM, Tohoku University

Spin-Flop Multicritical Phenomena
in Mn2AS4 (A = Si and Ge)
Y. Ueda GroupWe have succeeded in elucidating anomalous magnetic
properties of Mn2AS4 (A = Si and Ge) by combining themagnetic field-temperature (H-T) phase diagram associatedwith spin-flop transition and the representation analysis ofmagnetic structures [1].
Figures 1(a)-1(c) represent temperature dependence ofthe magnetization (M) divided by the external magnetic field(H) for various magnetic fields for A = Si. As clearly seenin Fig. 1(c), weak ferromagnetism along the c-axis with aspontaneous magnetic moment of 0.042 µB/Mn2+ emerges ina very narrow temperature range just below Néel temperature(TN = 86 K). On the other hand, the M/H along the b-axis at
0.1 T steeply decreases with lowering temperature, whichindicates a uniaxial anisotropy with the b-direction as an

Fig. 1. (a)-(f) Temperature (T) dependence of the magnetization (M)divided by the external magnetic field (H) at various magnetic fields
along the a- [(a) and (d)], b- [(b) and (e)], and c-axes [(c) and (f)] for
Mn2SiS4 [(a)-(c)] and Mn2GeS4 [(d)-(f)]. M is detected along the same
direction as H. (g), (h) The magnetic field-temperature (H-T) phase
diagram for Mn2SiS4 (g) and Mn2GeS4 (h). H is applied along theb-axis. The Ci-C'i (i = a, b, and c) denote antiparallel arrangements ofeach ferromagnetic chains with each spin parallel to the i-th axis.
The phase boundaries between the paramagnetic and magnetic phases
represent the second order transition, whereas those between different
types of magnetic phases indicate the first order spin-flop transition.

ISSP Activity Report 200525

line, but it terminates as a bicritical point on the second-orderline of antiferromagnetic transition. The enhancement of
M/H along the c-axis near TN is most likely a reflection offluctuations of Ca-C'a type spin arrangements, which can bestabilized by the application of small magnetic field. We can
explain the enhancement of M/H along the a-axis in a similarmanner by assuming that the spin easy axis is the c-axis in
the spin-flopped phase at low temperatures; both Fa and F’atype components belong to the same irreducible representation as Cc-C'c type spin arrangements, which fluctuations canbe observed even at zero magnetic field owing to a smallnessof HSF. The boundary between Ca-C'a and Cc-C'c type spinarrangements may be a first-order line, however, its locationin the H-T plane is unclear at present.
We need further studies to pursue intriguing aspects
of present compounds, for example, (1) what kind of
microscopic scenario takes part in a sudden decrease of HSF
near TN; (2) what is the criticality of the phase transition at
the unusual multicritical point, where first-order lines andsecond-order lines converge on the H = 0 line.
References[1] K. Ohgushi and Y. Ueda, Phys. Rev. Lett. 95, 217202 (2005).
Authors
K. Ohgushi and Y. Ueda

Crystal Growth of 3d Heavy
Fermion Compound, LiV2O4
Y. Ueda Group
Normal spinel oxides show a rich variety of electromagnetic properties which are closely related to the strong
geometrical frustration of a three-dimensional network withcorner-sharing magnetic tetrahedra, the so-called pyrochlore
lattice. LiV2O4 with a mixed valence of V3+ and V4+ is also amember of spinel oxides and very interestingly shows heavy
fermion behavior despite its d-electron system. The
mechanism of such heavy fermion behavior in LiV2O4 has
not been resolved, partly owing to the lack of high-quality
single crystals. We succeeded in growing single crystals of

LiV2O4 using an effective solvent system for mixed-valence
vanadium (V3+ and V4+) oxides [1]. We selected a
LiCl-Li2MoO4-LiBO2 system with 69:26:5 molar ratio, as aflux. The pellet of polycrystalline LiV2O4 with flux materials
was put into a platinum crucible with a carbon lid and loaded
into a fused quartz tube, the inside of which was coated witha carbon film formed by pyrolysis of acetone, thus avoiding
tube breakage by reaction with lithium compounds. The tube
was sealed under a 10-4 torr atmosphere and then placed in a
furnace. The best growth condition at 1:1 ratio of
LiV2O4/flux was as follows: the sample was soaked at 800°C
for 10 h, cooled to 450°C in 72 h and then cooled to room
temperature in 10 h. The grown crystals have a well-developed octahedral form bounded by {111} faces, with highcrystallographic quality and maximum size of almost
1.0-1.0-1.0 mm, as shown in Fig. 1. The crystals havestoichiometric composition and show heavy fermion behavior
with an extremely large γ value of ~460 mJ mol-1 K-2.
By using thus grown crystals, various measurements
were done. A sharp peak structure in the density of states at~4 meV above the Fermi level (EF) was observed in a
vacuum ultraviolet laser excited photoemission spectroscopy[2]. The evolution of the peak height corresponds well withthe crossover behavior to the heavy-fermion-like state as
observed in the thermal and transport properties. The
position, shape, and temperature dependence of the peakstructure is quite similar to the Kondo resonance observed inconventional f-electron heavy Fermion compounds. On theother hand, the muon Knight shift measured in single crystalsof LiV2O4 strongly suggests that the anomalous properties of
LiV2O4 originate from frustration of local magnetic moments[3]; the presence of inhomogeneous local magnetic moments
is demonstrated by the broad distribution of the Knight shift
at temperatures well below the presumed Kondo temperature( T* - 30 K) and moreover, a significant fraction ( - 10 %)of the specimen gives rise to a second component which isvirtually non-magnetic.
References[1] Y. Matsushita, H. Ueda, and Y. Ueda, Nature Materials 4, 845(2005).
[2] A. Shimoyamada, S. Tsuda, K. Ishizaka, T. Kiss, T. Shimojima,
T. Togashi, S. Watanabe, C. Q. Zhang, C. T. Chen, Y. Matsushita,
H. Ueda, Y. Ueda, and S. Shin, Phys. Rev. Lett. 96, 026403 (2006).
[3] A. Koda, R. Kadono, K. Ohishi, S.R. Saha, W. Higemoto,
Y. Matsushita and Y. Ueda, J. Phys.: Condens. Matter 17, L257 (2005).
Authors
Y. Matsushita, H. Ueda, and Y. Ueda

A New Spin-1/2 Dimer System
Ba3Cr2O8 with Rare Cr5+
Y. Ueda Group

Fig. 1. Scanning electron micrograph of grown LiV2O4 single crystals
with 1.0 × 1.0 × 1.0 mm size in the cross edge direction.

26

ISSP Activity Report 2005

Ba3Mn2O8 with the valence state of Mn5+ (3d2, S = 1)
has drawn much interest in its gapped ground state andmagnetization plateau at half of saturated magnetization [1].
The crystal structure is made up of isolated tetrahedral
MnO43- and Ba2+ ions, where Mn5+ ions set up double-layered
triangular lattices which are stacked along the c-axis withthreefold periodicity, as shown in the inset of Fig.1. The
antiferromagnetic intradimer interaction J0 (see Fig. 1) isdominant, which is the origin of the spin gap. Moreover, theinterdimer interactions J1, J2 and J3 are also antiferromagnetic
and their total is almost equivalent to a magnitude of J0.

‘From-Edge-to-Interior’ Spin Freezing
in a Magnetic Dipolar Cube
Takayama Group

Fig. 1. Temperature dependence of magnetic susceptibility for
Ba3Cr2O8. The circles and line are the observed and calculated
magnetic susceptibility, respectively. The inset shows the crystal
structure and magnetic interactions.

We have successfully synthesized the isostructuralcompound, Ba3Cr2O8 with very rare Cr5+ (3d1, S = 1/2) andthe compound was found to be a three-dimensional quantum
spin system with a spin singlet ground state of dimer origin[2]. From the analysis of magnetic susceptibility (χ) andhigh-field magnetization data, excitation gap is evaluated as∆/kB = 16.1 K. The calculated χ well reproduces theobserved χ in the temperature range of 6 ~ 300 K, as shownin Fig. 1. The parameters obtained from the best-fit are
g = 1.975(6), J0/kB = 25.04(3) K, J’(= 3J1 + 6J2 + 6J3)/kB =7.69(9) K and χ0 = -1.9(2) × 10-4 emu/mol. Thus, Ba3Cr2O8
can be regarded as a coupled antiferromagnetic dimer system
similar to Ba3Mn2O8, where the interdimer interactions work
effectively. The obtained interactions of Ba3Cr2O8 arestronger (weaker) in the intradimer (interdimer) than those of
Ba3Mn2O8; J0/kB = 17.4 K and J’/kB = 24.9 K. Because it isthought that the magnitudes of intra- and interdimer interactions are proportional to their corresponding distances dJi, we
compare dJi between Ba3Cr2O8 (dJ0 = 3.934 Å, dJ1 = 4.599
Å, dJ2 = 5.739 Å and dJ3 = 6.958 Å) and Ba3Mn2O8 (dJ0 =3.984 Å, dJ1 = 4.569 Å, dJ2 = 5.711 Å and dJ3 = 6.963 Å).
dJ0 related to the intradimer interaction is shorter by 1.27 %in Ba3Cr2O8, whereas in the interdimer interactions, dJ1 anddJ2 are longer by 0.65 % and 0.49 %, respectively, and dJ3 isshorter by 0.07 % in Ba3Cr2O8 than those in Ba3Mn2O8.
These results suggest that Ba3Cr2O8 has a stronger
intradimer interaction and weaker interdimer interactions
than those of Ba3Mn2O8, which are in agreement with theresults obtained from the analysis of magnetic susceptibility.

In recent years, systems consisting of arrayed ferromagnetic nanoparticles have attracted much attention as apossible element with a high storage density. To understandmagnetic properties of such systems the dipole-dipole
interaction is considered to be important as well as themagnetic anisotropic energy of each nanoparticle moment.
The former interaction has been well known since long ago,but cooperative aspects of dipolar systems, i.e., those of
spins interacting only via the dipole-dipole interaction, havelittle investigated [1]. This is particularly the case for dipolarsystems of a finite size. We have recently investigatedvarious finite-size dipolar systems whose magnetic dipole
moments are represented by classical Heisenberg spins, Si’s.
Let us discuss here a dipolar cube, which is cut out froma simple cubic lattice with the edges in parallel to the latticeaxes. The corresponding bulk dipolar system has the groundstate with the global continuous O(3) symmetry (see Fig. 1a),or it is given by any linear combination properly normalizedof x-, y-, and z-antiferromagnetically aligned ferromagnetic
chains (af-FMC) orders where the last one is shown in Fig.
1b [1]. We have investigated the freezing behavior of the
diploar cube during cooling process by the standard molecule
dynamics simulation, and have found a peculiar finite size
effect, which we call ‘from-edge-to-interior’ freezing [2].
Figure 2 shows patterns of the local freezing parameters,
i’s, the magnitudes of the time-averaged Si’s, which
represent the degree of freezing of each spins. At T=0.625(a), a little higher than T* at which the system’s specific heat
C exhibits a rather sharp peak, spins on some edges start
freezing. As T further decreases, the freezing extends fromedges to the interior and spins form domains (b-d), eachof which is in one of the three af-FMC orders whose ferromagnetic alignment is in parallel to the edge. The groundstate consists of 12 domains extending from the edges asshown in Fig. 1c.
We attribute a possible origin of the peculiar freezing
process described above to the reduction of symmetry of theinteraction Hamiltonian from O(3) in bulk (or interier) to Z2
on edges. In the interior, short-ranged orders with thecontinuous O(3) symmetry change easily their order as awhole. On the edges, on the other hand, the symmetry is
broken into the Z2 due to the peculiar anisotropic nature of
the dipole-dipole interaction, which connects the anisotropy
in the spin space with that in the real space, combined withthe existence of the boundary. The consequence is afree-energy barrier of the order of the system linear
dimension L against an overturn of ferromagnetically aligned
spins on the edge.

References[1] M. Uchida, H. Tanaka, M. I. Bartashevich, and T. Goto, J. Phys.
Soc. Jpn. 70, 1790 (2001); M. Uchida, H. Tanaka, H. Mitamura,
F. Ishikawa, and T. Goto, Phys. Rev. B 66, 54429 (2002).
[2] T. Nakajima, H. Mitamura, and Y. Ueda, J. Phys. Soc. Jpn. 75,
054706 (2006).
Authors
T. Nakajima, H. Mitamura, and Y. Ueda

Fig. 1. Spin configurations in the ground state. (a) One of the bulk
orders, (b) the z-af-FMC order, and (c) the 12-domain structure in low
energy states of the dipolar cube.

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Fig. 1. A contraction process. Circles are vertices (or spins) and lines
are edges (or bonds). Solid lines are those which have not been contracted yet and dashed lines are the ones that have been contracted
already.

Fig. 2. Freezing patterns i’s of the dipolar cube with L=16 attemperatures indicated whose unit is a characteristic magnitude of thedipole-dipole interaction of interest. The system’s specific heat C andorder parameter parameter S representing the degree of the bulk O(3)symmetry are shown in the center.

So far we have described the role of the peculiar
anisotropic nature of the dipole-dipole interaction on themagnetic properties of a dipolar cube. This anisotropic
feature is apparently concentrated near the boundaries, or ofshort-ranged in nature. We have to mention, however, thatthe long-ranged nature of the interaction is indispensable forthe system to reach the lowest-energy multi-domain state [2].
Similar as well as different types of boundary-dependent
magnetic orderings are observed in finite dipolar systems cut
out from FCC and HCP lattices and from a 2-dimensionalsquare lattice.
References[1] J. M. Luttinger and L. Tisza, Phys. Rev. 70, 954 (1946).
[2] K. Matsushita et al, J. Phys. Soc. Jpn. 74, 2651 (2005).
Authors
K. Matsushita, R. Suganoa, A. Kuroda, Y. Tomita, and H. Takayama.
aAdvanced Research Laboratory, Hitachi Ltd.

not take too long, however, for researchers to realize thedifficulty of the problem from the computational point ofview. Even a mathematically rigorous statement was made:the problem of finding the ground-state of a given sample of
the Edwards-Anderson spin glass model is NP-hard if thelattice is three dimensional or higher. Today's largest systemsize that can be handled with conventional methods does notexceed L=30 for three-dimensional models, which is notenough to answer the question beyond reasonable doubts.
We here propose another way of approaching the
problem [2]. Namely, we discuss an approximate numericalmethod. One of the most well-known examples for methodsof this kind is the Migdal-Kadanoff (MK) real space
renormalization, which yields a reasonably accurate result
for the ferromagnetic Ising model and was applied laterto spin glasses. The MK method yields results that support
the droplet picture against the mean-field picture. Our
approximation procedure consists of repeated application ofan elementary operation ("contraction") to the model with agiven lattice geometry and the system size. As depicted in
Fig.1, a contraction is performed as reconnection of bonds
and recalculation of the bond strength. Starting from thegraph of the same structure as the finite lattice on which
the model is defined, we gradually deform the graph bycontraction until it becomes a tree with no loop. Once thelattice has been deformed into a tree, it is easy to computethe correlation functions because of the absence of loops. In
this way, we can treat much larger systems than we can withconventional numerical methods such as the Monte Carlosimulation.
For the three dimensional system, the spin glass susceptibility shows a drastic change near T=J, which is presumably

Tree Approximation for Spin Glass
ModelsKawashima GroupThe spin glass problem was brought forth by experiments
on random magnets, and the Edwards-Anderson model was
subsequently proposed. Since then, the simplicity of themodel and the fundamental nature of the problem have beenencouraging a great deal of effort to be invested on its study.
While the existence of the spin-glass phase transition wasestablished by Monte Carlo simulations [1], the nature of thespin glass phase is a subject of active debates even today.
Early theoretical works produced two paradigms that areconsistent with experimental findings: the mean-field picture
and the droplet picture. Investigations in the last two decades
have been focused on the question as to which paradigm iscorrect for the three-dimensional model. For solving thisproblem, the principal tool was numerical calculations. It did

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Fig. 2. The Binder parameter for the three-dimensional model ofsystem sizes of L=4, 8, 16, 32, 64 and 128 (from top to bottom at hightemperature). The arrows indicate the locations of the crossings
between two successive system sizes. The inset shows the finite size
scaling for the largest three sizes, L= 32, 64 and 128. (The other sizes
are not shown in the inset.)the transition temperature. This is confirmed by Fig.2, whichshows the temperature dependence of the Binder parameterand the finite-size-scaling plot based on it. Curves of various
system sizes cross each other and the crossing between larger
systems takes place at lower temperature. The crossing pointin the thermodynamic limit is located around T = 1.0 J andg = 0.9. We can also produce a finite-size-scaling plot thatlooks reasonable, as shown in the inset. The plot yields theestimates of the critical exponent; ν=1.85. The agreementof these values with the results of Monte Carlo simulation[3], Tc = 0.98(5) and ν= 2.00 (15), are remarkably goodconsidering that the crudeness of the present approximation.
This is also an improvement on the MK result [4], Tc = 0.39
and ν=2.8.
The variance of the distribution of the squared overlap,
q2, is also computed. The variance is a decreasing function of
the system size at any temperature. The decrease is rapid
both at the high and low temperature ranges and is somewhat
more moderate in the middle. However, even at the middle
range, the variance is extremely small and converges to zeroin the thermodynamic limit, which suggests that the overlap
distribution function P(q) has a trivial structure, consistent
with the droplet picture.
References[1] N. Kawashima and A. P. Young, Phys. Rev. B 53, R484 (1996).
[2] N. Kawashima, J. Phys. Soc. Jpn. 25, (2006) in press.
[3] E. Marinari, G. Parisi and J. J. Ruiz-Lorenzo, Phys. Rev. B 58,
14852 (1998).
[4] B. W. Southern and A. P. Young J. Phys. C 10, 2179 (1977).
Authors
N. Kawashima

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