Highlights of Joint Research
Supercomputer CenterAll staff members of university faculties or publicresearch institutes in Japan are invited to propose research
projects (called User Program). The proposals are evaluatedby the Steering Committee of SCC. Pre-reviewing is done by
the Supercomputer Project Advisory Committee. In school
year 2013 totally 245 projects were approved. The total
points applied and approved are listed on Table. 1 below.
The research projects are roughly classified into thefollowing three (the number of projects approved):

The Supercomputer Center (SCC) is a part of theMaterials Design and Characterization Laboratory (MDCL)of ISSP. Its mission is to serve the whole community ofcomputational condensed-matter physics of Japan providing
it with high performance computing environment. In particular, the SCC selectively promotes and supports large-scalecomputations. For this purpose, the SCC invites proposals forsupercomputer-aided research projects and hosts the Steering
Committee, as mentioned below, that evaluates the proposals.
The ISSP supercomputer system consists of three subsystems: System A, which is intended for a parallel computation with relatively smaller number of nodes connected
tightly, and System B, which is intended for more nodes withrelatively loose connections. In July, 2010, the SCC replaced
the two supercomputer subsystems. The current system B is
SGI Altix ICE 8400EX, which consists of 30 racks or 15360
cores whereas the system A is NEC SX-9, which consists of
4 nodes or 64 cpus. They have totally 200 TFlops. System C- FUJITSU PRIMEHPC FX10 was installed in April, 2013.
It is highly compatible with K computer, the largest supercomputer in Japan. System C consists of 384 nodes, and eachnode has 1 SPARC64TM IXfx CPU (16 cores) and 32 GB ofmemory. The total system achieves 90.8 TFlops theoreticalpeak performance.
The hardware administration is not the only function of
the SCC. The ISSP started hosting Computational Materials
Science Initiative (CMSI), a new activity of promoting
materials science study with next-generation parallel supercomputing. This activity is financially supported by the
MEXT HPCI strategic program, and in CMSI, a number ofmajor Japanese research institutes in various branches ofmaterials science are involved. The SCC supports the activities of CMSI as its major mission.

Class

Max/Min
PointsApplication

Numberof
Projects

All the three involve both methodology of computationand its applications. The results of the projects are reported
in 'Activity Report 2013' of the SCC. Every year 3-4 projects
are selected for “invited papers” and published at the beginning of the Activity Report. In the Activity Report 2013, thefollowing three invited papers are included:"One-hundred-million-atom electronic structure calculations on the K computer", Takeo HOSHI"Strong correlation of electrons studied by computational
approaches—Physics of superconductors and topologicalphases", Shiro SAKAI, Takahiro MISAWA, Youhei
YAMAJI, Mayuru KURITA, and Masatoshi IMADA"Coarse-Grained Simulation of Surfactant Membrane",
Hayato SHIBA and Hiroshi NOGUCHI

Total Points
AppliedApproved

System A

System B

System C

System A

System B

System C

350

850

500

350

850

500

A

100

any timeB

2k, 1k, 500

twice a year

56

40.7k

45.0k

6.6k

28.7k

29.8k

5.8k

C

20k, 10k, 2.5k

twice a year

130793.5k1043.0k146.9k

486.5k

391.0k98.6k

D20k, 10k, 2.5k

any time

3

0

22.0k

0

0

19.0k

0

E

0, 30k, 2.5k

twice a year24-

653.0k

43.8k-283.5k36.6k

twice a year

0

0

0

0

0

0

0

CMSI

22

-

-

-

-

-

92.5k

Total

245

834.6k

1,763.9k

197.8k

515.6k724.2k

144.0kS

10First-Principles Calculation of Materials Properties (107)
Strongly Correlated Quantum Systems (30)
Cooperative Phenomena in Complex, Macroscopic
Systems (86)

Table 1. Research projects approved in 2013
The maximum points allotted to the project of each class are the sum of the points for the two systems; Computation for
1 CPU • hour corresponds to 0.32, 0.022, and 0.042 points for System-A, System-B, and System C, respectively.

26

ISSP Activity Report 2013

Neutron Science Laboratory
The Neutron Science Laboratory (NSL) has been playing
a central role in neutron scattering activities in Japan since
1961 by performing its own research programs as well as
providing a strong General User Program for the universityowned various neutron scattering spectrometers installedat the JRR-3 (20MW) operated by Japan Atomic Energy
Agency (JAEA) in Tokai (Fig. 1). In 2003, the Neutron
Scattering Laboratory was reorganized as the Neutron
Science Laboratory to further promote the neutron science
with use of the instruments in JRR-3. Under the General
User Program supported by NSL, 14 university-group-owned
spectrometers in the JRR-3 reactor are available for a wide
scope of researches on material science, and proposals close
to 300 are submitted each year, and the number of visiting
users under this program reaches over 6000 person-day/year.
In 2009, NSL and Neutron Science Laboratory (KENS),
High Energy Accelerator Research Organization (KEK)
built a chopper spectrometer, High Resolution Chopper
Spectrometer, HRC, at the beam line BL12 of MLF/J-PARC(Materials and Life Science Experimental Facility, J-PARC).
HRC covers a wide energy and Q-range (10μeV < ℏω < 2eV
and 0.02Å-1 < Q < 50Å-1), and therefore becomes complementary to the existing inelastic spectrometers at JRR-3.
HRC started to accept general users through the J-PARC
proposal system in FY2011.
Triple axis spectrometers, HRC, and a high resolution powder diffractometer are utilized for a conventional
solid state physics and a variety of research fields on hardcondensed matter, while in the field of soft-condensed matterscience, researches are mostly carried out by using the small
angle neutron scattering (SANS-U) and/or neutron spin echo
(iNSE) instruments. The upgraded time-of-flight (TOF)
inelastic scattering spectrometer, AGNES, is also available
through the ISSP-NSL user program.
On March 11, 2011, a great earthquake with Magnitude
9.0 hit North East Coast of Japan. Fortunately, JRR-3 wasunder regular inspection and no serious accidents or damages
were reported. However, the lifeline of Tokai Village area
was lost for more than two weeks, and it took more than twomonths before damage inspection of JRR-3 could be started.
As of May of 2014, JRR-3 has not restarted yet. General
User Programs of 2012 and 2013 were cancelled and that of2014 has been suspended so far. In order to compensate theFig. 1. The reactor of JRR-3. The eight neutron scattering instruments
are attached to the horizontal beam tubes in the reactor experimental
hall. Two thermal and three cold guides are extracted from the reactor
core towards the guide hall located to the left.

Fig. 2. The U.S.-Japan spectrometer, CTAX, installed at the cold guideline CG4, High Flux Isotope Reactor (HFIR), in Oak Ridge National
Laboratory. Members who contributed the relocation project of the
U.S.-Japan spectrometer celebrate the completion of the project in
October 2010.)

loss of the activity of NSL, a number of proposals acceptedin 2011 - 2013 were transferred to overseas owing to kindoffer from the major facilities, namely, ORNL, ILL, ANSTO,
and HANARO.
The NSL also operates the U.S.-Japan Cooperative
Program on neutron scattering, providing further research
opportunities to material scientists who utilize the neutron
scattering technique for their research interests. In 2010,
relocation of the U.S.-Japan triple-axis spectrometer, CTAX,
was completed, and it is now open to users (Fig. 2).
http://neutrons.ornl.gov/instruments/HFIR/CG4C/
The activity report on Neutron Scattering Research in
JFY2011 is given in NSL-ISSP Activity Report vol. 18 (2011),
http://quasi.issp.u-tokyo.ac.jp/actrep/actrep-18-2011/indexpub_vol18.html.

International MegaGauss
Science Laboratory
The objective of this laboratory (Fig. 1) is to study thephysical properties of solid-state materials (such as semiconductors, magnetic materials, metals, insulators, superconducting materials) under ultra-high magnetic field conditions.
Such a high magnetic field is also used for controlling the
new material phase and functions. Our pulse magnets, atmoment, can generate up to 87 Tesla (T) by non-destructive
manner, and from 100 T up to 760 T (the world strongest as

Fig. 1. Building view of the International MegaGauss Science Laboratory (C-building) at ISSP.

ISSP Activity Report 201327

Fig. 4. Fig.4. A photo of the V-type single-turn coil equipped with
40 kV, (A:100+B:100=200 kJ) fast operating pulse power system.
Measurements are carried out from room temperature down to 2 K by aspecially designed cryostat.

Fig. 2. The building for the flywheel generator (left hand side) and along pulse magnet station (right hand side). The flywheel giant DC
generator is 350 ton in weight and 5 m high (bottom). The generator,
capable of a 51 MW output power with the energy storage 210 MJ, isplanned to energize the long pulse magnet generating 100 T without
destruction.

an in-door record) by destructive methods. The laboratoryis opened for scientists both from Japan and from overseas,
especially from Asian countries, and many fruitful results are
expected to come out not only from collaborative research
but also from our in-house activities. One of our ultimate
goals is to provide the scientific users as our joint researchwith magnets capable of a 100 T, milli-second long pulses ina non-destructive mode, and to offer versatile physical precision measurements. The available measuring techniques now
involve magneto-optical measurements, cyclotron resonance,
spin resonance, magnetization, and transport measurements.
Recently, specific heat and calorimetric measurements arealso possible to carry out with sufficiently high accuracy.
Our standard non-destructive-type pulse magnets areenergized by single capacitor bank and can generate fields up
to 75 T for ordinary use. Their simple sinusoidal waveforms
are advantageous for precise and reliable measurements ofvarious physical properties. Several on-demand magnets

Fig. 3. (Build. C) The building for the electro-magnetic flux
compression, generating over 700 T. 1000 T project started since 2010,
and finally condenser banks of 9 MJ (5 MJ + 2 MJ +2 MJ) as a main
system with the 2 MJ sub bank system for the seed field have been
installed, and completed in the year of 2014.

28

ISSP Activity Report 2013

having irregular shapes and sizes are developed for someparticular experiments. We open six magnet cells for parallel
experiments and accept more than 50 research projects per
year in 2013.
A 210 MJ flywheel generator (Fig. 2), which is the world
largest DC power supply (recorded in the Guinness Book ofWorld Records) has been installed in the DC flywheel generator station at our laboratory, and used as an energy source oflong pulse magnets. The magnet technologies are intensively
devoted to the quasi-steady long pulse magnet (an order of
1-10 sec) energized by the giant DC power supply. The latest
long-pulse magnet can generate fields up to 36 T with its
pulse half-period of 1 sec.
Our interests cover the study on quantum phase transitions (QPTs) induced by high magnetic fields. Fieldinduced QPTs have been explored in various materials
such as quantum spin systems, strongly correlated electron
systems and other magnetic materials. Direct thermodynamic evidences of QPTs are obtained through magnetization and recently developed caloric measurements. For some
QPTs, changes in symmetry at the transitions are sensitively
resolved through measurements of electric polarization or
optical imaging using a polarizing microscope. High resolution of electrical measurements realized the observationof quantum oscillations in high quality crystals throughmeasurements of electrical resistivity, contactless impedance,
and torque magnetometry.
Magnetic fields higher than 100 T can only be obtainedwith destructing a magnet coil, where ultra-high magnetic
fields are obtained in a microsecond time scale. Our destructive techniques have undergone intensive developments.
The project, financed by the ministry of education, culture,
sports, science and technology, is now in progress, and goal
is to generate 1000 T by the electromagnetic flux compression (EMFC) system (Fig. 3). The system which is unique to
ISSP in the world scale is comprised of a power source of 5
MJ main condenser bank and 2 MJ condenser bank and has
been accomplished its installation. Two magnet stations areconstructed and both are energized from each power source.
Both systems are fed with a 2 MJ condenser bank used for aseed-field coil, of which magnetic flux is to be compressed.
As an easy access to the megagauss science andtechnology, we have the single-turn coil (STC) systemcapable of generating the fields of up to 300 T by afast-capacitor of 200 kJ. We have two STC systems, one isa horizontal type (H-type) and the other is a vertical type

Building C
Room
101-113

Building C
Room
114-120

Building C
Room 121

Building K

Alias

Type

Bmax

Pulse width
BorePower source

Applications

OthersElectro- Magnetic
Flux Compression

destructive

730 T

μs
10 mm

5 MJ, 40kV

Magneto-Optical
Magnetization

5K–
Room temperature

Horizontal
Single-Turn Coildestructive

300 T
200 T

μs
5 mm
10 mm

0.2 MJ, 50 kV

Magneto-Optical measurements
Magnetization

5 K – 400 K

VerticalSingle-Turn Coildestructive

300 T
200 T

μs
5 mm
10 mm

0.2 MJ, 40 kV

Magneto-Optical
Magnetization

2K–
Room temperature

0.9 MJ, 10 kV

Magneto-Optical measurements
Magnetization
Magneto-Transport
Hall resistance
Polarization
Magneto-Striction
Magneto-Imaging
TorqueMagneto- Calorimetry
Heat Capacity

60 T

40 ms
18 mm

70 T

40 ms
10 mm

Mid-Pulse Magnet Non-destructive

PPMS

Steady State

14 T

ResistanceHeat Capacity

MPMS

Steady State

7T

Magnetization

Non-destructive

87 T(2-stagepulse)

Short-Pulse
magnet85 T
Long-Pulse magnet Non-destructive

36 T

5 ms
10 mm

Lowest temperature
0.1 K

Down to 0.3 K

0.5 MJ, 20 kV

Magnetization
Magneto-Transport

2K –Room temperature

210 MJ, 2.7 kVResistance
Magneto-Calorimetry

2K –Room temperature

5 ms
18 mm
1s
30 mm

Independent
Experiment in 5 site

Table 1. Available Pulse Magnets, Specifications(V-type, Fig. 4). Various kinds of laser spectroscopy experiments such as the cyclotron resonance and the Faraday
rotation using the H-type STC are available. On the other
hand, for very low-temperature experiments, a combinationof the V-type STC with a liquid helium bath cryostat is very
useful and the magnetization measurements at temperature
as low as 2 K can be performed up to 120 T with high precision.

Center of Computational Materials Science
K-computer at Kobe won the title of the world-fastest
computer at TOP500 ranking announced at ISC11. Though
it is in the 4th place in the list as of today, it is still providing
the Japanese scientific community with an incomparable
amount of computational resources. With the advancement of hardware and software technologies, large-scale
numerical calculations have been making important contributions to materials science and will have even greater
impact on the field in the near future. Center of Computational Materials Science (CCMS) is a specialized research
center for promoting computer-aided materials science withmassively parallel computers, such as K-computer. The
center also functions as the headquarters of Computational
Materials Science Initiative (CMSI), which is an inter-institutional organization for computational science of a broad
range of disciplines, including molecular science, quantum

chemistry, biological materials, and solid state physics. ISSP
made contracts with 9 universities and 2 national institutes
for supporting the activities of CMSI in which nearly 100
research groups are involved. The main purpose of CMSI isto establish a new community of computational science inwhich researches from different backgrounds work together
on grand challenge problems, thereby developing computational infrastructures (new algorithms, coding styles,
standard software packages, etc) and inspire young scientists.
CCMS has a branch office in the RIKEN AICS buildingon the Port Island Kobe, where K-computer is located,for supporting CMSI researchers getting together at the
K-computer site to exchange idears of computational
science, fine-tune various applications software, and develop
better contact with staff members of RIKEN, the operating

Fig. 1. CMSI International Satellite Meeting 2013 in Kobe

ISSP Activity Report 201329

Laser and Synchrotron Research Center(LASOR Center)Fig. 2. Workshop: Programming Techniques for K-Computer (Mishima)

institute of K-computer. Another mission of the Kobe
branch of CCMS is exchanging ideas and techniques withresearchers from other fields of computer science. (There
are 5 major fields in the HPCI strategic program of MEXT,“biology”, “materials and energy” (our field), “seismology,
oceanography and meteorology”, “industrial applications”,
and “high-energy physics and cosmology”.)
The following is the selected list of meetings organized
by CMSI and CCMS in SY2013:• “CMSI Kobe Hands-On: xTAPP Tutorial”(Apr 23, Jul 30/2013, Kobe)• “Workshop: Programming Techniques for K-Computer”(Jul 1-Jul 3/2013, Mishima)• “Internaitional Workshop: EQPCM2013”(Jun 3-Jun 21/2013, Kashiwa)• “CMSI Application Developments Seminar TOKKUN!”(Jun 5, Aug 5, Sep 3/2013, Kobe)• “CMSI Kobe Hands-On: ALPS Tutorial”(Jul 10, Nov 6/2013, Kobe)• “CMSI Kashiwa Hands-On: Machikaneyama2002
Tutorial” (Jul 26/2014, Kashiwa)• “CMSI Division 1 Summer School”(Aug 12-Aug 16/2013, Zao)
• “CMSI Kobe Hands-On: Version Control System
Tutorial” (Aug 22/2013, Kobe)• “CMSI International Satellite Meeting 2013 in Kobe”(Oct 16-Oct 18 /2013, Kobe)• “CMSI International Satellite Meeting 2013 in Nagoya”(Oct 17-Oct 19 /2013, Nagoya)• “CMSI International Satellite Meeting 2013 in Tokyo”(Oct 18-Oct 19/2013, Hongo)• “CMSI International Symposium 2013”(Oct 21-Oct 22/2013, Hongo)• “CCMS Symposium” (Nov 19-Nov 20/2013, Kashiwa)• “ISSP supercomputer - CMSI Joint Symposium”(Dec 10-Dec 13/2013, Kashiwa)• “CMSI Kashiwa Hands-On: Version Control System
Tutorial” (Jan 14/2014, Kashiwa)• “Workshop: Programming Techniques for K-Computer”(Jan 28-Jan 30/2014, Atami)• “CMSI Kashiwa Hands-On: xTAPP Tutorial”(Feb 26/2014, Kashiwa)• “Joint Symposium of Elements Strategy Initiative,
CMSI, SPring-8, J-PARC and KEK”
(Feb 28-Mar 1/2014, Kashiwa)• “CMSI Kobe Hands-On: Rokko Tutorial”(Mar 11/2014, Kobe)

Laser and Synchrotron Research (LASOR) Centerstarted from October, 2012. LASOR Center aims to promotematerial sciences using advanced photon technologies at
ISSP by combining the “Synchrotron Radiation Laboratory” and “Advanced Spectroscopy Group”. These twogroups have long histories since 1980’s and have kept strong
leaderships in each photon science fields for a long timein the world. In the past several decades, the synchrotronbased and laser-based photon sciences have made remarkable progresses independently. However, recent progresses
in both fields make it feasible to merge the synchrotronbased and laser based technologies to develop a new direction of photon and materials sciences. In the LASOR Center,extreme laser technologies such as ultrashort-pulse generation, ultraprecise control of optical pulses in the frequency
domain, and high power laser sources for the generation ofcoherent VUV and SX light are intensively under development. The cutting edge soft X-ray beamline is also developed
at the synchrotron facility SPring-8.
LASOR center aims three major spectroscopic methods[ultrafast, ultra-high resolution, and operand spectroscopy]
by three groups [extreme laser science group, soft-X-ray
spectroscopy and materials science group, and coherent
photon science group], as illustrated in Fig.2. Under thisframework, various advanced spectroscopy, such as ultrahigh resolution photoemission, time-resolved, spin-resolved

Fig. 1. Open ceremony of LASOR center on October 2012.

Fig. 2. Developments of advanced spectroscopy at LASOR center by
three groups

30

ISSP Activity Report 2013

Fig. 3. Close look of a high-peak-power ultrashort-pulse laser

Fig. 5. 10-MHz high harmonic generation in an enhancement cavity.

spectroscopy, diffraction, light scattering, imaging, microscopy and fluorescence spectroscopy are in progress byemploying new coherent light sources based on laser andsynchrotron technologies that cover a wide spectral range
from X-ray to terahertz. In LASOR Center, a variety ofmaterials sciences for semiconductors, strongly-correlatedmaterials, molecular materials, surface and interfaces, andbio-materials are studied using advanced light sources andadvanced spectroscopy. Another important aim of LASOR
Center is the synergy of photon and materials sciences. Mostof the research activities on the extreme laser development
and their applications to material science are performed in
the ISSP buildings D and E at Kashiwa Campus where large
clean rooms and the vibration-isolated floor are installed.
On the other hand, the experiments utilizing the advanced
synchrotron source are performed at beamline BL07 in
SPring-8 (Hyogo).

tion extending to a photon energy of ~330 eV. The simulation predicts the soft-X-ray field consists of single isolated
attosecond pulses. For ultra-high resolution spectroscopy,
fiber-laser-based light sources are intensively developed
for producing EUV pulses for high resolution and timeresolved photoemission spectroscopy as well as extending
the frequency comb to ultraviolet or infrared for various
applications. The spectral range of intense optical pulses arebeing extended from visible to IR, MIR and THz ranges.
Various types of high-repetition-rate ultrastable light sources
are developed for laser-based ultrahigh resolution photoemission spectroscopy, high-average-power EUV generation in
an enhancement cavity, and frequency comb spectroscopy
for atomic physics, astronomical application, and frequency
standards.

• Extreme Laser Science GroupThe advancement of ultrashort-pulse laser technologies
in the past decade has transformed the laser development
at ISSP into three major directions, (i) towards ultrashortin the time domain, (ii) ultra high resolution in the spectral
domain, and (iii) the extension of the spectral range, withextreme controllability of the laser sources. For ultrafast
spectroscopy, we have developed carrier-envelope phasestable intense infrared light source that can produce sub-two
cycle optical pulses for high harmonic and attosecond pulse
generation. So far we observed coherent soft-X-ray radia-• Soft-X-ray and Materials Science Group
Recently, VUV and SX lasers have been developed. They
become very usuful for the materials science, if we develop
the SX spectroscopy, such as the cutting-edge photoemission spectroscopy. Laser has a lot of fruitful properties. For
examples, by using monochromatic light, ultra-high resolution angle-resolved photoemission spectroscopy (ARPES) isdeveloped. The achieved resolution of 70-μeV is the highest
resolution of the world. Fermiology of the materials science
with μeV resolution is improved drastically by using thelasers. On the other hand, when we use pulsed laser light, thetime resolved ARPES becomes possible. The time resolved
photoemision in fs region is powerful to know the relaxation
process of photo-induced phase transition. Furthermore, byusing CW and circulary light in VUV region, the photoelec-

Fig. 4. Phase-dependence of high harmonic spectra in soft X rays.

Fig. 6. Pump-probed photoemission system using 60-eV laser

ISSP Activity Report 201331

Fig. 1. 3D nano ESCA at SPring-8 BL07LSU
Fig. 7. Photonics devices under study: (left panel) semiconductor
quantum wires and (right panel) firefly-bioluminescence system
consisting of light emitter (oxyluciferin) and enzyme (luciferase)tron microscopy(PEEM) is developed. The spatial resolutionof nm region is very powerful for the nanomaterials.
• Coherent Photon Science GroupThe coherent-photon science group has main interests in exploring a variety of coherent phenomena andnon-equilibrium properties of excited states in condensed
matters, in collaborations with research groups in chargeof photoemission, operand-spectroscopy and extreme laser
science. This group covers a wide range of materials, fromsemiconductors, ferromagnets, complexes and superconductors to biomaterials. Various ultrafast optics technologies
such as femtosecond luminescence and pump-and-probetransmission/reflection spectroscopy are applied to studies on
wavepacket dynamics, photo-induced phase transitions andcarrier dynamics. Coherent control and observation of spin
dynamics in magnetic materials and metamaterial structuresby using high power terahertz radiation source is extensively
studied. Advanced photonics devices are intensively studied,such as quantum nano-structure lasers with novel low-dimensional gain physics, low-power light-standard LEDs, very
efficient multi-junction tandem solar cells for satellite use,
and wonderful bio-/chemi-luminescent systems for widebio-technology applications.

netic nanofilms.
The SRL staffs have joined the Materials Research
Division of the Synchrotron Radiation Research Organization (SRRO) of the University of Tokyo and they haveplayed an essential role in constructing a new high brilliant
soft X-ray beamline, BL07LSU, in SPring-8. The lightsource is the polarization-controlled 25-m long soft X-ray
undulator. The monochromator is equipped with a variedline-spacing plain grating, which covers the photon energy
range from 250 eV to 2 keV. At the downstream of thebeamline, four experimental stations have been developed forfrontier spectroscopy researches: the three-dimensional (3D)

Fig. 2. XES station at SPring-8 BL07LSU

Synchrotron Radiation Laboratory
The Synchrotron Radiation Laboratory (SRL) was established in 1975 as a research division dedicated to solid statephysics using synchrotron radiation (SR). In 1989, SRL
started to hold the Tsukuba branch, in the Photon Factory(PF), High Energy Accelerator Research Organization(KEK). SRL maintains a Revolver undulator, two beamlines
and three experimental stations; BL-18A for angle-resolved
photoemission spectroscopy with SCIENTA electron
analyzer, while undulator beamline BL-19A and BL-19B,
for spin- and angle-resolved photoelectron spectroscopy(SARPES) and soft X-ray emission spectroscopy experiments, respectively. Recently, a high-yield spin detector,
using very low energy electron diffraction, was developed at
BL-19A. SAPRES measurements have now been performed
with high-resolution and the experiments at the beamline
have become important for exciting topics of surface/solidstate physics such as topological insulators and ferromag-

32

ISSP Activity Report 2013

Fig. 3. TR-SX station at SPring-8 BL07LSU

nano-ESCA station, the soft X-ray emission spectroscopy(XES) station, the time-resolved soft X-ray spectroscopy(TR-SX) station, and the free-port station for any experimental apparatus. The beamline construction was completed
in 2009 and SRL established the Harima branch laboratory
in SPring-8. At SPring-8 BL07LSU, each end-station has
achieved high performance: the 3D nano-ESCA reaches thespatial resolution of 70 nm, the XES station obtains spectrawith energy resolving power E/∆E larger than 5,000, andthe TR-SX have established the laser-pump and SR-probe
method with the time-resolution of 50 ps which corresponds
to the SR pulse-width. The four end-stations have now been
opened fully to outside users. In 2013, 153 researchers madetheir experiments during the SPring-8 operation time of 3400
hours.

ISSP Activity Report 201333

Experimental Realization of a Quantum
Breathing Pyrochlore Antiferromagnet
K. Kimura and S. Nakatsuji
Exploring novel and exotic phenomena associatedwith spin degrees of freedom has been central subject
in condensed matter physics [1]. One of the most attractive systems in three dimension is a pyrochlore latticemagnet, which consists of corner-sharing regular tetrahedraof magnetic ions. The inherent geometrical frustration
suppressing a conventional magnetic order often leads toa variety of unusual properties [1]. A key building unit ofpyrochlore magnets is a single spin tetrahedron. Besides
being a good starting point for considering the essence ofphysics of full pyrochlore lattice, it has been theoreticallyshown to possess interesting properties such as a doubly
degenerate singlet state which can be labelled by scalar spinchirality [2]. To our knowledge, however, there has been no
experimental realization of the S = 1/2 quantum spin tetrahedral system, partly because typical 3d transition metal ionswith S = 1/2 (like Cu2+) are difficult to keep the tetrahedral
symmetry due to the inherent Jahn-Teller instability.
Here, we report the new Yb-based quantum spin system
Ba3Yb2Zn5O11 [3]. Polycrystalline samples were synthesized
by the standard solid state reaction method. The Rietveld
refinement on powder X-ray diffraction pattern has revealed
that this material crystallizes into the cubic structure withspace group F-43m, and Yb3+ ions form a so-called breathing
pyrochlore lattice [4] characterized by an alternating arrayof small and large Yb3+ regular tetrahedra (Fig. 1(a), inset).
Analyses of the crystalline electric field (CEF) scheme of
Yb3+ ions (4f13) and magnetic susceptibility data down to
30 K show that the CEF ground state is approximated bya magnetic Kramers doublet with the isotropic effective
g-factor geff = 2.66 for pseudospin-1/2. The very large gap(>500 K) to the excited states ensures that the low-temperature properties are described by pseudospin-1/2.
The temperature dependence of the magnetic susceptibility below 30 K is shown in Fig. 1(a). A broad maximum
at around 4 K suggests a formation of a quantum spin singlet
state due to antiferromagnetic interactions in a small Ybtetrahedron. This is supported by the magnetization curves
at selected temperatures (Fig. 1(b)). Though linear above 4
K, the magnetization curves show a clear non-linear increaseat B ~ 3 T below 4 K, a signature of the singlet-triplet crossover. These results are analyzed based on the pseudospin-1/2
single tetrahedron model with Heisenberg interactions J.
The good agreement is obtained with J = -6.43 K and geff =2.569, as indicated by the solid lines in Figs. 1(a) and 1(b),suggesting a formation of the doubly degenerate singlet stateFig. 1. (a) The temperature dependence of the magnetic susceptibility
below 30 K. Inset: Breathing pyrochlore lattice formed by Yb ions.
(b) The magnetization curves at selected temperatures. Solid lines in(a) and (b) represent calculated values based on the single tetrahedronmodel with Heisenberg J = -6.43 K and geff = 2.569.

34

ISSP Activity Report 2013

Fig. 2. The temperature dependence of (a) the magnetic specific heat
CM and (b) corresponding magnetic entropy SM. Solid lines in (a)and (b) are fit to the single tetrahedron model. The dashed lines in(b) denote the entropy for a two level system Rln(2) and for a doubly
degenerate singlet state (3/4)Rln(2).

with chirality fluctuations predicted by the model [2]. Solidevidence for the double degeneracy is provided by magnetic
specific heat CM measured at ISSP (Fig. 2(a)). CM down
to 0.38 K exhibits a broad peak associated with the singlet
formation without any signs of long-range order. Corresponding magnetic entropy SM is shown in Fig. 2(b). The
saturated value at 20 K is close to 75% of the value expected
for a standard two level system Rln(2), and 25% of magnetic
entropy remains below 0.38 K. This value is fully consistent
with the doublet degeneracy of the singlet state expectedfrom the single tetrahedral model. Moreover, as indicated by
the red line of Figs. 2(a) and 2(b), CM and SM are well reproduced by the model.
All the data presented in this study therefore uncover theunique doubly degenerate singlet state with chirality fluctuations at T = 0.38 K. To the best of our knowledge, this is a
new quantum state of matter which has never been established in existing materials. The mechanism for the liftingof the degeneracy is highly interesting and left for the future
study.
References[1] L. Balents, Nature 464, 199 (2010).
[2] H. Tsunetsugu, J. Phys. Soc. Jpn. 70, 640 (2001).
[3] K. Kimura, S. Nakatsuji, and T. Kimura, arXiv: 1404.6439[4] Y. Okamoto, G. J. Nilsen, J. P. Attfield, and Z. Hiroi, Phys. Rev.
Lett. 110, 097203 (2013).
Authors
K. Kimuraa, S. Nakatsuji, and T. KimuraaaOsaka University

Entanglement Spectra between
Coupled Tomonaga-Luttinger Liquids
R. Lundgren, Y. Fuji, and S. Furukawa
Quantum entanglement has emerged as a new tool tocharacterize quantum phases. While the entanglement isnothing but “quantum correlation” that has been a central
issue in condensed matter physics for a long time, newquantitative descriptions of the entanglement stimulated byquantum information theory have proved useful. The mostfrequently used quantity to describe the entanglement inquantum many-body problem is “entanglement entropy”.
However, there are other quantities which can be useful, andentanglement spectrum is one of them. Its relation to thespectrum of physical edge states is conjectured [1], and has
been confirmed in several examples.
Here we report on a recent joint research [2] on entanglement spectra between two coupled Tomonaga-Luttinger

Fig. 1. The dispersion of entanglement spectra of a spin ladder model
when a linear combination of two TL fields remains gapless while theother acquires a gap. The numerically obtained dispersion relationagrees well with the peculiar square-root dependence predicted by thetheory.

liquids (TLLs). TLL is a ubiquitous effective field theory forquantum many-body problem in one dimension, and many
physical systems such as quantum spin ladders and carbon
nanotubes are described as coupled TLLs. There are varietyof phases in coupled TLLs. In a class of gapless phases ofcoupled non-chiral TLLs, we find an entanglement spectrumwith a dispersion relation proportional to the square root of
the subsystem momentum. This result was derived using thefield theory, and was then confirmed numerically for a spinladder model. Such a dispersion is generally not expected
for a spectrum of physical excitations, including those ofedge states. We relate the unusual dispersion relation in theentanglement spectrum to a long-range interaction in theentanglement Hamiltonian. Our result sheds new light on thecorrespondence between the entanglement spectrum and thespectrum of the edge states.
This work was performed during a visit by R. Lundgren,a Ph. D. student at University of Texas, under the NSF
East Asia and Pacific Summer Institutes for U.S. Graduate
Students and the JSPS Summer Program, hosted by ISSP in
cooperation with Department of Physics, the University ofTokyo.
References[1] H. Li and F. D. M. Haldane, Phys. Rev. Lett. 101, 010504 (2008).
[2] R. Lundgren, Y. Fuji, S. Furukawa, and M. Oshikawa, Phys. Rev. B
88, 245137 (2013).

by the surface [2], and thus is suitable for investigating the
TSS characteristics.
The elastic-scattering vector within the TSS on thecleaved surface was obtained from a quasiparticle interference (QPI) pattern, which is a Fourier-transformed constantvoltage dI/dV image measured using scanning tunnelingmicroscopy at 5 K (Fig. 1). The observed elastic scattering
amplitude within the TSS was critically suppressed beyond
certain scattering-vector lengths, both in the Γ̅ -M
̅ and in Γ̅ -K̅
directions. That is, there is a critical scattering-vector length,

Fig. 1. (a) Topographic STM image of a cleaved surface of
Bi1.5Sb0.5Te1.7Se1.3. (b) dI/dV image at the sample bias voltage Vb= 0.28 V. (c) Amplitude map of the elastic scatterings, or QPI map,derived by Fourier transformation (FT) of the dI/dV image shown in(b). (d) FT amplitude image in Γ̅ - K
̅ and in Γ̅ -M
̅ directions as functionsof Vb. Critical scattering-vector lengths are plotted as red and blue
symbols.

Authors
R. Lundgrena, Y. Fuji, S. Furukawab, and M. OshikawaaUniversity of Texas at Austinb
The University of Tokyo

Robust Protection from Backscattering in
a Topological Insulator
Y. Ando, S. Shin and F. Komori
Surface of three-dimensional (3D) topological insulators (TIs) is a promising platform for novel device functions,whereon robust spin-polarized surface states occur due
to topological feature of the bulk wave functions. Highcarrier mobility is expected on 3D TI surface because 180ºbackscattering is forbidden under the spin-helical texture of
the topological surface states (TSSs). However, the prohibition range of backscattering has not been clarified yet. Here,
we study how the elastic scattering is suppressed as functionsof the scattering angle and electron energy in a single andunwarped upper Dirac cone of Bi1.5Sb0.5Te1.7Se1.3 [1]. The
quaternary compound Bi1.5Sb0.5Te1.7Se1.3 is identified to abulk-insulating TI, whose transport properties are dominatedFig. 2. (a,b) Intensity maps of ARPES before (a) and after (b)impinging a 170-fs pump laser pulse (1.5 eV). The dispersions above
EF are revealed by pumping the electrons into the unoccupied side.
An arrow in (b) indicates the bottom of the bulk conduction band. (c)Comparison between critical scattering-vector lengths qcx and qcy (redand blue symbols) obtained from QPI images and the dispersion of the
TSS (circles) from pump-probe ARPES. (d) Schematic of the elastic
scattering within a nearly ideal Dirac TSS. Scattering angle larger than
100º is effectively prohibited.

ISSP Activity Report 201335

qc, beyond which the scattering is effectively prohibited.
The critical length increased with the increase of energy
from the Dirac point as shown in Fig. 1(d). The band dispersions displayed in Fig. 2(a,b) were recorded by using angleresolved photoemission spectroscopy (ARPES) implemented
by a pump-and-probe method. Here, the dispersions above
the Fermi level were revealed by pumping electrons into
the unoccupied states (Fig. 2(b)). A nearly ideal Dirac-cone
dispersion was observed within the bulk band gap. The locusof the Dirac point, bulk-conduction-band minimum, andthe band velocity of the TSS were obtained. We find thatthe critical scattering vector length is 75 % shorter than thelength of the 180o backscattering vector (Fig. 2(c)). This
indicates that the scattering in the TSS is effectively prohibited in a wide angular range of 100-180o as schematicallyshown in Fig. 2(d). The robust protection from backscattering is good news for applications, but it poses a challenge
to the theoretical understanding of the transport in the TSS.
References[1] S. Kim, et al., Phys. Rev. Lett. 112, 136802 (2014).
[2] A. A. Taskin et al., Phys. Rev. Lett. 109, 066803 (2012).
Authorsaaa
S. Kim, S. Yoshizawa, Y Ishida, K. Eto , K. Segawa Y. Ando , S. Shin,andF. KomoriaOsaka University

Microscopic Origin of π Electronic
States in Silicene Revealed by
Scanning Tunneling Microscopy
Y. Yamada-Takamura, T. Ozaki, and Y. Hasegawa
Silicene, a monolayer of silicon atoms forming a
two-dimensional honeycomb lattice, has attracted significant attention in condensed matter physics because it has
electronic states similar with its carbon counterpart, grapheneand shares almost all the remarkable properties of graphene[1], such as massless Dirac fermion, pseudo spin, and the K/
K' valleys. There are, however, some differences between
the two ultimately thin materials; while graphene has aplanar structure, silicene is buckled [1]; the atoms in the two
sub-lattices have different heights, providing us a possibilityof inducing a staggered potential by an application of an
external electrical field [2]. Different from graphene, silicene
exhibits significant spin-orbit coupling [3], making it bear
topologically nontrivial electronic structure, which realizes
the quantum spin Hall effect or two-dimensional topologicalinsulator. The staggered potential can lift up degeneracy of
the K and K' valleys, opening up a possibility of an effective
spin polarized electron source [4].
Silicene has been formed so far on metal substrates, suchas Ag [5-7] by depositing Si on the substrate. It can also
be formed epitaxially on ZrB2 thin film grown on Si(111)substrate [8]; by annealing silicon atoms segregate from thesubstrate to form the one-monolayer silicon thin film on the
ZrB2(0001). Because of the lattice—matching— between the 2×2
unit cell of ZrB2(0001) and the √ 3 × √ 3 unit cell of silicene,— —the silicene on ZrB2 exhibits a √ 3 × √ 3 reconstruction.
Using a low-temperature scanning tunneling microscopy
and spectroscopy (STM/STS), we investigated atomic andelectronic structures of the silicon layer [9]. By comparing
the experimental results with those of first-principles density
functional theory calculations, we determined the atomic

36

ISSP Activity Report 2013

Fig. 1. (a) atomic structure of epitaxial silicene on ZrB2(0001) derived
from DFT calculations. Si atoms are colored in blue and Zr atoms arecolored in grey. (b) STM image (2 nm × 2 nm) (c-f) tunneling conductance (dI/dV) images at the sample bias voltages of -0.47 V, -0.36 V,-0.12 V, and -0.02 V, respectively.

structure and discussed the electronic states and the bonding
nature of each Si atoms within the unit cell.
Figure 1 shows results taken by STM/STS. Figure 1(a)is an atomic structural model determined in the present
study. Figure 1(b) is an STM image, and (c-f) are tunnelingconductance (dI/dV) images taken at various bias voltages,that is, local density of states (LDOS) mappings at the corresponding energy level with respect to the Fermi energy.
The structural model (Fig. 1(a)) and the STM image (b)indicate that the protrusions observed in the STM image
come from the atoms, marked C in the schematic, sitting
on top of Zr atoms. Because of the local configuration, thebuckling of the atom C is suppressed, making its orbitals
hybridize in planar sp2 manner. The tunneling spectra takenon the atoms indicate significant contribution of the pz orbital
to π/π* valence/conduction bands, which was observedwith angle-resolved photoemission (ARPES) [10]. The— —STS results also confirm the band gap due to the √ 3 × √ 3
buckled reconstruction observed by the ARPES study. On theother hand, atom A, which belongs to the same sub-lattice
as atom C, exhibits buckling larger than the free-standing
silicene, and possesses sp3-like hybridized orbitals. The
STM/STS results evidenced a clear correlation between
hybridization of the orbitals of the Si atoms and the buckling.
References[1] S. Cahangirov, M. Topsakal, E. Akturk, H. Sahin, and S. Ciraci,
Phys. Rev. Lett. 102, 236804 (2009).
[2] M. Ezawa, New J. Phys. 14, 033003 (2012).
[3] C.-C. Liu, W. Feng, and Y. Yao, Phys. Rev. Lett. 107, 076802 (2011).
[4] W.-F. Tsai, C.-Y. Huang, T.-R. Chang, H. Lin, H.-T. Jeng, and
A. Bensil, Nat. Commun. 4, 1500 (2013).
[5] P. Vogt, P. De Padova, C. Quaresima, J. Avila, E. Frantzeskakis,
M. C. Asensio, A. Resta, B. Ealet, and G. Le Lay, Phys. Rev. Lett. 108,
155501 (2012).
[6] L. Chen, C.-C. Liu, B. Feng, X. He, P. Cheng, Z. Ding, S. Meng,
Y. Yao, and K. Wu, Phys. Rev. Lett. 109, 056804 (2012).

[7] C.-L. Lin, R. Arafune, K. Kawahara, N. Tsukahara, E. Minamitani,
Y. Kim, N. Takagi, and M. Kawai, Appl. Phys. Express 5, 045802(2012).
[8] A. Fleurence, R. Friedlein, T. Ozaki, H. Kawai, Y. Wang, andY. Yamada-Takamura, Phys. Rev. Lett. 108, 245501 (2012).
[9] A. Fleurence, Y. Yoshida, C.-C. Lee, T. Ozaki, Y. Yamada-Takamura,and Y. Hasegawa, Appl. Phys. Lett. 104, 021605 (2014).
[10] R. Friedlein, A. Fleurence, J. T. Sadowski, and Y. YamadaTakamura, Appl. Phys. Lett. 102, 221603 (2013).
AuthorsaaaaA. Fleurence , Y. Yoshida, C.-C. Lee , T. Ozaki , Y. Yamada-Takamura ,andY.
HasegawaaJapan Advanced Institute of Science and Technology

Multiferroic Nanopillar Composites
Y. Matsumoto and M. Lippmaa
Composite materials can be used to elastically couple
ferroelectrics and ferromagnets, producing a multiferroic
material where the magnetization can be controlled by anelectric field or the dielectric polarization by a magnetic
field. In this work, a nanoscale pillar-and-matrix composite,
consisting of a layered Bi5Ti3FeO15 (BTFO) perovskite
matrix and nanoscale pillars of a ferrimagnetic spinel
CoFe2O4 (CFO), were used to study the effectiveness ofnanoscale strain-mediated elastic coupling. The main advantage of this approach is that it may be possible to develop
new types of multiferroics that can work close to room
temperature and do not contain lead.
A three-dimensional model of the composite materialis shown in Fig. 1(b), together with a surface electron
microscope image in Fig. 1(a). The pillars form spontaneously during thin film growth due to immiscibility of theperovskite and spinel phases and extend throughout the
thickness of the thin film, as shown in the cross-sectionaltransmission electron microscope image in Fig. 1c. Composition mapping shows nearly perfect segregation of Fe/Co and
Bi/Ti in pillars with an average diameter of about 50 nm.
From the point of view of achieving elastic multiferroic
coupling in a nanoscale composite, strain fields along thewalls of the nanopillars are critically important. The behavior
was analyzed by high-resolution electron microscopy, whichshowed that the nanostructure-matrix interface is heavily
strained. The strain relaxes gradually in the BTFO matrix,

Fig. 2. (a) Magnetization comparison of a pure BTFO matrix and the
BTFO/CFO nanocomposite film, showing the presence of ferromagnetism in the nanopillars. Inset shows the magnetization of a pure
CFO sample. (b) A ferroelectrically poled pattern on the surface of ananocomposite film, measured with a piezoresponse force microscope.

as illustrated by the red radial arrows in Fig. 1(b). Ferroelectric switching in the BTFO matrix material was studied by
piezoresponse force microscopy, showing that the polarization response depends on the volume fraction of nanopillars,reaching a maximum for a CFO volume of 35%. A bipolar
poling pattern for this sample is shown in Fig. 2(b). Analysis
showed the presence of both in-plane (ab plane) and out-ofplane (c-axis) polarization components, which is unusual forlayered bismuth perovskite ferroelectrics where the polarization direction is in the ab plane. Thermodynamic LandauGinzburg-Devonshire (LGD) calculations showed that this isa direct result of the strain fields surrounding the nanopillars
and may also be caused by the presence of shear domains
at pillar boundaries that are not perfectly c-axis oriented.
Magnetization analysis of the composites at 10 K showedthat the saturation magnetization of CFO is not significantly
affected by the 50 nm diameter of the nanopillars (Fig. 2a),reaching nearly the expected 4 µB/f.u. of bulk CFO.
The work showed that systematic strain control ispossible in spontaneously-formed nanopillar composites
consisting of a spinel ferromagnet and a perovskite ferroelectric. The piezoresponse analysis showed that the threedimensional nanoscale structuring and the associated strain
fields are critical for obtaining suitable polarization behavior
in the layered bismuth ferroelectrics. Work is now underway
to determine the multiferroic response in such nanocomposite systems.
Reference[1] A. Imai, X. Cheng, H. L. Xin, E. A. Eliseev, A. N. Morozovska,
S. V. Kalinin, R. Takahashi, M. Lippmaa, Y. Matsumoto, and
V. Nagarajan, ACS Nano 7, 11079 (2013).
Authorsa,bbcddA. Imai , X. Cheng, H. L. Xin , E. A, Eliseev , A. N. Morozovska,ef
S. V. Kalinin,
R.
Takahashi,
M.
Lippmaa,Y.
Matsumoto
,
and
b
V.
NagarajanaTokyo Institute of Technologyb
University of New South Walesc
Brookhaven
National LaboratorydNational Academy of Sciences of Ukrainee
Oak Ridge National Laboratoryf
Tohoku University

Fig. 1. (a) Scanning electron microscope image of the nanocomposite
film surface and (b) a three-dimensional model of the composite
material. The elastic strain field surrounding the nanopillars is marked
with red arrows. (c) Cross-sectional electron microscope image and (d)Co/Fe and (e) Bi/Ti EDS mapping images.

ISSP Activity Report 201337

Development of High Pressure, High Field
and Multi-Frequency ESR System
Using Hybrid Type Pressure Cell
T. Sakurai, H. Ohta, and Y. Uwatoko
Pressure has been recognized as one of the most important parameters to explore novel phenomena around thequantum critical point for quantum spin systems. As thehigh field and multi-frequency ESR is a powerful means tostudy quantum spin systems from the microscopic point ofview, it is very useful to introduce the parameter of pressure
to the ESR measurement. We developed the high pressure,high field and multi-frequency ESR apparatus by combining
the single-pass transmission type ESR apparatus using thepulsed high magnetic field up to 55 T and the unique piston
cylinder pressure cell previously [1]. The most characteristicfeature of this pressure cell is that all inner parts are made ofzirconium oxide which has relatively good transmittance ofelectromagnetic wave. It enables us to observe ESR underpressure. However, the pressure range is limited below 1
GPa at most because the single layer cylinder is used andits inner and outer diameters are 3 and 8 mm, respectively.
The limited sample space also affects the signal intensity.
The signal to noise ratio is not enough to study spin states indetail. Moreover, several compounds have been suggested tohave the critical pressures above 2 GPa recently [2]. Therefore, the new ESR apparatus which has higher pressure range
and sensitivity is required. In this study, we have developed
new ESR apparatus which uses the combination of thehybrid type pressure cell and the superconducting magnet to
improve these two points [3].
Figure 1 shows the newly developed pressure cell for
multi-frequency ESR measurement. The hybrid type cylinder
which consists of inner NiCrAl cylinder and outer CuBe
sleeve is used to achieve both larger sample space and higher
pressure range above 2 GPa. The inner and outer diametersof the cylinder are 5 and 28 mm, respectively. The innerparts are all made of zirconium oxide. We confirmed thatthe pressure can be generated over 2.5 GPa at low temperature. However, they are sometimes cracked when the load isapplied to generate the pressure over 2.5 GPa at low temperature. We have also developed new ESR apparatus using
cryogen free superconducting magnet with wide bore. Gunn
oscillator and backward wave oscillator which cover thefrequency region from 50 to 400 GHz are used as the lightsource. The ESR signal is detected by an InSb detector andFig. 2. Pressure dependence ESR spectra of CsCuCl3 for H||c at
105 GHz.

the signal is amplified by the lock-in technique. The larger
sample space and the use of the lock-in technique makethe sensitivity higher than previous apparatus successfully.
Figure 2 shows typical ESR spectra obtained by the developed high pressure ESR apparatus. CsCuCl3 is a well known
ABX3 type antiferromagnet with TN = 11 K. Figure 2 shows
the antiferromagnetic resonance of this compound obtained
at 4.2 K for H||c. It clearly shows that the resonance field
shifts to the higher field side as the pressure is increased.
This corresponds to the increase of the antiferromagnetic
gap on applying the pressure [4]. The maximum pressure
obtained in this measurement is 2.7 GPa as shown in Fig. 2.
This result also shows that this high pressure ESR apparatus
is promising tool to clarify the spin states of novel pressure
induced phenomenon for quantum spin system.
References[1] T. Sakurai et al., Rev. Sci. Instrum. 78, 065107 (2007).
[2] T. Sakurai et al., J. Phys. Conf. Ser. 150, 042171 (2009).
[3] K. Fujimoto et al., Appl. Magn. Reson. 44, 893 (2013).
[4] T. Sakurai et al., J. Phys. Conf. Ser. 215, 012184 (2010).
Author aaaT.
Sakurai , S. Okubo , H. Ohta , K. Matsubayashi, and Y. UwatokoaKobe University

One-Hundred-Million-Atom Electronic
Structure Calculations on the K Computer
T. HoshiFig. 1. Cut view of the hybrid type pressure cell for multi-frequency
ESR measurement. Red lines show the electromagnetic wave.

38

ISSP Activity Report 2013

In the current (peta-scale) or next generation (exa-scale)computational physics, a crucial issue is ‘ApplicationAlgorithm-Architecture co-design’ or inter-disciplinary
collaborations among physics, applied mathematics and thehigh-performance computation field.
Here we report that one-hundred-million atom (100-nmscale) electronic structure calculations were realized on the
K computer [1,2]. The methodologies were based on the
co-design. In particular, the novel iterative (Krylov-subspace)algorithms were constructed for the generalized shifted linear
equations ((zS-H)x=b), instead of the conventional generalized eigen-value equation (Hy=eSy) (See [3,4] and reference
therein). The methods give the real-space Green’s function,
instead of eigen states. These solvers were implemented in
our order-N calculation code ELSES (http://www.elses.jp/)
with modeled (tight-binding) systems based on ab initio

calculations. These solver algorithms are purely mathematicaland applicable to large-matrix problems in many computational physics fields beyond electronic structure calculations.
In Fig. 1, the calculations show high parallel efficiency(‘strong scaling’) with up to the full core calculations of the
K computer for one-hundred-million-atom (100-nm-scale)systems [1,2]. The calculated systems are amorphous-likeconjugated polymer (aCP), poly-(9,9 dioctyl-fluorene) with2
3
N=102,238,848 atoms and sp -sp nano-composite carbon
solid (NCCS) with N=103,219,200 atoms. The high parallel
efficiency stems not only from the fundamental mathematicaltheory but also from detailed techniques, such as programing
techniques for saving memory and communication costs anda parallel file I/O [4].
An application study with NCCS is picked out [4].
The study is an early-stage one on the formation processof the nano-polycrystalline diamond (NPD), a novel ultrahard material [5]. NPD is obtained by direct conversion
sintering process from graphite under high pressure and hightemperature and has characteristic 10-nm-scale lamellar-likestructures. NPD is of industrial importance for its extreme
hardness and strength and Sumitomo Electric Industries.
Ltd. began commercial production from 2012. Our simulation is motivated by the investigation of possible precursor
structures in the formation process of NPD and the struc2
tures should be nano-scale composites of sp (graphite-like)3

and sp (diamond-like) domains. Figure 2 shows the nanodomain analysis on the NCCS. The structure is a result of
our simulation. The π-type crystalline orbital Hamiltonian
population (πCOHP) analysis [4], an analysis method based2on the Green’s function, was used so as to distinguish the sp
3
and sp domains. The analysis clarifies shapes of domains
and structure of domain boundaries from huge electronic
structure data and gives theoretical foundations of thecomposite.
General future aspects are (i) large-scale calculation
methods for optical and transport properties and (ii) applications to various systems, such as organic materials.
The fundamental mathematical theory was constructedin the collaboration with applied mathematics researchers;
T. Sogabe (Aichi Prefectural University) and S.-L. Zhang(Nagoya University) [3]. The code for massive parallelism
was developed with the supercomputers, the systems B andC, at ISSP and the K computer was used in the research
proposals of hp120170, hp120280 and hp130052.
References[1] T. Hoshi et al., JPS Conf. Proc. 1, 016004 (2014).
[2] T. Hoshi et al., Proceedings of Science, in press (Preprint : http://
arxiv.org/abs/1402.7285/)[3] T. Hoshi et al., J. Phys.: Condens. Matter 21, 165502 (2012).
[4] T. Hoshi et al., J. Phys. Soc. Jpn. 82, 023710 (2013).
[5] T. Irifune et al., Nature 421, 599 (2003).
Author a,b
T.
Hoshi
a
Tottori
Universityb
JST-CREST

Rolled Lamellar Structure of Surfactant
Membranes Induced by Shear Flow
H. Shiba, H. Noguchi, and G. Gompper

Fig. 1. Benchmark for the parallel efficiency (‘strong scaling’) on the K
computer with one hundred million atoms. [1,2] The calculated systems
are amorphous-likeconjugated polymer (aCP), poly-(9,9 dioctyl23fluorene) and sp -sp nano-composite carbon solid (NCCS).

Fig. 2. Nano-domain analysis of NCCS visualized with the πCOHP [4].
The sp2 (graphite-like) and sp3 (diamond-like) domains are visualizedin (a), while only the sp2 domains are visualized in (b). A closeup of an
sp2- sp3 domain boundary is shown in (c).

Surfactant molecules in water self-assemble into various
structures such as micelles and bilayer membranes, whichdisplay a rich variety of rheological properties. It is known
that under shear flow, lamellar membranes can be oriented
parallel or perpendicular to the shear-gradient direction. Surprisingly, under high shear flow, the membranes
transform the planar lamellae to a closely-packed multilamellar vesicle structures, so-called the onion phase.
The onion radius is reversible and can be described by aunique decreasing function of the shear rate. Although thisonion phase was experimentally discovered 20 years ago,

Fig. 1. Snapshot of surfactant membranes in simple shear flow. (a) A
planar lamellae structure is formed at a low shear rate. (b) At a highshear rate, a rolled lamellar structure is formed perpendicularly to theflow direction. Top and bottom arrows represent the flow directions.
The membranes are viewed from the flow direction.

ISSP Activity Report 201339

the formation mechanism is not understood so far. Since
the onion size is in a micro meter scale, it is far beyond
applicable length scales for molecular simulations. Thus,
we employed a highly coarse-grained membrane model, ameshless-membrane model, in which a membrane particle
represents not a lipid molecule but a patch of the bilayer
membrane. The particles self-assemble into membranes via
multibody potential interactions.
At low shear rates, planar lamellae structures are formedat large membrane volume fractions (see Fig. 1a). At highshear rates, lamellar states exhibit undulation instability,
leading to rolled or cylindrical membrane shapes oriented
in the flow direction (see Fig. 1b). At even higher shear
rate, the planar lamellae structures are formed again. The
spatial symmetry and the structure factor of this rolled stateagree with those of intermediate states during lamellar-toonion transition measured by time-resolved small angle
neutron and X-ray scattering experiments. A cylindricalor wavy lamellar structure was speculated to be this intermediate structure, but could not be distinguished from thescattering pattern alone. Our simulation results revealed that
it is a rolled structure instead of regular cylindrical or wavy
structures. The planar membranes become unstable in theflow gradient direction so that the membranes are rolled inperpendicular to the flow direction. We have not reproduced
the onion structure itself. We will investigate the effects ofdefects and finite system size to pursue understanding of theonion phase.

0.42(2). So far no experimental study has been reported
because of absence of the model compound. Recently wefound that RCrGeO5 is a rare experimental realization of thespin-3/2 alternating spin chain. In this study we performed
inelastic neutron scattering experiments on the polycrystalline sample of YCrGeO5 to observe the magnetic excitationand to identify the spin Hamiltonian.
INS spectrums are shown in Figs. (a) and (b). The excitations are observed in the energy range of 8 meV ≤ ℏω ≤ 23
meV at 4.0 K and the intensity decreases with the increaseof Q. The intensity is suppressed at higher temperature of
199 K. The results mean that the observed excitations are
dominated by magnetic scattering. No excitation is observedat ℏω ≤ 8 meV and this reveals the existence of the spin
gap. The obtained powder INS spectrum was converted tothe one-dimensional S(q,ω) [5] as shown in Fig. (c). The
obtained profile was reproduced by empirical dispersion
formula ℏω = √ A2(sin k)2 + Δ2 with the parameters Δ =10 meV and A = 20 meV. Figure (d) shows the dynamical
structure factor calculated by DMRG with δ = 0.75 andΔ/J=1.1 and the calculation is consistent with the experiment.
The measured magnitude of the spin gap is Δ = 10 meV

Reference[1] H. Shiba, H. Noguchi, and G. Gompper, J. Chem. Phys. 139,014702 (2013).
AuthorsaH. Shiba, H. Noguchi, and G. Gomppera
Forschungszentrum Juelich

Neutron Scattering Study on Alternating
Spin-3/2 Chain Compound YCrGeO5
M. Hase, T. Masuda, and K. Kindo
Neutron scattering technique is known as a powerful tool
to probe the spin correlation in magnetic materials. Among
various types of neutron instruments the chopper spectrometer is designed particularly for collecting inelastic neutron
scattering (INS) spectrum. Combination of position sensitive
detectors and accumulative data-acquiring system leads to astate-of-art instrument for measuring the dynamical structure
factor S(q,ω). Neutron Scattering Laboratory operates theHigh Resolution Chopper (HRC) spectrometer installed in
J-PARC/MLF. The instrument covers the incident neutronenergy range 10 meV ≤ Ei ≤ 500 meV and the scattering
angle range 0.3º ≤ ψ ≤ 40º, which meets typical magnetic
scattering in most of magnetic materials. In this highlight
we introduce a recent study on an alternating spin-3/2 chain
compound by using the HRC spectrometer.
The phase diagrams of the alternating spin chain
systems for various magnitudes of spin were extensively
studied theoretically [1-4]. The model Hamiltonian is= JΣ i(1-(-1)iδ)Si.・Si+1. In case of spin-3/2 chain thespin alternation δ induces a spin gap at 0 < δ < 0.42(2) and0.42(2) < δ ≤ 1, and the ground state is non-magnetic singlet.
Tomonaga-Luttinger liquid (TTL) state exists at δ = 0 and

40

ISSP Activity Report 2013

Fig. 1. (a) Inelastic neutron scattering (INS) spectrum of YCrGeO5
at 4.0 K. (b) INS spectrum of YCrGeO5 at 199 K. (c) INS spectrum
converted to one-dimensional dynamical structure factor. (d) Calculated
dynamical structure factor by DMRG method.

and this leads to J = 11meV. By using these parameters themagnetic susceptibility was quantitatively reproduced. Thus
our study revealed that YCrGeO5 is the first experimental
realization of the spin-3/2 alternating spin chain.
As shown in Figs. (a) and (b) INS spectrum with highexperimental resolution can be efficiently collected by using
HRC spectrometer. Although the polycrystalline sample
form, one-dimensional S(q,ω) can be obtained by the elaborated analysis. Combination of chopper spectrometer andpolycrystalline sample will be a defacto standard in forthcoming era of neutron science.
References[1] Y. Kato and A. Tanaka, J. Phys. Soc. Jpn. 63, 1277 (1994).
[2] M. Kohno, M. Takahashi, and M. Hagiwara, Phys. Rev. B 57, 1046
(1998).
[3] M. Yajima and M. Takahashi, J. Phys. Soc. Jpn. 65, 39 (1996).
[4] S. Yamamoto, Phys. Rev. B 55, 3603 (1997).
[5] K. Tomiyasu, M. Fujita, A. I. Kolesnikov, R. I. Bewley, M. J. Bull,and S. M. Bennington, Appl. Phys. Lett. 94, 092502 (2009).
Authors abb
M. Hase , M. Soda, T. Masuda, aD. Kawana, T.Yokoo , S. Itoh ,A.
Matsuo,K.
Kindo,
and
M.
Kohno
a
National Institute for Materials Science (NIMS)bHigh Energy Accelerator Research Organization (KEK)

Detection of Berry’s Phase in a
Bulk Rashba Semiconductor
H. Murakawa, M. Tokunaga, and Y. Tokura
The geometrical information of the system parametercan be encoded in the wave function as a Berry’s phase andgoverns the system properties. In quantum systems, Berry’s
phase can be a source for various emergent phenomena.
Despite its ubiquity, there are few experimental observationsof Berry’s phase of bulk states. Here, we report detection of a
nontrivial π Berry’s phase in the bulk Rashba semiconductor
BiTeI via analysis of the Shubnikov-de Haas (SdH) effect.
BiTeI has a polar crystal structure made up of stacked
layers of bismuth (Bi), tellurium (Te) and iodine (I) atoms.
Because of the absence of inversion symmetry and thestrong polarity of the system, accompanied by the strong
spin-orbit interaction of Bi, an extremely large Rashba
spin splitting occurs in a bulk scale (Fig. 1A). As shown in
Fig. 1B, two coaxial spin polarized Fermi surfaces verging

Fig. 2. (A) Magnetoresistivity of BiTeI sample. (B) The oscillatorycomponent as a function of 1/B. (C) Landau index plot of the outerFermi. (inset) Magnified view around the intercept.

at Dirac point, namely, inner and outer Fermi surfaces (IFSand OFS, respectively) are formed. Theoretical prediction indicates that electrons on both FSs with the circularspin texture acquire π Berry’s phase (øB = π) and varies thesystem energy from the trivial case (øB = 0). Under magnetic
field, Landau levels in a Dirac system with øB = π locate at the
middle of those in a system with øB = 0. Thus, π Berry’s phasecan be detected as a π phase shift in quantum oscillation.
The extremely large Rashba spin splitting in BiTeI
has great advantages on observing the Berry’s phase.
The large ratio of the extremal cross-sectional areas (AN)of IFS and OFS enables the clear separation of the two
sets of SdH oscillations. Moreover, the giant Rashba
energy can dominate the Zeeman effect and preserve thecircular spin texture even in a high magnetic field region.
According to the Lifshitz-Onsager quantization rule,ANℏ/eB = 2π(n + 1/2 - øB/2π) = 2π(n + γ) , Berry’s phase
is obtained as an intercept value γ in the linear relation
between 1/B and Landau index number n. For precise
evaluation, observation of the SdH oscillation down tolower Landau level is important. Therefore, high-magnetic
field measurement with use of the non-destructive pulse
magnet can be a powerful tool. Figure 2A shows the SdH
oscillation from the OFS up to 56 T. We assign integer
indices to the ρxx peak positions in 1/B and half integer
indices to the ρxx valley positions (Fig. 2B). As shown in
Fig. 2C, the interpolation line has almost zero intercept (γ =1/2 – øB/2π = 0), confirming the existence of π Berry’s phasein the 3-dimensional Rashba semiconductor BiTeI.
Reference[1] H. Murakawa, M. S. Bahramy, M. Tokunaga, Y. Kohama, C. Bell,Y. Kaneko, N. Nagaosa, H. Y. Hwang, and Y. Tokura, Science 342,
1490 (2013).
AuthorsabcH. Murakawa, M. S. Bahramy, M. Tokunaga,
Y. Kohama,
C. Bell ,
a
a,b
a,c
a,b
Y.
Kaneko,N.
Nagaosa
,
H.Y.
Hwang
,
and
Y.
TokuraaRiken CEMSb
The University of Tokyoc
Stanford University

Fig. 1. (A) Energy band dispersion of the Rashba spin split band in
BiTeI. (B) The inner and outer Fermi surfaces and helical spin textures(arrows) in BiTeI.

ISSP Activity Report 201341

Discovery of Novel Phase of Solid
Oxygen in Ultrahigh Magnetic Fields
T. C. Kobayashi and Y. H. Matsuda
Molecular oxygen has the spin S = 1 and the relation
between the magnetic interaction and the crystal structure
has been attracting attention of material scientists for along time [1]. In the present work, we discovered a novel
phase of solid oxygen as a result of the first-order structural phase transition by applying an extraordinarily strong
magnetic field of up to 193 Tesla [2]. The change in the
crystal symmetry due to the rearrangement of O2 molecules
is strongly suggested, which can make the new oxygen phaseferromagnetic. Since all known seven phases of solid oxygen
have the antiferromagnetic nature, it is remarkable to obtain
ferromagnetic oxygen.
The magnetization and magneto-transmission measurements have been performed in ultrahigh fields using adestructive manner, the single-turn coil technique. The
magnetic field was applied in the α-phase of solid oxygen at
low temperatures (T < 24 K). As shown in Fig. 1, we foundthat the magnetization rapidly increased at around 125 T.
This is a clear evidence of the field-induced magnetic phasetransition. Since the magnetization process has the large
hysteresis, the transition is expected to be of first order.
Figure 2 shows the time dependence of the magnetic field
and two-dimensional magneto-transmission image. You can
see that something drastic happens at very high magnetic
fields. The field where this phenomenon is seen almost corresponds to the critical field of the magnetization jump shownin Fig. 1.
The structural phase transition along with the magnetictransition explains the experimental findings; the significant
increase of the light transmission is due to the reduction of
the classical light scattering at the domain boundaries. In thestrong magnetic field, the crystal structure becomes isotropic(cubic) from the anisotropic one (monoclinic). The origin of
the phase transition is suggested to be the rearrangement of
O2 molecules, which is driven by the change in the exchangeinteraction depending on the spatial arrangement of the O2
molecules.
The discovered novel phase is the eighth phase of solid
oxygen, which has the different geometry of O2 moleculesfrom other seven phases. The state-of-the-art high magnetic
field technique was essential in this discovery. Nowadays, inmaterials science, many treasures waiting for discovery are
expected at the frontier of extreme high field.
References[1] Y. A. Freiman, and H. J. Jodl, Phys. Rep. 401, 1 (2004) andreferences therein.
[2] T. Nomura, Y. H. Matsuda, S. Takeyama, A. Matsuo, K. Kindo,
J. L. Her, and T. C. Kobayashi, Phys. Rev. Lett., Editors’ Suggestion,(2014) in press.
AuthorsaT. C. Kobayashi , T. Nomura,Y. H. Matsuda, S. Takeyama, A. Matsuo,bK.
Kindo,andJ.
L. Hera
Okayama Universityb
Chang Gung University

Structure Determination of Silicene on
Ag(111) by Low-Energy Electron Diffraction
K. Kawahara, T. Shirasawa, and N. Takagi

Fig. 1. The magnetization curve in α-oxygen at 9 K up to 129 T.

Fig. 2. (a) The waveform of the pulsed magnetic field. (b) The
two-dimensional magneto-transmission image of α-oxygen.

42

ISSP Activity Report 2013

Silicene, a 2-dimensional honeycomb lattice of Si,
attracts much attention since it is predicted to acquire exotic
features such as massless Dirac fermions and high electron
mobility [1]. As the spin orbit coupling of Si is 1000 times
larger than that of C, silicene is one of the most interesting
candidates for 2-dimensional topological insulator [2]. In
addition, silicene should match with the current Si based
technology. These characteristics make silicene promising in
the next generation devices.
Several groups reported synthesis of silicene on Ag(111)[3-8]. Silicene forms 4×4 superstructure on the Ag(111)
surface [4-8]. On a fundamental question ‘Does 4×4 silicene
have Dirac fermions?’, the arguments conflict among the
research groups [4,7]. The electronic structure is connectedwith the atomic arrangement. Thus, by determining theatomic position, we can answer the above question. However,
the geometric structure of 4×4 silicene has not been established. Several groups proposed the structural models of 4×4
silicene based on the result of scanning tunneling microscopy(STM) [4-8]. The reasonable model is a buckled structuralmodel constructed by the STM and density functional theory(DFT) calculations [4-7]. In contrast, Feng et al. claimed
that the corner-hole like features observed in the STM image
are assigned to missing Si atoms and proposed a model inwhich hydrogen atoms terminate the dangling bonds of Si
atoms [8]. The discrepancy arises from that the STM image
is convolution of the geometric structure together with theelectronic density of states. Hence we cannot determine thepositions of the individual atoms in the 4x4 silicene only by

Observing Hot Carrier Distribution
in an n-type Epitaxial Graphene
on a SiC Substrate
H. Fukidome and I. Matsuda

Fig. 1. The top and side views of the best-fit structure of 4×4 silicene.
The side view is the cross section along the AA’. Green and blue balls
represent Si atoms. The green Si atoms are moved up to the vacuum.
White and Gray balls represent Ag atoms in the first and second layer,
respectively.

STM measurements.
Electron diffraction is one of the most reliable andpowerful techniques to determine the geometric structure.
We have determined the geometric structure of 4×4 siliceneby using low-energy electron diffraction [9]. We examined
the regularly-buckled silicene, the buckled structure where
six Si atoms are displaced and the structural model proposedby Feng et al. The optimized reliability factor (R-factor)of the buckled structure shown in Fig. 1 was 0.17. The
optimized R factor of the model proposed by Feng et al. [8]was 0.48. The buckled structural model as Fig. 1 reproduced
the experimental results very well. Figure 1 shows the top
and cross sectional views of the best-fit structural modelfor 4×4 silicene. The green Si atoms are displaced perpendicularly to the surface and silicene forms into a buckled
structure. The Si-Si bond lengths range from 2.29 to 2.31 Å,
which are shorter than that for the bulk diamond structure.
Substrate Ag atoms are displaced vertically at the interface.
The R-factor is 0.52 for the model without the displacement of Ag, indicating that the displacement of Ag atoms
is a key factor to determine the structure of 4×4 silicene.
The displacement of Ag also reflects the strong interaction
between silicene and Ag substrate, supporting the experimental results showing absence of the Dirac fermions [7].
In addition, the experimentally determined structure of 4×4
silicene matches quite well with that optimized by DFT
calculations [4,5]. Thus, the geometric structure of 4×4
silicene on Ag(111) was completely clarified.
References[1] S. Cahangirov et al., Phys. Rev. Lett. 102, 236804 (2009).
[2] M. Ezawa, Phys. Rev. Lett. 109, 055502 (2012).
[3] B. Lalmi et al.,Appl. Phys. Lett. 97, 223109 (2010).
[4] P. Vogt et al.,Phys. Rev. Lett. 108, 155501 (2012).
[5] C.-L. Lin et al., Appl. Phys. Express 5, 045802 (2012).
[6] R. Arafune et al., Surf. Sci. 608, 297 (2013).
[7] C.-L. Lin et al., Phys. Rev. Lett. 110, 076801 (2013).
[8] B. Feng et al., Nano Lett.12, 3507 (2012).
[9] K. Kawahara et al., Surf. Sci. 623, 25 (2014).
AuthorsabaK. Kawahara, T. Shirasawa,
R. Arafune , C.-L. Lin , T. Takahashi,aaM.
Kawai , and N. TakagiaThe University of Tokyob
IMANA-NIMS

A monolayer graphene is "supermaterial" that is thethinnest, lightest, and strongest material with ultrahighelectrical and thermal conductivity. Recently optical properties of the graphene have also attracted interests in the fieldof developing optoelectronic, plasmonic and nanophotonic
devices. However, detailed mechanisms of the photo-induced
phenomena of a graphene, such as the multiple carrier generation, have not been understood and, thus, the direct investigation of the non-equilibrium carrier dynamics has beenstrongly called for.
In the present research, we carried out femtoseconds(fs)time- and angle-resolved photoemission experiment toobserve the temporal variation of the Dirac electrons after the
optical pumping in real time. The measurement was carriedout using high-harmonic generation (HHG) laser system
based on the Ti:Sappire laser (hν=1.57 eV). Photon energy of
28.26 eV for the HHG laser pulse is generated by irradiating
the second harmonic laser pulses (hν =3.14 eV) at Ar gas
and it is high enough to cover the electron momentum of the
Dirac band at the K point in angle-resolved photoemission
measurement. The time-resolved data were obtained using
the pump (3.14 eV laser) and probe (HHG) method with arepetition rate of 1 kHz.
Figure 1(a) shows a series of angle-integrated spectra
taken with a laser pumping power of 2.5 mW at various
delay times. Dispersion of the Dirac bands, observed byangle-resolved photoemission at the K point, is shown in
the inset. With laser pumping (t=0), the photoemission
intensity of the upper Dirac band (UDB) decreases significantly while that of the lower Dirac band (LDB) remains
constant. Around the energy corresponding to the Fermi
energy, the spectral tail extends in energy. The slope of thespectral edge also becomes small. At each delay time, theFig. 1. (a) Comparison of the spectra at different delay times. The
experimental spectra were taken at a photon energy of 28.26 eV atvarious delay times with pumping laser irradiation (hν=3.14 eV). The
intensity variation for the UDB is clearly reproduced by the simulated
result (d) while the intensity for the LDB is unchanged. The inset
shows dispersion of the Dirac bands near the Fermi level (EF). The
upper Dirac band (UDB) and lower Dirac band (LDB) are labeled in
the figure and the Dirac Point (DP) is indicated by an arrow. (b)–(d)Simulation of energy spectra for two cases of Dirac free electrons. The
inset shows their two-dimensional band dispersion. (b) density of states,(c) Fermi–Dirac function, and (d) resulting spectra.

ISSP Activity Report 201343

raw experimental data, drawn with thin colored lines, arecurve-fitted with a function that is a product of density ofstates of graphene, Fig. 1 (b), and the Fermi-Dirac function,
Fig. 1(c). In the spectral simulation of Fig. 1 (d), the Fermiedge shows a peak-like structure, and the peak top shifts
toward higher binding energy at higher temperature. In
Fig. 1(a), the fitting results are drawn through raw experimental data with bold colored lines. The temporal evolution
in Fig. 1(a) is described by comparing it with the intensities
simulated in Fig. 1(d) , and it reflects the time evolution of
the electronic temperature, that is, the transient distributionof the hot carriers (electrons).
Reference[1] T. Someya, H. Fukidome, Y. Ishida, R. Yoshida, T. Iimori,R. Yukawa, K. Akikubo, Sh. Yamamoto, S. Yamamoto, T. Yamamoto,T. Kanai, K. Funakubo, M. Suemitsu, J. Itatani, F. Komori, S. Shin, andI. Matsuda, Appl. Phys. Lett. 104, 161103 (2014).
Authors
I. Matsuda, H. Fukidomea, J. Itatani, S. Shin, and F. KomoriaTohoku University

Electronic Structure of Carbon-Related
Catalysts During Fuel Cell Operation by
Soft X-Ray Emission Spectroscopy
H. Niwa, Y. Harada, and M. Oshima
Polymer electrolyte fuel cells (PEFCs) are expected to bepromising power sources of high efficiency, low operating
temperature, and low pollution for transportation andresidential applications. Recently, carbon-based catalysts
are getting considerable attention since adequately designed
carbon-based catalysts show high ORR activities and are
expected to be cathode catalysts alternative to conventional
Pt-based catalysts [1,2]. The origin of their high ORR activities should be elucidated to further enhance the activities forcommercialization. Previous electronic structure studies oncarbon-based catalysts were mostly done at ex situ condition[3]. However, the results were not always conclusive
because they did not observe the chemical state of eachelement under PEFC working condition. To further explore
the ORR mechanism of carbon-based cathode catalysts,

Fig. 2. Smoothed Fe 2p SXE spectra compared with the open circuit
voltage (lower panel) and the same gas condition (upper panel). Blacksolid lines are difference spectra.

operando observation of the electronic structure is strongly
required.
Here we report a novel electrochemical cell systemdeveloped for operando soft X-ray emission spectroscopyof cathode catalysts for polymer electrolyte fuel cells at
BL07LSU of SPring-8[4]. Incorporating a membraneelectrode assembly on a vacuum compatible flange(Fig. 1), the system enables direct observation of elementspecific electronic structure that changes with gaseousand potential conditions for electrochemical reaction. We
have successfully observed the electronic structure of ironin the iron phthalocyanine-based cathode catalyst undervarious working conditions by the operando soft X-ray
emission spectroscopy. At open circuit voltage it is foundthat an oxidized iron site exists even after reductive hightemperature pyrolysis and is active for oxygen adsorption(Fig. 2), which is not expected from ex situ results for
powder samples where metallic iron site inactive for ORR
dominates.
References[1] W. Y. Wong, W. R.W. Daud, A. B. Mohamad, A. A. H. Kadhum,
E. H. Majlan, and K. S. Loh, Diam. Relat. Mater. 22, 12 (2012).
[2] F. Jaouen, E. Proietti, M. Lefèvre, R. Chenitz, J.-P. Dodelet, G. Wu,
H. T. Chung, C. M. Johnston, and P. Zelenay, Ene. Environ. Sci. 4, 114(2011).
[3] U. I. Kramm, I. Abs-Wurmbach, I. Herrmann-Geppert, J. Radnik,
S. Fiechter, and P. Bogdanoff, J. Electrochem. Soc. 158, B69 (2011).
[4] H. Niwa, H. Kiuchi , J. Miyawaki, Y. Harada, M. Oshima, Y. Nabae,
and T. Aoki, Electrochem. Commun. 35, 57 (2013).
Authorsaab
H. Niwa, H.c Kiuchi , J. Miyawaki, Y. Harada, M. Oshima , Y. Nabae ,
and
T.
Aoki
a
The University of Tokyob
Tokyo Institute of TechnologycToshiba Fuel Cell Power Systems Corporation

Clarification of Mechanism of
Ferromagnetism inFerromagnetic Semiconductor
M. Kobayashi, A. Tanaka, and Y. HaradaFig. 1. (a) Schematic view of the MEA cell for operando SXE measurements. (b) Photograph of the MEA cell on a vacuum flange. (c) Photograph of a XYZ manipulator and a main chamber.

44

ISSP Activity Report 2013

A ferromagnetic semiconductor (FMS), in which ahost semiconductor is doped with a low concentration ofmagnetic ions, has attracted much attention in spintronics,
which exploits the properties of magnets for electronics

3+

states mainly consist of the Mn electronic configuration
5
6 2composed of the charge-transferred states (d L and d L ), in
5
which the ligand hole is weakly bound to the Mn 3d state(which can be explained by the magnetic polaron model:
2+
Fig. 2(b)), rather than the pure Mn state (which can be
explained by the Zener’s p-d exchange model: Fig. 2(a)). In
order to reproduce the experimental broadening of the Mn
d-d excitation, not only the (Gaussian) broadening by theenergy resolution but also the 0.5 eV Lorentzian broadening
is required. The additional Lorentzian broadening in the
RIXS spectra can be attributed to the lifetime broadening in
the final state of the RIXS process where fast decay of thed-d excitations to an electron-hole pair in the host valence
and conduction bands occurs because of the hybridization
between the Mn 3d orbital and the ligand band.
Fig. 1. Soft X-ray emission spectrumof GaMnAs and simulation
3+
results. The simulation result for Mn (red) is in good agreement withthe experimental spectrum (black).

indispensable in industry. These semiconductors have bothelectrical properties of a semiconductor and magnetic
properties of a doped magnetic element. Ga 1-xMn xAs(GaMnAs) is a typical ferromagnetic semiconductor obtainedby doping a small amount of manganese (Mn) into gallium
arsenide (GaAs). Its practical application as a spintronics
material is being examined because it exhibits ferromagnetism at a relatively high temperature. However, the mechanism of ferromagnetism in GaMnAs has not been conclusively determined, and various physical models have been
proposed.
In this study, to address the electronic structure of
the doped Mn ions, we report the results of Mn L3 x-ray
absorption spectroscopy (XAS) and resonant inelastic x-ray
scattering (RIXS) measurements of Ga1−xMnxAs (x = 0.04)
at SPring-8 BL23SU and BL07LSU [1], respectively. RIXS
is a powerful tool to investigate electronic excitations inelement- and symmetry-specific ways including d-d andcharge-transfer (CT) excitations for open shell 3d orbitals[2], and magnetic excitations for spin or charge-orderedsystems [3]. These excitations are sensitive to electron correlation, crystalline symmetry, and the strength of hybridization with the ligand band. The RIXS spectra obtained for
Ga1−xMnxAs are compared with configuration-interaction(CI) cluster-model calculations [4], and the electronic structure parameters are estimated.
The obtained XAS spectrum was compared with the
CI cluster model calculations. Both the calculated spectra2+3+
for the Mn and Mn states well reproduce the experimental XAS spectrum and we cannot determine the effective
charge on the Mn site. On the other hand, the RIXS spectra
provide definitive information: the RIXS spectra in Fig. 1
show a broad profile even at energy resolutions high enoughto distinguish individual d-d excitation peaks of MnO. An
analysis by the CI calculations indicates that the Mn groundFig. 2. Models for mechanism of ferromagnetism in Ga1-xMnxAs: (a)Zener's p-d exchange model and (b) magnetic polaron model. The
direction of the arrows in the upper figure indicates the direction ofmagnets.

References[1] G. Ghiringhelli, M. Matsubara, C. Dallera, F. Fracassi,
A. Tagliaferri, N. B. Brookes, A. Kotani, and L. Braicovich, Phys. Rev.
B 73, 035111 (2006).
[2] M. L. Tacon, G. Ghiringhelli, J. Chaloupka, M. M. Sala,
V. Hinkov, M. W. Haverkort, M. Minola, M. Bakr, K. J. Zhou,
S. Blanco-Canosa, C. Monney, Y. T. Song, G. L. Sun, C. T. Lin,G. M. D. Luca, M. Salluzzo, G. Khaliullin, T. Schmitt, L. Braicovich,and B. Keimer, Nat. Phys. 7, 725 (2011).
[3] A. Tanaka and T. Jo, J. Phys. Soc. Jpn. 63, 2788 (1994).
Authorsaabac
M. Kobayashi, H.d Niwa , Y.a Takeda , aA. Fujimoria, Y. Senba , H.
c
Ohashi , A.a Tanaka , S. Ohya , P. N. Hai , M. Tanaka , Y. Harada, andM.
Oshimaa
The University of TokyobJapan Atomic Energy AgencycJapan Synchrotron Radiation Research Institute (JASRI)dHiroshima University

Selective Probing of the OH or OD
Stretch Vibrations in Liquid Water Using
Resonant Inelastic Soft X-ray Scattering
Y. Harada, T. Tokushima, and S. Shin
The unique property of water, such as high boilingand melting point compared to molecules like non-metal
hydride, or less density in solid form than in liquid form, areFig. 1. The water vibronic structure observed in the RIXS spectrum.
Dotted red curve is a standard Raman spectrum of liquid water in thesame experimental configuration x(y, x+z)y [2] as the O 1s RIXS.

ISSP Activity Report 201345

M. Kobayashi, M. Oshima, Y. Senba, H. Ohashi, K. T. Wikfeldt, A.
Nilsson, L. G. M. Pettersson, S. Shin, Phys. Rev. Lett. 111, 193001(2013).
[2] G. E. Walrafen, M. S. Hokmabadi, and W.-H. Yang, J. Chem. Phys.
85, 6964 (1986).
[3] J.D. Smith, C. D. Cappa, K. R. Wilson, R. C. Cohen, P. L. Geissler,
R. J. Saykally, Proc. National Acad. Sci. (USA) 102, 14171 (2005).
[4] Y. Nagata, R.E. Pool, E.H.G. Backus, M. Bonn, Phys. Rev. Lett.
109, 226101 (2012).
AuthorsabcY. Harada, T.c Tokushima ,c Y. Horikawa,O. Takahashi, H. Niwae,f,ddM. Kobayashi,
M.
Oshima
,
Y.
Senba
,
H.
Ohashi,K.
T.
Wikfeldt,
g,hhA.
Nilsson,L.
G.
M.
Pettersson
,
and
S.
Shin
a
RIKEN/SPring-8b
Hiroshima Universityc
The University of Tokyod
JASRI/SPring-8eUniversity of Icelandf
NORDITA, AlbaNova University Centerg
SLAC National Accelerator Laboratoryh
AlbaNova University Center, Stockholm University
Fig. 2. Isotope effect on the multiple vibrational excitations of water.
For each spectrum the background level is indicated by a dashed line.
The extracted HDO spectrum is fitted with a linear combination of the
H2O and D2O spectra in the range from 531–534 eV.

explained by an attractive force between water molecules,
called 'hydrogen bond'. There are many proposed local structural models that describe the water network. Among them,a continuum model, where hydrogen bonds in water are
distorted, broken and reformed continuously, but water itself
is composed of a single component, or a mixture (microheterogeneity) model, where the network is considered asa mixture of various hydrogen bond configurations, arewell known. However, the model that describes better thehydrogen bond property of liquid water, is still under debate.
In this work, we have succeeded in revealing validity ofthe mixture (micro-heterogeneity) model by selective observation of hydrogen-bond-broken water molecules and in
detecting with high sensitivity the difference in the degreeof hydrogen bond strength between light (normal) waterand heavy water (in which both hydrogen atoms have been
replaced with deuterium) using soft X-ray resonant inelastic
scattering at BL07LSU and BL17SU of SPring-8[1]. Highresolution O 1s resonant inelastic X-ray scattering spectraof liquid H2O/D2O/HDO, obtained by excitation near the
pre-edge resonance (Fig. 1) show, in the elastic line region,
well-separated multiple vibrational structures corresponding
to the internal OH stretch vibration in the ground state ofwater. The energy of the first-order vibrational excitation isstrongly blue-shifted with respect to the main band in the
Infrared/Raman spectra of water [2], indicating that watermolecules with a highly weakened or broken donatinghydrogen bond are correlated with the pre-edge structure in
the X-ray absorption spectrum. As shown in Fig. 2, the vibrational profile of pre-edge excited HDO water is well fittedwith 50±20% greater OH-stretch contribution compared to
OD, which strongly supports a preference for OH being theweakened or broken H-bond in agreement with the wellknown picture that D2O makes stronger H-bonds than H2O[3,4]. Accompanying path-integral molecular dynamics
simulations show that this is particularly the case for strongly
asymmetrically H-bonded molecules, i.e. those that are
selected by pre-edge excitation. These results are expected
to lead to the clarification of the role of water in various
chemical and catalytic reactions as well as water in biological organisms where hydrogen bond plays an important role.
References[1] Y. Harada, T. Tokushima, Y. Horikawa, O. Takahashi, H. Niwa,

46

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ISSP Activity Report 201347