Research Highlights
Multiband Superconductivity
with Unexpected Deficiency of
Nodal Quasiparticles in CeCu2Si2
Sakakibara GroupThe gap symmetry of unconventional superconductors has attracted much attention of scientific community
because it is closely related to the exotic pairing mechanism.
CeCu2Si2 (Tc ~ 0.6 K) is a historically important compoundthat made a breakthrough in the field of superconductivity;
in 1979, exotic superconductivity was discovered for the
first time in this strongly-correlated heavy-electron system[1]. Until quite recently, its gap symmetry was believed to
be a nodal d-wave pairing mediated by spin fluctuations[2]. The presence of line nodes had been suggested fromthe power-law temperature dependence of various physicalquantities, such as the NMR relaxation rate 1/T1 [3] and thespecific heat, both of which were measured in the intermediate temperature region above 0.1 K. Recent interest has
been focused on the location of line nodes: whether the gap
symmetry is dx2−y2 or dxy type [4,5].
To elucidate the gap symmetry of CeCu2Si2, we performed
specific-heat measurements at low temperatures down
to 40 mK using a high-quality single crystal (S-type)[6].
Quite unexpectedly, the zero-field specific heat Ce exhibits
exponential temperature dependence below 80 mK (Fig.
1(a)), reminiscent of full-gap superconductivity. In addition,
we provide thermodynamic evidence for nodeless superconductivity that the low-temperature specific heat is proportional to the magnetic field at low fields for all field orienta-

Fig. 2. The calculated Fermi surfaces of CeCu2Si2 colored by themagnitude of the Fermi velocity νF.

tions and does not change with the c-plane field rotation. We
found that the temperature dependence of the zero-field Ce /T
including the linear behavior in the intermediate-T region canbe reproduced on the basis of a phenomenological two-gap
model within the conventional BCS framework using two
BCS gaps, Δ1 = 1.76kBTc and Δ2 = 0.7kBTc, whose weights
are 65% and 35% of the total density of states, respectively(Fig. 1(b)). We also found anomalous behaviors in the field
variations of the specific heat as well as the magnetization at
low temperatures in the high-field regime (Fig. 1(c)), which
can be interpreted as a strong Pauli paramagnetic effectoccurring in a multiband superconductor. All the present
results match with the prediction of multiband superconductivity in the absence of nodal quasiparticles.
One may suspect that line nodes might exist in the gap
on light-mass bands, since the specific heat measurement
mainly probes the heavy-mass band. To get an insight into
the band structure of CeCu2Si2, we have performed firstprinciples calculations by the LDA+U method. As shown in
Fig. 2, CeCu2Si2 has a flat electron band around the X pointwith the heaviest mass and two hole bands around the Zpoint. In this Fermi-surface topology, the deficiency of nodal
quasiparticles requires the negligibly small effective mass ofthe two light-mass bands, which contradicts the multiband
feature detected from the specific-heat measurement. This
leads to a conclusion that the pairing symmetry of CeCu2Si2
is not the anticipated d-wave state, but the multiband full-gap
state, including an unconventional s-wave such as s±-wave,
a conventional s-wave, or a fully-gapped d+id state. Thesefindings would open a new door into electron pairing in
CeCu2Si2 and help understand the pairing mechanism of
exotic superconductors.
References[1] F. Steglich et al., Phys. Rev. Lett. 43, 1892 (1979).
[2] O. Stockert et al., Nat. Phys. 7, 119 (2011).
[3] Y. Kitaoka et al., J. Phys. Soc. Jpn. 55, 723 (1986).
[4] H. A. Vieyra et al., Phys. Rev. Lett. 106, 207001 (2011).
[5] I. Eremin et al., Phys. Rev. Lett. 101, 187001 (2008).
[6] S. Kittaka et al., Phys. Rev. Lett. 112, 067002 (2014).

Fig. 1. (a) Temperature dependence of the electronic specific heat of
CeCu2Si2 divided by temperature, Ce /T, for H // [100]. (b) Temperaturedependence of Ce /T at zero field, where the T-dependent normal-state
contribution is subtracted. The solid line is a best fit to the two-gap
BCS model. (c) Field-temperature phase diagram for H // [100] and acontour plot of the superconducting contribution to Ce /T. Anomalous
behavior that can be ascribed to the Pauli paramagnetic effect is clearlyseen in the high-field and low-temperature region below 0.1 K.

2

ISSP Activity Report 2013

Authors
S. Kittaka, Y. Aoki, Y. Shimura, T. Sakakibara, S. Seiroa, C. Geibela,
F. Steglicha, H. Ikedab, K. MachidacaMax-Planck-Institute for Chemical Physics of SolidsbKyoto Universityc
Okayama University

Gapless Quantum Spin Liquid State ina Purely Organic Spin-1/2 Triangular
Lattice κ-H3(Cat-EDT-TTF)2
Mori GroupA quantum spin-liquid (QSL) state is an exotic groundstate where interacting spins continue to fluctuate without
any formation of long-range magnetic order (LRMO) evenat a sufficiently low temperature. A variety of QSL states
have been theoretically predicted, but nevertheless, a systematic understanding of the elementary excitation from theexperiments remains an arduous challenge. This is mainly
because of the rareness of the experimental candidates,
which are still restricted to the several spin-frustrated lattices,such as triangular, kagome, and hyperkagome lattices.
Recently, Mori group has discovered gapless quantum spin
liquid state in a purely organic spin-1/2 triangular latticeκ-H3(Cat-EDT-TTF)2 (abbreviated as κ-H)[1,2]. In thisarticle, the results of SQUID and torque magnetometry
suggesting the QSL state with gapless magnetic excitations in purely organic single-component molecular Mott
insulator, κ-H, have been demonstrated [3].
Samples were prepared by the electrochemical oxidation of H2Cat-EDT-TTF molecules in the presence of a base[1, 2]. A characteristic structural feature of this material isthat, in a 2D layer, two face-to-face (H2Cat-EDT-TTF)0.5+
molecules form a strongly dimerized molecular unit, asshown in Fig. 1(a). Because of the strong dimerization, a
dimerized unit can be treated as one site, resulting in an
effective spin 1/2 per site. As schematically illustrated in Fig.
1(b), each spin is arranged on the triangular mesh with theanisotropy parameter t’/t ~ 1.48 at T = 50 K, where t and t’
are the hopping integrals around the sides of rhomboids andalong one diagonal, respectively. It is specific that this Mott
insulator contains only conducting layers linked by hydrogenbonds without anion layers (Fig. 1(c)), namely purelyorganic single-component quantum spin liquid crystal.
Firstly, we measured the static magnetic susceptibility
at 1 T employing a magnetic property measurement system(Quantum Design) in the temperature region from 2 to 300 K

Fig. 1. (a) Molecular arrangement in a two-dimensional layer (b-cplane) of κ-H3(Cat-EDT-TTF)2. The dotted ellipsoids denote thestrongly dimerized molecules. (b) A schematic of the anisotropic triangular lattice with transfer integrals t’ and t. The closed circles and thearrows on them represent the sites of the triangular lattice composedof the dimerized molecules and the S = 1/2 spins, respectively. (c) The
interlayer packing structure viewed in the a-c plane. The adjacent layers
are connected by hydrogen bonds. The dotted ellipsoids represent
dimerized molecules similar to those described in (a).

Fig. 2. Magnetic susceptibility as a function of temperature χ(T) on
a semilogarithmic plot. The black diamonds denote the result of the
SQUID measurement at 1 T. The red squares represent χ(T) estimatedfrom the torque at 10 T for a–b rotation. The susceptibility χ(T) for thea–b rotation is normalized to χ(50 K) from the SQUID measurement.
The χ(T) is nearly independent of temperature below T ~ 3 K, possibly
being attributed to the Pauli paramagnetic contribution and suggesting
the presence of gapless magnetic excitations in the QSL state.

using poly-crystalline samples of ~16 mg (Fig. 2). The entire
temperature dependence of χ is roughly described by theS = 1/2 Heisenberg antiferromagnetic model of an isotropic
triangular lattice, with an antiferromagnetic exchangecoupling constant J/kB ~ 80–100 K. To shed light on themagnetic properties at lower temperatures, we measured themagnetic torque. As the magnetic torque only detects theanisotropic susceptibility in principle, the isotropic contribution from impurity spins is naturally eliminated, providing
us with the intrinsic low temperature magnetic properties.
For our system, it should be noted that sinusoidal behavior
with two-fold periodicity is observed up to H = 17 T anddown to T = 50 mK. Moreover, the observed amplitude of
the sinusoidal torque curve is precisely proportional to thesquare of the magnetic field. These observations indicatethat the system remains paramagnetic even at T = 50 mK.
The above argument about the torque together with thesusceptibility χ(T) reveals that spin frustration on the triangular lattice strongly suppresses the formation of long-range
antiferromagnetic order even at J/kB / T ~ 103, suggesting thedevelopment of the QSL state as the ground state. Secondly,
we focus on the excitation spectrum of the QSL state. In
Fig. 2, χ(T) from torque magnetometory is normalized usingχ determined from the SQUID measurement. What is notablein Fig. 2 is that χ(T) is nearly independent of temperature below T ~ 3 K, possibly being attributed to the Pauliparamagnetic contribution. This suggests the presence ofgapless magnetic excitations in the QSL state.
In conclusion, we report the results of SQUID and torquemagnetometry of an organic spin-1/2 triangular-latticeκ-H3(Cat-EDT-TTF)2. Despite antiferromagnetic exchange
coupling at 80–100 K, we observed no sign of antiferromagnetic order down to 50 mK owing to spin frustration on thetriangular lattice. In addition, we found nearly temperatureindependent susceptibility below 3 K associated with Pauliparamagnetism. These observations suggest the development of gapless quantum spin liquid as the ground state. On
the basis of a comparative discussion, we point out that thegapless quantum spin liquid states in organic systems share apossible mechanism, namely the formation of a band with a
Fermi surface possibly attributed to spinons.

ISSP Activity Report 20133

References[1] H. Kamo, A. Ueda, T. Isono, K. Takahashi, and H. Mori,
Tetrahedron Lett. 53, 4385 (2012).
[2] T. Isono, H. Kamo, A. Ueda, K. Takahashi, A. Nakao, R. Kumai,
H. Nakao, K. Kobayashi, Y. Murakami, and H. Mori, Nature Commun.
4, 1344 (2013).
[3] T. Isono, H. Kamo, A. Ueda, K.Takahashi, M. Kimata, H. Tajima,
S. Tsuchiya, T. Terashima, S. Uji, and H. Mori, Phys. Rev. Lett. 112,
177201 (2014).
Authors
T. Isono, H. Kamo, A. Ueda, K. Takahashia, M. Kimata, H. Tajima, S.
Tsuchiyab, T. Terashimab, S. Ujib, and H. MoriaKobe UniversitybNational Institute for Materials Science

Quantum Criticality in a Metallic Spin
Liquid System Pr2Ir2O7
Nakatsuji GroupAt finite temperatures, electronic magnetic moments inmagnetic materials are thermally fluctuating and one may
have magnetic phase transitions accompanied by critical
thermal fluctuations. This transition temperature can belowered by controlling external parameters like pressure andmagnetic field. In temperature near absolute zero, thermal
fluctuations will be reduced, allowing quantum fluctuations
to take effect. When the quantum fluctuations are strong
enough, one may have a phase transition at absolute zero,
namely quantum phase transition. In the vicinity of thequantum phase transition point, anomalous magnetic andmetallic behaviors may be observed, such as high-Tc superconductivity in the cuprates and iron pnictides, and unconventional superconductivity in the heavy fermion systems.
These phenomena are stemmed from the anomalous metallic
state around the quantum critical point.
Intensive studies on quantum critical phenomena havebeen done on the groups of compounds so called heavy
fermion systems. It is well known that the tuning of theexternal parameters by applying pressure or magnetic field
suppresses the magnetic order down to absolute zero, leading
to the emergence of quantum criticality along with the evolution of exotic phase such as anomalous superconductors.
Here we have revealed the existence of quantum criticality ina metallic spin liquid system Pr2Ir2O7, which may provide a
key insight on the mechanism of large spontaneous anomalous Hall effect observed in its spin liquid state [1-4]. In
this material, Pr ions form the pyrochlore structure, wherethe vertices of the corner sharing network of tetrahedra areoccupied by Pr ions' Ising type spins, and the ferromagnetic
interaction between them causes geometrically frustrated

Fig. 1. Schematic phase diagram of temperature (T) vs. external parameters in strongly correlated electron systems.

4

ISSP Activity Report 2013

Fig. 2. (a) Temperature dependence of the magnetic Grüneisenratio ΓH of Pr2Ir2O7. The divergent behavior of ΓH appears withdecreasing of a magnetic field. (b) Critical scaling of the magnetic
Grüneisen ratio ΓH for Pr2Ir2O7. This analysis evidences a zero-field
quantum critical point in the material.

spin ice state. As a result, the ground state is considered to
be spin liquid, having no dipolar magnetic order. In addition,
the spontaneous Hall effect appears in this spin liquid phase,suggesting the emergence of a chiral spin liquid state which
has finite spin chirality.
In this study, we performed the precise magnetocaloric effect measurements on Pr2Ir2O7 [1]. The magnetic
Grüneisen ratio, which measures the change of temperature
with magnetic field under adiabatic conditions, generally
is known to diverge at the quantum critical point, and thus
this physical quantity is very sensitive to the existence ofquantum criticality.
The results show the divergence of magnetic Grüneisenratio, indicating the existence of quantum criticality. In
addition, the critical scaling which examines the position of aquantum critical point, suggests that Pr2Ir2O7 has a quantum
critical point at zero magnetic field. Therefore, this system islocated at a zero-field quantum critical point without tuningof any external parameter.
In summary, we have found the zero-field quantum criticality in Pr2Ir2O7 as indicated by the divergent Grüneisenratio and zero-field quantum critical point. All these results
suggest that the chiral spin liquid phase accompanied withthe spontaneous Hall effect emerges under the influenceof the quantum criticality led by geometrical frustration.
This supports the manifestation of novel type of "quantum
critical spin liquid" states. Our study highlights the spin ice
as the parent state of the chiral spin liquid state, inducing the
spontaneous Hall effects and many other intriguing quantum
magnetic phenomena as represented by coherent propagation of monopoles. Our findings of quantum criticality which
emerges in a spin liquid state of a highly frustrated metal

require further studies, both experimental and theoretical, on
the group of frustrated metals.
References[1] S. Nakatsuji et al., Phys. Rev. Lett. 96, 087204 (2006).
[2] Y. Machida, S. Nakatsuji, Y. Maeno, T. Tayama, T. Sakakibara, and
S. Onoda, Phys. Rev. Lett. 98, 057203 (2007).
[3] L. Balicas, S. Nakatsuji, Y. Machida, and S. Onoda, Phys. Rev.
Lett. 106, 217204 (2011).
[4] Y. Machida, S. Nakatsuji, S. Onoda, T. Tayama and
T. Sakakibara, Nature 463, 210 (2010).
[5] Y. Tokiwa, J. J. Ishikawa, S. Nakatsuji, and P. Gegenwart, Nat.
Mater. 13, 356 (2014).
Authors
Y. Tokiwaa, J. J. Ishikawa, S. Nakatsuji, and P. GegenwartaaUniversity of Augsburg

respect to the ppv structure. We then demonstrated that thecompound exhibits superconductivity below 4.2 K (Fig. 1).
This is the first report of superconductivity in an isostructuralcompound. Even though the critical temperature is limited to
low temperatures at present, it should be possible to increase
the critical temperature by optimizing chemical composition.
Indeed, the critical temperature reaches the maximum valueof 5.6 K in a slightly N-deficient compound, V3PNx with x= 0.9. This discovery stimulates further explorations of new
superconducting materials with ppv and anti-ppv structures.
References[1] M. Murakami, K. Hirose, K. Kawamura, N. Sata, and Y. Ohishi,
Science 304, 855 (2004).
[2] B. Wang and K. Ohgushi, Scientific Reports 3, 3381 (2013).
Authors
B. Wang and K. Ohgushi

Superconductivity in Anti-Post-Perovskite
Vanadium Compound
Ohgushi Group
Superconductivity, which is a quantum state induced byspontaneous gauge symmetry breaking, frequently emerges
in quasi-two-dimensional materials. The typical examples are
high-critical-temperature cuprate with the layered-perovskite(pv) structure, and other examples include a ruthenate
Sr2RuO4, boride MgB2, hafnium nitride chloride, cobaltate
NaxCoO2•yH2O, an intercalated graphite C6Ca, and ironbased pnictides and chalcogenides. Hence, the layered
character of the host crystal structures is widely believed tobe essential in producing superconductivity. This provides animportant hint for exploring new superconducting materials.
Recently, the post-perovskite (ppv) structural transition of
MgSiO3 was discovered under extremely high-pressure(~120 GPa), and captures great interests because the ppv
MgSiO3 is considered to be the main constitute of the Earth’s
lowermost mantle, D" layer [1]. The ppv structure has apeculiar two-dimensional character, and is expected to be agood platform for superconductivity. However, up to now,
no superconductivity has been observed in isostructural
materials.
We here report the discovery of superconductivity in theanti-post-perovskite (anti-ppv) compounds [2]. We focusedon the vanadium compound V3PN with an anti-ppv structure, where the anion and cation positions are reversed with

Fig. 1. The resistivity of an anti-post-perovskite compound V3PN. The
resistivity drops to zero at low temperature, indicating the onset ofsuperconductivity. The inset shows the crystal structure.

Quantum Phase Transitions of the
Hubbard Model on the 1/5-Depleted
Square Lattice
Ueda Group
Quantum phase transition is one of the central issues ofthe present-day condensed matter physics. One of the routes
to approach quantum criticality is to introduce geometricalfrustration. Another way is to deplete lattice sites periodically, leading to weaker connectivity. A typical example
is depletion from the triangular lattice: One can obtain thehoneycomb lattice by the 1/3-depletion and the kagome
lattice by the 1/4-depletion. It is interesting to note that thetight-binding models on the depleted lattices often show
peculiar dispersions like flat bands and/or Dirac cones.
In the present study [1, 2, 3] we investigate Hubbard
model on the 1/5-depleted square lattice, Fig. 1. Although
the lattice structure looks very artificial, this structure canbe found in nature in CaV4O9 or K0.8Fe1.6Se2. The unit cellof this lattice contains four sites and thus the energy band
structure of the non-interacting system has four bands. At thesymmetric point where t1 = t2 the lowest band and the third
one touch at the Γ point, forming a Dirac cone. The second
lowest band intersects the apex of the Dirac cone. Therefore
this characteristic dispersion is relevant to quantum phasetransitions of the Hubbard model on this lattice at quarter
filling [1, 2]. With the Coulomb interaction the ground statephase diagram obtained by the mean field theory changes
from the non-magneteic insulating phase to the paramagnetic
metallic phase, then to an antiferromagnetic metallic phase,and finally to an antiferromagnetic insulating phase, as theratio t2/t1 is increased. Since the Dirac cone is embedded inthreefold degenerate electronic states the effective theory
around the Γ point is SU(3) Dirac electrons. The rich phase
diagram of the Hubbard model is understood by using thiseffective theory.
The quantum phase transitions at half-filling are also very
interesting [3]. It has been known that the Heisenberg model,
which is the effective Hamiltonian in the strong correlation
limit, shows two quantum phase transitions, one from thedimer singlet phase to the insulating phase with the antiferromagnetic long range order and then onto the plaquette singlet
phase. We have looked at the quantum phase transitions of
the Hubbard model as a function of Coulomb interaction

ISSP Activity Report 20135

References[1] Y. Yamashita, M. Tomura, Y. Yanagi, and K. Ueda, Phys. Rev. B 88
195104 (2013).
[2] Y. Yamashita, M. Tomura, Y. Yanagi, and K. Ueda, JPS Proceedings,
SCES2013.
[3] Y. Yanagi and K. Ueda, JPS Proceedings, SCES2013.
Authors
Y. Yamashitaa, M. Tomura, Y. Yanagib, and K. UedaaNihon UniversitybTokyo University of Science

Structural Evolution of 1D Spectral Function
from Low- to High-Energy Limits
Takada GroupThe concept of spin-charge separation plays a central
role in describing low-energy physics near Fermi points ina one-dimensional (1D) interacting electron gas, a typicalexample of the spin-1/2 Luttinger liquid (LL). This concept
may be confirmed in real materials by various experiments,
including the recent high-resolution angular resolved photoemission spectroscopy in which the one-electron spectral
function A(p,ω) can be directly measured in the wide rangeof momentum p and energy ω.
If p is not restricted to the region near the Fermi
momentum pF, the linear spectrum approximation, usuallyadopted in the LL theory, is not sufficient in appropriately
obtaining A(p,ω). In fact, the effect of the nonlinear spectrumon A(p,ω) has been intensively studied in recent years.
According to those studies on integrable systems, A(p,ω)
has singularities for arbitrary p in proportion to |ω-εν(p)|-μν(p)with ν=s and c, where εs(p) and εc(p) are energies of spinand charge collective excitations, respectively. In the usual
LL theory, the exponent μν(p) is independent of p, but thenonlinearity in the electron dispersion makes it depend on
p. Since the edge of support of A(p,ω) is located at ω=εs(p),μs(p) determines the power of the threshold singularity inA(p,ω) and its actual value has been given from the finitesize spectrum obtained by the Bethe-ansatz method. For

6

ISSP Activity Report 2013㻝㻠㻜㻰㼕㼟㼜㼑㼞㼟㼕㼛㼚㻌㼛㼒㻌㼍㻌㼒㼞㼑㼑㻌㼑㼘㼑㼏㼠㼞㼛㼚㻝㻞㻜㼜㻛㼜㻲㻌㻙㻝

under the assumption of paramagnetic states. The nature of
the metal-insulator transition is different depending on theratio of t2/t1. On the dimer side, it is shown by applying thecluster dynamical mean field theory that the metal-insulatortransition is continuous. This continuous Mott transition ischaracterized as a Lifshitz transition driven by the Coulombinteraction.

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Fig. 1. The 1/5-depleted square lattice. One can define either theunique plaquette covering or the dimer covering. We consider the
Hubbard model on this lattice with different hopping matrix elements
for the plaquette bonds or the dimer bonds.

nonintegrable systems, this threshold singularity remains
intact, but the singularity at ω=εc(p) is smeared into a broadpeak.
In those preceding works, only the singularities atω=εs(p) and ω=εc(p) are discussed on the belief that theelectron nature will not sustain in the spin-charge separated
system. For p far away from pF, however, the effect of interactions becomes so weak that we would naively expectthat the nature of an injected electron to measure A(p,ω)
manifests itself as a main peak in A(p,ω). Then a natural
question arises: Does an electron-like excitation mode
actually exist in the 1D interacting electron gas for p muchlarger than pF? If yes, a related and more intriguing question
is: How does the electron-like mode reconcile with thephysics of spin-charge separation for p near pF?
In pursuit of answers to those questions, we havecarefully studied the 1D one-electron Green's function G(p,t)
in momentum space and time and found that for p~pF, its
long-time asymptotic form is composed of three independentmodes of power-law decay [1]. Two of them correspond towell-known spinon and (anti)holon excitations, but the rest
describes the mode of an electron-like particle (pseudoelectron) which may be regarded as an electron dressed with a“cloud” of low-lying spin and charge collective excitations.
This pseudoelectron does not appear as a main structurein A(p,ω) for p~pF and never leads to a finite jump in the
momentum distribution function n(p). As p goes away frompF, the pseudoelectron structure gets broader, but with thefurther increase of p, it becomes less broad and eventually for p>>pF, it evolves as a main and divergent peak inA(p,ω) by swallowing the antiholon mode. Concomitantly,
its dispersion relation approaches the one of a free electron,
allowing us to regard the pseudoelectron as a free electron,
but actually it is not quite, nor the Landau's quasiparticle,
basically because this excitation is accompanied by powerlaw decay. Those results clarify the generic feature of A(p,ω)
in a 1D metal and answer the aforementioned two questions.
As an illustration of the overall behavior of A(p>pF,ω)with the change of p and ω, we adopt the Yang-Gaudin modelin the weak-coupling region in order to explicitly computeA(p,ω). In Fig. 1, the obtained result is displayed withincreasing p from pF to show its complete structural evolution
in the 1D weakly-interacting electron gas with the quadratic
dispersion ξp (=p2/2m-pF2/2m). Since we focus on the regionof ω in the very vicinity of ξp, only the pseudoelectron mode
appears as a singular structure in A(p,ω) in Fig. 1.

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㻜
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㻲
㻞
㻞
ξ 㼜㻌㻩㻔㼜㻌㻌㻙㻌㼜㻌㻌㻕㻛㻞㼙㻌㻲㻜㻚㻜㻞Λ: 㼏㼡㼠㼛㼒㼒㻌㼙㼛㼙㼑㼚㼠㼡㼙㻌

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㻙㻜㻚㻝

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Fig. 1. Structural change of the pseudoelectron peak in the one-electron
spectral function A(p,ω) for the Luttinger liquid (Yang-Gaudin model).
Inset: The entire structure of the spectral function near the Fermi point.

The pseudoelectron introduced here very much resembles
a quasiparticle in higher-dimensional Fermi-liquid systems,
although it is not quite the same, reflecting the specialty of
1D physics. We hope that this concept of a pesudoelectron
will be confirmed in the future through experiment and/or
large-scale numerical calculation with deliberately-chosenparameters so as to avoid its overdamping regime.
Reference[1] H. Maebashi and Y. Takada, Phys. Rev. B 89, 201109 (R) (2014).
Authors
H. Maebashi and Y. Takada

Particle Statistics, Frustration,
and Ground-State Energy
Oshikawa Group
Quantum statistics of identical particles is one of the mostfascinating aspects of quantum mechanics. The wavefunction
does not change when two identical bosons are exchanged,
while it acquires a negative sign when two identical fermionsare exchanged. This fundamental distinction leads to various
differences in physical properties between a system of manybosons and that of many fermions. For example, fermionsobey Pauli exclusion principle, which forbids more than
one particles to occupy the same single-particle state. In
contrast, any number of bosons can occupy the same singleparticle state, concerning the quantum statistics. In fact, at
sufficiently low temperatures, a macroscopic number ofbosons may occupy the same single-particle state. This is
the celebrated Bose-Einstein Condensation, which has been
directly observed in ultracold atoms.
Let us consider two systems with the “identical” Hamiltonian, but the one consists of many bosons and the otherconsists of many fermions. By “identical” Hamiltonian
we mean the single-particle eigenstates and their energy
eigenvalues are identical. Our question is: which system
has the lower ground-state energy, the one of bosons or thatof fermions? It might appear a trivial question, since in theFig. 1. Comparison of the ground-state energies of fermions and hardcore bosons on the square lattice of 26 sites. The horizontal axis is
the number of particles ne per site, and the vertical axis is the flux per
plaquette Φ in unit of the flux quantum Φ0. The ground-state energyof fermions is lower than that of bosons in the colored region, andthe difference is most significant in the red-colored region, where
ne = Φ/Φ0 or an equivalent relation approximately holds.

ground state all the bosons will occupy the lowest-energy
single-particle eigenstate, while most of the fermions will
occupy higher energy single-particle eigenstates because of
the Pauli exclusion principle. Then the ground-state energyof fermions would be always higher than that of bosons.
However, this simple argument is only valid when there isno interaction. When there is an interaction between theparticles, as is the case in realistic systems, the Bose-Einstein
Condensation is not perfect. Thus the comparison is a
nontrivial question in the presence of interaction.
We first proved [1] a theorem which states: even in
the presence of interaction, bosons have a lower groundstate energy if all the hopping amplitudes are positive. This
theorem can be understood in terms of frustration among
quantal phases; if all the hopping amplitudes are positive,
there is no such frustration in the system of bosons whilefermions are subject to frustration due to their quantum
statistics. We can violate the assumption of the theorem
by introducing negative and/or complex hopping amplitudes. This makes it possible that the inequality between
the ground-state energies to be reversed. This again canbe understood in terms of frustration introduced among
hoppings. In fact, we succeeded in constructing several
concrete examples, in which the ground-state energy offermions is lower than that of corresponding bosons [1].
This includes a rigorous proof on the delta-chain case, forwhich the reversal was reported earlier on small clusters [2].
References[1] W. Nie, H. Katsura, and M. Oshikawa, Phys. Rev. Lett. 111, 100402(2013).
[2] S. D. Huber and E. Altman, Phys. Rev. B 82, 184502 (2010).
AuthorsaW.
Nie, H. Katsura , and M. OshikawaaGakushuin University

K-computer Simulation forElectrochemical Energy Conversion
Sugino Group
Energy flows through matters in various forms andcontrolling the energy flow is an important subject of interdisciplinary material research. The energy flow dynamics
is complex but is particularly rich near the interface, where
the energy carrier (particle or field) switches from one toanother. The electrochemical interface offers a field forconverting an ionic flow to the electronic flow, and thisconversion process is used as the principle of a battery.
The corresponding basic process is the electron-transfer
reaction dynamics, which has now attracted renewed attention not only from electrochemistry but also from many otherfields. In this context, this group is intended to describe thedynamics, with the help of the K-computer power, unambiguously within the first-principles molecular dynamics(FPMD) scheme. A simulation team was organized fromresearch groups of Osaka/Tohoku/Nagoya University andnational laboratories (AIST and NIMS). The joint simulationteam has recently made progress toward the goal.
The team modeled the system using a metal slab, asolution slab, and the dielectric continuum slab as shown in
Fig. 1. This model, called the effective screening medium(ESM) model, was originally developed by the present groupin 2006 [1] but was recently improved to enable a precise

ISSP Activity Report 20137

References[1] M. Otani and O. Sugino, Phys. Rev. B 73, 115407 (2006).
[2] N. Bonnet, T. Morishita, O. Sugino, and M. Otani, Phys. Rev. Lett.
109, 266101 (2012).
[3] I. Hamada, O. Sugino, N. Bonnet, and M. Otani, Phys. Rev. B 88,
155427 (2013).
[4] N. Bonnet, O. Sugino, and M. Otani, J. Chem. Phys. 140, 044703(2014).
[5] N. Bonnet, O. Sugino, and M. Otani, to be published in J. Phys.
Chem. C.
AuthorsaaaO. Sugino, b N. Bonnet,b I. Hamada, M. Otani, T. Ikeshoji
,
b
c
d
Y.
Morikawa,
K.
Inagaki,H.
Kizaki
,
K.
Akagi
,
M.
AraidaiaAdvanced Industrial Science and TechnologybOsaka Universityc
Tohoku UniversitydNagoya University

Fig. 1. The ESM modeling of the electrochemical interface. The model
consists of the electrode slab (Pt(111) in the present case), solution slab
(liquid water plus a hydronium ion) and the dielectric continuum. The
continuum is characterized by the dielectric constant. The bias potential
is controlled with the potentiostat scheme [2].

control of the bias potential [2,3]. By this technique one
can describe a subtle imbalance of chemical equilibrium
between the electrons and the ions, essential for studying anelectrochemical process. This algorithm was implemented to
an FPMD code, called STATE-senri, which had been highly
parallelized for K-computer.
One of the first target of this new FPMD scheme wasto investigate how the solvent fluctuation would affect thecatalytic activity. The solvent fluctuation causes the bias
potential to fluctuate, whereby assisting the catalytic reaction
to occur. The fluctuation is small for the bulk catalysts but isincreasingly enhanced as the catalyst is reduced in size. By
combining the simulation with the classical Marcus theory,
the exchange current for the reaction, which is a measure ofcatalytic activity, was found to be enhance by 15 when thesize of nano-particle is reduced to 3 nm in diameter [4]. Thisenhancement factor is indeed a large value which is comparable to (or larger than) that achievable with the nano-shellmethod, i.e., a technique to enhance the activity by alloying
the subsurface Pt. The fluctuation effect has not been
seriously studied so far, but the present calculation suggests
further room for enhancing the catalytic activity by controlling the structure of the interface.
This group has been performing a large number of firstprinciples simulations, with or without the ESM, and theresults have accumulated. With those data, this group hasreconsidered how the reduction of an oxygen moleculewould occur, which is the central issue of electrochemistry
and many efforts have been devoted. When the simulation results were analyzed together with recently available
experimental results, it was found that some of the reaction
pathways can be excluded and accordingly narrowed down
the possibilities [5]. It was then concluded that the reaction
proceeds mainly through a pathway called the associative
pathway in the steady state, while the other one, called thedissociative pathway, is chosen prior to steady state. This
analysis emphasizes importance of the dynamical effect indiscussing the dominant mechanism.
More extensive simulations are now conducted in
K-computer. The team is preparing further for the coming
exa-flops supercomputers to establish a microscopic theoryof electrochemical energy conversion.

8

ISSP Activity Report 2013

Kondo Signature in Heat Transfer
via a Local Two-State System
Kato Group
Heat and electric transport have several similarities aswell as dissimilarities. Fourier's law in heat transport corresponds to Ohm's law in electric transport, and these laws
are commonly categorized as diffusive transport. Ballistictransport leads to the quantization of conductance in electricas well as heat transport. The conductance quantum wasmeasured in mesoscopic electric conduction in 1988, andmuch later, the version of heat transport was also measured[1]. Recently, the concept of thermal diode has also been
discussed, and an experiment has been conducted for demonstrating this [2]. Recent progress in transport studies strongly
indicates that heat transport analogue exists for many categories of electric transport.
We have studied the Kondo effect in heat transport via a
local two-state system [3]. We consider a two-state system
coupled to phonon environments. The two-state system is,for example, obtained from truncation from a symmetricdouble-well system with a small tunneling matrix element
Δ. The present system is described by the spin-boson Hamiltonian with Ohmic dissipation. The cut-off energy of thephonon bath and the dimensionless system-environment
coupling are denoted with ωc and α, respectively. It is known
that the spin-boson model can be mapped onto the Kondo
model with anisotropic exchange coupling. In order toFig. 1. (Main panel) Two different regimes expected in the spin-bosonmodel. (Inset) The thermal conductance calculated by the quantum
Monte Carlo method is shown as a function of the temperature.

study the Kondo-like effect in the phonon systems, we havederived the exact formula of thermal conductance, and haveevaluated it by the quantum Monte Carlo method.
Summary of the results is shown in Fig. 1. The main
figure shows the Kondo regime and the incoherent tunnelingregime in the space of temperature T versus dimensionless coupling strength α. At the bottom of the main figure,mapping onto corresponding anisotropic Kondo model isshown: the antiferromagnetic Kondo (AF-Kondo) and theferromagnetic Kondo (F-Kondo) region are respectively
mapped onto the region of 0<α<1 and 1<α<4 in the spinboson Hamiltonian. Heat transport via the Kondo effect isrealized in the Kondo regime (the orange region).
In the inset of the figure, we show temperature dependence of the thermal conductance κ calculated by thequantum Monte Carlo method. Different legends in the graphdenote with different ratios of Δ/ωc. As seen in the figure, thedata for a fixed α fall onto one curve by appropriate scalingof the temperature and the thermal conductance with the
Kondo temperature TK. This behavior is characteristic of the
Kondo effect. Below the Kondo temperature, conductancefollows the universal temperature dependence proportional
to T3. The obtained thermal conductance is much larger thanthe one expected from stochastic transition of the two-statesystem [4]. This is a manifestation of strong correlation
between system and reservoirs, which is analogous to the
Kondo effect in electric transport. On the other hand, attemperatures higher than the Kondo temperature TK, thethermal conductance becomes proportional to T2α-1. Thisbehavior is understood by a simple approximation based on
the Fermi's golden rule. We note that the thermal conductance is always proportional to T2α-1 for α>1 since the Kondo
temperature is zero.
In summary, we have investigated the Kondo effect inthermal transport via a local two-state system, by utilizing
the spin-boson model with ohmic dissipation. We hopethat our study motivates further research on low-energy
heat transfer via local systems. Other types of dissipation(superohmic/subohmic dissipation) and far-from-equilibrium effect will be intriguing subjects in this direction.
References[1] B. J. Wees et al., Phys. Rev. Lett. 60, 848 (1988); K. Schwab et al.,Nature (London) 404, 974 (2000); H.-Y. Chiu et al., Phys. Rev. Lett.
95, 226101 (2005).
[2] N. Li, J. Ren, L. Wang, G. Zhang, P. Hänggi, and B. Li, Rev. Mod.
Phys. 84, 1045 (2012); C. W. Chang, D. Okawa, A. Majumdar, and
A. Zettl, Science 314, 1121 (2006).
[3] K. Saito and T. Kato, Phys. Rev. Lett. 111, 214301 (2013).
[4] T. Ruokola and T. Ojanen, Phys. Rev. B 83, 045417 (2011).

Fig. 1. Scanning electron micrograph of the gate configuration for the
sample. The gray regions are metallic Schottky gates.

splits electronic spin states according to the direction of
momentum. Electronic states in a QPC structure are a set ofone-dimensional bands with discrete edges due to quantization transverse to the one-dimensional motion. As a result,ordinary conductance of a QPC is quantized with the unit
2of 2e /h, which is called conductance quantum. There
are a number of experimental reports on the anomalousquantization of conductance at a half conductance quantum,
2i.e. e /h, which phenomenon, called 0.5 conductance plateau
has been attributed to the result of perfect spin polarization[1] though direct evidence was not obtained. Such a highspin polarization was also predicted on the 1.0 plateau (i.e.
22e /h) [2] but due to the lack of detection method, there havebeen no experimental confirmation.
We have developed a new technique for the detection of
spin-polarization with time-domain spin blockade. For thatthe two-electron tunneling process from the target device toa quantum dot (QD) is used. When the process is to a single
orbital level, the probability is suppressed while it is not for
the tunneling into two orbital levels. Hence the degree ofsuppression has a direct relationship to the spin-polarizationin the target with the spin-relaxation time in the QD as aparameter.
As the base system, two-dimensional electron gas(2DEG) in an (In, Ga)As quantum well was adopted. The
sample consists of a QPC with a side-coupled QD withanother QPC for the remote charge detection (Fig. 1). The
conductance of the QPC as a function of the gate voltage
shown in Fig. 2 clearly exhibits 0.5 anomalous plateau struc-

Authorsa
K.
Saito and T. Katoa
Keio University

Nature of Spin Polarization
in a Quantum Point Contact
Katsumoto Group
Creation of spin current in quantum structures of
non-magnetic materials is a key technique in semiconductor spin-electronics (spintronics). The use of spin-orbit
interaction (SOI) and quantum point contact (QPC) structure is a promising candidate. So called Rashba-type SOI

Fig. 2. QPC conductance as a function of the gate voltage. The unit
is the quantum conductance Gq=2e2/h. The insets show bias voltage(VQPC) dependence of spin-polarization of 0.5 Gq and 1.0 Gq plateaus
respectively.

ISSP Activity Report 20139

2

ture in addition to the ordinary quantized plateau at 2e /h.
Spin-polarization measured with the QD detector is shown
as a function of the QPC bias voltage VQPC in the insets ofFig. 2, both for 0.5 and 1.0 plateaus. Here the definition of
spin-polarization P is P = (Nu–Nd) / (Nu+Nd), where Nu andNd correspond to the concentration of up-spin electrons anddown-spin electrons respectively. At zero-bias voltage, P ishigh on 0.5 plateau while it decreases with increasing VQPC.
The differential conductance of the QPC, on the contrary,increases with VQPC indicating that the mechanism of thespin-polarization is a kind of spin-filtering, which cuts
the flow of spin-down electrons. On the other hand at 1.0
plateau, P gradually increases with VQPC, indicating that themechanism here is spin-rotation, which turns up the down
spin of electrons. There is a small dip structure in P near thezero-bias at 0.5 plateau, which reflects the formation of the
Kondo cloud. As just described we have demonstrated thepowerfulness of the present method and revealed the mechanisms of spin-polarization in a QPC with Rashba-type SOI.
References[1] P. Debray et al., Nature Nanotechnology 4, 759 (2009).
[2] M. Eto, T. Hayashi, and Y. Kurotani, J. Phys. Soc. Jpn. 74, 1934
(2005).
[3] M. Kohda et al., Nature Communications 3, 1082 (2012).
Authors
S. Katsumoto, S.W. Kim, T. Nakamura, and Y. Hashimoto

Spin Injection into a Superconducting Niobium
Otani Group
Spintronics is a spin counterpart of electronics and its
potentiality for future applications as well as for novel
physics has boosted a number of researches in recent years.
Among diverse materials, superconductors are promising
for their distinct properties of spin transport; because of asuperconducting energy gap, spins are mediated by quasiparticles and it is theoretically predicted that the spin relaxation
time (τs) is enhanced owing to the reduced group velocity
of quasiparticles. However, previous experimental studies
are not conclusive due to the problems of device structures.
Thus, for both basic physics and applications, it is highly
desirable to estimate τs in superconductors correctly without
spurious effects. In this work [1], we aimed at injecting a

Fig. 2. The relation between τs and I. τs is normalized by that in thenormal state (τsnormal = 2.3 × 10−13 s). As I decreases, τs dramaticallyincreases.

pure spin current into superconductors with a large spin-orbit(SO) interaction and estimating τs appropriately. Through theuse of such superconductors, we can expect more complex
phenomena for further studies where superconductivity,
magnetism and SO interaction are closely coupled.
We fabricated a typical lateral spin valve structure witha superconductor Nb, consisting of two ferromagnetic
Permalloy (Ni81Fe19; Py) wires bridged by a nonmagnetic Cu
wire and a Nb wire inserted below the Cu bridge in between
the two Py wires. By flowing a charge current (I) from oneof the Py wires to the Cu, a pure spin current is generated in
the Cu bridge and partly absorbed into the Nb due to its large
SO interaction. The pure spin current can be detected by theother Py wire, and the detected nonlocal spin valve (NLSV)
signal is reduced when the spin absorption occurs. In ourstudy we performed spin absorption measurements bothabove and below the critical temperature of Nb (TC = 5.5 K).
In Fig. 1, we show a result of the spin absorption
measurements taken at 370 mK, much lower than TC. The
NLSV signal shown here is normalized by I. Above TC, it isindependent of I (not shown here), as expected for normal
metals. Far below TC, on the other hand, the NLSV signal
obviously increases as I decreases, indicating the suppressionof the spin absorption at the Nb wire. In order to evaluate theamount of the pure spin current absorbed from Cu into Nb,
we calculated the density of states of the superconducting
Nb using the Usadel equation where the superconducting
proximity effect between Cu and Nb is taken into account.
In the Usadel equation, τs of the superconducting Nb istreated as a free parameter. With this parameter, we canreproduce the NLSV signal for each I, and simultaneously
obtain τs. The obtained data are shown in Fig. 2. τs is clearlyenhanced with decreasing I. To know the relation between I
and temperature, we also measured the interface resistance
between Cu and Nb as functions of I and temperature, andfound that the effective temperature (T) at the interface
is increased by I. These results clearly demonstrate theenhancement of τs in a superconductor with decreasing T
consistent with the theoretical prediction [2].
References[1] T. Wakamura, N. Hasegawa, K. Ohnishi, Y. Niimi, and Y. Otani,
Phys. Rev. Lett. 112, 036602 (2014).
[2] T. Yamashita, S. Takahashi, H. Imamura, and S. Maekawa, Phys.
Rev. B 65, 172509 (2002).

Fig. 1. NLSV signals Rs measured at 370 mK (< TC) with two different
currents I = 20 μA (red) and 100 μA (blue).

10

ISSP Activity Report 2013

Authors
T. Wakamura, N. Hasegawa, Y. Niimi, and Y. Otani

One-dimensional Fermi Surface
is Stabilized on Ge Surface Even
at Low Temperature
Komori group
In solid-state physics, we learn that electrons in
one-dimensional metals undergo metal-to-insulator transition induced by the Peierls instability due to electron-phonon
interaction or exhibit a Luttinger liquid behavior due toelectron-electron interaction with decreasing temperature.
Atomic nanowires formed on semiconductor surfaces offer
an opportunity to investigate such intriguing phenomena of
one-dimensional metals. So far, however, their manifestations are largely disturbed by the presence of defects in the
nanowires. Highly-ordered atomic nanowires at surfaces
are thus essential not only for clarifying if they exhibit thePeierls instability or a Luttinger liquid behavior, but also forunderstanding universal nature of one-dimensional electronicsystems. On the Ge(001) surface, defect- and kink-free
Pt-induced atomic nanowires (Pt/Ge(001) NWs) can beformed, and are very suitable for the study of one-dimensional electrons. [1,2,3] We have investigated the groundstate electronic properties of Pt/Ge(001) NWs at the Fermi
level (EF) in situ by angle-resolved photoelectron spectroscopy at 6 K, a sufficiently lower temperature than its structural phase transition at 80 K, using single-domain samples
shown in Fig. 1.
Figure 2(a) shows the Fermi-surface mapping at 6 K. Twometallic surface bands labeled S1 and S2 are clearly identified. The Fermi surface of S1 consists of straight lines in the
ky direction, indicating that the S1 band disperses only inthe nanowire direction. It is therefore an ideal one-dimensional metallic state which is decoupled from neighboring
electronic systems. As shown in Fig. 2(b,c), it exhibits asteep dispersion and clearly crosses EF in the substrate bulk
band gap. Obviously, there is no gap opening at EF. Thus,
the S1 band does not directly contribute to the structuralphase transition at 80 K and is stable against the Peierls
instability if it exists. The electron-phonon interaction could
not be strong enough to induce any structural transition with
the nesting vector for S1 by the Peierls instability scheme.
In order to examine a Luttinger liquid behavior, the energy
distribution curve of S1 was analyzed as shown in Fig. 2(d).
The observed spectral shape agrees with the normal FermiDirac-type distribution function without any indication of
the power-law behavior predicted for a Luttinger liquid.
These novel results provide not only a valuable contribution

Fig. 1. STM images of Pt/Ge(001) nanowires (NW) on a vicinal
Ge(001) substrate. The substrate was tilted toward the [̅110] directionby 2˚. Most of wide terraces are covered by the nanowires parallel tothe step edges. The areas surrounded by purple lines in (left) represent
minority domains. In both images, the bias voltage is 1.5 V and thetunneling current 0.5 nA.

Fig. 2. (a) Constant energy ARPES intensity map at EF of Pt/Ge(001)
NWs with single-domain samples formed on the vicinal Ge(001)substrate. The photoelectron_ _intensity is represented by color scale. (b)ARPES image taken along ΓJ. Dashed curves represent the band structures of S1 and S2, which were fitted to the data. (c) Schematic illustration of the S1 and S2 bands. (d) Integrated energy distribution curve of
S1 near EF over ky in the surface Brillouin zone is compared with theresults of a Ta foil.

to the phenomenology of nanowires/semiconductor systems
but also a major advance for understanding the intriguing
physics of the one-dimensional electrons on solid surfaces.
References[1] Gürlu et al., Appl. Phys. Lett. 83, 4610 (2003).
[2] I. Mochizuki et al., Phys. Rev. B 85, 245438 (2012).
[3] K. Yaji et al., Phys. Rev. B 87, 241413(R) (2013).
Authors
K. Yaji, I. Mochizukia, S. Kim, Y. Takeichia, A. Harasawa, Y. Ohtsubob,
P. Le Fèvreb, F. Bertranb, A. Taleb-Ibrahimib, A. Kakizaki, andF. KomoriaHigh Energy Accelerator Research Organizationb
Synchrotron SOLEIL

Fabrication, Spectroscopic Characterization
and Transport Properties of Aromatic
Monolayers Covalently bonded to Si(111)
Yoshinobu Group
Fabrication of self-assembled monolayers (SAMs) oforganic molecules on semiconductor surfaces is one of
the central issues in surface science and device chemistrybecause of its ability to impart renewed functionalities on thesurfaces. Since well-ordered and dense organic SAMs can be
fabricated without severe reaction conditions and expensive
equipment, organic SAMs on a semiconductor have become
attractive materials that are easily accessible model systems
for both fundamental scientific research and development ofpractical molecular devices. Among various semiconductor–
organic interface structures, Si-organic molecule systems arepromising candidates for future applications because of theactual proven performance of Si in today’s electronics.
In this study [1], SAMs composed of aromatic moleculeswith different anchor groups were fabricated on the Si(111)
surfaces by wet chemical reactions. We investigated thebonding structures and transport properties by spectroscopic
and electrical measurements, respectively. By using simple

ISSP Activity Report 201311

Fig. 1. (left) Schematic representations of the aromatic monolayers on
Si(111) in this study [Si(111)–O–Ph (green), Si(111)–C–C–Ph (blue),and Si(111)–C=C–Ph (red)]. (right) Transmission FT-IR spectra ofthese Si(111)-aromatic SAMs measured at an incident angle of 60°.
Copyright © 2013 American Chemical Society.

aromatic molecules (phenol, styrene, and phenylacetylene) as
initial precursors, we successfully fabricated aromatic SAMs
covalently bonded to Si(111) surfaces through different
anchor structures (Si–O–, Si–CH2–CH2–, and Si–CH=CH−).
Transmission infrared spectra clearly indicate that the phenyl
rings in the SAMs are oriented almost perpendicular to theSi surfaces. High-resolution X-ray photoelectron spectrausing synchrotron radiation (KEK-PF BL13) reveal thatthe aromatic molecules attach to the Si surface with thesurface coverage of ~ 0.5. These experimental results lead toa conclusion that the aromatic SAMs form densely packed
monolayers on Si(111) using the present wet chemical
methods. Judging from the current density–voltage measurements of Hg/aromatic SAM–Si(111) sandwiched structures,the “Si(111)–O–Ph” (SAM by using phenol) system showshigher conductivity compared with the long-chain alkyl
SAM on Si(111).

a hole whose phase is conjugated with that of the electron.
Elastic scatters redirect reflected electrons/holes toward
the interface, making a loop in their trajectories with theinterface. Because of constructive quantum interference dueto the phase conjugation, the looped trajectories enhance
the probability of the Andreev reflection and consequently
proximity effect. As the Andreev-reflected quasiparticles cantunnel though the interface without reflection, the enhancement is called reflectionless tunneling.
Using scanning tunneling microscopy (STM), we studied
the proximity effect and the roles of the atomic-scale defects
in real-space with nano-meter scale spatial resolution. The
measurement was performed at 2.15 K in ultrahigh vacuum.
The proximity effect was characterized via tunnelingspectroscopy of a gap formed by the pair correlation at the
Fermi energy. The investigated super/normal interface wasformed in situ by creating superconducting Pb islands on a
Pb-induced two-dimensional (2D) normal metal formed ona Si substrate. In Fig. 1, a topographic STM image (left) anda mapping of the corresponding conductance at the bottomof the gap, that is, zero bias conductance (ZBC) mapping(right), are shown.
In the ZBC mapping, all Pb islands are colored green,
which indicates zero ZBC and good superconductivity there,
and the normal metal far from the Pb islands is coloredyellow, indicating no gap there. The area surrounding the Pb
islands has the color of blue to red, implying a gap formation by the proximity effect. The gap propagates from thesuperconducting Pb islands and decays into the normal metal
with the coherence length ξ of 40 nm. It is found from themapping that the proximity effect strongly depends on theatomic structure of the interface; it is strong at the place
where a Pd island is directly sitting on the 2D layer, andReference[1] Y. Harada, T. Koitaya, K. Mukai, S. Yoshimoto, and J. Yoshinobu,
J. Phys. Chem. C 117, 7497 (2013).
Authors
Y. Harada, T. Koitaya, K. Mukai, S. Yoshimoto, and J. Yoshinobu

Visualization of "Reflectionless Tunneling"in the Superconducting Proximity EffectFig. 1. (left) STM image of Pb islands on a Pb-induced two-dimensional normal metal formed on a Si substrate (1.0 μm square). The
edges of the Pb islands and the steps of the normal metal layer are
highlighted with white and black dotted lines, respectively. (right) The
zero-bias conductance (ZBC) color map of the same area as in the
topographic image.

Hasegawa and Kato Groups
When a superconducting material is brought into contact
with a normal metal, superconducting properties penetrate
into the metal. The phenomenon called proximity effecthas been utilized to induce the pair correlation into various
materials, including topological insulators to realize elucidating Majorana fermions.
The propagation of the proximity effect depends on the
distribution of scattering centers in the normal metal, as you
may expext. But curiously, it is enhanced by the presenceof elastic scatters. The proximity effect is caused by the
Andreev reflection at the super/normal interface, where
injected electrons from the normal metal is retro-reflected as

12

ISSP Activity Report 2013

Fig. 2. (left) 400 nm × 400 nm ZBC color map taken on a confined areasurrounded by the Pb island and a step edge of the normal metal layer.
(right) Normalized negative ZBC profiles across the super/normal interface and the step edges measured along the colored lines drawn in the
topographic image. The length written for each plot is the terrace width
measured along the corresponding line.

basically no gap formation is observed at the edge of theisland that coincides with the downward step of the 2D layer.
The observed structural dependence can be explained by thevariation of the conductivity at the super/normal interface.
We also found that the surface steps severely block the
propagation, which indicate they behave as a strong potential
barrier or an elastic scatter for the two-dimensional electrons.
In order to find the role of the steps on the proximity effect,
we investigated an area where the step is very close to thesuper/normal interface shown in Fig. 2 (the upper left ofFig. 1). In the wedged area whose terrace width ranges fromalmost 0 to 2ξ we observed the enhancement of the gap
and found that deeper gap is formed on narrower terrace,
as demonstrated in the right panel of Fig. 2, which shows
ZBC profiles for various width terraces. Because of the realspace and cross-sectional observation, we can probe ZBC
throughout the system while eliminating unwanted effectsof structural defects at the super/normal interface and in thenormal metal. The observed enhancement is overall consistent with the reflectionless tunneling. The dependence of theenhancement on the terrace width is, however, not explained
quantitatively with the calculation based on the Usadel
equation, a quasi-classical Green's function formalization.
Some phenomena that are not considered in the calculation
are probably involved in the enhancement of the proximity
effect.
References[1] H. Kim, S.-Z. Lin, M. J. Graf, T. Kato, and Y. Hasegawa,
arXiv:1401.2602
AuthorsaaH.
Kim, S.-Z. Lin , M. J. Graf , T. Kato, and Y. HasegawaaLos Alamos National Laboratory

Ferromagnetic Josephson Junctions
Lippmaa Group
Ferromagnetism and superconductivity are generally
incompatible because in conventional superconductors
Cooper pairs are formed by two electrons with opposite
spins, while in ferromagnets, exchange interactions align thespins parallel to each other. In our recent work, we study thepossibility of a long-range supercurrent surviving in highly
spin-selective materials, such as half-metals in spin filter
tunnel junctions. In particular, our interest is in determining
the role of magnetic disorder in promoting a spin-triplet
supercurrent in a superconducting tunnel junction with aferromagnetic insulator barrier.
The tunnel junctions (Fig. 1a) were grown by pulsed
laser deposition and consisted of superconducting
La1.85Sr0.15CuO4 (LSCO) and Au/Nb/Au electrodes at thebottom and the top of the device. The gold layers were usedas oxygen diffusion barriers but were superconducting due
to the proximity effect. The tunnel barrier material was
Pr0.8Ca0.2Mn0.9Sc0.1O3 (PCMSO), which is a weakly ferromagnetic insulator. The PCMSO layer was doped with Ca
to obtain the ferromagnetic insulating phase, while partial
Mn substitution by Sc was used to tune the coercivity of thebarrier layer and to introduce intentional magnetic disorder.
An array of lithographically patterned junctions can be seenin Fig. 1b.
The tunnel junctions showed characteristic step-like

Fig. 1. (a) Schematic view of the La 1.85 Sr 0.15 CuO 4 (LSCO) /Pr0.8Ca0.2Mn0.9Sc0.1O3 (PCMSO) / Au / Nb / Au junction. (b) Imageof a patterned junction array. (c) Current-voltage characteristic of ajunction at 4.2 K.

current-voltage characteristics below 6.9 K (Fig. 1c), limitedby the superconducting transition temperature of the Au/
Nb top electrode. Differential conductance spectra of thejunctions showed three characteristic peaks that correspondto various quasi-particle tunneling maxima, as illustrated inFig. 2. The spectra measured at various temperatures yieldedthe expected gap values for LSCO (8 to 14 meV) and Au/Nb(1.25 meV) above 4.5 K. Below this temperature, an anomalous increase of the gap energies was observed. This gap
energy change was visible as an increase of the normalized
critical current, ICRN of the devices. Since the contributionof a direct leak between the s-wave Nb/Au top electrode andthe d-wave LSCO bottom electrode is unlikely, we consider
the possibility of scattering from magnetic disorder inducedin the PCMSO barrier by the Sc doping. Magnetic disorderat a c-axis-oriented cuprate superconductor and a ferromagnet may give rise to spin-triplet supercurrent, effectively
increasing the junction critical current.
The devices developed in this work allow us to probe
the role of magnetic disorder at superconductor – ferromagnet interfaces, since it is possible to adjust the disorderby suitable level of Sc doping in PCMSO, while the junction
characteristics can be probed as a function of an external
magnetic field. Our future interest is to discern if singlet

Fig. 2. (a) Differential tunneling spectra at various temperatures and (b)proposed band diagrams explaining the origins of the three peaks, A(top), B (middle), and C (bottom) in the tunneling spectra.

ISSP Activity Report 201313

pairs may be stabilized by local antiferromagnetism inthe barrier or if scattering into spin-triplet Cooper pairs
dominates.
Athors ab
T.
Harada , M. Matvejeff , R.Takahashi, and M. LippmaaaMax
PlanckInstitutefor
Solid State Researchb
Aalto University

Possible Kondo Physics Near a
Metal-Insulator Crossover in the
A-site Ordered Perovskite CaCu3Ir4O12
Uwatoko GroupThe interplay between localized and itinerant electronsis at the heart of many intriguing phenomena such as theheavy-fermion behavior and the unconventional superconductivity associated with a magnetic quantum critical point[1]. The archetype systems for studying such interplay areconcentrated on the 4f/5f-based intermetallic compounds,
which can be effectively described as a Kondo latticeconsisting of a dense periodic array of f electrons interactingwith the conduction-band electrons [2]. Here, we reported
the realization of an analog d-electron Kondo lattice in anovel A-site-ordered perovskite CaCu3Ir4O12, which canbe synthesized only under high-pressure conditions, e.g. at
9 GPa and 1250 °C [3].
As shown in Fig. 1(b), the A-site-ordered perovskite

Fig. 1. (a) The Cu-O bond length in the coplanar CuO4 versus B-Obond length in the octahedral BO6 for A-site ordered perovskites
CaCu3B4O12; (b) The inverse magnetic susceptibility of several
CaCu3B4O12 perovskites; inset: schematic crystal structure of the A-siteordered perovskite.

14

ISSP Activity Report 2013

Fig. 2. Temperature dependence of (a) magnetic susceptibility χ(T) and
its inverse χ-1(T) measured under H = 1 T after zero-field cooling, (b)resistivity ρ(T) under H = 0 T, (c) thermopower S(T), and (d) specificheat C(T) under H = 0 and 9 T.
2+

CaCu3B4O12 can accommodate the Cu and the transition4+
metal B ions, respectively, in the square-planar CuO4 andoctahedral BO6 units, in which the d electrons can be coupled
2by strong covalent mixing with shared O ions. Depending
4+2+
on the B ions, the Cu 3d electrons can be tuned to beeither localized or itinerant. A plot of Cu-O versus B-O bond2+
length in Fig. 1(a) illustrated that the Cu 3d electronic
states in CaCu3Ir4O12 is located right at the itinerant-tolocalized boundary. In consistent with this observation, themagnetic susceptibility of CaCu3Ir4O12 shown in Fig. 1(b)demonstrated a crossover from the high-temperature CurieWeiss (CW) behavior to a low-temperature enhanced Pauliparamagnetism.
Figure 2 summarizes the physical properties of
CaCu3Ir4O12. The χ(T) measured at H = 1 T shows no signature of a magnetic ordering transition down to 1.8 K, but*a clear deviation from the CW law below T ≈ 80 K. The*
susceptibility anomaly below T is echoed in ρ(T), and also*
in the thermoelectric power S(T). Above T , ρ(T) follows a*perfect linear T-dependence; below T , ρ(T) exhibits a strong
downward trend with no sign of a curvature change down
to 0.5 K, strongly suggestive of non-Fermi-liquid behavior.
Significantly, S(T) also behaves anomalously, changing sign*
at T and remaining large as T approaches zero. The large
thermoelectric power at low temperatures signals strongly
energy-dependent quasiparticle properties. The absenceof a corresponding anomaly in the specific heat C(T) rules*out the possibility of a second-order phase transition at T .
Interestingly, C/T of CaCu3Ir4O12 exhibits a remarkableenhancement below ~ 20 K. The absence of a corresponding
enhancement in the closely related SrIrO3 perovskite, which
is a Pauli paramagnetic metal, indicates that this low-temper2+
ature anomaly must be associated with the Cu 3d electrons
or with interactions between Cu:3d and Ir:5d.
*
The presence of a characteristic temperature at T = 80

K is reminiscent of the low-temperature behavior in typicalheavy-Fermion materials, and is suggestive of Kondo-like2+
hybridization between primarily localized Cu 3d electrons*
and the itinerant Ir 5d electrons below T . In this picture,electrons on the two sublattices are weakly coupled above*
T . The initial reduction in χ(T) relative to the extrapolated*
T > T CW fit, may signal the onset of hybridization of Cu
4+electrons with the itinerant Ir 5d electrons. The quick*
drop of ρ(T) and the sign crossover of S(T) at T appear to
confirm the emergence of a hybridization between more
localized electrons on Cu and more itinerant electrons on Ir.
2+
The incorporation of the Cu 3d electrons into the itinerant
bands would not only enhance the density of states, but also
reduce the scattering from local magnetic moments. Further
evidence for the coherent incorporation of Cu local momentsin the Ir bands can be found in the specific-heat measurement, leading to a strong enhanced electronic specific-heat2
coefficient γ = 173 mJ/mol K for CaCu3Ir4O12. Our results
demonstrated that the A-site-ordered perovskites can host abroad spectrum of interesting phenomena that deserve further
explorations.
References[1] O. Stockert and F. Steglich, Annu. Res. Condens. Matter. Phys. 2,
79 (2011).
[2] Y.-F. Yang, and D. Pines, PNAS 109, E3060 (2012).
[3] J.-G. Cheng, J.-S. Zhou, Y.-F. Yang, H. D. Zhou, K. Matsubayashi,
Y. Uwatoko, A. MacDonald, and J. B. Goodenough, Phys. Rev. Lett.
111, 176403 (2013).
AuthorsaJ.-G. Cheng,
K. Matsubayashi,
Y. Uwatoko,
J.-S.c Zhou , A.
aab
MacDonald,
J.
B.
Goodenough
,
Y.-F.
Yang
,
H.
D.
Zhou
a
University of Texas at AustinbInstitute of Physics Chinese Academy of Sciencesc
University of Tennessee

Stability of Quantum Hall Ferromagnetic
Phase under High Magnetic Fields in the
Organic Dirac Fermion System
Osada Group
A layered organic conductor α-(BEDT-TTF)2I3 underhigh pressure (>1.5 GPa) is a multilayer Dirac fermion
system, in which 2D massless Dirac fermion layers stackwith weak interlayer coupling, and the Fermi level is fixed at
the Dirac point. Such an undoped 2D Dirac fermion systemshows the ν = 0 quantum Hall (QH) effect at sufficiently highFig. 2. High-field interlayer magnetoresistance of 2D organic Diracfermion system α-(BEDT-TTF)2I3. No sign of the QHF-QHI transition
was observed.

magnetic fields resulting from the spontaneous symmetry
breaking (SSB) of four-fold (spin and valley) degeneracy ofthe singular n = 0 Landau level. Generally, two kinds of theν = 0 QH state are possible in the Dirac systems (Fig. 1):
One is the QH ferromagnet (QHF) state with the metallic
helical edge state consisting of a pair of n = 0 QH chiral edge
states with opposite spin and chirality. The other is the QH
insulator (QHI) state with no edge state and some real-spacestructure. In this way, the high-field ground state is one of the
key issues of the physics of the Dirac fermion system.
In graphene, the typical 2D Dirac fermion system, thehigh-field ground state (the ν = 0 QH state) is believed to be
the QHI phase. In contrast, in α-(BEDT-TTF)2I3, the highfield ground state is considered to be the QHF phase. Itsinterlayer magnetoresistance shows anomalous saturation
at high fields. We illustrated that this saturation resistanceis scaled not by the sectional area of sample crystals but by
sample perimeter. This fact directly means that the metallic
surface transport is dominant in the saturating region, andit strongly suggests that the QHF phase with the metallic
helical edge state is realized. However, recently, Kanoda
et al. observed the fine structures of the NMR lineshape athigher fields (B > 15 T), which suggests the appearance ofsome real-space structure accompanied by the QHI. Theirresult shows the possibility of transition from the QHF to the
QHI in higher field region.
To confirm this possibility experimentally, we performed
high-field transport measurement up to 31 T using the
National High Magnetic Field Laboratory at Tallahassee,
USA. Since no surface transport exists in the QHI phase,
rapid increase of the interlayer resistance can be expected at
the QHF-QHI transition. As shown in Fig. 2, the interlayer
resistance shows increase reflecting the activated bulk transport at low fields, and it switches to weak increase at higher
fields up to 31 T. This saturation is caused by the surface
transport via helical edge state. We can see no anomaly in thesaturating region, especially around 15 T. This result meansthat the QHF state survives up to 31 T with no QHF-QHI
transition suggested by the NMR study.
The pure QHF phase is spatially uniform state. To
explain the NMR result, it might be necessary to consider the
mixture of spin and valley degree of freedom.
Authors
T. Osada, M. Sato, T. Konoike, and K. Uchida

Fig. 1. Schematic energy dispersion of QH edge states in the QHF (left)
and QHI (right) states. If the valley splitting becomes larger than thespin splitting in high magnetic fields, the QHF-QHI transition mightoccur. The helical edge channel (indicated by a circle) in the QHF
phase disappears at the QHI phase.

ISSP Activity Report 201315

Parallelizable Multi-Worm Algorithm
Kawashima Group
Massively parallel computations open the door to manyotherwise intractable problems in condensed matter physics,
in particular, quantum many-body problems. Recent highperformance computers, symbolized by K-computer in Kobe,
rely on a large number, as many as a million, of processors(or more precisely "cores"). Therefore, we must split thecomputational tasks into many bits and assigning each toone of the numerous processors. This is by no means a trivial
thing to do. Often we must significantly alter algorithms tosplit the task in an efficient way. In other words, we must
develop new parallelizable algorithms. The quantum Monte
Carlo method with the worm update algorithm [1, 2] has abroad range of applicability and is one of the standard tools
to treat quantum many-body problems. However, in thisalgorithm, update of configurations is realized via the motionof a single-point object in the whole space-time (or thelattice), making it hard to split the task.
Based on the directed-loop algorithm [2], one of thevariants of the worm algorithm, we proposed a parallelizable
worm algorithm, which we named the parallelizable multiworm algorithm (PMWA). In PMWA, multiple worms are
introduced by an artificial source field η and the space-timeis decomposed into many domains, as illustrated in Fig. 1.
Each domain is assined to a processor and typically contains
10-100 worms. We estimate physical observables by extrapolation to the η = 0 imit. Introducing multiple worms requires
a different procedure from the conventional worm algorithm
so as to satisfy the detailed balance condition. The migration of the worms over the domain boundaries is effectively
realized through communications between domains so thatthe ergodicity condition is satisfied. The PMWA is applicableto the soft-core Bosons and the quantum spins as the conventional worm algorithm.
We applied the PMWA to the extended Hard-core BoseHubbard model on square lattice defined by
H = -t bi+bj + V ninj - μ ni -η (bi++ bi)

Σ
(ij)

Σ
(ij)

Σi

Σi

where bi is the annihilation operator at site i, t is the hopping
energy, V is the nearest neighbor repulsion, and μ is thechemical potential. We established the extrapolation procedure to obtain physical quantities in the η = 0 limit. Then,
we confirmed that the extrapolated PMWA result agreed
with the result obtained by the conventional DLA algorithm.
Figure 2(a) shows the order parameter of the Bose-Einstein

Fig. 2. (a) The order parameter of the Bose-Einstein condensate as afunction of η;. (b) The standard error as a function of the number ofdomains with the fixed number of Monte Carlo steps. (c) The standard
error as a function of the number of domains with the computationaltime (wall-clock time) being fixed.

condensate Q ≡ 〈b〉 as a function of η. Here we carried out
simulations of the size of L × L × β = 10,240×10,240×16
using 3,200 processing cores, where L is the system's linear
dimension and β is the inverse temperature. This size exceeds
the maximum reachable size of the conventional worm
algorithm. To evaluate the parallelization efficiency of thealgorithm, we measured the standard error as a function of
the number of domains N. We found a very weak N-dependence for the relaxation time in the simulation with fixed
number of the Monte Carlo steps (Fig. 2(b)). In addition,in comparison with the conventional DLA results, PMWA
results show better accuracy in fixed wall-clock time with
N>8 (Fig. 2(c)), and the difference increases as the system
becomes larger.
References[1] N. Prokof'ev, B. Svistunov, and I. Tupitsyn, Sov. Phys. JETP 87,
310 (1998) etc.
[2] O. F. Syljuasen and A. W. Sandvik, Phys. Rev. E 66, 046701 (2012).
[3] A. Masaki-Kato, T. Suzuki, K. Harada, S. Todo, and N. Kawashima,
arXiv:1307.0328 (2013), to be published in Phys. Rev. Lett. (2014).
Authorsab
A.
Masaki-Kato, T. Suzuki , K. Harada , S. Todo, and N. Kawashimaa
University
of
Hyogo
b
Kyoto University

Dynamic Light Scattering Microscope:
Accessing Opaque Samples
with High Spatial Resolution
Shibayama Group

Fig. 1. Worms and worldlines in PMWA. The yellow dashed lines are
the domain boundaries.

16

ISSP Activity Report 2013

Dynamic light scattering (DLS) is a technique forobtaining the size distribution of particles in solution.
However, it cannot be applied to opaque samples. There
are two types of opaque samples. One is a black sample,
which is a strong light-absorbing material such as ink. In thiscase, scattered light is completely absorbed by the sample
itself and one cannot obtain the signal. The other type ismilky samples, which are strong light-scattering materials
such as milk. In this case, multiple scattering is inevitable.
In addition to this, poor spatial resolution sometimes also
becomes a problem. There is a growing demand to investi-ered not to aggregate. One of the possible explanations forour result is the existence of attractive interactions between
colloidal particles. In other words, colloidal particles show
collective motion rather than Brownian motion at higher
concentrations. This consideration is supported by the factthat the viscosity of neat ink is approximately five times
higher than that of water. Taking advantage of its high spatial
resolution, this technique can also be readily applied to othermedia such as biological cells and gels.
References[1] T. Hiroi and M. Shibayama, Opt. Express. 21, 20260 (2013).
[2] P. Navabpour et al., Colloid Polym. Sci. 283, 1025 (2005).
Authors
T. Hiroi and M. Shibayama
Fig. 1. Photograph of the dynamic light scattering microscope.

gate the dynamic behavior of samples with a higher spatial
resolution. One such example is a tracking of material transportation in biological cells. To realize such applications,the spatial resolution needs to be near the diffraction limit.
Here, we propose a new DLS technique, “DLS microscope”
that mitigates the above-mentioned disadvantages (Fig. 1)[1]. The proposed DLS microscope obtains signals with abackscattering geometry. This enables us to measure opaquesamples with high spatial resolution.
By using the DLS microscope, we measured the size
distribution of opaque samples such as polystyrene latex andChinese ink with changing their concentrations. Figure 2
shows the results for the polystyrene latex suspension andChinese ink. In the case of polystyrene latex suspension, thedata show the monodisperse nature. The calculated diametershows good agreement with the nominal diameter (100 nm).
Since the result for the polystyrene latex suspension shows
peaks at almost the same position in all concentrations, we
can conclude that the polystyrene latex suspension shows
no morphology change through varying the concentration.
This is consistent with the following molecular interpretation: each particle has a negative charge on the surface, andthe particles repel each other keeping their form unchanged
and showing Brownian motion. This result corresponds to
the result obtained by using diffusing wave spectroscopy [2].
In contrast, the result for Chinese ink seems to show thatthe average particle size increases and the size distribution
broadens at higher concentrations. However, as Chinese ink
is a protective colloidal solution, the particles are consid-

Fig. 2. (Left side) Concentration dependence of the size distribution ofa polystyrene latex suspension. The nominal diameter of the polystyrene latex particles is 100 nm. The 1 - 0.01 wt%, as measured by the
DLS microscope, is represented by the solid lines. The 0.01 - 0.0001
wt%, as measured by the typical DLS system, is represented by thedashed lines. (Right side) Concentration dependence of the size distribution of Chinese ink. The 10 - 0.05 wt%, as measured by the DLS
microscope, is represented by the solid lines. The 0.05 - 0.001 wt%,
as measured by the typical DLS system, is represented by the dashedlines.

Magnetic Relaxation in a
Tb-based Single Molecule Magnet
Yamamuro GroupA single-molecule magnet (SMM) is a metal complex
that behaves as an individual nanomagnet. Each molecule,containing several metal centers with unpaired electrons,possesses a giant resultant spin. Given that the giant spin
exhibits easy-axis anisotropy (D < 0), the magnetization reversal between the ground states with Sz = +/-S is2hindered by the potential barrier of DSz . The traditional
research subjects have been SMMs containing multiple
transition metal atoms such as Mn, Fe, and Ni, while thecurrent mainstream is shifting to lanthanide SMMs. Owing
to a large contribution of angular momentum, lanthanide
complexes can become SMMs containing only one or twomagnetic ions. One of the central issues in lanthanide SMMs
is the quantum tunneling mechanism for the magnetization reversal. In most cases, the tunneling occurs between
ground states (pure tunneling process) and/or excited states(thermally assisted tunneling process). The origins of thetunneling have not been fully understood yet.
We have investigated the spin dynamics of Tb-Cu
dinuclear SMM by means of neutron scattering techniques.
The chemical formula of the sample is TbCuC19D20N3O16
and its molecular structure is schematically shown in theFig. 1. Dynamic structure factors of TbCu-SMM. The data at 3.5 K
represent the resolution function. The other data are fitted to the combination of delta and Lorentz functions. The inset shows the molecular
structure of TbCu-SMM.

ISSP Activity Report 201317

momentum of the Tb ion, Jz (see Fig. 2(a)), assuming thespin Hamiltonian including ligand field anisotropy, exchange
3+2+interaction between Tb and Cu , and hyperfine interaction. The excitation of 1.7 meV is the transition between the
ground and the first excited states. We thus conclude that themagnetization reversal occurs through quantum tunnelingbetween the pairs of degenerated excited states at 1.7 meV.
This is called thermally activated tunneling process, in otherwords the Orbach mechanism of spin-lattice relaxation. On
the other hand, the relaxation 2 detected by QENS has quite-12
different Ea and τ0; Ea(2) = 174 K and τ0(2) = 1.5 × 10 s.
The relaxation 2 is also of the Debye type and gets activatedupon heating according to the Arrhenius law. The simplest
scenario for the origin of the relaxation 2 is that the relaxation 2 takes place through the tunneling between higher
excited states. The activation energy of 174 K (= 15 meV)roughly corresponds to the higher energy level as seen in theenergy diagram in Fig. 2(a).
This is the first successful QENS work to observe
magnetic relaxation in SMM. The use of QENS can open
a new window into the study of relaxation processes in theclass of SMMs.

Fig. 2. (a) Calculated energy diagram as a function of z-component of
the total angular momentum of Tb ion. Dashed lines represent possible
relaxation processes. (b) Arrhenius plot of the relaxation times observedon the five spectrometers. The results from the ac susceptibility
measurements are also shown.

inset of Fig.1. In order to reduce the contribution fromthe strong incoherent scattering of H atoms, the sample
3+
was fully deuterated. Only two magnetic ions, Tb (J = 6)
2+
and Cu (S = 1/2), are involved in a molecule. Following
inelastic neutron scattering studies to determine the energy
scheme [1], we investigated the magnetic relaxation bymeans of quasielastic neutron scattering (QENS) experiments using various spectrometers with different energy
resolutions; AGNES (∆E = 49 µeV) at JRR-3, AMATERAS(∆E = 120 µeV) at J-PARC, TOFTOF (∆E = 8 µeV) at FRM
II (Germany), HFBS (∆E = 0.8 µeV) and neutron spin-echo
NSE (Fourier time, 0.007 < t < 12 ns) at NCNR (USA) [2].
The use of five spectrometers enables us to investigate relaxations over a wide time range between 1 ps and 100 ns.
Figure 1 shows the dynamic structure factors of Tb-Cu-1
SMM at Q = 0.5Å taken on the TOFTOF spectrometer.
Clearly, QENS broadening was observed above 30 K. The
QENS spectra were well fitted to the combination of deltaand Lorentz functions. The linewidth of the Lorentzian,
Γ(Q), exhibits no pronounced Q-dependence, indicating thatthe relaxation is of a local origin. The relaxation times wereobtained from the relation τ = 1/Γ. In Fig. 2(b), we show
the Arrhenius plot of relaxation times estimated from the
QENS together with those from the ac magnetic susceptibility measurements. This provides a clear evidence of theexistence of two distinct relaxation processes. We designate
the slower relaxation as the relaxation 1 and the faster one
as relaxation 2. Both relaxation processes follow the Arrhenius relation τ = τ0exp(∆Ea/kBT). As for the relaxation 1,
the activation energy Ea and prefactor τ0 are estimated to be-8
16.0 K and 4.1 × 10 s, respectively. It should be emphasized that Ea corresponds to the excitation energy of 1.7 meV
observed in the INS study [1]. We have calculated an energy
diagram as a function of the z-component of the total angular

18

ISSP Activity Report 2013

References[1] M. Kofu, O. Yamamuro, T. Kajiwara, Y. Yoshimura, M. Nakano,
K. Nakajima, S. Ohira-Kawamura, T. Kikuchi, and Y. Inamura, Phys.
Rev. B 88, 064405 (2013).
[2] M. Kofu, T. Kajiwara, J. S. Gardner, G. G. Simeoni, M. Tyagi,
A. Faraone, K. Nakajima, S. Ohira-Kawamura, M. Nakano, andO. Yamamuro, Chem. Phys. 427, 147 (2013).
Authorsab
M.
Kofu, T. Kajiwara , G. G. Simeoni , and O. YamamuroaNara
Wemen’s
UniversitybTechnical University of Munich

Nano-magnetism in
Relaxor Magnet LuFeCoO4
Masuda Group
Dielectric property exhibiting an enhanced permittivity
in wide temperature range is known as relaxor ferroelectricity and it has attracted great interest in the field of basic
and applied physics. In the relaxor system Polar Nanoregions (PNRs), where coherent polarizations in nano-scaledomains are randomly oriented in the bulk crystal, play
important role. So far disordered perovskite oxides suchas Pb(Mg1/3Nb2/3)O3, Pb(Sc1/2Ta1/2)O3, etc., have been
extensively studied [1] and all of them are purely dielectric and non-magnetic. Here natural question arises: what
would happen if the relaxor system contains magnetic ions?
One of our group members answered to the question in his
pioneering work on a relaxor magnet, 2/3BiFeO3-1/3BaTiO3[2]; appearance of a new type of nano-magnetism where
superparamagnetic moments are induced limitedly inside
the PNRs. For further understanding of relaxor magnets
we study a new compound LuFeCoO4 by combinationof bulk properties measurements and neutron diffraction
technique. Our study reveals the relationship between PNRs
and magnetic correlation, and establishes the magnetic anddielectric phase diagrams of the relaxor magnet as shown in
Fig. 1(a) [3].
LuFeCoO4 is a two-dimensional (2D) triangular spin

3D Liner Imploding Process in theElectro-Magnetic Flux Compression
Takeyama Group

Fig. 1. (a) Magnetic phase diagram (upper panel) and dielectric phase
diagram (lower panel). PM, SM, NR, ER, and PE mean paramagnetism,superparamagnetism, nonergodic relaxor, ergodic relaxor, and paraelectric, respectively. (b)-(g) The relationship between PNR and magnetic
correlation at various temperatures. White arrows are electric polarization and yellow arrow are weak-ferromagnetic moments. (h) Temperature dependence of magnetic and nuclear diffuse intensities. (i) The
temperature dependencies of magnetic and nuclear correlations.
3+2+system where Fe and Co ions are randomly positioned on
the vertices of the triangles. Temperature dependence of thepermittivity constant showed a typical behavior of a conventional relaxor. The result suggested the existence of PNRs
and it is supported by neutron diffuse scattering. Magnetic
susceptibility showed a weak-ferromagnetic component
and magnetization showed superparamagnetic behavior
at T < 190 K. With further decreasing the temperature thesusceptibility showed a well-defined anomaly at 70 K andthe magnetization showed hysteretic behavior at the basetemperature, indicating a realization of magnetic order with aweak-ferromagnetic component.
The nuclear scattering results at T > 190 K in Fig. 1(h)show typical behavior of relaxor systems. At 190 K < T <275 K, the PNRs are freezed and static. With decreasing thetemperature at T = 190 K the nuclear diffuse intensity begins
to decrease and simultaneously the magnetic peak appears.
These results mean that the size of the PNRs increases and,synchronizedly, magnetic correlation develops. The magnetic
correlation length at 190 K is 7 nm and it is the same as thatof nuclear scattering as shown in Fig. 1(i).
Based on the bulk property measurements and neutron
diffraction experiments, we propose a superparamagnetic
model that a weak ferromagnetic correlation is, in thetemperature range of 190 K < T, developed in the crystallographic c plane inside the PNRs as schematized in Fig. 1(d).
Outside the PNRs the nuclear correlation is incoherent andso is the magnetic correlation. Spin moments in the PNRs
behave as superparamagnetic moments and those outside the
PNRs do as conventional paramagnetic spins. At the temperature TMD ~ 190 K the magnetic correlation achieves longranged inside the PNRs. With decreasing the temperature thedomain size of PNR increases and consequently the accompanied magnetic domain increases as described in Fig. 1(c).
At the TN' = 70 K a magnetic correlation in 2D and dielectric correlation in 3D are both in quasi-long-range order as
schematized in Fig. 1(b). Meanwhile at T ≥ TMD the magnetic
order is absent as shown in Figs. 1(e)-(g). The proposedmodel consistently explains both bulk properties and neutron
scattering results.
References[1] A.A. Bokov and Z.-G. Ye, J. Mater. Sci. 41, 31 (2006).
[2] M. Soda et al., J. Phys. Soc. Jpn. 80, 043705 (2011).
[3] M. Soda et al., submitted to Phys Rev. B.

The electromagnetic flux compression (EMFC) techniquein ISSP has been established to generate magnetic fields up
to 730 T [1], which is the world strongest magnetic field in
an indoor experiment. There, a metallic cylinder called the“liner” is subjected to the inductive electromagnetic force
from a primary coil, and the initial magnetic field (~4 T) is
compressed by an imploding liner within a few microseconds
before the magnet destruction. Owing to the high reproducibility in its operation of the EMFC technique, solid-statephysics experiments are nowadays available in ultra-highmagnetic fields of up to 700 T at room temperature and up to
600 T at temperatures as low as 5 K. For solid-state physics
experiments performed under ultra-high magnetic fields,
the particularly important issues are (a) high precision of
the magnetic field intensity, (b) the spatial uniformity of themagnetic field, and (c) appropriate bore at a peak magnetic
field. For issue (a), the magnetic field intensity was recently
verified to have a precision of 3 % up to 500 T, by calibratingwith the Faraday rotation measurement on optical glasses[2]. For issues (b) and (c), detailed knowledge of dynamicalmagnetic-flux compression processes is required, for which
computer simulation can be a powerful tool. Thus, we clarified the relationship between the liner’s imploding process
and the spatiotemporal distribution of the magnetic field
intensity, by comparing with the simulation [3].
In Fig. 1(a), the solid curve is the magnetic field intensity
curve (hereafter termed as the B-t curve) at z = 0. We adopted
the liner of 50 mm length (Llin, illustrated in the inset) and of
1.5 mm thickness. A multi-probe with an eight pick-up coils
were set along the magnetic field axis (defined as the z-axis)
at the center of the imploding liner, and the spatial distribution of the magnetic field intensity was simultaneously
measured in a single shot The magnetic field’s profile in thez direction, B(z), was shown in Fig. 1(b). The field homogeneity was thus successfully determined for the first time in
the EMFC experiment. Up to 500 T, the field homogeneity
was estimated as good as the measurement error (~3 %) atz = ±1 mm, which has proven to be sufficiently good for thegeneral solid-state physics measurements.

Fig. 1. (a) Magnetic field curves of the experiment (the solid curve, Llin
= 50 mm) and the simulation (the dashed curve, Llin = 35 mm). The
inset illustrates the liner’s shape. For t = 0 μs, we show the liner used in
the experiment (Llin = 50 mm). For t = 37 μs, the simulated imploding
liner in case of Llin = 35 mm deforms into a barrel-like shape. The color
map indicates the secondary current intensity, Is, induced in the liner.
(b) Spatial profiles of magnetic field intensity along the magnetic fieldaxis at each time, which corresponds with the open symbols in (a). The
solid-curves are the fitting of two Gaussian functions for the experimental data, and the dashed curves are the simulated spatial profiles incase of Llin = 35 mm.

ISSP Activity Report 201319

Two-hump structure of B(z) in Fig. 1(b) is hardly
explained by the mesh modeling of liner’s dynamics developed by Miura and Nakao [4]. In Ref. 4, the liner’s thickness
was assumed to keep uniform along the z direction during
the implosion. However, their model is far from accuracy indescribing the real situation and is oversimplified. Therefore,
we revised the simulation by independently calculating the
dynamics of the liner mesh with its respective z coordinate,
which allows us to obtain a nonuniform liner deformation.
The dashed curves in Figs. 1(a) and 1(b) are the simulation result in case of Llin = 35 mm. The length of the primarycoil (Lpri) is set to be 45 mm. The simulated liner’s shape at
37 μs is illustrated in the inset of Fig. 1(a), and the color map
indicates the secondary current intensity, Is, induced in theliner. The barrel-like deformation of liner’s inner wall shownin Fig. 1(a) induces the two hump structure of B(z) in Fig.
1(b). Note that the liner’s barrel-like deformation only takes
place when Llin < Lpri, whereas Llin (50 mm) > Lpri (45 mm)
at t = 0 μs in the real experiment. This fact implies a threedimensional implosion process associated with a compression of the liner along the z-axis. The present work has
pointed out that a three-dimensional analysis of an imploding
liner is of particular importance for further record-breaking
higher magnetic field in the EMFC, and for an increased
controllability as well as a higher precision in the solid-statephysics measurements.
References[1] S. Takeyama and E. Kojima, J. Phys. D: Appl. Phys. 44, 425003(2011).
[2] D. Nakamura et al., Rev. Sci. Instrum. 84, 044702 (2013).
[3] D. Nakamura, H. Sawabe and S. Takeyama, Rev. Sci. Instrum. 85,
036102 (2014).
[4] N. Miura and K. Nakao, Jpn. J. Appl. Phys. 29, 1580 (1990).
Authors
D. Nakamura and S. Takeyama

Heat Pulse Measurements of Specific Heat
in Pulsed Magnetic Fields
Kindo Group
Measurements of the specific heat (Cp) in magnetic fields
have been widely used to investigate fundamental propertiesof a material. Although several attempts to provide specificheat data at high magnetic field have been made, experimentsin pulsed magnets are subject to strong constrains that arise
from the short timescale of the measurements. Therefore,
calorimetric studies have all been limited to the magnetic
field range accessible to continuous magnets (dc magnets),with very few exceptions of pulsed magnets having an
unusually long (>250 ms) pulse field duration [1,2]. In thiscontext, we have developed, for the first time, a calorimeterthat can operate in short pulse duration of less than 50 ms[3].
We attempt to obtain a quick thermal response by
reducing the physical size of the calorimeter. Our calorimeterconsists of a Au16Ge84 thin-film thermometer, a Ni50Cr50
thin-film heater, 100 μm diameter Constantan relaxation
wires, and a quartz thermal bath. The eliminations of a bulk
thermometer and a bulk heater lead to a quick monitoringof the temperature and enable us to study thermodynamic
properties, such as Cp and magnetocaloric effect (MCE), in

20

ISSP Activity Report 2013

Fig. 1. Time dependence of the temperature (black dots), the power ofthe heat pulse (red), and the field profile (blue) during the 36 ms pulsed
magnetic field. Gray curve represents a fit obtained with a simple heat
equation.

the 36 ms pulsed magnet that is commonly used to reachfields above 50 T. As a test of our thin-film based calorimeter, we have measured the Cp of slab-shaped Cu3Mo2O9
single crystalline samples (0.10 mg (sample 1) and 0.0285
mg (sample 2)) between 3 and 20 K in pulsed magnetic fields
up to ~56 T.
Figure 1 shows the time dependence of sample temperature (black curve) during a specific heat measurement in thepresence of 56 T pulsed magnetic fields. During the application of square heat pulse (red curve), the sample temperature
linearly increased, and immediately after the heat pulse aclear step in temperature was observed. The short time scaleof measurement (~0.5 ms) significantly reduces the uncertainty in magnetic field (blue curve); 0.2% of the appliedfield (56.0 ± 0.1 T) for observing a single temperature step.
Since the step-like temperature change is the ideal situation
for the heat pulse calorimetry, the change of the temperature(ΔT) can be estimated by the linear extrapolation (green
lines in Fig. 1(a)), and the ΔQ is obtained by integrating theP(t) curve. The molar specific heat was calculated by Cp =-1 -1
ΔQΔT m M, where m and M are the sample mass and themolar weight, respectively, and its temperature dependence
was plotted in Fig. 2.
Figure 2 shows the Cp(T) of sample 1 (black dots) andsample 2 (blue crosses) in various magnetic fields are shown
together with PPMS data measured at 0, 7 and 13 T. Cp(T)Fig. 2. A comparison of the specific heat data from the heat-pulse
calorimeter (sample 1; black squares and sample 2; blue crosses)with those from the PPMS (red curves) near the phase transition in
Cu3Mo2O9. Arrows indicates the peak temperature in specific heat.

show λ-type pronounced peak and its peak temperature andthe size of the anomaly varies as the applied magnetic fieldchanges. Although the point-to-point scatter obtained fromthe present set-up is more than that of PPMS (red curve),
the over-all temperature dependence of Cp, including thepeak shape and its absolute value, agree well with the datameasured in PPMS. We estimate that the absolute accuracyof the measurement is better than ±10% for temperature
range from 3 to 20 K. The quantitative agreement between
two sets of data shows that the experiment we are doing inpulsed field is the same as the experiments we have been
doing in dc fields. The resultant Cp(T) data can be analyzed
in the conventional way to determine the entropy, specificheat exponent, and possibly the order of the phase transition.
We have developed an effective calorimeter for use in apulsed magnetic field up to ~56 T and tested their performance with Cu3Mo2O9 single crystalline samples. For
Cu3Mo2O9, this system exhibits an overall accuracy of better
than ±10% for entire temperatures range of 3 to 20 K. This
technique is now open to users of International Megagauss
Science Laboratory.
References[1] M. Jaime et al., Nature 405, 160 (2000).
[2] Y. Kohama, C. Marcenat, T. Klein, and M. Jaime, Rev. Sci. Instrum.
81, 104902 (2010).
[3] Y. Kohama, Y. Hashimoto, S. Katsumoto, M. Tokunaga, andK. Kindo, Meas. Sci. Technol. 24, 11505 (2013). (IOP select)
Authors
Y. Kohama, Y. Hashimoto, S. Katsumoto, M. Tokunaga, and K. Kindo

Electronic Phase Transitions of Graphite
in the Quasi-Quantum Limit State
Tokunaga and Kindo Groups
The quantum limit state, where all carriers populate thelowest Landau level, exhibits various anomalous phenomena
because of the strong correlation acting on the carriers
confined by magnetic fields. This state can be realized only
in extremely high magnetic fields, the order of 104 T, in
ordinary metals.
Graphite is a semimetal having a small number of18-3electrons and holes ( ~ 3 × 10 cm for each) and goes
into the quasi-quantum limit state, where only the lowest
electron-like (Landau index n = 0, spin↑and↓) and holelike (n = −1, ↑and↓) subbands are populated in magnetic
fields above 7.4 T applied along the c-axis. With further
increasing the field, emergence and collapse of a density
wave state has been claimed through measurements ofin-plane (ρ xx) and out-of-plane (ρ zz) resistivity [1]. In
addition, recent magnetoresistance measurements up to 80 T
suggested emergence and collapse of another density wave
state at 53 T and 75 T, respectively [2]. The actual nesting
vector in these density wave states and also the underlying
structure of the subbands, however, remain unclear as yet
because of the limited experimental technique available
above 25 T.
We studied Hall resistivity (ρxy) and magnetization ofgraphite single crystals in pulsed high magnetic fields [3]. As
the main feature of the results, the ρxy becomes almost zeroas the field approaches 53 T, where ρxx and ρzz show anoma-

Fig. 1. (upper) In-plane (ρxx) and out-of-plane (ρzz) resistivity ofgraphite single crystals as a function of magnetic field applied along
the c-axis at temperature of 1.4 K. (lower) Magnetic field dependence
of in-plane Hall resistivity (ρxy) of graphite at 1.4 K. The colored areas
represent different electronic phases suggested by transport measurements. The inset of the lower panel shows schematic illustration of thedispersion curves of the populated subbands above 53 T.

lies as shown in Fig. 1. In the microscopic point of view, theHall resistance is given by summation of the matrix elementsof the current operator between the states having different
Landau indices ∆n = 1 and, hence, sensitive to the wholedispersion curves of the relevant subbands. The almost zeroHall resistance seems consistent with the existence of two
populated subbands (n = 0↓ and n = −1↑) [1] rather than theless-symmetric three subband model (n = 0↑ , n = 0↓ andn = −1↑) [2]. Thereby, the quantum limit state likely
emerges above 53 T. In this quantum limit state, density
wave can be formed by the nesting between the Fermi pointsof the n = 0↓ and n = −1↑ subbands, which is equivalent
to the BCS-like pairing state of excitons. Further studies areneeded to clarify the nature of the additional phase transitionat 75 T in this quantum limit state.
References[1] H. Yaguchi and J. Singleton, J. Phys.: Condens. Matter 21, 344207(2009).
[2] B. Fauqué et al., Phys. Rev. Lett. 110, 266601 (2013).
[3] K. Akiba et al., submitted to J. Phys. Soc. Jpn.
Authorsa
K. Akiba, A. Miyake, H. Yaguchi , A. Matsuo, K. Kindo, andM.
TokunagaaTokyo University of Science

Ultrafast Photo-Induced Transitionof an Insulating VO2 Thin Film
into a Non-Rutile Metallic State
Shin Group
Vanadium dioxide (VO2) is an exemplary strongly correlated material known for its dramatic insulator-to-metaltransition around room temperature (Fig. 1(a)). The phasetransition can also be triggered by external stimuli such as
electric gating and light irradiation. VO2 thus provides aninteresting platform for novel functionalities. By using time-

ISSP Activity Report 201321

Development of the Polarization Control
Segmented Cross Undulator
at SPring-8 BL07LSU
I. Matsuda, Harada, and Shin Groups

Fig. 1. Insulator-to-metal transition of VO2. (a) Structural change frommonoclinic insulating phase to rutile metal phase upon the thermaltransition.
(b) Time-resolve spectrum at delay = 0.95 ps (F ~ 9 mJ/
2
cm , 270 K) compared with a static photoemission spectrum of a rutile
metallic state recorded at 350 K. (c) Time evolution of spectral weightover [-0.2, 0.2 eV]. The inset shows the unoccupied side of the spectra
for delay = 0.95, 4.05, 9.07 ps in the logarithmic scale.

resolved photoemission spectroscopy (TrPES) implemented
by a high-harmonic generation method, we investigated
the ultrafast photo-induced transition of VO2, and provide
insights into the non-equilibrium dynamics governed bystrong correlations.
The valence-band spectrum of a VO2 thin film wasmonitored in a pump-and-probe TrPES configuration. A
metallic state emerged when the sample was impinged
by a 170-fs pulse above a critical fluence F = 6 mJ/cm2,
which was directly evidenced by the ultrafast increase of
the spectral weight at the Fermi level (EF). The transition
was accompanied by a spectral-weight transfer on a 1-eV
scale characteristic of strong electron correlations. The
photo-induced metallic state exhibited a unique spectrum
that tails up to ~0.4 eV above EF. This indicates that the
emergent metallic state is different from the rutile state (Fig.
1(b)). After the impulsive impact, the spectral weight at EF
further increased until ~3 ps (Fig. 1(c), inset), indicatingthat the metallic state grew proliferatively. The ~0.4-eV tailalso persisted for > 3 ps, the behavior of which cannot be
explained by the photo-response of typical Mott insulatorsthat relaxes within ~1 ps. These observations emphasize
the importance of non-electronic degrees of freedom in thephoto-induced dynamics of VO2.

Synchrotron-based soft X-ray spectroscopy has provento be a powerful experimental technique for studying theatomic, molecular and electronic structures of materials.
The energy range of soft X-rays covers the K-edge absorption of the most abundant elements on Earth (e.g. C, N, O,
Si) and the L-edge absorption of the industrially importanttransition metals (Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn).
Soft X-ray spectroscopy has many unique characteristics,
such as element specificity, chemical specificity and surface
sensitivity. Furthermore, responses of a matter with different
light polarization determine orbitals of the electronic states
in solids, configurations of molecules on a surface, and spin/
orbital magnetic moments of magnetic atoms. These properties make them versatile for application in a wide range ofscientific fields including surface chemistry, environmental
science and magnetism. Therefore, developments of polarization control soft X-ray sources have been one of the significant issues in condensed matter physics.
Recently, we have constructed a new soft X-ray
beamline, BL07LSU, at SPring-8 to perform advanced
soft X-ray spectroscopy for materials science. To realize
the state-of-the-art performance, a novel segmented crossundulator (SCU) was developed and adopted as a soft X-ray
light source. Figure 1 shows schematic drawing and photograph of the SCU. It consists of eight undulator segments
and seven phase shifters (PS), and its total length is 27 m.
Four segments generate horizontally linearly polarized radiation at the fundamental radiation (Figure-8 undulator), andthe other four segments generate vertically linearly polarizedradiation (Figure-∞ undulator). The horizontal and verticalundulator segments are placed alternately. Circular or tilted
linearly polarized light can be obtained by superposing
horizontally and vertically linearly polarized radiation, andthe helicity of the circularly polarized radiation, for example,
can be changed by the PSs. The PSs adjust the relative
phase of the undulator radiation emitted from each segment
by changing the path length of the electron orbit between
segments with a local orbit bump. Linear polarization, PL,and circular polarization, PC, are evaluated by the rotating
analyzer ellipsometry using the multilayer mirrors. The
degrees of linear polarization (PL) from four horizontal orfour vertical segments are both 1.00. The degree of circularpolarization (PC) from four horizontal and four vertical

Reference[1] R. Yoshida, T. Yamamoto, Y. Ishida, H. Nagao, T. Otsuka, K. Saeki,
Y. Muraoka, R. Eguchi, K. Ishizaka, T. Kiss, S. Watanabe, T. Kanai,J. Itatani, and S. Shin, Phys. Rev. B 89, 205114 (2014).
Authorsaaa
R. Yoshida, aT. Yamamoto,
Y. Ishida, H. Nagao , T. Otsuka , K. Saeki ,aY. Muraoka , R. Eguchi , K. Ishizaka, T. Kiss, S. Watanabe, T. Kanai,J.
Itatani, and S. Shina
Okayama University

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ISSP Activity Report 2013

Fig. 1. Schematic and photograph of a novel segmented cross undulator
at SPring-8 BL07LSU that generates polarization controlled soft X-ray.

segments are - 0.94 and 0.93 for left- and right-handed circularly polarized light, respectively.
The degree of polarization at SPring-8 BL07LSU is highenough to perform various soft X-ray spectroscopy using
light polarization, such near-edge X-ray fine structure, X-ray
magnetic circular dichroism, X-ray magneto-optical Kerr
effect, and resonant soft X-ray diffraction. Moreover, thebeamline optics and the experimental stations allow users tocarry out the experiments also at small region (70 nm), withtime resolutions (50 ps), and with high energy resolution (>10,000).
Reference[1] S. Yamamoto, Y. Senba, T. Tanaka, H. Ohashi, T. Hirono,
H. Kimura, M. Fujisawa, J. Miyawaki, A. Harasawa, T. Seike,S. Takahashi, N. Nariyama, T. Matsushita, M. Takeuchi, T. Ohata,
Y. Furukawa, K. Takeshita, S. Goto, Y. Harada, S. Shin, H. Kitamura,
A. Kakizaki, M. Oshima, and I. Matsuda, Journal of Synchrotron
Radiation 21, 352 (2014).
Authors
I. Matsuda, S. Yamamoto, Y. Harada, and S. Shin

Discovery of a Giant Kerr Rotation in a
Ferromagnetic Transition Metal by M-edge
Resonant Magneto-Optic Kerr Effect
I. Matsuda and Shin Groups
Magneto-optical effects are one of the central topics
in condensed matter physics. The effects have been used
to study magnetism and spin transport in materials. Manyexperiments with the magneto-optical Kerr effect (MOKE)
have been performed using linearly polarized laser light ofa single wavelength typically in the visible region. Recently,
there has been remarkable progress in new-generation
light sources, such as the X-ray free-electron laser (FEL)
and the high-harmonic generation (HHG) laser. These new
monochromatic lasers have ultra-short pulse widths andtunable photon energies ranging from the extended violet to
the X-ray region. Thus, MOKE measurements with photon
energy tuned at the absorption edge (so-called resonant
MOKE or RMOKE) are of considerable interest. Measurements with the new-generation light sources are expected tobe element-specific and to trace spin dynamics in real time
via time-resolved measurements.

In the present research, we have studied the Kerr
rotation angle and ellipticity in the RMOKE at photon
energies corresponding to the M-shell absorption edge of

Fig. 1. Photon energy dependence of the Kerr rotation angle, θk, of
the Ni film calculated with the resonant scattering theory (sim.) andmeasured experimentally with VUV synchrotron radiation (exp.).
Measurement was performed at room temperature under a magnetic
field of B = ± 0.47 T.

transition metal. We investigated the RMOKE in the wellknown nickel film both theoretically and experimentally.
The simulation was carried out with resonant scattering
theory based on the Kramers-Heisenberg formula. The
predicted Kerr rotation angle indicated a large MOKE (>10˚). Then, the large Kerr rotation angle was confirmedby rotating analyzer ellipsometry using vacuum ultraviolet (VUV) synchrotron radiation at KEK PF BL-18A.
Figure 1 compares experimentally measured and calculated
variations of θk with photon energy. The experiment reproduced large θk values (~10 degree) at the absorption edge:
the overall features are in good agreement with the calculations, where the model parameter set has been independently
determined with other core-level spectroscopies.
Since the RMOKE measurement is inherently element
specific and is a photon-in and photon-out experiment, it isable to probe spin states of selected elements in complicatedmagnetic materials under external electromagnetic fields.
Moreover, we observed a large Kerr rotation angle (> 10˚)in the RMOKE, which is much larger than angles (<1˚)
typically found for the MOKE with visible light. Conventional MOKE measurement has always required lock-in
amplifier techniques to obtain a high enough signal-to-noise
ratio. However, the large Kerr rotation angle in the RMOKE
we observed does not require such techniques. This advantage allows us to detect the magnetism or spin dynamics ofa very small amount of the magnetic element in a material,
such as a dilute magnetic semiconductor, or to measure verysmall structure, such as the magnetic nanostructure on asurface. This feature makes it possible to trace these ultrafast
spin dynamics in terms of individual chemical species inreal time when combined with FEL or HHG lasers.
Reference[1] Sh. Yamamoto, M. Taguchi, M. Fujisawa, R. Hobara, S. Yamamoto,K. Yaji, T. Nakamura, K. Fujikawa, R. Yukawa, T. Togashi, M. Yabashi,
M. Tsunoda, S. Shin, and I. Matsuda, Phys. Rev. B 89, 064423 (2014).
Authorsa
I.
Matsuda, Sh. Yamamoto, T. Nakamura , and S. ShinaJapan Synchrotron Radiation Research Institute/SPring-8

Ultra-Broad Mode-Spacing
Optical Frequency Comb
Kobayashi GroupA mode-locked oscillator produces bunch of regularly
spaced longitudinal modes in an optical frequency region. It
is called as an optical frequency comb. When the frequency
comb was invented, it was regarded as a dream light source
since one oscillator produces tens of thousands modes, corresponding to thousands of ultra-narrow linewidth cw lasers.
Many people said that cw lasers were no longer necessary
since making the mode-locked laser was easier than making
very stable cw lasers. However, this story was not correct.
One could not chose one comb tooth from neighboringmodes because the mode spacing was too small to pick byusing any dispersive optics. The mode-spacing is a repetition
rate of the mode-locked (ML) oscillator, the high-repetition
rate then makes broad mode spacing. The repetition rate of
the ML laser is determined by the cavity length since only
one pulse is propagating in a cavity. The smaller cavity then

ISSP Activity Report 201323

Fig. 1. Schematic of the experimental setup. DMs, dichroic mirrors;
CM, chirp-compensation mirror; HR, high reflection mirror; L1 and L2,
lenses.

produces the broader mode-spacing optical frequency comb.
Another direction is to make higher resolution spectrometeror spectrograph to resolve each mode. The highest frequency
resolution in a commercially available spectrometer is about
4 GHz in an optical frequency, then the higher repetition rate
than 4 GHz is crucial to resolve comb teeth.
Here we report on our recent progress of high-repetitionrate ML oscillator [1]. We are concentrating on the laserdiode (LD) pumped Kerr-lens ML oscillator. There are somereasons to study this type of lasers. Typical high-repetitionrate comb is made by using Ti:sapphire laser pumped by alarge green laser which is not easy to keep running for manydays. However, many applications of the frequency comb
require long-term operation. A single-mode-fiber (SMF)
coupled LD can be kept running as long as we need without
any degradation or miss alignment. The main drawback is
its poor power. The highest power of the SMF coupled LD islimited below 1 W. Lower power pumping makes it difficult
to realize high-repetition-rate ML oscillator. We invented
new scheme of the ML oscillator with high finesse. This
makes the intra-cavity power high enough to realize the Kerrlens ML. By adopting Yb-doped ceramic as a gain medium,
we have realized 6-GHz repetition rate oscillator with femtosecond pulse duration.
Figure 1 shows the schematic of the small oscillator.
ML oscillator consists of two concave mirrors and twoplane mirrors. One of the plane mirror is specially coatedFig. 2. Absorption spectroscopy of meta-stable He. Red curve shows
the laser spectrum without He atoms. The modulation correspond comb
structure. Blue curve was obtained with meta-stable He atoms. We cansee clear two absorptions that correspond to the triplet S to triplet P
transitions in a meta-stable state. 1S-2S transition is strictly prohibitedso that the metastable life time is long enough to observe these transitions.

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ISSP Activity Report 2013

to compensate a chirp in the cavity. 1-mm-thick Yb:Lu2O3
ceramic was used for the laser gain. Obtained spectrum
width was about 8 nm, and the measured pulse duration was
160 fs. By using commercially available optical spectrum
analyzer, each comb mode was clearly resolved. This comb
is applied for the precision spectroscopy of meta-stable He
atoms, which has three absorption lines in the spectrum of
this laser. Figure 2 shows the absorption spectroscopy of
He obtained with 5.2-GHz repetition rate at this time. Three
3
3
3
3
3
3
lines correspond 2 S1- 2 P0, 2 S1- 2 P1 and 2 S1- 2 P2
3
3
transitions. The energy spacing of 2 P0- 2 P1 is 30 GHz, and33that of 2 P1- 2 P2 is 2 GHz, which is too small to resolve inthis setup. The absolute frequency could be determined if theoffset frequency is controlled, although it is not realized yet.
We have demonstrated the multi-GHz repetition-rate
Kerr-lens ML oscillator with LD pumping for the first time
to our best knowledge. It would open new applications withmode-resolved spectroscopy. The astronomical applicationis one of the promising candidate, in which this kind of laser
could be used in order to find extra-solar planets.
Reference[1] M. Endo, A. Ozawa, and Y. Kobayashi, Opt. Lett. 38, 4502 (2013).
Authors
M. Endo and Y. Kobayashi

Generation of Attosecond Soft-X-Ray
Bursts in the Water Window
Itatani GroupHigh harmonic generation using intense laser pulses is
the most promising method to produce attosecond opticalpulses. In the past decade, attosecond science has been
extensively exploited using the advanced Ti:sapphire laser
technology that can produce attosecond pulses in the extreme
ultraviolet (EUV) below 200 eV in photon energies. Recent
development of intense infrared sources that are based on
optical parametric chirped-pulse amplification (OPCPA)
has opened the opportunities to extend the spectral range ofhigh harmonics from the EUV to the soft X ray region, but
the lack of well-controlled reliable IR sources has been the
problem to realize attosecond soft-X-ray spectroscopy.
In the present research [1], we have developed a novel IR
light source that is based on OPCPA using BiB3O6 crystals
pumped by Ti:sapphire lasers. This OPCPA system canamplify an octave-spanned bandwidth from 1100 to 2200
nm, which enables the phase-stable direct amplification of
sub-two-cycle IR pulses. The light source produces 0.5-mJ,

Fig. 1. (a) Experimentally obtained high harmonic spectra recorded atrelative carrier-envelope phases in steps of 0.1π rad. (b) Simulated highharmonic spectra 14
assuming
10-fs optical pulses at 1600 nm with an2
intensity of 4 × 10 W/cm .

9-fs pulses with stable carrier-envelope phases (CEPs) at
1-kHz repetition rate. We have achieved excellent stabilities
in the output energy and CEP, which are comparable to thoseof commercial Ti:sapphire lasers.
Figure 1(a) shows observed high harmonic spectra
around the cutoff photon energy as a function of the CEP.
The maximum photon energy reaches ~330 eV that is wellbeyond the carbon K edge at 284 eV. The shifting structuresof the spectral peak is called the half-cycle cutoff, which is aclear signature of the generation of single isolated attosecond
pulses. Figure 1(b) shows simulated results that are basedon the strong-field approximation assuming the experimental parameters described in the caption of Fig. 1(a). The
good agreement between the experimental and simulation
results suggests the generation of isolated attosecond bursts
in the water window. This result is an important milestone
to extend the attosecond spectroscopy into the soft X ray
region where element-specific ultrafast spectroscopy as well
as novel attosecond techniques using high-energy electron
wavepackets can be explored in the future.
Reference[1] N. Ishii, K. Kaneshima, K. Kitano, T. Kanai, S. Watanabe, and
J. Itatani, Nature Comm. 5, 3331 (2014).
Authors
N. Ishii, K. Kaneshima, K. Kitano, T. Kanai, S. Watanabea, and
J. ItataniaTokyo University of Science

ISSP Activity Report 201325