Activity Report
2003
Preface1

Research Highlights

2 - 25Highlights of Joint Research

26 - 45

ISSP Workshop

46 - 48

ISSP Activity Report 2003

Preface

The ISSP was established in 1957 based on the recommendation of the National Council of Japan to set up a new institutefor condensed matter physics. The idea was to establish a topclass research institute for experimental and theoretical studies of condensed matter and to promote cooperative researchin the field throughout the country. Soon after the inauguration, the name of ISSP became recognized world-wide in thecommunity of condensed matter physics.
Since then there have been two major changes in the historyof the institute. The first was the reorganization of the institute in the mid-eighties. The system of divisions consisting of several groups was introduced and the
project of the "Physics in extreme conditions", which includes Ultra-High Magnetic Fields, Laser
Physics, Surface Physics, Ultra-Low Temperatures, and Very High Pressures, was started. The second major change was the relocation of the ISSP to the new Kashiwa campus which was completed
in 2000.
To make the status of the research activities public, new forms were introduced two years ago:
Activity report A for research mostly by the staff of the ISSP and Activity report B for joint research
conducted by the ISSP. This is the Activity Report for the fiscal year 2003 in the style of single volume with all the research activities at the ISSP. The system of national universities in Japan has madea profound change from April first this year by the so called "houjinka", which means that national
universities, once directly belonged to the ministry of education, have become independent administrative entities. In the new scheme importance of the Activity Report will be enhanced.
We are happy to receive any comments on the Report for the possible improvement of the research
activities of the ISSP.

June 18, 2004

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

ISSP Activity Report 2003

1

Research Highlights

Pressure Induced Phase
Transformation in Hollandite-type
KAlSi3O8
T. Yagi
We have found a pressure induced phase transformation
in KAlSi3O8 at around 20 GPa and room temperature.
Hollandite structured KAlSi3O8 is believed to be one of theimportant constituent minerals of subducting slab in the
Earth. In spite of the close packing of atoms, hollanditetype structure can accommodate large cations such as potassium. Among various silicates, there are not so many structures which can accommodate such large cation under pressure. Therefore its behavior under deep Earth condition has
geophysical importance for the discussion of the distributionof potassium and hence the thermal history of the Earth,because the radioactive decay of potassium is the main heatsource in the deep Earth. Moreover, its unique structure
attracts attention from a view point of materials science as
well.
We have carried out high pressure in situ X-ray diffraction study on hollandite structured KAlSi3O8 up to 28 GPa
at room temperature, using helium as a pressure transmitting
medium. Helium provides nearly hydrostatic environment
and, as a results, high quality X-ray diffraction profiles wereobtained under pressure. Starting sample was prepared froma stoichiometric mixture of oxides heated to 1800K at about
20 GPa using multi-anvil apparatus and the X-ray diffraction experiments were carried out at BL13 of the Photon
Factory. Fig. 1 shows series of X-ray diffraction profiles
obtained with increasing pressure. Contrary to the previous
report [1], the existence of a phase transformation was foundat around 20 GPa, which occurs reversibly at room tempera-

Fig. 2. Change of the unit cell angle c of hollandite-type KAlSi3O8
under pressure. At the transition, c starts to deviate from 90 deg andthe distortion increases with increasing pressure .

ture. Analysis of the diffraction pattern above 20 GPa indicates the transformation from tetragonal to monoclinic structure. Hollandite-type structure has several modifications.
Original hollandite, BaMn8O16, has monoclinic symmetry
while hollandite-type KAlSi3O8 has tetragonal symmetry at
low pressure. Above 20 GPa, however, it distorts into monoclinic structure. In Fig. 2, the change of the angle c of theunit cell is shown as a function of pressure. It is clear fromthis figure that the tetragonal unit cell starts to distort at
about 20 GPa. If we look at the unit cell dimensions, a-axis
splits into a- and b-axis at the transition while c-axischanges almost continuously across the transition. No
meaningful discontinuous change in volume was found at
the transition and no hysteresis of the transition pressure wasfound in increasing and decreasing pressure cycle. All thesefacts suggest that the nature of this transition is a second
order structural transition. Hollandite structure has one
dimensional "tunnel" along c-axis in which large cation issitting and this tunnel is deformed at the transition.
Therefore, although the density is affected only little by thistransition, it is likely that the partitioning of large elements
which is contained in this tunnel may be affected considerably below and above the transition. Various physical properties may be affected by this transition as well and further
studies are required to clarify these problems.
Reference[1] F. Tutti, L.S. Dubrovinsky, S.K. Saxena, Geophys. Res. Lett., 28,2735 (2001)
Authors
T. Ferroir, T. Onozawa, T. Yagi, S. Merkel, N. Miyajima
M. Nishiyamaa, T. Irifunea, and T. Kikegawaba Ehime University
b Photon Factory, KEK

Fig. 1. X-ray diffraction profiles of hollandite-type KAlSi3O8 underpressure. Many peaks of the original tetragonal structure splits into
two ore more peaks above about 20 GPa.

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ISSP Activity Report 2003

The Antiferro-Quadrupole Order andthe Field-Induced Heavy-Fermion
State in the Filled Skutterudite
PrFe4P12
M. Takigawa
In some rare-earth compounds, high-order multipoles
such as electric quadrupoles and magnetic octupoles of the
4f-electrons play important roles in determining the electricand magnetic properties. Recently, intermetallic compoundswith the filled skutterudite structure, RT4X12 (R = rare earth,
T = transition metal, X = pnictogen), have attracted muchattention because of a variety of intriguing phenomena in thecommon lattice structure. Among them, PrFe4P12 is uniquein that the ground state is nonmagnetic but ordered, with thecritical temperature TA = 6.5 K at zero field. It transforms
into a non-ordered heavy-fermion state by applying magnetic field [1]. The low-field phase is suggested to be an antiferro-quadrupole ordered state, however, there is little evidence for the associated change of Pr-4f orbitals. Nuclear
magnetic resonance (NMR) is a powerful tool to detect sucha local change of the electronic state via the hyperfine fieldat the probed nuclear sites. In order to know more about theordered phase as well as the heavy-fermion phase at highfields, we carried out NMR measurements at the P site in
PrFe4P12.
Figure 1(a) shows 31P NMR spectra above and below TA.
The splitting of the NMR lines in the ordered phase indicates existence of two kinds of P sites with different hyperfine fields below TA. This cannot be due to an ordinary antiferromagnetic (dipole) order because the splitting appears tovanish at zero-field as shown in Fig. 1(b). It should also be
noted that the field dependence of the splitting is different
for different sites. This suggests that the transition at TA isnot structural but electronic.
These observations are understood in terms of the antiferro-quadrupole (AFQ) order accompanying alternation of
the electronic state of Pr atoms on the corner and body-center sites of the bcc lattice. Since this leads to a loss of thetranslational symmetry from the corner to the body center,the presence of two inequivalent P sites is naturally
explained. The Pr-4f magnetization induced by the field
takes different values on different sites in the AFQ state,giving rise to a field-induced staggered magnetization. The
hyperfine couplings between P and Pr atoms also alternate

Fig. 2. Temperature and field dependences of the 31P nuclear apinlattice relaxation rate 1/T1 in PrFe4P12 for the field H parallel to<100> and <111>. Open and solid symbols represent 1/T1 for
H//<100> and <111>, respectively. The arrow indicates the criticaltemperature of the antiferro-quadrupole order at 2 T.

because they depend on the symmetry of Pr-4f orbitals. This
alternation, as well as the staggered magnetization, result inthe staggered component of the hyperfine fields in addition
to the uniform one, leading to the field-induced line splitting.
We found that the main contribution to the line splitting
comes from magnetization parallel to the external field H.
This allows us to deduce the AFQ order parameter to be
dominantly O20 of the C3 quadrupoles. Further quantitative
analyses revealed that the line splitting is determined predominantly by the field-induced staggered octupoles rather
than dipoles, which again confirms the AFQ order in the
ground state.
On the dynamical side, the 31P nuclear spin-lattice relaxation rate 1/T1 decreases rapidly below TA as shown in
Fig. 2 consistent with the nonmagnetic ground state.
Anisotropy of 1/T1 is relatively small in the AFQ phase. On
the other hand, remarkable anisotropy of 1/T1 was observedin the high-field heavy-fermion phase. For H//<100>, 1/T1
exhibits power-law behavior 1/T1 " Tp at low temperatures.
The exponent p is field-dependent and approaches one athigh fields, suggesting that the Fermi-liquid state is recovered in the high-field limit. In contrast, 1/T1 for H//<111>remains enhanced and is almost temperature-independent
down to 1.5 K. The enhancement of 1/T1 may be related tonear degeneracy of the crystal-field levels for H//<111>.
This implies a non-Fermi-liquid state due to the quadrupolar
Kondo effect, the overscreening process of 4f-electron
quadrupoles by the conduction electrons [2].
References[1] H. Sato et al., Physica B 328, 34 (2003).
[2] D. L. Cox, Phys. Rev. Lett. 59, 1240 (1987).

Fig. 1. (a) 31P NMR spectra in PrFe4P12 for the field along <111>
above and below the transition temperature. Each line splits to a pairof lines (indicated by circles and crosses) in the ordered phase. (b)Field dependence of the line splitting at 2 K for the field along<100>. The dotted line indicates the critical field.

Authors
J. Kikuchi, M. Takigawa, H. Sugawaraa, and H. Satoa
a Tokyo Metropolitan University

ISSP Activity Report 2003

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Field-angular Oscillation of the
Zero-Energy Density of States in the
Anisotropic Heavy-Fermion
Superconductor CeCoIn5
T. Sakakibara
While the conventional BCS-type superconductors show
the superconducting gap opened over the whole Fermi surfaces, anisotropic superconductors are generally characterized by zeros in the gap (either point or line nodes) which
break the symmetry of the crystal lattice. Identification of
the gap-node structure is therefore of fundamental importance in understanding anisotropic superconductivity and its
pairing mechanism. Regarding this issue, it has recently
been pointed out that the zero-energy density of states(ZEDOS) in the superconducting mixed state exhibits acharacteristic oscillation with respect to the angle between
the magnetic field and the nodal directions [1,2].
Experimentally, the angular dependent ZEDOS can beprobed by the thermal conductivity or the specific heatmeasurements in rotating magnetic field at low temperatures. While the former measurement has high sensitivity to
the nodal quasiparticles relevant to the phenomenon [3], thelatter experiment has an advantage over the former in the
point that the interpretation of the result is more straightforward. We have performed the angle-resolved specific heatmeasurements in the superconducting mixed state of theanisotropic superconductor CeCoIn5 (Tc=2.3 K), andobserved a characteristic fourfold angular oscillation in thebasal plane.
Figure 1 shows an example of the specific heat data for asingle crystal of CeCoIn5, obtained at 0.38 K. The magnetic
field of 3 T is rotated in the (001) plane of the tetragonal
crystal. As can be seen from the figure, the specific heatshows a characteristic fourfold angular oscillation in thec-plane, with the minima (maxima) appearing along the[100] ([110]) directions. The solid line is a fit to a sinusoidal
function D CH/CH=A4cos(4 i ), where CH denotes the fielddependent part of the specific heat. We have done the similar measurements at various temperatures and fields, andfound that the relative amplitude A4 of the angular oscillation persists to low field region H/Hc2~0.18, where Hc2

Fig. 1. Specific heat of CeCoIn5 measured at 0.38 K in a magnetic
field of 3 T rotated within the (001) plane. The fourfold angular oscillation comes from the gap-node structure of d-wave superconductivity. The field angle is measured with respect to the [100] direction.

4

ISSP Activity Report 2003

denotes the upper critical field. We confirmed that A4 is adecreasing function of magnetic field, and rapidly decreases
by raising temperatures up to ~0.5 Tc. These results strongly
suggest that the observed angular oscillation comes from the
ZEDOS of nodal quasiparticles of d-wave superconductivity. From the angular position of the specific-heat minima([100] directions), we may conclude that the gap symmetryof CeCoIn5 is most likely to be of dxy type.
References[1] I. Vekhter et al.: Phys. Rev. B 59 (1999) R9023.
[2] P. Miranovic et al.: Phys. Rev. B 68 (2003) 052501.
[3] K. Izawa et al.: Phys. Rev. Lett. 87 (2001) 057002.
Principal Publication and Authors
J. Phys. Condens. Matter 16 (2004) L13.
H. Aoki, T. Sakakibara, H. Shishido, R. Settaia, Y. Onukia, P.
Miranovica, and K. Machidab.
a Osaka Univ.
b Okayama Univ.

Origin of the Mid-infrared Band in
Molecular Conductors
H. Tajima
Optical conductivity spectra of molecular conductors
exhibit an intense absorption band in the mid-infrared region(mid-infrared band) around room temperature, where theirelectrical behavior is that of metals or semiconductors with anarrow band gap. This phenomenon has aroused interest of
many researchers [1]. In order to understand the mechanismof this phenomenon, we have proposed the following
hypothesis [2]:(i) In some molecular conductors, the original crystal
symmetry is broken even at room temperature due tocharge ordering (or spin polarization) resulting from the
Coulomb repulsion.
(ii) The symmetry breaking due to the Coulomb repulsion is followed by domain formation. Electric current iscarried by charged domain walls rather than by single
particles.
(iii) The fast time scale of the optical measurement
enables one to detect fluctuating order parameters ofcharge separation or spin polarization. Such parameters
are hardly detected by other methods.
This hypothesis relates the mid-infrared band in the conductivity spectra to the charge-ordering phenomena (and to
the associated metal-insulator transition) observed in various
molecular conductors [3].
Figure 1 shows the optical conductivity spectra ofb -(ET)2PF6 above and below the metal-insulator transition
temperature (TM-I =297 K) [4]. The decrease in the conductivity below 1500 cm-1 associated with the M-I transition islarger for the polarization parallel to the a-axis than for theother polarization. In the insulating phase, the conductivity
spectra show a gap feature for the E//a polarization below
1000 cm-1 and a hump appears around 2000~3000 cm-1 inthe E//c spectra.
In order to explain the spectral features shown in Fig. 1,
we have calculated the optical conductivity spectra by
applying the mean-field approximation to the extended
Hubbard model. We assumed charge-ordering pattern(shown in Fig. 2a) proposed by Y. Nogami & T. Mori [5].

Construction of Highly Correlated
System by the SupramolecularInteraction:l -type C5DTP-TTF Salts
H. Mori

Fig. 1. Infrared conductivity spectra of b -(ET)2PF6 for the polarization parallel to the a-axis (top panel) and to the c-axis (bottom panel).
Note that the spectra exhibit drastic changes associated with themetal-insulator transition at 297 K (=TM-I).

Fig. 2. (a) A schematic pattern of the charge-ordering assumed in thecalculation for the insulating phase. The characters N and I denoteneutral and ionic molecules, respectively. (b, c) Conductivity spectra
calculated for the charge-order (insulating) and metallic phases.

Generally the electronic states of organic conductors
strongly depend upon the molecular arrangements. In thecase of ET (bis(ethylenedithio)tetrathiafulvalene) salts, thel -type structure is very important due to the high superconducting transition temperatures (Tc). [1] According to thephysical insight, the half-filled band structure, a dimeric
structure of 2:1 composition, with the two-dimensional
Fermi surface is important to afford the high Tc. Then, thedonor molecules (CnDTP-TTF in Fig. 1) have been
designed for the dimeric structures, so that l -type and b "type donor arrangements have been obtained by means of
the steric hindrance of the cycloalkylenene group [2] and theintermolecular CHfN interactions of the pyrazino group.
Figure 2 shows the donor arrangement of thel -(C5DTP-TTF)4GaCl4. The short CHfN contacts of ahydrogen bond (< 2.75 Å) and the steric hindrance of thecyclopentylene group construct the l -type donor arrangement.
The similar behavior was observed forl -(C5DTP-TTF)2X [X = PF6, AsF6, SbF6].
In order to elucidate the electronic states of the dimericl -type and b "-type salts, the electrical resistivities were
measured as shown in Fig. 3. A series of the l -type salts[X = PF6, AsF6, SbF6] show the behavior of Mott insulatorswith an effective half-filled band. Their spin susceptibilities
were also remained even in the insulating state. The metallic regions above 150 K for the PF6 salt and 250 K for the

Fig. 1. Molecular structures of CnDTP-TTF.

The spectral features below TM-I are well explained by thiscalculation (Fig. 2b). Interestingly, the peak in the E//cpolarization appears even though charge-ordering does notexist (Fig. 2c). These calculations show that the midinfrared
band in the E//a polarization reflects charge-ordering, while
the peak in the E//c spectra does not. This conclusion is consistent with the observed spectra shown in Fig. 1.
References[1]. For example, see C. S. Jacobsen, in Semiconductors andSemimetals, edited by E: Conwell (Academic, London, 1988), Vol. 27,
Chap. 5.
[2]. T. Yamamoto, H. Tajima, J. Yamaura, S. Aonuma, and R. Kato, J.
Phys. Soc. Jpn., 68, 1384 (1999); H. Tajima, S. Kyoden, H. Mori, and
S. Tanaka, Phys. Rev. B 62, 9378 (2000).
[3]. For example, see Seo and H. Fukuyama, J. Phys. Soc. Jpn., 66,
1249 (1997).
[4] Y. Ding and H. Tajima, Phys. Rev. B 69, 115121 (2004).
[5] Y. Nogami and T. Mori, J. Phys. IV 12, Pr9-233 (2002).
Authors
H. Tajima and Y. Ding
Fig. 2. Crystal structure of l -(C5DTP-TTF)4GaCl4. The intermolecular CHfN contacts (< 2.75 Å) indicated by the dotted lines and thesteric hindrance of the cyclopentylene group construct the l -type
donor arrangement.

ISSP Activity Report 2003

5

Fig. 3. Temperature dependences of electrical resistivitiesl -(C5DTP-TTF)2X [X = PF6, AsF6, SbF6], l -(C5DTP-TTF)4GaCl4,
and b "-(C6DTP-TTF)3GaCl4. A series of the l -type salts [X = PF6,
AsF6, SbF6] are Mott insulators with an effective half-filled band.
These resistivity behaviors indicate the chemical pressure effect fromthe X = SbF6, AsF6, to PF6 salt. In contrast, b "-(C6DTP-TTF)3GaCl4
with 5/6-filled band structure shows the metallic behavior down to
40 K.

AsF6 salt are originated from the thermal excitation beyond
the Mott insulating gap. These temperature dependences
move parallel to the low resistivity from X = SbF6, AsF6, toPF6, indicating the chemical pressure effect. On the other
hand, b "-(C6DTP-TTF)3GaCl4 with the 5/6-filled band
structure shows the metallic behavior down to 40 K.
References[1] H. Urayama-Mori, H. Yamochi, G. Saito, K. Nozawa, T. Sugano,
M. Kinoshita, S. Sato, K. Oshima, A. Kawamoto, and J. Tanaka,
Chem. Lett., 1988, 55.
[2] H. Suzuki, K. Yamashita, M. Suto, T. Maejima, S. Kimura, H.
Mori, Y. Nishio, K. Kajita, and H. Moriyama, Synthetic Metals, 144,
89 (2004).

demanded new comprehension about perovskite manganeseoxides. The A-site disordered form (R0.5Ba0.5)MnO3 withthe same constituent elements is crucial to deepen the understanding of the structural and physical properties of perovskite manganites. Very recently the A-site disordered Babased manganite R0.5Ba0.5MnO3 has been synthesized andinvestigated in its structures and electromagnetic properties.
The disordered form R0.5Ba0.5MnO3 has a primitive cubic
perovskite cell with no tilt of MnO6 octahedra; neverthelessmagnetic glassy states instead of the electronic states characteristic of perovskite manganites dominate the groundstate of R0.5Ba0.5MnO3. The magnetic glassy states could be
due to the disorder effect that hinders the long-range magnetic ordering. Since the ionic radius of Ba2+ is much larger
than that of Sr2+ and also R3+, R0.5Ba0.5MnO3 must include
a spatial heterogeneity on a nanometer scale, which leads tothe magnetic nonhomogeneous state.
Novel magnetoresistance behaviors, as shown in Fig. 1,
have been observed in Pr0.5Ba0.5MnO3. The resistivity
decreases stepwise as the magnetic field increases, while themagnetization increases stepwise with the close relation to
the resistivity behaviors. These behaviors are not reversible
in the magnetic field. The stepwise behaviors in the magnetization and resistivity were observed up to 4.9 K but they
vanished dramatically at 5.0 K. Similar behaviors were previously reported in Pr0.5Ca0.5MnO3 doped with a few percent of other cations such as Sc, Ga or Co on the Mn site.
This is the first observation of ultrasharp magnetization andresistivity change in the nondoped system. A model basedon ordinal two-phase mixture cannot explain the behaviors.
We have no explanation for such multi-step magnetization
and resistivity change at present. However we would like toemphasize a close relation between the observed behaviors
and any spatial heterogeneity on a nanometer scale.
Principal publication and authors
Y. Ueda and T. Nakajima, J. Phys.: Condens. Matter, 16 1 (2004).

Authors
S. Kimura, K, Yamashita, S. Ichikawa, H. Suzuki, Y. Nishioa, K.
Kajitaa, and H. Mori,aToho Univ.

Novel Magnetoresistance Behaviors
Observed in Pr0.5Ba0.5MnO3
Y. UedaRecently we successfully synthesized the A-site orderedBa-based manganite, RBaMn2O6 (R = Y and rare earth elements). The electronic phase diagram was reported as ahighlight of Activity Report 2002. Compared with the A-site
disordered R0.5A0.5MnO3 (A = Sr, Ca), RBaMn2O6 displays
remarkable features: (1) relatively high charge and orbital
order (CO) transition temperatures (TCO) far above 300 K,(2) a new stacking variation of the CE-type CO with a 4-foldperiodicity along the c-axis (4CE-CO), (3) the presence ofstructural transition above TCO, possibly accompanied by
the dx2-y2 type orbital order, and (5) the electronic phase segregation in the end member LaBaMn2O6.
The discovery of such novel structural and physicalproperties in the A-site ordered manganite RBaMn2O6 has

6ISSP Activity Report 2003

Fig. 1. Magnetic field dependence of magnetization and resistivity at
2 K in Pr0.5Ba0.5MnO3.

The Second Example ofPressure-Induced Superconductivity inb -Vanadium Bronzes
Y. UedaPreviously we reported the first observation of superconductivity in vanadium oxides as a highlight of Activity
Report 2001. The compound is b -Na0.33V2O5 which is amember of b -vanadium bronze family with the chemical
formula of b -AxV2O5 (the stoichiometric composition x =1/3 or 0.33). There are six compounds in the family, as listed in Table 1. The monoclinic crystal structure consists ofa characteristic V2O5-framework and A-cations. The
V2O5-framework is formed by sharing the corners and edgesof (V1)O6, (V2)O6 and (V3)O5 polyhedra, where V1, V2
and V3 are the three crystallographically independent vanadium atoms. The (V1)O6, (V2)O6 and (V3)O5 polyhedra
form three kinds of infinite chain along the b-axis; zigzag
chains, two-leg ladder chains and zigzag chains, respectively. The A-cations are located in the tunnels surrounded by
the three kinds of chain. The b -vanadium bronzes show aquasi-1D conductivity along the b-axis. All members except
b -Pb1/3V2O5 undergo metal-insulator transitions accompanied by the charge ordering. The ground states of the lowtemperature insulator phases are antiferromagnetic orderedstates in b -A+1/3V2O5 (A+ = Li, Na and Ag) and spin gap
states in b -A2+1/3V2O5 (A2+ = Ca and Sr). These properties
are summarized in Table 1.
It is very interesting what happens when the charge ordermelts under high pressure. Since the discovery of the pressure-induced superconductivity in b -Na1/3V2O5 we havestudied the presence/absence of superconductivity underhigh pressure in b -vanadium bronze family. Inb -A2+1/3V2O5, the metal-insulator transitions (charge ordertransitions) are suppressed under high pressure but no superconductivity has been observed at least down to 2 K.
Recently we observed the superconductivity inb -Ag1/3V2O5. The charge ordered phase is suppressed withincreasing pressure and superconductivity appears around

Table 1. b -Vanadium bronze family
Compounds

V-valence TM-I (K)b -Li1/3V2O5

V4++5V5+

Ground state

Ground state (HP-ex.)b -Na1/3V2O5 V4++5V5+

200

AFI (TN =7 K)

SC (TC = 24 K ?)135

AFI (TN = 24 K)

SC (TC = 8 K)

V4++5V5+90

AFI (TN = 24 K)

SC (TC = 6 K)

b -Ca1/3V2O5 2V4++4V5+

150

Spin gap

PM (nonsuper)b -Sr1/3V2O5 2V4++4V5+

170

Spin gap

PM (nonsuper)

2V4++4V5+-PM

PM (nonsuper)b -Ag1/3V2O5

b -Pb1/3V2O5

TM-I: metal-insulator transition temperature
AFI : antiferromagnetic insulator
TN : antiferromagnetic transition temperature
PM : paramagnetic metal
SC : superconductivity
TC : superconducting transition temperature
7 GPa. The superconducting phase (S.C.) adjoins the chargeordered phase (C.O.) with the coexistence region of the twophases, as shown in Fig. 1. This means the first order transition from the charge ordered phase to the superconducting
phase, although the nature of charge order transition on temperature seems to be a second order under ambient pressure.
The supreconducting transition temperature about 6 K
for b -Ag1/3V2O5 is somewhat lower than 8 K forb -Na1/3V2O5. The presence/absence of superconductivity
in b -A+1/3V2O5/ b -A2+1/3V2O5 suggests the optimum
doping level of V4+/V5+ = 1/5 for the superconductivity inb -vanadium bronze family. Very recently we haveobserved a sign of superconductivity with a possible
TC = 24 K in b -Li1/3V2O5. A detailed experiment is now inprogress.

Instability of the Spin-Glass Phase in a
Static Magnetic Field
H. Takayama

Fig. 1. The pressure-temperature (P-T) phase diagram ofb -Ag1/3V2O5. TCO (squares) charge-ordering temperature, TSC(circles) superconducting transition temperature.

The subject titled has been one of the fundamental issues
which has remained unsettled yet in spite of extensive
debates since the early stage of the spin-glass (SG) study.
The mean-field theory predicts its stability up to a certain
critical magnitude of a field h, while by the droplet theory
the SG phase is unstable even in an infinitesimal h. Its difficulty is considered to lie on the following fact: spin dynamics governed by thermally-activated processes is so slowthat even in the time-window of real experiments, we cannot
distinguish whether a SG-state we are observing is in an
asymptotic regime close to equilibrium or far from it. In
such circumstances, a possible strategy is to perform a literalnumerical experiment on nonequilibrium dynamics of model
spin glasses and to compare the results with real experiments.
Following this strategy, we have examined the field-shift
aging protocol on the 3D Ising Edwards-Anderson model bymeans of the standard Monte Carlo simulation[1,2]. In the
protocol, a system is quenched to a temperature T below thetransition temperature Tc (- 0.95 in unit of the variance ofinteractions), it is aged in zero field by a period of tw, andthen h is switched on and the induced magnetization M(t) is

ISSP Activity Report 2003

7

Fig. 1. Scaling plots of RcrLh vs Rw/Lh of the h-shift processes at
T=0.4,0.5,0.6 and 0.8. The line represents y=x. In the inset we plot
tcr vs. tw in the h-shift processes at T=0.6 with h=0.05, 0.1, 0.2, 0.3,
0.4, 0.5, 0.6, and 0.75 from top to bottom.

measured. The system is expected to crossover from the h=0
aging state to the h>0 one around t=tcr which is specified bya peak position of the logarithmic time-derivative of M(t).
The sets of (tcr,tw) for various values of h thus obtained at
T=0.6 are shown in the inset of Fig. 1.
Associated with tw and tcr, we introduce the two characteristic length scales: Rw / bT t1w/z (T) , which is the mean sizeof SG domains grown in h=0 up to the field shift, and1/z (T) - aT h
Rcr / bT tcr, which is the one grown under the new
field h (>0) by a period of tcr after the shift. The two lengths
are then scaled by the so-called field overlap length Lh. It isa key quantity in analyzing the field effect on the SG phaseby the droplet theory; droplet excitations of a size L with
L> (<) Lh are governed by the Zeeman (SG stiffness) energy. Surprisingly it is found that, with a proper choice of thevalues of aT , bT and z (T) in the above expressions, thescaled plots of Rcr/Lh vs Rw/Lh of all the data obtained for
different T, tw and h lie on a single curve as shown in Fig. 1.
The consequence of the obtained scaling is clear; theh-shift aging process is nothing but a dynamical crossoverfrom the SG state in h=0 to the paramagnetic state in h>0,
or, the SG phase is unstable under h>0 in the equilibrium
limit. Furthermore, when the simulated results are simply
extended to the time window of the experiment on a real
Ising spin glass Fe0.5 Mn0.5 TiO3 , our picture of the dynamical
crossover turns out to be consistent with the experimental
data of the occurrence of irreversibility seen by the deviation
the field-cooled magnetization (FCM) from the zero-FCM,
though they were interpreted before as an evidence of the
AT phase transition predicted by the mean-field theory [3].
2higher transition temperature Tc since the discovery of copper oxide superconductor by Bednorz and Muller in 1986.
Many copper oxide superconductors have been synthesized,
and the world record of Tc reached 135 K at ambient pressure, which was reported in the mercury containing copperoxide in 1993. After 10 years, it becomes more and moredifficult to find a new copper oxide superconductor evenwith low Tc's. The playground of these superconductors is a
CuO2 sheet which consists of square arrangement of Cu
atoms. However, there is another class of lattices in nature
made of triangles such as triangular or kagome lattice.
Recently, effort to search for a new superconductor in suchtriangle-based lattices has been done extensively, and isalready successful in the triangular lattice of CoO2 in
NaxCoO2 : yH2O with Tc = 5 K by Takada [1] and in thethree-dimensional pyrochlore lattice made of tetrahedra in
Cd2Re2O7 with Tc = 1 K by our group [2]. Although the Tc
is rather low, the mechanism of superconductivity seems
very interesting on the basis of uniqueness of the trianglebased lattices. In 2003 we discovered a new family of
pyrochlore oxides AOs2O6 called the b -pyrochlore oxide
which shows superconductivity with Tc up to 10 K [3], tentimes higher than in the a -pyrochlore oxide Cd2Re2O7.
The a -pyrochlore oxide has the general chemical formula A2B2O7 or A2B2O6O', where A is a larger cation and B isa smaller transition metal (TM) cation. The ideal structure iscomposed of two types of cation-oxygen sublattice: one is acorner-sharing tetrahedral network composed of A atoms
with an O' atom in the center of each tetrahedron, and theother is that of B atoms with each B atom coordinated quasioctahedrally by six O atoms. This type of tetrahedral network is called the pyrochlore lattice, and has been studied
extensively in order to elucidate the effect of geometricalfrustration on the properties of localized spin and itinerant
electron systems. In contrast, the b -pyrochlore oxide has
the general chemical formula AB2O6 and crystallizes in asimilar cubic structure with space group Fd 3 m as in thea -pyrochlore oxide, but with a large, monovalent alkaline
metal ion located at the O' site of the a -pyrochlore structure, as shown in Fig. 1. This means that the A2O' unit of
the a -pyrochlore is replaced by a large A atom in theb -pyrochlore. We synthesized three b -pyrochlore osmatesfor A = K [3], Rb [4], and Cs [5].

References[1] H. Takayama, J. Magn. Magn. Mater. 272-276P1 (2004) 256.
[2] H. Takayama and K. Hukushima, to appear in J. Phys. Soc. Jpn.,
preprint cond-mat 0307641.
[3] H. Aruga Katori and A. Ito, J. Phys. Soc. Jpn. 63 (1994) 3122.
Authors
H. takayama and K. Hukushimaa
aDept. Basic. Sci. Univ. Tokyo

New Superconducting Pyrochlore
OxidesZ. Hiroi
We have been searching for a new superconductor with a8

ISSP Activity Report 2003

Fig. 1. Structural view of the b -pyrochlore oxide AOs2O6. The redoctahedron with six light blue balls on the vertices represents theOsO6 unit, and the big ball does an A atom that is coordinated by six
oxygen atoms from surrounding OsO6 octahedra. The crystallographic sites are 8b, 16c and 48f in the space group Fd-3m for A, Os
and O atoms, respectively. The pyrochlore lattice emerges from thestructure by extracting only osmium atoms.

that the Little-Parks oscillation a superconducting wire network exhibits fine structures at commensurate values of a(average flux per plaquette), and that D Tc(H) reproduces theedge shape of the Hofstadter spectrum for the corresponding
2D lattice. These and the majority of subsequent studies
were conducted in a uniform magnetic field. In the present

Fig. 2. Temperature dependence of resistivity ( t ), showing a superconducting transition at Tc = 9.6 K, 6.3 K, and 3.3 K for threeb -pyrochlore oxides with A = K, Rb, and Cs, respectively. That ofa -pyrochlore oxide Cd2Re2O7 with Tc = 1.0 K is also shown, after
multiplied by 10.

Figure 2 shows the temperature dependence of resistivity
measured on polycrystalline samples. The Tc is 9.6 K, 6.3
K, and 3.3K for A = K, Rb, and Cs, respectively. As the lattice is expanded with increasing the ionic radius of the Aions from K+ to Cs+, the Tc decreases gradually. This suggests that the Tc decreases with imposing negative chemicalpressure upon the Os pyrochlore lattice, which is in contrast
to the case of conventional BCS superconductor where theTc may increase under negative pressure with increasing thedensity-of-state at the Fermi level. The mechanism of superconductivity in the b -pyrochlore oxides is not known at the
moment, but we believe that interesting physics is involved
there on the triangle-based lattice.

Fig. 1. Hofstadter butterfly spectra under spatially modulated magnetic field of checkerboard (left) and stripe (right) patterns.

References[1] K. Takada, et al., Nature 422, 53 (2003).
[2] M. Hanawa, et al., Phys. Rev. Lett. 87, 187001 (2001).
[3] S. Yonezawa, et al., J. Phys.: Condens. Matter 16, L9 (2004).
[4] S. Yonezawa, et al., J. Phys. Soc. Jpn. 73, 819 (2004).
[5] S. Yonezawa, et al., J. Phys. Soc. Jpn., in press.
Authors
S. Yonezawa, Y. Muraoka, and Z. Hiroi

Superconducting Network with
Magnetic Decoration --- Hofstadter
Butterfly in Spatially Modulated
Magnetic Field
Y. IyeEnergy spectrum of tight-binding electrons on a twodimensional (2D) square lattice under a magnetic field perpendicular to the plane has an exquisite structure, known as
Hofstadter butterfly. Experimental verification of the full
Hofstadter butterfly spectra is a formidable task, although
recent attempts using semiconductor lateral superlatticeseem to attain a certain degree of success. Superconducting
wire network constitutes an experimental system intimately
related to the Hofstadter problem. It has been demonstrated

Fig. 2. (Top) Superconducting network decorated with a ferromagnetic array. Experimental traces of Little-Parks oscillation for the
checkerboard (left) and stripe (right)

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9

study, we address ourselves to the case of spatially varying
magnetic field,.
Figure 1 shows the evolution of the Hofstadter spectra in
the presence of a spatially alternating external flux pattern,whose amplitude is denoted by the parameter b . The spectrum at the top is the original Hofstadter butterfly corresponding to b =0. The route on the left side is for a checkerboard
pattern, and that on the right side is for a stripe pattern. The
spectra for the two cases become identical again for b =1/2,
The corresponding experimental system was realized byuse of mesoscopic superconductor/ferromagnet hybrid structures. Typical structure is shown in Fig. 2. A superconducting wire network is decorated by a ferromagnetic array. The
latter is magnetized by an external magnetic field applied
parallel to the network plane. The stray magnetic field generated by the ferromagnetic array creates a checkerboardpattern flux arrangement. The amplitude of the spatiallyvarying field can be continuously varied by changing theazimuthal angle of the parallel field.
The lower left panel of Fig. 2 shows the Little-Parks
oscillation as a function of perpendicular magnetic field forthe checkerboard field. With increasing amplitude of thecheckerboard field, the dips at half-integer flux develop, andtake over those at integer flux. At a higher value of thecheckerboard field, the original oscillation pattern isrestored, and the cycle is repeated. The lower right panel
shows the corresponding date for a stripe field.
A conspicuous difference from the checkerboard case is thatthe oscillation amplitude becomes much reduced at b =1/4.
This and other features are in line with the behavior of thespectral edge shown in Fig. 1.
Authors
Y. Iye, E. Kuramochi, M. Hara, A. Endo, and S. Katsumoto

Non-Democratic Nature of
Wavefunctions in Quantum Dots

Fig. 1. (a) Coulomb resonant “dips” appeared in the transport througha quantum dot, which is in between a Coulomb blockade and an open
dot. Sold curves are the results of fitting by Fano formula. (b)“Reversed” Coulomb diamonds appeared in a color plot of the conductance against the plane of source-drain bias and gate voltage.

tunneling path is so high that the opening of the resonant
path causes negative interference and results in decrease in
the total conductance. This is also confirmed by the fact that
the Coulomb diamonds appear as high conductance regions
as shown in Fig. 1(b). As indicated by solid curves, every
dip has the Fano-type distortion.
What is the origin of such non-democratic social structure in quantum dots? It is known that in some energy region
in non-integrable potential, wavefunction “scarring” occurs,in which the probability amplitude is high along corresponding classical trajectory. If such “scar” connects the inlet
and the outlet of the dot, the transmission coefficientthrough the level is very high producing an SCS.
Then the magnetic field applied perpendicular to the dot
causes Lorentz bending of the scarred trajectory and focusing of other scarred wavefunction, hence series of transitions
occurs. Figure 2 shows such properties in the ridges of conductance. When the field is low and the cyclotron diameteris larger than the dot size, the transitions of the ridges are

S. Katsumoto
In an elementary course of quantum mechanics, students
learn that the parity of wavefunction in a quantum wellchanges alternatively with the energy eigenvalue. Howeverin the transport through semiconductor quantum dots, it was
found that long trains of in-phase resonant peaks appear, inother words, many neighboring states have the same parity,
which has been a long standing problem in the transport
through quantum dots [1].
We have clarified that this is due to a non-democratic
nature of wavefunctions in quantum dots, i.e., there exist
some special states with anomalously strong couplings withthe electrodes, which dominate the parity of surrounding
ordinal states.
The existence of such strongly coupled states (SCSs) canbe known from the interference between an ordinal resonant
tunneling path and a path through an SCS by co-tunneling.
The interference results in a characteristic lineshape of theresonant peaks, known as the Fano effect [2].
Figure 1(a) shows the conductance around zero-bias versus the gate voltage. In order to magnify the effect of SCSs,
the dot is in a region of parameters between Coulomb blockade and open dot. Coulomb resonant “dips” appear insteadof ordinal peaks. This is because the conductance of the co-

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ISSP Activity Report 2003

Fig. 2 Color plot of the conductance of a quantum dot versus theplane of the gate voltage and the magnetic field. Small oscillation
along the gate voltage is the Coulomb oscillation.

significantly disordered. On the other hand, at high fields,
the shrinkage of the cyclotron motion leads to the formationof edge states, and the transitions become quite regular,
where the SCS is the edge state placed closest to the electrodes.
References[1] R. Schuster et al., Nature 385, 417 (1997).
[2] K. Kobayashi, H. Aikawa, S. Katsumoto and Y. Iye, Phys. Rev.
Lett. 88, 256806 (2002).
Authors
S. Katsumoto, H. Aikawa, K. Kobayashi, A. Sano, and Y. Iye

Strain-dependent Dissociation of
Molecules and Diffusion of Adsorbates
on a Nitrogen Adsorbed
Cu(001) Surface

Fig. 2. Successive STM images for (a) wide and (b) narrow areas on
the clean Cu surface of the nitrogen modified grid pattern after oxygen molecule adsorption. The latter area is connected only to the narrowest clean Cu lines. Black areas are nitrogen-adsorbed surface.
Yellow dots are oxygen atoms adsorbed on the clean Cu surface. The
numbers at the left-bottom corners show the times (minutes) after thescanning of the surface started. The image sizes are (a) 8.0 #10.4 nm2, and (b) 5.6 # 5.6 nm2 .

F. Komoriface at RT except on the narrowest lines. The adsorbates onnarrow areas diffuse more rapidly than on wide areas asshown in Fig.2. On a connected broad Cu area of the grid
the more oxygen adsorbates are seen on the wider region
where the lattice is less compressed. The diffusion barrier ofthe oxygen adsorbates is smaller at the more compressedsurface area on the Cu grid. These results are consistent withthe predictions by the d-band model that the dissociation
probability of oxygen molecule and the diffusion barrier ofthe oxygen adsorbates are small on the highly compressedarea.

Dissociative adsorption of molecules and diffusion ofadsorbates on strained metal surfaces have been studied bothexperimentally and theoretically [1]. The observed preferential dissociation and small diffusion at the stretched surfaces
are attributed to the changes in the electronic energy levels
and the width of d-bands due to the lattice deformation on
the basis of the density-functional calculations.
We have studied adsorption of oxygen molecules on agrid pattern of the partly nitrogen-covered Cu(001) surface
at room temperature (RT) by scanning tunneling microscopy(STM) [2]. On this surface, nitrogen-adsorbed patches withaverage size of 5 # 5 nm2 are regularly arranged on clean Cu
surface. The Cu-N short-range attractive interaction makes
the local c(2 # 2)N structure while the nanometer-scale grid
pattern is caused by the minimization of the total elastic
energy including the substrate. The Cu lattice at the patch is
stretched by the adsorption while that at the clean surface isinhomogeneously compressed up to 5 %. The STM image
shown in Fig. 1 demonstrates the lattice distortions at bothclean Cu surface and the nitrogen-adsorbed patch near theboundary.
Oxygen molecules are dissociatively adsorbed on theclean Cu(001) surface of the grid pattern without changing
the nitrogen-adsorbed surface. The adsorbate density issmall on the narrow surface where the Cu lattice is highly
compressed, indicating the strain-dependent dissociation
probability. Especially, there are few adsorbates at the intersections of the Cu grid connected only to the narrowest Culines. The oxygen adsorbate can migrate on the clean sur-Fig. 1. An STM image showing a c(2 # 2)N patch (red-yellow) surrounded by (1 # 1) Cu surface (purple). The protrusions of the Cu
atoms on the clean Cu surface are enhanced separately from those of
the nitrogen atoms inside the c(2 # 2)N patch. The latter at the hollow
site of the Cu lattice is imaged yellow. The average coverage of nitrogen on this surface is 0.1 ML. The size of the image is 11.9 #11.1 nm2.

References[1] for example, M. Gsell, P. Jakob and D. Menzel, Science 280(1998) 717.
[2] S. Ohno, K. Yagyuu, K. Nakatsuji and F. Komori, Surf. Sci. 554(2004) 183.
Authors
S. Ohno, K. Yagyuu, K. Nakatsuji, and F. Komori

Lateral Displacement by Transient
Mobility in Chemisorption of CO
on Pt(997)
J. Yoshinobu
When a gaseous molecule enters in a chemisorption
potential well and chemisorbs on the surface, the adsorptionenergy has to be dissipated. This energy is transferred to not
only various motions of the molecule on the surface but also
electron-hole pair and phonon excitations of the substrate. A"hot precursor" species in such energy dissipation processes
is transiently mobile on the surface, which plays an important role not only in thin film growth, surface chemical reaction but also molecular manipulation of adsorbates.
However, the lateral displacement by the transient mobilityof an incoming molecule has not been accurately determinedyet. In this study [1], the adsorbed states of CO on Pt(997) at
11K have been investigated by means of infrared reflection
absorption spectroscopy (IRAS). At 11K thermal migration
is suppressed and thus the initial chemisorption at terracesites and step sites is controlled by the transient mobility of

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anymore. (4) The initial kinetic energy of gaseous CO isneglected since it is very small as compared with the adsorption energy (~1.3eV) and the diffusion barrier (0.19 eV).
Using this simple model, the occupation probability at stepsites, P(step), is easily calculated to be 2l/ r L, where L is the
distance between steps. In the present experiment, P(step)=1 /(3.6+1)=0.217 and L = 2.0nm, we can estimate the lateral displacement of transient mobility for CO on Pt(997) tobe 0.68nm.
Reference[1] J. Yoshinobu, N. Tsukahara, F. Yasui, K. Mukai and Y.
Yamashita, Phys. Rev. Lett. 90 (2003) 248301.
Authors
J. Yoshinobu, N. Tsukahara, F. Yasui, K. Mukai, and Y. Yamashita
Fig. 1. A series of IRAS spectra of CO on Pt(997) at 11K. (a)0.005ML, (b) 0.010ML and (c) 0.015ML. (d) After heating the0.015ML CO adsorbed surface above 200K and cooled to 11K.

adsorbing molecule.
Figures 1(a)-(c) show a series of IRAS spectra foradsorbed CO on Pt(997) at 11K. Two peaks are clearlyobserved at 2067 cm-1 and 2090 cm-1, and they are ascribedto atop CO at step and atop CO at terrace, respectively. With
increasing the coverage from 0.005, 0.010 to 0.015 ML(monolayer), two peaks do not shift, and the integrated
absorbance of each peak is proportionally increased. Theseexperimental results indicate that adsorbed CO species do
not interact each other, i.e., they are singleton CO species.
By heating the adsorbed surface, thermal migration is activated and all CO species are chemisorbed at step sites (Fig.
1(d)). Since CO does not desorb by 200K heating, all CO
species become adsorbed at step sites. From these results,
the initial occupation ratio between atop CO at terrace andatop CO at step is directly estimated to be 3.6:1.
In order to estimate the lateral displacement of transientmobility for CO on Pt(997), we introduce a simple model
which includes following assumptions (see Fig. 2). (1)Gaseous CO molecules uniformly collide with the surface.
(2) The transient migration occurs randomly on Pt(997). (3)Once CO moves across a step during the transient migration,
it sticks at a nearby atop site on the step and never moves

Kondo Transport Studied by the Finite
Temperature Density
Matrixrenormalization Group Method
K. UedaTransport properties through quantum dots have been of
great interest in recent years. At high temperatures the conductance through a dot shows peaks as a function of the gate
voltage when the number of electrons in the dot changes,
which are known as Coulomb oscillation peaks. When thenumber of electrons is odd, the conductance becomes larger
as temperature is lowered below a characteristic temperatureand reaches to the unitarity limit (2e2 /h) in the low temperature limit. This effect of Kondo transport was actuallyobserved a few years ago [1]. One of the interesting features of the Kondo transport compared with the usual Kondo
effect of a magnetic impurity in a metal is that various
parameters are tunable in quantum dots. A typical example
is that the number of electrons in the quantum dot is easilyvaried by changing the gate voltage.
Recently we have developed a numerical scheme to calculate the conductance through a quantum dot by using thefinite temperature density matrix renormalization group(FT-DMRG) method [2]. A minimal model for the transport
thorough a quantum dot is the one dimensional tight binding
model with an impurity site in the middle.

H := -

iv

c0@v c0v + Uc0@ - c0 - c0@ . c0.

(1)v when i = 0, i + 1 = 0,t=1others,(2)+ edti, i+1 := *Fig. 2. (a) Schematic model of adsorbed CO on Pt(997) at 11K and(b) that after heating above 200K (~0.016ML). (c) An isotropic
migration model. ri: the first impact point. rf: the initial chemisorption
site. l: the mean lateral displacement of the transient mobility.

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ISSP Activity Report 2003

ti, i+1 (ci@v ci+1v + h.c.)v

where ed is the energy level of the state in the dot and U is
the Coulomb interaction when two electrons are put into thedot. The energy level ed may be varied by changing thegate voltage. The coupling constant between the dot and theleads is given by v.
Conductance through the dot is expressed by the Greenfunction at the impurity site, G0 ( ~ ). The Green function in
the frequency space can be calculated by the analytic continuation from the thermal Green function as a function of theimaginary time, G0 ( x ) obtained by FT-DMRG. The

Fig. 1. Conductance g/( 2e2 /h) as a function of the gate voltage Vg
for v = t/ 2 and U=4t at various temperatures calculated by the
FT-DMRG.

numerical analytic continuation generates a main error of
G0 ( ~ ), but its integrated value like the conductance showsa better behavior than G0 ( ~ ) itself.
Figure 1 is an example of the conductance through thequantum dot as afunction of the gate voltage which isdefined by ed = -U/2 +Vg. One can see the crossover fromthe Coulomb oscillation behavior at high temperatures to the
Kondo resonance below the Kondo temperature.
An advantage of the FT-DMRG is that once the eigen
vectors for the bulk part is obtained the Greens function at
the impurity site can be calculated simultaneously for various impurity parameters like the gate voltage. In this sense,
the FT-DMRG is a very convenient and powerful tool todiscuss transport properties through quantum dots when the
Kondo temperature is not so small.
References[1] W. G. van der Wiel, et. al.: Science 289 (2000) 2105.
[2] X. Wang and T. Xiang: Phys. Rev. B 56 (1997) 5061; N. Shibata:J. Phys. Soc. Jpn. 66 (1997) 2221.
Authors
I. Maruyama, N. Shibataa, and K. Uedaa Dept. of Basic Science, Univ. of Tokyo.

A Unified Approach to Filling- andBandwidth-Control Mott Transitions
M. Imada
The nature of the system in which quantum fluctuations
and electron correlations play essential roles is one of themain subjects in condensed matter physics. When the kinetic
energy and the Coulomb repulsion compete severely, theground state of many-body electron systems can be highly
nontrivial. Metal-insulator transitions driven by the electron
correlation postulated by Mott [1] provide a typical example
of such a nontrivial behavior.
In the transition to the Mott insulator, it is known that
there exist two different basic routes to control the competition of the interaction and kinetic energies. One is the control by bandwidth (relative to Coulomb repulsion) and theother is filling (averaged number of carrier electrons in aunit cell). Controls by these two parameters can be found ina lot of examples in real materials including transition metalcompound, organic materials and 3He systems. In spite ofplenty of the experimental results, phase diagrams of theMott insulator and metals have not been fully elucidated in

microscopic theoretical descriptions.
The Hubbard model with nearest- and next-nearestneighbor transfers is a minimal model to describe theessence of the Mott insulator and metals with their transitions on the bandwidth- and filling-control routes. The filling-control Mott transition (FCMT) was studied at zero temperature by the quantum Monte Carlo method in the
Hubbard model on a square lattice [2]. The transition showsa continuous character with a singular divergence of thecompressibility and critical divergence of the antiferromagnetic correlation length. The bandwidth-control Mott transition (BCMT) was studied also at zero temperature by thepath integral renormalization group method [3]. In contrast
to the FCMT, the BCMT shows a first-order transition,although a naive expectation is that the continuous FCMT
anticipates also the continuous BCMT. This contrast is
essentially consistent with the trend of the experimental
observations cited above.
Mott has originally proposed a first-order BCMT
because of the role of the long-ranged part of the Coulombinteraction [1]. However, the numerical result shows that thefirst-order transition takes place even with the onsite interaction only. In this circumstance, it has been greatly desired toclarify basic properties of the BCMT and the FCMT in a
unified way to further elucidate the contrast.
In this study, we have developed the grand-canonicalpath-integral renormalization group method [4] which isuseful to calculate chemical potential dependence of physical quantities. The path-integral renormalization groupmethod has been originally developed in the canonicalensemble. This algorithm does not suffer from negative-sign
problem and can be applied to any lattice structure with any
boundary conditions. Extension of this algorithm to the
grand-canonical framework makes it possible to study thecorrelated electron systems whose control parameters arefilling, bandwidth and lattice structure in a unified way. In
the grand-canonical path-integral renormalization group calculations, metal-insulator transitions are carefully examinedwith finite-size scalings and extrapolations to the thermodynamic limit.
By using the newly developed method, the ground-statephase diagram of the Hubbard model with next-nearestneighbor transfer scaled by nearest-neighbor transfert’/t =-0.2 on the square lattice has been determined in theplane of chemical potential n and Coulomb interaction U[4] (Figure 1). The Mott-insulator phase is drawn as the pinkarea and the metallic phase is drawn as the yellow area. The
remarkable result is that the V-shaped Mott insulator phaseappears. At the corner of the V-shaped metal-insulator
boundary, the BCMT occurs and at the edges except the corner, the FCMT occurs. We have confirmed the continuous
character of the FCMT with diverging charge compressibility and the first-order BCMT with a jump of the double occupancy.
To analyze the relation between the shape of the phaseboundary and the order of the metal-insulator transition, we
have analytically derived a general relation of the slope ofthe metal-insulator transition line in the n -U phase diagramand physical quantities [4]: In the case of the first-order
metal-insulator transition, DU/dn is expressed by the ratioof the jumps in the filling and the double occupancy
Dn/DD . In the case of the continuous transition, DU/dn isexpressed by the ratio of the compressibility and dn/dU in
the metallic phase. These relations can be regarded as generalizations of Clausius-Clapeyron and Ehrenfest equations in

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From Chemical Bonding to
Confinement Due to Nonadiabatic
Processes by the Enhancement of
Quantum Fluctuations
Y. TakadaFig. 1. Ground-state phase diagram in the plane of chemical potential
n and Coulomb interaction U for nearest-neighbor transfer t=1.0 and
next-nearest-neighbor transfer t'=-0.2 on the square lattice in the thermodynamic limit [4]. Open circles represent the metal-insulator
boundary extrapolated to the thermodynamic limit by using the data
of N=4 # 4, 6 # 6, 8 # 8, 10 # 10 lattice systems. The solid black lines
represent the least-square fit of metal-insulator boundary for U=4.0,
5.0, 6.0, 7.0 and 8.0. The V-shaped Mott insulator phase is drawn aspink area and the metallic phase is drawn as the yellow area. The corner of the V-shaped metal-insulator boundary is the bandwidth-control Mott transition point and the edges are the filling-control Mott
transition points. The red diamonds represent chemical potentials forhalf filling for U=0.0 and U=2.0 in the thermodynamic limit. By connecting those points to the bandwidth-control Mott transition point,
the blue-dashed line represents the half-filled density, which is the
bandwidth-control route in the metallic phase.

the first- and second-order transitions at finite temperature,
respectively, to the quantum phase transitions.
These relations support that the V-shaped phase boundary is resulted from the first-order BCMT coexisting withthe continuous FCMT with diverging compressibility.
Namely, it is shown that the V-shaped metal-insulator transition line together with the first-order BCMT is not compatible with the presence of the first-order FCMT near the
BCMT. When the V-shaped Mott insulator phase appears,
the charge gap D c opens at U=Uc and shows marked linear
dependence on U for U>Uc, namely, D c~U-Uc. It is notedthat the linear opening of the Mott gap has been actuallyobserved in the perovskite compounds, R1-xCaxTiO3 [5],which is consistent with our results.
We have also shown the possible and impossible shapesof the metal-insulator boundary with first-order and continuous transitions by using the thermodynamic relations
derived above: The-shaped structure of the insulator
phase with the first-order BCMT coexisting with the firstorder FCMT can be realized in contrast with the V-shapedcase. On the other hand, in the case of the U-shaped insulator phase (for example, the case with an essential singularform of the charge gap as D c ~ exp[at/(U-Uc)]), if the firstorder BCMT exists at the corner, the first-order transition
cannot be retained at the FCMT, which is classified to thesame class as the V-shaped case.

The physical nature of chemical bonding has been studied in the adiabatic approximation in which only electronic
motion contributes to the formation of covalent bonds.
Although this approximation may well apply to most realistic molecules and solids, a more comprehensive understanding of chemical bonding will be obtained by investigating
the roles of quantum fluctuations of nuclei in the bond formation.
We have commenced such an investigation by considering a hydrogen-like molecule, (M+M+m-m-), composed of
two unit-positive charge particles (‘protons’) with mass M
and two electrons with mass m. The mass ratio m/M ischanged continuously from zero (the adiabatic limit) tounity (the limit of dipositronium), covering the issue of abiexciton or a bound complex of two electrons and twoholes in between. We have employed the diffusion Monte
Carlo (DMC) simulations at zero temperature in order toevaluate the exact values for various ground-state properties
such as the total energy E0 and the average separation of‘protons’ (or the bond length), together with the corresponding ones in the adiabatic approximation.
In Fig. 1, the calculated result for E0 in DMC is shown
as a function of m/M by the solid curve. The obtained E0
increases monotonically with increasing m/M, but it isalways smaller than the upper limit of the energy for the
existence of a bound state of this four-body problem (thedotted curve), namely, E0 = -M/(m+M) in hartree units. The
adiabatic approximation provides a very accurate value forE0 with relative errors less than only 1% as long as m/M issmaller than 0.1, convincing us that the binding mechanism

References[1] N. F. Mott and R. Peierls: Proc. Phys. Soc. London. A 49 72(1937).
[2] N. Furukawa and M. Imada: J. Phys. Soc. Jpn. 61 3331 (1992).
[3] T. Kashima and M. Imada: J. Phys. Soc. Jpn. 70 3052 (2001).
[4] S. Watanabe and M. Imada: J. Phys. Soc. Jpn. 73 1251 (2004).
[5] T. Katsufuji, Y. Okimoto and Y. Tokura: Phys. Rev. Lett. 75 3497
(1995).
Authors
S. Watanabe and M. Imada

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Fig. 1. Ground-state energy of a hydrogen-like molecule(M+M+m-m-) in both DMC and the adiabatic approximation as afunction of the mass ratio m/M.

Superconductivity and Abelian ChiralAnomalies
M. Kohmoto

Fig. 2. Average distance between two ‘protons’ <Rp> in uints of the
Bohr radius as a function of m/M. The inset shows the relative variation D Rp/<Rp>.

for this molecule in this range of m/M is nothing but thechemical bonding in the usual argument [1].
The decrease of the binding energy with increasing m/M
is associated with the rapid increase of the bond length asshown in Fig. 2. Thus the vibrons or the zero-point quantum
fluctuations of ‘protons’ are found to hamper this chemicalbonding by weakening the binding energy and widening thebond length.
For m/M larger than about 0.2, the adiabatic approximation becomes too crude to support any bound states in thesystem. Concomitant with this result for E0, the variation of‘proton’ positions is as large as the average separation oftwo ‘protons’. This implies that proton motion has changed
its character with the increase of m/M; at m/M<<1, ‘protons’
are localized and separated to each other by the bond length,
while for m/M larger than about 0.2, they overlap in thewhole molecule in just the same way as electrons.
Since the adiabatic potential cannot afford an enoughattractive energy to bind ‘protons’ for m/M>0.2, the existence of a bound state (or the confinement of this four-bodysystem) as demonstrated by DMC suggests that nonadiabatic
processes or the effects due to retardation of electron
response to ‘proton’ motion should play a crucial role in thebinding mechanism for m/M in this region [2].
Finally we note that the parameter m/M controls the twocompeting effects on the binding mechanism; one is to
weaken the binding force through enhancing quantum fluctuations and the other is to strengthen it through promoting
nonadiabatic processes. The crossover takes place at m/Maround 0.2.

The importance of quantum-mechanical phases in condensed matter physics has been recognized and emphasized
for recent several decades. The fundamental character of avector potential is evident in the Aharonov-Bohm effectwhere the U(1) gauge structure is essential and a magnetic
field in itself plays only a secondary role [1]. Topologicalstructures in quantum gauge field theories have also beenstudied and extensive knowledge has been accumulated [2].
Quantum mechanics itself supplies a fundamental gauge
structure [3]. It is known as geometrical phases in many different contexts, where gauge structures emerge by restricting physical spaces. The quantum Hall effect is one of the
key phenomena to establish the importance of geometricalphases [4].
The topological character of the Hall conductance was
first realized by the Chern number expression, where theBloch functions define "vector potentials'' in the magnetic
Brillouin zone accompanied with a novel gauge structure[5]. Further the ground state of the fractional quantum Hall
effect is a complex many-body state where another kind ofgauge structure emerges [6].
These quantum states with nontrivial geometrical phases
are characterized by topological orders which extend an ideaof order parameters in statistical mechanics to the quantum
states without spontaneous symmetry breaking [7].
Recently, following a prediction of flux phases for correlated electron systems, spin Hall conductance is defined forsuperconductors based on the Bogoliuvov-de Gennes (BdG) equation [8].
In Ref.9, we established a topological characterization ofgeneral superconductivity based on the BdG equation on
lattices. A map to a parameter space which represents the
BdG hamiltonian is considered. In the parameter space, ananalogue of the Dirac monopole exists and the Chern num-

References[1] K. Ruedenberg, Rev. Mod. Phys. 34, 326 (1962).
[2] Y. Takada and T. Cui, J. Phys. Soc. Jpn. 72, 2671 (2003).
Authors
Y. Takada and T. Cuia
aJilin University.

Fig. 1. Formally, we can treat a three-dimensional superconductivity
as a collection of two-dimensional systems parametrized by, say, kz .
The superconducting node can be considered as the critical point of
the quantum phase transition of the two dimensional systems between
two states with different topological orders.Here we illustrate a example of the two-dimensional systems at (a) kz = 0 and (b) kz = - 2r/5 .
A point node exists between these kz 's.

ISSP Activity Report 200315

bers are analyzed. As for the unitary superconductors, condensed matter realizations of chiral anomalies for nonAbelian connections are given explicitly.Topological consideration is useful to distinguish superconductivities withthe same pairing symmetry. Our analysis also clarified nodal
structures of superconducting gaps with various anisotropic
order parameters, which is closely related to the quantum
Hall effect in three-dimensions. Various types of the nodal
structures are not accidental but have fundamental topological origins.
References[1] Y.Aharanov and D.Bohm, Phys. Rev. 115, 485 (1959).
[2] T.Eguchi, P.B.Gilkey, and A.J.Hanson, Phys.Rep. 66, 213 (1980).
[3] M.V.Berry, Proc.R.Soc. A 392, 45 (1984).
[4] D.J.Thouless, M.Kohmoto, P.Nightingale, and M.den Nijs, Phys.
Rev. Lett. 49, 405 (1982).
[5] M.Kohmoto, Ann. Phys. (N.Y.) 160, 355 (1985).
[6] R.B.Laughlin, Phys. Rev. Lett. 50, 1395 (1983).
[7] X.G.Wen, Phys. Rev. B40, 7387 (1989).
[8] T.Sentil, J.B.Marston, and M.Fisher, Phys. Rev. B60, 4245 (1999).
[9] Y.Hatsugai, S.Ryu, and M.Kohmoto, cond-mat/0308332.

Movie of the Nuclear Wave-packets
T. Suemoto
Since the pioneering work for NaI by Zewail's group [1],observations of the wave-packet (WP) oscillations havebeen reported for a number of molecules. One of the interesting challenges is to capture directly the time-evolving
shape of the WP propagating on the potential surface.
However, in most of the experiments up to now, the oscillatory behavior has been observed only at one or few points
on the adiabatic potential surface. Therefore, the shape of
the wave-packet had to be estimated from the temporal
behavior of the signal observed at certain point with an aidof theoretical model.
Here, we present a demonstration of making a movie ofthe WP, showing the time development of entire WP shape

Fig. 1. The upper panel shows the adiabatic potential curves of STE
in the Pt-Br complex. The abscissa corresponds to the displacementof the Br- ion. The lower panel is a two dimensional display of thetime evolution of the luminescence intensity which can be regarded
as the amplitude of the wave-packet. The ordinate is the luminescence photon energy, which approximately corresponds to the spatial
position of the Br- ion.

16

ISSP Activity Report 2003

Fig. 2. A movie of the wave packet. The pictures in the column a are
the vertical cross-sections cut from the 2D-plot shown in Fig. 1. The
frames are taken every 40 fs. The curves in the column b are the calculation based on a harmonic oscillator model. The scales are adjusted to match the experimental data in the column a. The area notobserved in our experiment is covered by yellow color.

moving on the potential surface [2]. In contrast to previous
experiments, our method is direct and completely modelfree.
We used single crystals of the halogen-bridged mixedvalence platinum complex [Pt(en)2] [Pt(en)2Br2] (ClO4)4(en=ethylenediamine), which is known as a prototype of thequasi-one-dimensional system. The adiabatic potential
curves for the ground state and the self-trapped-exciton(STE) on a configuration coordinate are shown in Fig. 1. By
observing the hot luminescence, we can obtain the shape of
the wave-packet directly. The time evolution of the luminescence was investigated at ambient temperature using a frequency up-conversion technique with a time resolution of 50
fs. After a careful correction against the spectral response of
the measurement system, we plotted the 7 # 240 data on aplane of the photon energy and the time delay in a color
scale as shown in the lower part of Fig. 1. We can clearly
see the sinusoidal motion of the WP around 0.8 eV.
From vertical cross-sections of this plot, we can obtain amovie, which is the first demonstration of real time capturing of the wave form within our knowledge. Here we canshow some impressive snapshots taken every 40 fs on theleft column in Fig. 2. We can see not only the oscillation,
but also the change of the shape. That is, the peaks are flat at
the center of oscillation (246 fs) and become sharp andasymmetric at the turning points (126, 286 fs). These features are well reproduced in the calculation based on a harmonic oscillator as shown on the right column in Fig. 2.
This reliable method of capturing the wave-packet form
will be useful for investigating the dynamical behavior of
the atomic motions including the de-coherence process andalso for developing coherent control method of chemical
reactions in molecules and lattice rearrangements in solids.

References[1] A. H. Zewail, J. Phys. Chem. A 104, 5660 (2000).
[2] T. Matsuoka,J. Takeda, S. Kurita and T. Suemoto, Phys. Rev. Lett.
91, 247402 (2003)
Authors
Taira Matsuoka, Tohru Suemoto, Jun Takedaa, and Susumu Kuritaa

a Yokohama National University

Structural Study of theSi(111) 3 # 3 -Ag Surface at Low
Temperature by X-ray Diffraction
T. Takahashi
The Si(111) 3 # 3 -Ag surface has been attracting
interest since the frist observation of the surface by low
energy electron diffraction in 1960s. Its structure had been
controversial until a honeycomb-chained triangle (HCT)model as shown in Fig. 1(a) was proposed by surface X-ray
diffraction [1]. Recently, however, it is suggested that themost stable structure of the Si(111) 3 # 3 -Ag surface is
not the HCT structure but an inequivalent triangle (IET)
structure shown in Fig. 1(b) by the study of first-principles
calculations and scanning tunneling microscopy (STM) at
low temperatures [2].
The IET model is characterized as a structure in whichlarge triangles of Ag atoms indicated by red broken lines inthe HCT model rotate slightly around their centers, losing
the mirror symmetry. The proposal of the IET model also
casts some question about the structure at room temperature(RT), and suggests a possibility of a model in which Ag
atoms thermally fluctuate at RT between two IET structures
rotated in opposite directions, illustrated in the right downpart of Fig. 1(b).
In this work [3], we investigate the in-plane structure of
the surface both at RT and at low temperature using surface
x-ray diffraction in the grazing incidence geometry. We
have observed the intensities of diffraction spots peculiar to
the 3 # 3 structure both at RT and at a low temperatureof 50K. Experiments were done at PF-BL15B2. Figures
2(a) and 2(b) show the Patterson maps calculated from theobserved intensities at RT and 50K, respectively. Sincepeaks in the Patterson map correspond to the interatomic
vectors between two atoms, peaks A observed at RT are
assigned to the interatomic vector between Ag atoms making large triangles in Fig. 1(a) as marked by A. In contrast,
the peaks A split into two peaks A1 and A2 at 50K as shownin Fig. 2(b). This split indicates the existence of twin

Fig. 1. Illustration of the HCT model (a) and IET model (b). Graylines show the unit cell. In the IET model, large triangles indicated by
red broken lines rotate slightly from the symmetric positions in the
HCT model. This rotation changes the size of small triangles indicated by light green. In the IET model, there exists the domain structurein which the large triangles are rotated in the opposite direction as
illustrated in the right down part.

Fig. 2. Patterson maps calculated from observed intensities at RT(a)and at 50K(b). Peaks A at RT, corresponding to interatomic vectors
between Ag atoms, split into two peaks A1 and A2 at 50K. Peaks B
correspond to interactomic vectors between the first-layer of Si
atoms.

domains in which triangles of Ag atoms are rotated in opposite directions. This result is also supported from the observation of diffuse scattering originating from twin domains at
low temperatures.
We have also determined the phase transition temperature to be about 150K from the temperature dependence ofthe diffuse scattering. Temperature dependence of some diffraction spots indicates that the phase transition is accompanied by structural changes. That means the structure at RT
is not explained by the model in which Ag atoms thermallyfluctuate between the two IET structures rotated in opposite
directions. The analysis of the data observed at RT also
supports the HCT model.
References[1] T.Takahashi et al., Jpn. J. Appl. Phys. Part 2 27(1988) L753.
[2] H. Aizawa et al., Surf. Sci. 429 (1999) L509.
[3] H. Tajiri et al., Phys. Rev. B68 (2003) 035330.
Authors
T. Takahashi, H. Tajiria, K. Sumitani, and K. Akimotoba JASRIb Nagoya University.

Intersubband Absorption Linewidth in
GaAs Quantum Wells
H. Akiyama
Effects of scattering mechanisms on intersubband
absorption linewidth in GaAs QWs are studied quantitatively. Figure 1 shows an intersubband absorption spectrum ofa modulation-doped 8-nm GaAs/AlAs single QW with asheet electron concentration NS of 9.8 # 1011 cm-2 (see theinset), measured at 4.5 K using a Fourier transform infrared
spectrometer with a microscope ( n -FTIR). The linewidth
2 Cop was 11.1 meV and this is about an order of magnitudelarger than the transport energy broadening 2 Ctr of 1.2
meV, to which mobility n of 2.9 # 104 cm2/Vs corresponds
through transport relaxation time. Temperature dependencesof the linewidth 2 Cop and transport broadening 2 Ctr areplotted in Fig. 2 by red and blue circles, respectively. Little
correlation [1] was found between them in the temperature
range from 4.5 to 300 K.
We calculated absorption linewidth using Ando’s microscopic theory [2], which is similar to the conventional
method of calculating transport mobility. In Fig. 2, calculated values of the linewidth 2 Cop and transport broadening
2 Ctr are shown by the various curves, where relevant con-

ISSP Activity Report 200317

References[1] K. L. Campman, H. Schmidt, A. Imamoglu, and A. C. Gossard,
Appl. Phys. Lett. 69, 2554 (1996).
[2] T. Ando, J. Phys. Soc. Jpn. 44, 765 (1978); 54, 2671 (1985).
[3] T. Unuma, M. Yoshita, T. Noda, H. Sakaki, and H. Akiyama, J.
Appl. Phys. 93, 1586 (2003); T. Unuma, T. Takahashi, T. Noda, M.
Yoshita, H. Sakaki, M. Baba, and H. Akiyama, Appl. Phys. Lett. 78,
3448 (2001).
[4] T. Unuma et al., unpublished.
Authors
Takeya Unuma, Masahiro Yoshita, Takeshi Nodaa, Hiroyuki Sakakia,
and Hidefumi Akiyamaa Institute of Industrial Science, University of Tokyo

Lasing in a Single Quantum Wire
Fig. 1. The intersubband absorption spectrum of a modulation-doped
8-nm GaAs/AlAs single QW measured at 4.5 K using a n -FTIR.
Note that its linewidth 2 Cop = 11.1 meV is an order of magnitudelarger than the transport energy broadening 2 Ctr = 1.2 meV, towhich the mobility n = 2.9 # 104 cm2/Vs corresponds. The inset
shows the structure of the QW.

tributions of interface roughness (IFR; with a mean height D
of 0.4 nm and a correlation length K of 4.3 nm), LO
phonon, and LA phonon scattering are included one by one(see legend). Satisfactory agreement between the experimental data and the theoretical calculations was obtained forboth absorption linewidth and mobility.
Therefore, the apparent lack of correlation [1] between
absorption linewidth and mobility is confirmed and consistently explained. At low temperatures, interface roughness
scattering dominates both 2 Cop and 2 Ctr , and the 2 Cop of
10.4 meV is about an order of magnitude larger than the2 Cop of 1.2 meV. This is because the contribution fromintrasubband scattering in the first excited subband is muchlarger than that in the ground subband [3]. As a result of thesignificantly different effects of interface roughness and LO
phonon scattering [3], absorption linewidth has little correlation with mobility.
Very recently, we have succeeded in measuring intersubband electronic Raman scattering in narrow GaAs-based single QWs such that the intersubband energy separation ismore than ~ 150 meV [4]. We are discussing a relation
between intersubband absorption and Raman scattering interms of transition energy and linewidth.

Fig. 2. Temperature dependences of the absorption linewidth 2 Cop
and transport broadening 2 Cop (or mobility n ). Measured values areplotted by circles, and calculated ones are shown by lines, in which
the contributions of interface roughness (IFR), LO phonon, and LA
phonon scattering are considered.

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ISSP Activity Report 2003

H. Akiyama
We fabricated a single quantum wire with a small crosssectional size of 14 nm by 6 nm and significantly reduced
size variation over the whole cavity length of 0.5 mm [1],and achieved lasing in the wire ground state. This is thethinnest laser in the world.
The single quantum wire laser is fabricated by anadvanced crystal growth method called the cleaved-edge
overgrowth method (CEO) with molecular beam epitaxy(MBE), in which two MBE growth steps are separated by anin situ wafer cleavage process. Figure 1 shows a schematiccross-sectional view of the single wire laser structure. In thefirst MBE growth, we grew a 14 nm quantum well (stemwell) at 600 oC on a non-doped (001) GaAs substrate. Then,
we cleaved the wafer, and grew a 6 nm GaAs quantum well(arm well) at 490 oC on an in situ cleaved fresh (110) edgeof this structure. After the growth of the arm well, we interrupted growth and annealed the GaAs surface for 10 minutes
at 600 oC. At a T-shaped intersection of a stem well and an
arm well, quantum-mechanical confinement of electrons
forms a quantum wire. The blue contour curves show the
probability of 1-D electrons in a quantum wire. This quantum wire has no higher 1-D subband, and is in the 1-D quan-Fig. 1. Schematic cross-sectional view of a single wire laser structure.
Percentages show Al-concentration x in AlxGa1-xAs. A quantum wireis formed at a T-intersection of a 14 nm thick Al0.07Ga0.93As stem
well and a 6 nm thick GaAs arm well, which is embedded in a core of
T-shaped optical waveguide formed by a 500 nm Al0.35Ga0.65As stem
layer and a 111 nm Al0.1Ga0.9As arm layer. The squared wave functions, or probability, of photons and electrons in the device are drawn
by contour curves, which show that photons and electrons are confined at the T-intersections. The thick arrows indicate [001] and [110]axes for the first and second MBE growth.

Fig. 2. Lasing spectra of the single quantum wire laser at 5K for thevarious input powers of Pin=8.3, 17, 33, 66, 130 and 260 mW, measured with spectral resolution of 0.2 meV. Lasing lines due to thequantum wire, the arm well, and the stem well are observed. The lasing of the quantum wire has excellent characteristics of a low threshold, a single lasing mode, and small red shifts.

tum limit.
The sample was pumped optically with an excitation
light focused via the top surface of the structure shown in
Fig. 1. Figure 2 shows laser emission spectra from thedevice at 5K for various excitation input powers Pin. At
input power Pin=8.3 mW, multi-mode laser emission of thequantum wire is observed at 1.578 eV. It changes to singlemode at Pin=17 mW, and shows slight red shifts with mode
hopping as the input power increases. The amount of red
shifts is 1.5 meV at Pin=260 mW. At higher energies, laser
emissions from the arm well and the stem well are also
observed. We observed lasing of this quantum wire laser at
5 – 60 K. Our 1-D quantum wire provided an opportunity to
study many interesting fundamental issues in 1-D systems.

Fig. 1. Time dependence of the measured vertical beam size after
injection from the injector linac to the ring. The beam started toshrink after the injection and finally reached the equilibrium state in
the ring. This resulted from the radiation damping.

Reference[1] Y. Hayamizu, et al. Appl. Phys. Lett. 81, 4937, (2002); Y.
Hayamizu, et al. Proc. 26th Int. Conf. on Phys. of Semiconductors(2003).
Authors
Y. Hayamizu, M. Yoshita, S. Watanabe, H. Akiyama, L. N. Pfeiffer a,and K. W. West a.
a Bell Laboratories, Lucent Technologies

Intrabeam-scattering andRadiation-damping Effects Observedwith a Developed Beam Profile
MonitorN. Nakamura
We have developed a high-resolution beam profile monitor based on an X-ray imaging optics with two FZPs in the
KEK-ATF damping ring in order to measure an electron
beam size less than 10 n m [1,2]. The synchrotron radiation(SR) from the electron beam at the bending magnet is monochromatized by a Si(220) crystal monochromator and theelectron beam profile is twenty-times magnified by two
FZPs and detected on an X-ray CCD camera. The spatial

Fig. 2. Current and coupling dependencies of beam size measured by
the beam profile monitor. The horizontal beam size increased withbunch current because of the intrabeam scattering. Four beam images
show that the horizontal and vertical beam sizes correlatively changewith the emittance coupling.

resolution is about 0.6 n m (1 v ) for the selected photon
energy of 3.235 keV. After successfully obtaining an electron beam image, we started to observe various beam behaviors in the ring with this monitor system.
Figure 1 shows time dependence of the measured vertical beam size after injection from the injector linac to thering, in addition to three transverse beam images. The X-ray

ISSP Activity Report 2003

19

CCD camera and the X-ray shutter were synchronized with
the injection trigger signals. The vertical beam size
decreased with time after injection and then reached theequilibrium state, as shown in Fig. 1. The beam shrink is
due to the radiation damping effect. The radiation damping
time was obtained by fitting the time dependence to an
exponential curve to be about 25 ms. This is in good agreement with the design value.
Low-emittance and low-energy rings such as the ATF
damping ring and the Super-SOR light source can be affected by intrabeam scattering, which causes increase of beam
size with bunch current. Figure 2 shows bunch current
dependences of the horizontal and vertical beam sizes measured by the beam profile monitor. The horizontal beam size
significantly increased with bunch current because themomentum spread increased due to intrabeam scattering and
the SR source point had a considerable horizontal momentum dispersion. The coupling dependences were also measured by changing currents of the skew-quadrupole coils
wound on two kinds of sextupole magnets, "SD" and "SF".
As shown in Fig. 2, the vertical beam size increased with theemittance coupling, while the horizontal one decreased. The
emittance coupling effect was clearly found in this experiment.
References[1] K. Iida et al., Nucl. Instr. and Meth. A 506 (2003) 41-49.
[2] N. Nakamura et al., Proc. of 8th International Conference on
Synchrotron Radiation Instrumentation: AIP Conference Proceedings
705, San Francisco, pp.545-548, 2003.
Authors
N. Nakamura, H. Sakai, H. Takaki, T. Mutoa, and H. Hayanoaa High Energy Accelerator Research Organization (KEK)

Development of Scanning TunnelingMicroscope Combined with
Synchrotron Radiation Light Source
for Elemental Analysis
T. Kinoshita and Y. Hasegawa
Scanning tunneling microcopy (STM) provides us information on atomic structure of material surfaces with highspatial resolutions. It is, however, not easy to obtain chemical or elemental information except a few special cases suchas inelastic tunneling spectroscopy. The reason is simply
because STM probes tunneling current which basicallyreflects valence electronic states of a sample. In order toaccess elemental information, one needs to probe core-level
electrons as Auger and x-ray photoelectron spectroscopy do.
In this project, we constructed an ultrahigh vacuum (UHV)
STM with which one can excite core-level electrons withsynchrotron radiation (SR) light source and pick them up by
the probe tip (SR-STM) (Fig. 1) [1]. Advantages of using
SR light source are, in addition to its high intensity, that theenergy of the light can be tuned so that one can excite coreelectrons of a specific element, advantageous for the elemental identification.
We first tested a performance of the system by using astandard sample of the Si(111)- 7 # 7 surface. All experiments were carried out at BL-19A, Photon Factory, KEK,
Japan. In spite of the mechanically and electronically noisy
conditions, we successfully took atomically resolved images

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ISSP Activity Report 2003

Fig. 1: schematic of SR-STM

of the surface, implying a stability of our system as a UHVSTM. We then checked a detection of photo-emitted electrons excited from core states with the tip. To test this, wetook STM images of the Si surface under the SR illumination with its photon energy varied from 96 eV to 106 eV.
The Si 2p adsorption edge (99 eV) is in the photon energy
range. When the photo-emitted electrons are detected by thetip, the tip is retreated from the surface, trying to keep thetotal current constant, and consequently the image looks
high (bright) (Fig. 2). We found a sharp increase in height at
the edge energy, indicating the detection of the excited coreelectrons. This result demonstrates the possibility of elemental identification using this technique.
References[1] T. Matsushima, T. Okuda, T. Eguchi, M. Ono, A. Harasawa, T.
Wakita, A. Kataoka, M. Hamada, A. Kamoshida, Y. Hasegawa and T.
Kinoshita, Rev. Sci. Instrum. 75 2149 (2004).
Authors
T. Matsushima, T. Okuda, T. Eguchi, M. Ono, A. Harasawa, T.
Wakita, A. Kataoka, M. Hamada, A. Kamoshida, Y. Hasegawa, and T.
Kinoshita

Fig. 2 STM image (right) of the Si(111)7 # 7 surface. During taking
this image, the sample was irradiated with a light coming from synchrotron radiation light source. The light energy was changed from
93 eV at the bottom of the image to 105 eV on the top. The averagedheight of the image was plotted in the left-side panel.

Surface Magnetism of Graphite in
Liquid 3He
H. Ishimoto
The adsorbed 3He (S=1/2) on the well defined surface isa fruitful system to study two-dimensional magnetism. In
particular 3He film adsorbed on graphite has extensively
been studied owing to the atomically flat surface and large
surface area of graphite. According to recent studies on thefilm, the layer by layer growth is confirmed up to 7 layers atleast and the first and the second atomic layer solidify on thegraphite surface, while the upper layers above the second
one exist as a liquid. The motion of 3He atoms in the solid
layer is considered to be restricted to a two dimensional
plane, since the binding energy is evaluated to be about 135
K and 65 K for the first and the second layer, respectively,
much larger compared with the temperature for measurements. Therefore, a magnetism for the first or the second
layer is understood as a consequence of an ideal two-dimensional magnet with multiple spin exchange (MSE) interactions. The exchange of even number of particles is anti-ferromagnetic (AFM), while that of an odd number is ferromagnetic (FM). The competition between them causes evolution from anti-ferromagnetism to ferromagnetism as afunction of the areal density. In addition to the intra layerinteractions, there is a long standing controversy on a possible indirect interaction mediated by 3He quasi-particle in theover-layer liquid. Theoretically various models are proposed
based on a second order perturbation theory, for example,
the quasi-localized-Fermion theory etc.. Most of the models
predict the existence of RKKY (Ruderman-Kittel-KasuyaYoshida)
type
indirectexchange
interaction.
Experimentally, the recent experiments on the adsorbed filmsuggest a rather small contribution of this type of interaction.
However on the boundary surface dipped in the bulk liquid 3He, there is no clear evidence so far. Here we present
the first systematic study with a cw NMR technique for thesolid 3He layer on graphite covered with bulk liquid 3He asa function of liquid pressure between 0.6 and 31.38 bar.
After subtracting the liquid contribution, the nuclear
magnetization corresponding to the solid layers on the
Grafoil surface shows a strong ferromagnetic tendency with

Fig. 1. Pressure dependence of the Weiss temperature for third andfourth layer.

a periodic behavior as function of liquid pressure. The firstlayer is known to be paramagnetic and the ferromagnetic
tendency in the second layer is too small to reproduce theobserved behavior. The other possibility is the solidificationof third and fourth layer with increasing the liquid pressure,analogous to the liquid pressure dependent epitaxial adsorption of solid 4He up to seven layers. The detailed analysis
gives the pressure dependence of the Weiss temperature asshown in Fig.1, indicating that the third layer is completed
at 19 bar and the fourth at 25 bar. The number of localized
spins estimated from the solid magnetization is almost doubled from 0 bar to 25 bar, being consistent with this scenario.

Rotation-induced 3D Vorticity inSuperfluid Monolayer 4He Films
Condensed in a Porous Glass
M. Kubota
Superfluid 4He films adsorbed in porous media provide a
unique possibility to study the interplay between 2D and 3D
physics [1]. Especially for the superfluid transition, the system shows similar behavior to the 2D film (the KosterlitzThouless transition [2]) such as the density proportional
super-fluid transition temperature Tc, with the energy dissipation peak around Tc [3]. Whereas the superfluid density’s
critical index is found close to 2/3 of 3D systems[1,4] and a
sharp cusp of the specific heat[5] appearing around Tc,
which is similar to the transition of the bulk 4He m transition.
A crucial role in the Kosterlitz -Thouless 2D transition[2] belongs to thermally excited vortex-antivortex pairs(VAPs). However multiple connectivity of the superfluid
film in porous media allows a variety of vortex configurations other than VAP, e.g., vortex rings and 3D corelesspore vortices [7 - 9]. For bulk liquid 4He 3D superfluid transition, a mechanism is proposed where the vortex rings playthe important role [6]. Therefore one may expect that theFig. 1. Energy dissipation peak under rotation (a), the number from 1
to 6 each corresponds to rotation speed 0, 0.79,1.57, 3.14, 4.71, 6.28
rad/sec respectively. It is realized that the data in (a) is actually thesummation of the two peaks,namely the static peak and and a rotationally induced peak,by subtracting the static peak 1 from the data
displayed in (a). Rotationally induced peak ( D Q –1 X ) is displayed in(b) together with the static peak with dashed number, each corresponding to the number in (a). Data are all for the film with the superfluid
transition temperature Tc=628mK, which is determined as in [3]. See
also Fig. 2 caption.

ISSP Activity Report 2003

21

Magnetotransport Studies under
Pulsed Ultra-High Magnetic Fields
Using RF Techniques
T. Osada

Fig. 2. The right side (high T) peak is the one appears in static condition. The group of the left peaks is rotation-induced ones for various
scaled (divided) by X . The fact that all the curves collapse more orless on the same curve proves linearity on X . In addition, superfluid
density scaled by its zero temperature value is displayed. The lagest
slope is extrapolated to zero density to Tc=628mK where the static
peak is also located[3].

response of the film on a porous substrate would be essentially different from that on a plane substrate, and observation under rotation should be quite interesting for understanding the role of these vortices on the superfluid transition.
With use of the world record rotational cryostat [10], thefirst clear evidence of rotationally induced vortex lines insuch thin film superfluid formed in a porous glass substrate
has been obtained. In ref.[11], a detailed study of torsional
oscillator experiments under steady rotation up to 6.28
rad/sec is reported for a 4He superfluid monolayer film
formed in 1 n m-pore diameter porous glass. We found a
new additional dissipation peak with the height being in proportion to the rotation speed, which is located to the lower
temperature than the vortex pair unbinding peak observed in
the static state. We propose that 3D coreless vortices (“porevortices”) appear under rotation to explain this new peak.
That is, the new peak originates from dissipation close to thepore vortex lines, where large superfluid velocity shifts thevortex pair unbinding dissipation to lower temperature. This
explanation is confirmed by observation of nonlinear effects
at high oscillation amplitude.
References[1] J. D. Reppy, J. Low Temp. Phys. 87 205 (1992) and references
therein.
[2] J. M. Kosterlitz and D. J. Thouless, J. Phys. C6, 1181 (1973);
Prog. Low Temp. Phys. B 7, 373 (1978).
[3] K. Shirahama et al., Phys. Rev. Lett. 64, 1541 (1990); Physica B
165-166, 545 (1990).
[4] N. P. Mikhin et al., Physica B 329-333, 272 (2003).
[5] S. Q. Murphy, J. D. Reppy, Physica B 165 Part 1, 547 (1990).
[6] G. A. Williams, Phys. Rev. Lett. 82, 1201 (1999); ibid 68, 2054
(1992).
[7] T. Minoguchi, Y. Nagaoka, Prog. Theor. Phys. 80, 397 (1988).
[8] J. Machta and R. A. Guyer, Phys. Rev. Lett. 60, 2054 (1988); J.
Low Temp. Phys. 74, 231 (1989).
[9] T. Obata, M. Kubota, Phys. Rev. B 66,140506 (2002).
[10] V. Kovacik et al., J. Low Temp. Phys. 101, 567 (1995).
[11] M.Fukuda, M. Zalalutdinov, V. Kovacik, T. Minoguchi, T. Obata,
M. Kubota, and E. Sonin, :cond-mat/0405092/May 2004.
Authors
M. Fukudaa, M. Zalalutdinovb, V. Kovacikc, T. Minoguchid, T. Obata,
M. Kubota, and E. Sonine
a Central Research Lab.Hitachi Ltdb Physics Dept. Cornell Univ.
c KOHZU Precision Co, Ltd.
d Inst. Physics, Univ. Tokyoe Racah Institute, Hebrew Univ. of Jerusalem.

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ISSP Activity Report 2003

(1) Fractional Quantum Hall Effect Observed in Ultra-High
Magnetic Field Range
Two dimensional (2D) electron systems has been expected to show various electronic states under ultra-high magnetic fields where the filling factor v is much smaller than 1;
fractional quantum Hall state, composite Fermion state, evendenominator state (pairing state), Wigner crystal state, Hall
insulator state, etc. To clarify the high field phase diagramof the 2D electron system, we have tried the transport measurement of 2D systems under pulsed ultra-high magnetic
fields employing the RF reflection coefficient method.
In this case, there is additional technical difficulty. The
impedance of 2D systems becomes too large (>> 1M X ) tomeasure using RF under ultra-high fields. Such high impedance is out of range of the sensitivity of RF reflection coefficient method, and it becomes easy to be affected by the stray
capacitance of the system. To avoid the above problems, wehave fabricated a Corbino-type device with modified electrode geometry as shown in the inset of Fig. 1. This device
has large electrode perimeter and short electrode distance todecrase the device impedance wherever possible.
Figure 1 shows the diagonal conductivity vxx of
GaAs/AlGaAs heterointerface with the mobility of~2.4x105 cm2/Vs measured under pulsed ultra-high field
generated by the single-turn coil method. We can see two
dip structures at field positions corresponding to v=1 andv=1/3. This is the highest field measurement which haveobserved the fractional quantum Hall effect. In spite of hightemperature T > 4.2 K, the v=1/3 structure could beobserved since the energy gap is enhanced ( D ~ 50 K) at
ultra-high magnetic fields (B ~ 80 T). The present result is
promising one for future studies on quantum Hall systems
under ultra-high magnetic fields.
(2) Carbon Substitution Effect of a Novel Superconductor
MgB2
We have investigated the carbon substitution effect on anovel two-gap superconductor MgB2. Since carbon substitution has strong influence on both of the electronic structure
and impurity scattering rate, its superconducting properties
are substantially modified. Microcantilever torque measurements revealed that the upper critical field Hc2 is greatly

Fig. 1. Diagonal conductivity vxx of two-dimensional electron gas in
the ultra-high magnetic field range. Inset: modified sample geometry.

Fig. 2: Surface impedance of Mg(B1-xCx)2 measured by the tunnel
diode oscillator method.

enhanced by a factor of two ( n 0Hc2 (0) > 33 T) by 3-5%
carbon substitution, though Tc monotonously decreases withcarbon content. The enhancement is explained by a reduction in the inplane coherence length, while the Tc reductionis due to carrier doping. We also observed the temperaturedependent Hc2-anisotropy at all carbon contents. This factstrongly suggests that the two-gap superconductivity is lessinfluenced by a small amount of carbon substitution, in contrast to theoretical predictions.
We have also studied the carbon substitution effect on
the vortex phase diagram of MgB2. Since carbon-substituted
MgB2 single crystals are very small (200-500 n m), we havedeveloped a high-sensitive RF technique with the use of atunnel diode oscillator (Fig. 2), from which the effective
penetration depth m eff is obtained. We demonstrate a systematic change of the peak effect with the carbon content.
This result is accounted for by an impurity effect of carbon
atom on the vortex order-disorder transition.

using a cubic-anvil or diamond-anvil pressure cell, and themeasurements were limited on the macroscopic level suchas resistivity or AC susceptibility measurement. A clamptype pressure cell is much more convenient and available tovarious methods in which a larger sample volume is needed.
However, usual clamp-type pressure cell is made of CuBe
alloy and the maximum pressure is 3 GPa at most. Hence,the superconductivity has not been studied on the microscopic level for 7 years after the discovery of the superconductivity in 1996. We have improved clamp-type pressure
cell using NiCrAl alloy, and succeeded in the microscopic
study for the first time using 63Cu nuclear magnetic resonance (NMR) [1]. This enabled us to study the pairing symmetry as well as the relation between the spin gap and thesuperconductivity.
The appearance of the superconductivity was confirmedby measuring resonance frequency of a NMR probe attachedto the pressure cell. The resonance frequency is given as
f " 1/ (LC) where L and C represent inductance and variable capacitance of the NMR probe, respectively. The sample is contained in a coil and the onset of superconductivity
is detected by the change of the resonant frequency, i.e. thechange of L value. This method corresponds to an AC susceptibility measurement. Tc at 3.5 GPa is obtained to be
4.7 K which is consistent with that obtained from the resistivity measurement. The T dependence of f at several fields
is shown in the inset of Fig. 1. If we probe the temperature
at which the resonance frequency starts to change against H,
this gives Hc2 vs. Tc characteristics as is shown in the main
panel of Fig. 1. The conventional Pauli limit is calculated as
8.65 T by using the value of Tc at zero field. It should beremarked that the superconductivity is quite stable evenunder high field close to Pauli limit.
The NMR measurement at 3.5 GPa was performed byapplying the magnetic field H parallel to the leg direction,
crystal c axis to avoid influence of vortex motion. As shown

Authors
T. Osada, E. Ohmichi, S. Ikeda, T. Inokuchi, K. Kobayashi, T.
Sekitani, M. Kobayakawa, A. Ogasawara, E. Komatsu, and K. Kajiki

Observation of Spin Gap andSuperconducting Gap in a Spin LadderCuprates Sr2Ca12Cu24O41: NMR Study under High Pressure of
3.5GPaY. Uwatoko
Sr14-xCaxCu24O41 (x=11.5-13.5) has been attracted anumber of researchers because the system is the only spinladder compound which exhibits superconductivity.
However, the superconductivity is realized only under highpressure above 3 GPa. This fact has been main hurdle toinvestigate the superconducting state, although investigation
with using a variety of experimental methods is required not
only for determination of the pairing symmetry but also forsolution of spin-gap problem as is the case of high-Tc
cuprates. High pressure above 3 GPa is usually attained by

Fig. 1. Hc2 –Tc characteristics obtained from resonance frequency (f)of a NMR probe. Conventional Hc-Tc curve is shown as a red broken
curve. Pauli limit is calculated to be 8.65 T from the value of Tc atzero field. The inset shows raw data of the resonance frequency. The
superconducting state is observed from the onset of f.

ISSP Activity Report 200323

around Tc as is shown in the precise data at low T. If theorbital part does not change drastically by applying pressure,
Korb is estimated to be 0.08% from the data at ambient pressure. Then, Kpara is estimated to be 0.17%.
In the present work, we found that superconductivity
with a full gap is realized and is quite stable even at highfield close to Pauli limit. The fact excludes the possibility
that conventional s-wave superconductivity is realized inthis system. As for existence of a full gap, both d-wave andp-wave cases are possible if no nodes across the Fermi surface. However, in the case of d-wave superconductivity twoquestions should be resolved, namely one is why the NMR
shift remains unchanged below Tc, and the other is why singlet pairing occurs again at low temperatures below Tc
although spin-singlet pairing is already formed at high temperatures. As far as NMR results is concerned, p-wave witha full gap may be preferred.
The present work was partially supported by Grant-inAid for the Ministry of Education, Science and Culture,
Japan, and by grants of Ogasawara Foundation for the
Promotion of Science and Engineering, Japan and Simadzu
Science Foundation, Japan.
Reference[1] N. Fujiwara, et. al., Phys. Rev. Lett. 90 (2003) 137001.
Authors
N. Fujiwara, N. Môri, Y. Uwatoko, T. Matsumoto, N. Motoyama, andS. Uchida
Fig. 2. The main panel shows relaxation rate (T1-1) of 63Cu nuclei forthe ladder site. Spin gap and superconducting gap are observed as anactivated behavior and a hump at high T above 50 K and low T
below 4K, respectively. The inset shows 63Cu-NMR shift for the ladder site. Red points represent data taken precisely in a narrow T range
between 1.4 K and 4 K, whereas Green points represent data taken ina wide T range. The blue curve which fit the green points represent
theoretical curve for the insulating spin-ladder system.

Developments of Micro Pressure Celland Specific Heat of Ce2RhIn8
I. Umehara and Y. Uwatoko

in the main panel of Fig. 2, T1-1 for the ladder site was measured at 70.0 MHz which corresponds to 6.2 T. (Tc at thisfield is about 2.8 K as is seen from Fig. 1. ) The activated T
dependence is observed at temperatures higher than 50 K,
i.e., T1-1 " exp ( - Dspin /T). The value of the gap Dspin is
estimated to be 173 K. It should be noted that the spin-activation behavior is seen in the state in which the chargetransport is metallic. By contrast, below 50 K T1-1 is dominated by a Korringa-type T-linear term followed by a peakdeveloped just below Tc. The appearance of the peak implies
that a finite gap exists in the quasi-particle excitation at all
wave vectors, namely no nodes across the Fermi surface.
The peak is still observed even at high field close to thePauli limit, which is not expected in conventional spin-singlet superconductors.
The inset of Fig. 2 shows T dependence of the NMR
shift. The shift is given as a sum of two components, theorbital and the spin parts (K=Korb + Kspin), and the spin partis proportional to the spin susceptibility. The T dependenceof K at high temperatures fits well the theoretical curve for aspin ladder system, K = K0 + K1 / T exp(- Dspin /T). The gap
Dspin is obtained to be 217K and is comparable with that
estimated from T1-1. The value of K0 is estimated to be
0.25%. K0 is expressed as K0 = Korb + Kpara where Korb
represents the orbital part and paramagnetic contribution
corresponding to the Korringa term in T1-1, , respectively.
Below Tc ( =2.8 K at 6.2 T ) the paramagnetic contribution
should diminish toward T = 0, if spin-singlet superconductivity is realized. However, no appreciable change is seen

24

ISSP Activity Report 2003

Strong correlated electron system under high pressureshows the variety of physical properties. Especially, specificheat measurement under pressure is the powerful tool tounderstand the ground state properties of strong correlated
electron system under high pressure. We have developed themicro pressure cell for the measurements of specific heat,
which can be also measured the magnetization in the commercial SQUID magnetometer. In this paper we report about
design of our micro pressure cell and present the data of specific heat of Ce2RhIn8 under pressure.
Figure 1 shows the schematic drawing of micro cell,which we designed. An important change from previous one[1] is using a Cu sample cell to consider about pressure
dependence of heat capacity in Teflon sample cell. Using
this type of sample cell, it can be measured free from subtracting such a pressure dependence of heat capacity. The
dimensions of the cylinder were 8.8, 2.7 and 21mm in outer
diameter, inner diameter and length, respectively. As a piston and piston backups, we used ZrO2. The mixture of
Fluorinert FC70:FC77=1:1 was used as a pressure transmitting media. It is estimated the pressures by the change ofsuperconducting transition temperatures of Sn with the commercial SQUID magnetometer.
The total heat capacities are measured, which includes
several parts of micro cell, pressure transmitting media andsample as a function of temperature below 4 K under thehigh pressure for single crystalline Ce2RhIn8. Ce2RhIn8 iswell known Kondo material, which shows superconductivity

Authors
Izuru Umeharaa, Fumiaki Tomiokaa, Tetsuya Fujiwara, Masato Hedo,
and Yoshiya Uwatoko
aDibision of Physics, Faculty of Engineering, Yokohama National
University, Yokohama 240-8501

Fig. 1. Schematic drawing of new micro pressure cellunder high pressure. [2] The applied pressures are 0, 0.28,
0.43, and 0.57 GPa, respectively. In the case of Ce2RhIn8
measurements, the weight of sample is only about 40 mg.
This value is less than 0.5% comparing to that of pressure
cell. It is subtracted heat capacities of micro cell and pressure transmitting media from total heat capacity in order toobtain net heat capacities of sample under pressure. For thissubtraction, we have already measured the heat capacities ofpressure transmitting media under various pressures.
Figure 2 shows the specific heat of Ce2RhIn8 under pressures. It is clearly seen Néel temperature (TN) of 2.8 K atambient pressure. TN decreases monotonically increasingpressure. Néel temperatures are estimated to be 2.5 K at 0.28
GPa, 2.2 K at 0.43 GPa, respectively. At 0.57 GPa, we can
not observe any transition above 1.7 K. Magnetic contribution of specific heat around TN is also decreases increasingpressure, which means the magnetic entropy decreases
under high pressures. Estimated magnetic entropy are 1.7
J/K mol-Ce at 0 GPa, 1.3 J/K mol-Ce at 0.28 GPa and 1.2
J/K mol-Ce at 0.43 GPa, respectively.
We are now expanding these measurements to lower
temperatures using 3He cryostat and higher pressures toobserve superconductivity of Ce2RhIn8.
References[1] I. Umehara et al. to be published.
[2]M. Nicklas et al., Phys. Rev. B. 67 (2003) 020506.

Fig. 2. The specific heat for Ce2RhIn8 as a function of temperature
under the various pressures.

ISSP Activity Report 2003

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