BL7U

Nodeless Superconducting Gap in AxFe2Se2 (A=K, Cs) Revealed byAngle-Resolved Photoemission Spectroscopy㩷

Y. Zhang1, L. X. Yang1, M. Xu1, Z. R. Ye1, F. Chen1, C. He1, H. C. Xu1, J. Jiang1, B. P. Xie1,
J. J. Ying2, X. F.Wang2, X. H. Chen2, J. P. Hu3, M. Matsunami4, S. Kimura4 and D. L. Feng1
1
State Key Laboratory of Surface Physics, Advanced Materials Laboratory, and Departmentof Physics, Fudan University, Shanghai 200433, China
2
Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics,
University of Science and Technology of China, Hefei, Anhui 230026, China
3
Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA
4
UVSOR Facility, Institute for Molecular Science and The Graduate University for Advanced
Studies, Okazaki 444-8585, Japaniron-based superconductor could mainly rely on theelectron Fermi surfaces near M. Thus, the sign change
in the s± pairing symmetry driven by the inter-band
scattering as suggested in many weak coupling
theories becomes conceptually irrelevant in describing
the superconducting state here. A more conventionals-wave pairing is probably a better description.

Pairing symmetry is a fundamental property thatcharacterizes a superconductor. For the iron-based
high-temperature superconductors, an s±-wave
pairingsymmetryhasreceived
increasing
experimental and theoretical support. Morespecifically, the superconducting order parameter is an
isotropic s-wave type around certain Fermi surface,but it has opposite signs between the hole Fermi
surfaces at the zone center and the electron Fermi
surfaces at the zone corners.
Recently,
a newseries ofiron-based
superconductors, AxFe2Se2 (A=K, Cs), has been
discovered with relatively high transition temperatureof ~30 K [1]. We have conducted angle-resolve
photoemission spectroscopy (ARPES) experiment on
AxFe2Se2 (A=K, Cs) at beamline BL7U[2].
Figure 1 (a) reveal the photo emission intensity
map along out-of-plane momentum (kz) in the *ZAM
plane. The cross-sections of the N and GFermi surfaces
clearly show weak dispersion along the kz direction,
indicative of a rather two-dimensional electronic
structure. The spectral weight of N increases from* toZ, and a small electron pocket could be clearlyobserved for the Nband around Z [Fig. 1 (b)]. This
gives an electron pocket around Z with its residual
spectral weight extending towards * However, weemphasize that the size of the N pocket is much smaller
than that of the G pocket, which is rather
kz-independent as illustrated in Fig. 1 (c). The
experimental Fermi surface topology clearly showsthat there is no hole Fermi surface near the zone
centre, and A0.8Fe2Se2 is indeed the most heavily
electron-doped iron-based superconductor by far.
The momentum distribution of superconducting gap
is deduced in Fig. 2. The gap of the band around the M
point is of the isotropic s-wave type within theexperimental uncertainty, which averagely is about
10.3 meV. Further data taken with different photon
energies in Fig. 2 (b) indicate such a gap does not varywith the out-of-plane momentum kz. Smaller gap
about 7 meV could be observed for the N band withlittle kz-dependence.
Our data show that the rather robust
superconductivity in such a highly electron-doped

Fig.1. The Fermi surface and band structure as afunction of kz for K0.8Fe2Se2.

Fig.
2.
Momentumdependence
superconducting gap of K0.8Fe2Se2.

of

the

[1] J. Guo et al., Phys. Rev. B 82 (2010) 180520.
[2] Y. Zhang et al., Nature Materials, 10 (2011) 273.

75

BL1A

Characterization of Magnesium in Huntite by an XAFS Method㩷
T. Kurisaki1, D. Tanaka1, S. Kokubu1 and H. Wakita1,2㪈Department of Chemistry, Faculty of Science, Fukuoka University,
Fukuoka 814-0180, Japan
2
Advanced Materials Institute, Fukuoka UniversityFukuoka 814-0180, Japan

HuntiteDolomite

Normalized Intensity / a.u.

A huntite was called ‘a hanto-ishi’ and was used asa pigment of the white in ancient Egypt. This mineralchemical formula is expressed in CaMg3(CO3)2. The
huntite has very few reports about locality, and it is arare mineral. However, this mineral is usedabundantly in ancient Egypt. On the other hand, thedolomite is a general carbonate that has a similar
chemical composition to the huntite. But, thedolomite was not used as a white pigment It is notapparent why the huntite which is a rare mineral wasused as a pigment of the white. It is reported that thehuntite is produced in Nagasaki. The X-ray
absorption spectroscopy is frequently used for stericand electronic structure analyses because it gives
good information on the local structure and oxidation
state of the absorbing atom [1]. We performed
electronic structure analysis of calcium in the huntite
using the XAFS method in last year.
In this work, we performed the XANES spectra
measurement about various magnesium salts andminerals such as huntite and dolomite. The obtained
experimental XANES spectra are analyzed using the
calculated theoretical spectra from DV-XĮ
calculations. The X-ray absorption spectra were
measured at BL1A of the UVSOR in the Institute of
Molecular Science, Okazaki [2]. The ring energy of
the UVSOR storage ring was 750 MeV. Mg K-edge
absorption spectra were recorded in the regions of1275-1380 eV by use of two Beryl crystals. The
absorption was monitored by the total electron yieldusing a photomultiplier. The samples were spread
onto the carbon tape on the first photodynode made of
CuBe of the photomultiplier.
Figure 1 shows the observed Mg K-edge XANES
spectra for the Huntite and Dolomite. The Mg-K
XANES spectra of Huntite and Dolomite show
different peak profiles at second peaks. This resultshows that the magnesium in these minerals has
different electronic states. We are going to try to
calculate the spectra by DV-XĮ molecular orbital
calculations. The calculated XANES spectra and theobserved XANES spectra of the Huntite and theDolomite are shown in Fig. 2.
The observed XANES spectra and the calculated
XANES spectra showed good agreement. The peak A
is estimated to the electron transition (mainly Mg 1s tounoccupied mixed orbital consisting of O 3d, O 3p, O
3s and O 4s). The obtained XANES spectra are
analyzed using the calculated spectra from DV-XĮ
calculations.

1300

1350
Energy / eV

Fig. 1. Observed Mg K-edge XANES spectra of
Huntite and Dolomite.

HuntiteCalc.

A

Dolomite
Calc.

Intensity / a.u.

Intensity / a.u.

A

1300

1350
Energy / eV

1300

1350
Energy / eV

Fig. 2. The comparison of observed XANES spectrawith calculated XANES spectra for Mg K-edge.

[1] T. Kurisaki, S. Matsuo, I. Toth and H. Wakita, Anal.
Sci. 24 (2008) 1385.
[2] S. Murata, T. Matsukawa, S. Naoè, T. Horigome,
O. Matsuodo and M. Watatabe, Rev. Sci. Instrum. 63(1992) 1309.

76

BL1B

Temperature Dependence of Polarized Fundamental Absorption-Edge
Spectra of PbMoO4

M. Fujita1 and M. Itoh2
1
Japan Coast Guard Academy, Kure 737-8512, Japan
2
Dept. Electrical and Electronic Engineering, Shinshu University, Nagano 380-8553, Japan
than the critical value ıc = 1.50. The present value of
ı0 is considerably smaller than this critical value,indicating that the exciton-phonon coupling is strong
in PbMoO4. In fact, the intrinsic luminescence due toself-trapped excitons in PbMoO4 is observed with apeak at around 2.3 eV when excited with light of thephoton energies above 3.3 eV. The large Stokes shift (a1.0 eV) of the intrinsic luminescence band is insatisfactorily agreement with the above conclusion on
the strong exciton-phonon coupling in PbMoO4.
(a) E//aí1

ABSORPTION COEFFICIENT (cm )Lead molybdate (PbMoO4) is well known as asuperior medium for acousto-optic devices. This
material also attracts attention because of its great
potential to be used as an effective cryogenic detector
for double ȕ decay experiments. The crystal has thescheelite structure with the optical axis along thec-axis. Recently we found remarkable dichroism of
the lowest exciton band at 3.6 eV in PbMoO4 [1]. In
the present study, we have measured the temperaturedependence of the fundamental absorption-edge
spectra of PbMoO4 crystals with use of polarized light.
PbMoO4 samples were obtained from Furukawa
Company. They were grown by the Czochralski
technique from the raw materials of PbO of 99.99%
purity and MoO3 of 99.9% purity in 1:1 molar ratio,
and was purified by three-time crystallization. The
thickness of the sample was 2.0 mm.
The absorption spectra for the polarization parallelto the a-axis (E//a) and c-axis (E//c) measured atvarious temperatures are shown in Figs. 1 (a) and (b),respectively. Distinct dichroism is observed at eachtemperature. For E//a, a weak absorption band isobserved as a shoulder superimposed on the tail above
150 K. The band shows significant red shift andbecomes remarkable with increasing temperature.
Such structure is not observed for E//c. It should be
noted that the exciton absorption band [1] and theshoulder structure in Fig. 1(a) are both observed
distinctly for E//a, while they are extremely weak or
not discernible for E//c. This correspondence suggeststhat the shoulder structure is attributed to the exciton
absorption perturbed by some lattice defects [2].
In contrast to the case of E//a, no additional
structure is observed in the tail region for E//c attemperatures between 6 and 300 K. In Fig. 2 areplotted the absorption coefficients for E//c in alogarithmic scale. The straight lines fitted to the linear
portions of the experimental curves for T t 100 K
converge to a point. This fact indicates that the Urbach
rule, Į = Į0exp{–ı(E0–E)/kBT}, holds for PbMoO4 asin the case of PbWO4 [3]. The parameters of theconversing point are obtained as Į0 = 1.5×104 cm–1
and E0 = 3.52 eV. The steepness parameters ı at T t
100 K are plotted by open circles in the inset in Fig. 2.
The solid curve in the inset shows the best fit of theequation, ı(T) = ı0(2kBT/ʄZ)tanh(ʄZ/2kBT) to thedata points with the high-temperature steepness
parameter ı0 = 0.67 and average photon energy ʄZ =26 meV.
An exciton in three-dimensional lattices becomes
self-trapped and gives rise to a luminescence bandwith large Stokes shift, when the value of ı0 is smaller

20100(b) E//c
20

300 K
250 K
200 K
150 K
100 K
50 K
6K

10
0

3

3.2
3.4
PHOTON ENERGY (eV)

10

5

10

4

10

3

10

2

10

1

10

0

10

ǻ

í1

ABSORPTION COEFFICIENT (cm )Fig. 1. Absorption-edge spectra of PbMoO4 at various
temperatures for (a) E//a and (b) E//c.

0.8
0.6
0.4
0.2
0
0
100 200 300
TEMPERATURE (K)

í1

3

300 K
250 K
200 K
150 K
100 K
50 K
6K

3.2
3.4
PHOTON ENERGY (eV)Fig. 2. Logarithmic plot of the absorption spectra for
E//c.
In the inset is shown the temperaturedependence of ı.
[1] M. Fujita et al., Phys. Status Solidi B 247 (2010)405.
[2] M. Fujita and M. Itoh, Phys. Status Solidi B 247(2010) 2240.
[3] M. Itoh et al., Phys. Status Solidi B 231 (2002)
595.

77

BL1B

Luminescence Properties of KCl:Ag- Crystals
under the Excitation around the Fundamental Absorption Edge

T. Kawai1 and T. Hirai2
Graduate School of Science, Osaka Prefecture University, Sakai 599-8531, Japan
2
Faculty of Science and Engineering, Ritsumeikan University, Kusatsu 525-8577, Japan1

6.70 and 7.60 eV. The excitation spectrum for the BG
luminescence band in KCl:I also has the three
responses in the same energy region [1-3]. The
similarity between them leads us to the idea that the2.60 eV luminescence band in KCl:Ag- has it’s origin
in the two-center type localized exciton such as [(Ag Cl)- + e-].

In KCl crystals containing iodine ions as hetero
halogen ions, the absorption bands due to the I- ionsare observed around the fundamental absorption edgeof the KCl crystals. The excitation on theseabsorption bands induces the several luminescencebands due to the localized exciton [1-3]. Among theseluminescence bands, the luminescence bands
observed in the visible energy region are called the
BG and B bands and are attributed to the radiative
decay from the relaxed exciton state which consists ofa hole localized on a pair of an iodine and chlorine
anion and a bound electron, [(Cl I)- + e-].
In this study, we have investigated luminescence
properties of KCl crystals doped with Ag- centers
under the excitation around the fundamental
absorption edge. Since the Ag- anions doped in alkali
halide crystals are substituted for the halogen ions,
the Ag- anions would be regarded as pseudo halogen
ions. Therefore, the localized excitons related with
the Ag- ions would be existed in the same way as the
localized excitons in KCl:I crystals.
Figure 1 shows the luminescence spectra of KCl:
Ag- crystals at 10 K, which are measured at BL1B of
the UVSOR facility. Under the excitation on thevarious energies between 6.42 and 7.21 eV, theluminescence band peaking at 2.91 eV is observed.
The 2.91 eV luminescence band is attributed to theradiative transition from the relaxed excited state of3
T1u in the Ag- center and is called the A' luminescenceband[4, 5]. The excitation on 8.26 eV above theintrinsic exciton band of KCl crystals brings about abroad luminescence band at 2.30 eV, which is ascribedto the intrinsic STE in KCl single crystals [6]. It should
be noted that a broad luminescence band having ahalfwidth of 0.52 eV is observed at 2.60 eV under theexcitation on the energy region between 6.66 and 7.21
eV, in addition to the A' luminescence band. The
excitation spectra for the 2.60 eV and STE
luminescence bands are shown in the insert of Fig.1.
The excitation spectrum for the STE luminescenceband exhibits the sharp response at 7.65 and 8.27 eV
and is consistent with that reported previously[6]. On
the other hand, the excitation spectrum for the 2.60 eV
luminescence band exhibits the sharp responses at
6.70, 7.14, and 7.59 eV.
The 2.60 eV luminescence band in KCl:Ag- is very
similar to the BG luminescence band in KCl:I in theenergy position and lineshape [1-3]. The excitation
spectrum for the 2.60 eV luminescence band exhibits
three sharp responses in the energy region between

700

Wavelength (nm)
500
400

600

Luminescence Intensity (arb. units)

200
180
Excitation

160
KCl:Ag
10 K

A' band

KCl:Ag 10 K

for STEfor 2.60 eV
6.0

7.0

8.0

STE

Exc. 8.26 eV
Exc. 7.21 eV

2.0

2.5
3.0
Photon Energy (eV)Exc. 6.81 eV
Exc. 6.66 eV
Exc. 6.42 eV
3.5

Fig. 1. Luminescence spectra of KCl:Ag- crystals
under the excitation around the fundamental
absorption edge. Inset is excitation spectra for the
STE and 2.60 eV luminescence bands.
[1] K. Kan'no, M. Itoh and Y. Nakai, J. Phys. Soc. Jpn.
47 (1979) 915.
[2] T. Higashimura, H. Nakatani, M. Itoh, K. Kan'no
and Y. Nakai, J. Phys. Soc. Jpn. 53 (1984) 1878.
[3] A. Ohno and N. Ohno, Phys. Status Solidi C 8(2011) 112.
[4] K. Kojima, S. Shimanuki, M. Maki and T. Kojima,
J. Phys. Soc. Jpn. 28 (1970) 1227.
[5] W. Kleemann, Z. Physik 249 (1971) 145.
[6] K. Kan'no, T. Matsumoto and Y. Kayanuma,
Pure & Appl. Chem. 69 (1997) 1227.

78

BL1B

Optical Study in the Vacuum-Ultraviolet Energy Region of BaFe2As2

M. Nakajima1,2,3, K. Kihou2,3, C. H. Lee2,3, A. Iyo2,3, H. Eisaki2,3 and S. Uchida1,3
1
Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
2
National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8568, Japan
3
JST, Transformative Research-Project on Iron Pnictides (TRIP), Tokyo 102-0075, Japan
Introduction
The parent compounds of iron-arsenide
superconductors, with BaFe2As2 as a representative
example, are unique metals which exhibit a structuralphase transition accompanied or followed by amagnetic phase transition. The magnetostructuralstate has attracted much attention as a proximate
phase to a superconducting phase. To elucidate themechanism of superconductivity in iron-based
compounds, it is important to investigate theelectronic properties in the normal state as well as
those in the superconducting state.
Optical spectroscopy is one of the most useful
methods for investigating the electronic dynamics insolids. If reflectivity is obtained for a wide energy
range, optical conductivity can be derived through the
Kramers-Kronig (K-K) transformation of thereflectivity spectrum. In this work, we measured
reflectivity in the vacuum-ultraviolet (VUV) regionof BaFe2As2. Reflectivity measurement in this energy
region is necessary to obtain reliable opticalconductivity for multi-band/orbital systems such asiron-pnictides or cuprates [1], since a number of
intraband and interband excitations overlap in thesame energy region and extend up to 10 eV or higher.

0.6
BaFe2As2

Reflectivity

0.5
0.4
0.3
0.2
0.1
0
0

10

20
Energy (eV)30

Fig. 1. Reflectivity spectrum of BaFe2As2 measured
at room temperature in the energy range up to 32 eV.

BaFe2As2-1

Conductivity (: cm )

4000
3000

-1

Experimental
Single crystals of BaFe2As2 were synthesized by aself-flux method [2]. The in-plane reflectivity
spectrum in the VUV region between 4 and 35 eV
was measured at room temperature using a
Seya-Namioka-type grating at BL1B of UVSOR
facility. The optical conductivity spectrum was
obtained via K-K transformation of the measured
reflectivity spectrum connected to the lower-energy
one.

2000
1000
0
0.01

measured value
const.
-2

Z-4
Z

0.1
Energy (eV)Fig. 2. Kramers-Kronig
conductivity spectra for
extrapolations.

Results and Discussion
The in-plane reflectivity spectrum up to 32 eV isshown in Fig. 1. The reflectivity shows goodcoincidence with the result measured in the laboratory.
Three distinct reflectivity edge appear at ~ 1, ~ 10,and ~ 25 eV. Each edge indicates the end point of aseries of intraband or interband excitations.
Figure 2 shows the conductivity spectra obtainedby various extrapolations above 4.5 eV. If we use
conventional extrapolation schemes for the K-K
transformation, the obtained conductivity appreciably
deviates from the present result, particularly in theregion higher than 0.3 eV. Thus, the measurement in
the VUV region is essential for quantitative analysis.

1transformed opticalvarious high-energy[1] S. Tajima et al., J. Opt. Soc. Am. B 6 (1989) 475.
[2] M. Nakajima et al., Phys. Rev. B 81 (2010)
104528.

79

BL1B

Identification of Cr3+ Impurity Contained in YAlO3 by Photoluminescence
K. Nakahara and Y. Ohki
Dept. Electrical Engineering and Bioscience, Waseda University, Tokyo 169-8555, Japan

Introduction
Yttrium aluminate (YAlO3) has been attracting
much attention as a promising candidate for a gateinsulator in advanced metal-oxide-semiconductor
devices [1]. By analyzing photoluminescence (PL)properties of YAlO3, we have been examining the
localized states present in the band gap that are
assumed to cause leakage current.
PL Intensity [a.u.]Single Crystal

Experimental
The samples examined are YAlO3 (100) single
crystals grown by the Czochralski method and YAlO3
thin films prepared by a spin-coating method. The
films were annealed in oxygen at designatedtemperatures between 600 and 1000 oC. Using
synchrotron radiation under multibunch operation at
the BL1B line of UVSOR Facility as a photon source,
PL spectra were measured at 10 K. Crystallization ofthin films were confirmed by in-plane X-ray
diffraction (XRD) measurements.

1000 ͠
900 ͠
800 ͠
700 ͠

Results and Discussion
FigWTG 1 shows PL spectra induced by 6.4 eV
photons, obtained at 10 K for the crystal and the films
deposited and annealed on Si substrates. The crystal
sample has PL peaks at 1.65 and 1.70 eV. On theother hand, the two PL peaks are similar to those of
R-lines of Cr3+ ions in Cr-doped YAlO3 [2]. The films
annealed at 600, 700 and 800 ºC have no PL peaks,
while those annealed at 900 and 1000 ºC have PL
peaks at 1.70, 1.75 and 1.79 eV. Similar differences inpeak energy between the crystal and the deposited
film are also seen in LaAlO3 thin films deposited on
different substrates [3], which is possibly due to thelattice distortion.
The XRD patterns shown in Fig. 2 indicate that thefilms annealed at 900 °C or higher are polycrystalline,
while those annealed at 600, 700 and 800 °C areamorphous. As indicated by the broken lines, the twolarge peaks almost agree with those that should be
assigned to YAlO3, and subtle differences would be
due to lattice distortion. However, the presence ofother small peaks suggests that Y3Al5O12 and Y4Al2O9
are included in the films.
These results indicate that the PL is caused by the
crystallization
and
that
the
corresponding
spin-forbidden transition in Cr3+ becomes possible
owing to spin-orbit mixing induced by the crystal
field [3]. Therefore, there is a possibility that thepresent YAlO3 samples contain Cr3+ ions as animpurity. Detection of Cr3+ ions by inductively
coupled plasma atomic emission spectrometry(ICP-AES) failed, indicating that the Cr3+ ion content
in the present samples is below 1 ppm.

600 ͠

1.6

1.7
1.8
Detected Photon Energy [eV]1.9

(2 0 2)(1 3 1)

Fig. 1. PL spectra induced by 6.4 eV photons in
YAlO3.

Intensity [a.u.]

1000 ͠
900 ͠
800 ͠
700 ͠
600 ͠
40

50

60

70

2ǰ [deg]
Fig. 2. In-plane XRD patterns of YAlO3 thin films.
[1] S. A. Shevlin et al., Phys. Rev. Lett. 94 (2005)
146401.
[2] M. Yamaga et al., J. Phys.: Condens. Matter 5(1993) 8097.
[3] E. Hirata et al., Jpn. J. Appl. Phys. 49 (2010)
091102.

80

BL1B

Relaxed Exciton Luminescence of KCl Heavily Doped with KI

A. Ohno and N. Ohno
Graduate School of Engineering, Osaka Electro-Communication University, Neyagawa,
Osaka 572-8530, Japan
Extensive studies have been made on luminescencearising from localized relaxed excitons in KCl
containing iodine impurities. Stimulation in iodine
absorption bands of dilute I–-doped KCl (KCl:I)
produces two characteristic luminescence bands
under excitation with UV light or with X-rays at low
temperatures. These luminescence bands have been
well explained in terms of localized relaxed excitons
at ICl– molecule (I– monomer) and a complex of two
iodine ions (I– dimer), respectively [1-3]. However,
optical and luminescence properties of heavily doped
KCl:I crystals have not been examined sufficiently
for applying to highly efficient scintillation material.
Single crystals of KCl:I were grown by the
Bridgeman method from reagent grade KCl addedwith an appropriate amount of KI. The obtained
crystals were transparent up to 5 mol %. The opticalmeasurements were made at 6 K.
The luminescence spectra excited with photons
near the exciton absorption region of KCl were
almost the same as those of the previous studies; twoluminescence bands peaking at 2.64 eV (monomer
emission) and 4.64 eV (dimer emission) at 6 K.
Photoexcitation spectra detected at 4.64 eV for various
KI mol % of KCl:I crystals are shown in Fig. 1. The
each spectrum has been normalized at unity at the
maximum. As can be seen clearly, the excitation peaks
at 6.46 and 7.07 eV for the 0.01 mol % crystal movetoward the lower energy side with increasing KI
concentration. The low energy shift of these excitation
bands suggests that there coexist dimers, trimers andlager sizes of I – ions (KI cluster) in heavily doped
KCl:I crystal [1].
The luminescence energies of the trimers andclusters of I – ions in heavily doped KCl:I crystals are
expected to be different from that of I– dimers. We
have examined the photoluminescence spectra excited
at various photon energies. The results for 3 mol %crystal are shown in Fig. 2. It is clearly confirmed thatthe peak energy of the luminescence band moves from4.62 eV excited at 6.46 eV to 4.32 eV at 5.90 eV.
These luminescence bands are supposed to be thecomposite bands due to radiative annihilation of therelaxed exciton at dimers, trimers and clusters of I –ions.
In KCl:I crystals containing KI above 1 mol %,there exists a small amount of dimer centers ascompared with monomers, and moreover the amountof trimers and clusters of KI is supposed to be
extremely smaller than those of monomers and dimers.
However, the luminescence intensities of suchlarge-sized centers are found to be comparable with

Intensity [arb. units]that of the dimer emission. This fact suggests that theexcitons trapped in KI clusters in heavily doped KCl:I
crystals would give the high luminescence efficiency.
The similar mechanism has been reported in CsI:Na
system [4], where the observed luminescenceenhancement originates from self-trapped excitons in
NaI nanoparticles in host crystals.

1

KCl:I 6 K
4.64eV

0.5

5mol%
3mol%
1mol%
0.01mol%

0678Photon Energy [eV]Fig. 1. Photoexcitation spectra of KCl:I crystals
detected at 4.64 eV for various KI mol % at 6 K.

Intensity [arb. units]

1.5

KCl:I 6 K
3mol%

1

6.46eV
6.36eV
6.20eV
6.05eV
5.90eV

0.5

0
4

4.2

4.4
4.6
4.8
Photon Energy [eV]5

Fig. 2. Photoluminescence spectra of KCl:I (3 mol %)excited at various photon energies at 6 K.
[1] N. Nagasawa, J. Phys. Soc. Jpn. 27 (1969) 1535.
[2] K. Toyoda, K. Nakamura and Y. Nakai, J. Phys.
Soc. Jpn. 39 (1975) 994.
[3] K. Kan’no, M. Itoh and Y. Nakai, J. Phys. Soc.
Jpn. 47 (1979) 915.
[4] M. Nakayama, N. Ando, T. Miyoshi, J. Hirai and
H. Nishimura, Jpn. J. Appl. Phys. 41 (2002) L263.

81

BL1B

Optical Study of the Current-Induced Mott Transition in Ca2RuO4
R. Okazaki1, Y. Nishina1, Y. Yasui1, I. Terasaki1, F. Nakamura2, Y. Kimura2,
M. Sakaki2 and T. Suzuki2
1
Department of Physics, Nagoya University, Nagoya 464-8602, Japan
2
ADSM, Hiroshima University, Higashi-Hiroshima 739-8530, JapanThe Mott insulating state, which is driven by the
Coulomb interaction U, has attracted increasing
research interests since it manifests a significance ofmany-body effects in condensed matter physics.
Various exotic electronic states emerge in the vicinityof the Mott insulating phase as seen in high-Tc
cuprates and organic conductors.
The 4d-electron transition metal oxide Ca2RuO4
provides various interesting phenomena around theMott insulating phase. Systematic isovalent Sr-doping
study revealed that the ground state of Ca2-xSrxRuO4changes from the antiferromagnetic Mott insulator(x = 0) to the spin-triplet superconductor (x = 2) [1].
Pressure study also indicates the wide variety ofelectronic ground states [2].
Very recently, Nakamura et al. reported theelectric-field-induced Mott transition in Ca2RuO4 byapplying relatively low electric field of ~ 50 V/cm atroom temperature [3, 4]. Furthermore the current-induced metallic state persists down to T = 4.2 K and
the ferromagnetic (FM) order emerges below T ~ 15
K, which is reminiscent of the pressure-induced FM
state [2]. The origin of this current-induced electronic
state is still an open question.
To shed further light on a nature of thisnonequilibrium electronic state, we have investigatedthe optical property of the Mott insulating phase in
Ca2RuO4 under external electrical currents. We used
the beam line BL1B in UVSOR facility to measure
the reflectivity spectra of Ca2RuO4 in an energy

region of 2 – 30 eV, which enables us to evaluate theoptical conductivity through the Kramers-Kronig(KK) analysis. The reflectivity spectra of Ca2RuO4
single crystals measured at room temperature withdifferent currents are shown in Fig. 1. We find anincrease of the reflectivity by the currents in
low-energy region, as well as several pronounced
peaks corresponding the interband transitions asreported in previous optical studies [5]. Figure 2
shows the optical conductivity spectra of Ca2RuO4transformed by the KK analysis. An optical gap
decreases as a function of the external currents,
implying a possible current-induced metallic state in
Ca2RuO4. We are now developing an evaluation ofself-heating effects under the large external currents.

Fig. 1. Reflectivity spectra of Ca2RuO4 single
crystal measured at room temperature with externalcurrents for I || ab. The electric field of incident lightis parallel to the ab planes.

Fig. 2. Optical conductivity V1 of Ca2RuO4 obtained
from the Kramers-Kronig transformation of thereflectivity spectra.

[1] S. Nakatsuji and Y. Maeno, Phys. Rev. Lett. 84(2000) 2666.
[2] F. Nakamura et al., Phys. Rev. B 65 (2002)
220402(R).
[3] M. Sakaki et al., JPS meeting (2010) 20pPSA-28;
Y. Kimura et al., (2010) 24aPS-3.
[4] F. Nakamura et al., submitted.
[5] J.S. Lee et al., Phys. Rev. Lett. 89 (2002) 257402;
J.H. Jung et al., 91 (2003) 056403.

82

BL1B

Metal-Insulator Transition in V1-xWxO2J. S. Lee1, K. Shibuya2, M. Kawasaki1,2 and Y. Tokura1,2,3
1
Department of Applied Physics and Quantum Phase Electronics Center (QPEC),
University of Tokyo, Tokyo 113-8656, Japan
2
Correlated Electron Research Group (CERG) and Cross-Correlated Materials ResearchGroup (CMRG), ASI, RIKEN, Wako 351-0198, Japan
3
Multiferroics Project, ERATO, Japan Science and Technology Agency (JST),Tokyo 113-8656, Japan
Here, we have investigated optical properties ofsolid-solution V1íxWxO2 epitaxial films in a wide
range of doping concentration (0 d x d 0.33). We
found there are systematic variations of the opticalconductivity spectra in the high energy region (asshown in Fig. 1). Detailed examination of the optical
gap reveals that the ground state changes from theinsulator, to metal, and to insulator as x (the W
content) increases (upper panels in Fig. 2). As asignature of the dimerization of V ions there appear
the splitting of the phonon around 300 cm-1 for thepure VO2. As x increases, the splitting becomes lessprominent and is absent for x=0.33 (lower panels in
Fig. 2). This indicates that while the insulating statefor x = 0.0 is closely related to the Peierls instability,that for x = 0.33 can be understood by consideringpurely the electron-correlation.

The metal-insulator transition in VO2 takes place ataround 340 K accompanying the structural transition
from high-temperature rutile to low-temperature
monoclinic phases. Dimerization of V ions occurs in
the latter with a tilting of the V–V bond from c-axis.
3d electrons localize on V sites forming spin singlet
dimers. The driving mechanism of the metal-insulatortransition has long been argued whether
Mott–Hubbard or Peierls transitions. It has been
recently suggested that both mechanisms are active
and thus the transition is viewed as a many-bodyPeierls type.
From the viewpoint of device application, dramaticchange in electrical resistivity as well as infraredtransmission across the transition above room
temperature makes VO2 potentially useful for optical,
electrical, and electro-optical switches by exploiting
the phase control through the electron injection into
pure VO2 by electric field or light absorption. There
has been an enduring interest in controlling themetal-insulator transition temperature (TMI) of VO2
by chemical substitution. Electron doping with use ofhigher valence elements such as W, Mo, and Nb wasshown to reduce the transition temperature. Amongthem, W is known to be the most effective, reducing
the transition temperature by 21–28 K/at.%. The
valence of W was confirmed to be 6+, whichcorresponds to adding two electrons per W atom.
However, physical properties of electron-doped VO2
are not well understood because only few studies
have been carried out in single crystals.

20 K

0.4

0.2

00

0.0

5000 10000

-1

800

-1

-1

V1(Z) (: cm )Wavenumber (cm )600 +110

0.1

0.2

0.3

0.0

x (W)

x=0.00 295 K
x=0.05 20 K
x=0.11 20 K
x~0.14 20 K
x=0.33 295 K

D

Egap (eV)

1000

1.0

E

400

0.5

+70

IE/ID

-1

V1(Z) (: cm )

-1

0.6

x=0.00 (295K)
x=0.05x=0.11x=0.14x=0.33 (295K)

200
0 250

300

350

-1

0.0

Wave number (cm )

0.1

0.2

x (W)

0.3

0.0

Fig. 2. Left panels: the optical conductivity spectra up
to the visible region (upper) and in the far-infrared
region (lower). Right panels: doping-dependences ofthe optical gap energy (upper) and the intensity ratioof two phonon modes (lower).
[1] F. J. Morin, Phys. Rev. Lett. 3 (1959) 34.
[2] J. B. Goodenough, J. Solid State Chem. 3(1971)
490.
[3] K. Shibuya et al., Appl. Phys. Lett. 96 (2010)
022102.

Fig. 1. Optical conductivity spectra V1(Z) of
V1-xWxO2 obtained at the room temperature.

83

BL1B

Analysis of 4f-5d Excitation Spectra of Pr3+-Doped SrY2O4

S. Watanabe1, Y. Ichikawa2, M. Yoshino2, T. Yamada2 and T. Nagasaki3
1
Venture Business Laboratory, Nagoya University, Nagoya 464-8603, Japan2
Department of Materials, Physics and Energy Engineering, Graduate School of Engineering,
Nagoya University, Nagoya 464-8603, Japan
3
EcoTopia Science Institute, Nagoya University Nagoya 464-8603, Japan
Trivalent praseodymium (Pr3+)-doped oxides havebeen widely studied for application as a luminescentmaterial in devices such as solid-state lasers andphosphors. Because of strong recent desire forluminescent materials in high-energy regions, the4f2-4f15d1 transitions of Pr3+ in wide band-gap host
crystals have drawn attention for potential application
as UV and VUV phosphors.
We make a report of the analysis of the emission
and excitation spectra for Pr3+-doped SrY2O4. The
Pr3+-doped SrY2O4 sample was prepared by thesolid-state reaction at 1323 K and identified as single
phase by x-ray diffraction measurement. The
concentration of Pr3+ ion in the sample was 3 mol%.
The excitation spectrum at 10 K was measured
monitoring the emission at 516 nm.
In addition, we also analyzed the spectra based on
first-principles calculations for multiplet energy andabsorption spectrum using the 4-component
relativistic configuration interaction (CI) method[1, 2]. This calculation method has been successfully
applied to analysis of the multiplet energy levels andoptical spectrum between the multiples for impurity
ions in host crystals. The SrY2O4 host crystal has
three cation sites, which are Y1 site, Y2 site and Srsite. The occupied sites of the Pr3+ ions were analyzed
by comparison between the experimental spectrum
and theoretical spectra. The solution energies of Pr3+
into the host crystal were also estimated byfirst-principles band calculations. These calculations
were performed using VASP code [3].
The calculated solution energies of Pr3+ into
SrY2O4 host crystal were 0.11 eV for Y1 site, 0.31 eV
for Y2 site and 0.64 eV for Sr site, respectively, atoxidation limits. For the substitution in Sr site by Pr3+
ion, the oxygen vacancy in host crystal, SrY2O4,
required for the charge compensation. Therefore it isnatural that the solution energy of Pr3+ into the Sr sitewas much higher than the solution energy of Pr3+ into
other two sites.
Figure 1 shows the experimental excitation
spectrum and theoretical absorption spectra. The
peaks A – F were observed in the experimental
spectrum. The peak E originates in the fundamental
absorption of the SrY2O4 host crystal. The theoretical
spectra were calculated with three models, whichwere occupied three sites mentioned above,
respectively (Y1, Y2 and Sr site). While the intensityof the experimental excitation spectrum is not directlycompared with the theoretical absorption spectra,

relative peak energy can compared between each
spectra. The absorption of the Pr3+ ions in all threesites , Y1, Y2 and Sr site, contributed to the peaks A,
B and C. On the other hand, the peak D can clearly
attributed the absorption of Pr3+ in Sr site and thebroad peak F can attributed the absorption of Pr3+ in
Y1 and Y2 site.
Consequently, we can conclude that the Pr3+ ionsoccupied in all three sites, Y1, Y2 and Sr site.

Fig. 1. Experimental excitation spectrum andtheoretical absorption spectra of Pr3+-doped SrY2O4
materials.
[1] K. Ogasawara, T. Iwata, Y. Koyama, T. Ishii,
I.Tanaka and H. Adachi, Phys. Rev. B 64 (2001) 143.
[2] S. Watanabe, K. Ogasawara, M. Yoshino and T.
Nagasaki, Phys. Rev. B 81 (2010) 125128.
[3] G. Kresse and D. Joubert, Phys. Rev. B 59 (1999)
1758.

84

BL4B

Charge State Analysis of Co Ions in Pr1-xSrxCoO3-G
T. Yoshioka, H. Kanamori and T. Yamamoto
Faculty of Science and Engineering, Waseda University, Tokyo 169-8555, Japan

Introduction
Perovskite-type cobalt oxides R1-xAxCoO3-G (R and
A are rare-earth and alkaline-earth ions, respectively)
have been extensively studied, because of theirunique electronic and magnetic properties such as
colossal magnetoresistance and metal-insulatortransition [1-3]. In order to understand such properties,
it is essential to know the charge state, i.e., valence,of Co ions in these materials. When the alkaline-earth
ion is incorporated into PrCoO3, i) valence of Co ionscontrols the system, i.e., from Co3+ to Co4+ or ii)
oxygen vacancy is created, to keep the systemelectrically neutral. However, these mechanisms ofcharge compensation have not yet been fully
understood. Then the charge state analysis of Co ionsin Pr1-xSrxCoO3-G are carried out here by the Co-L2,3
X-ray absorption near-edge structure (XANES)
measurements changing a concentration of Sr (x=0,0.1, 0.3, 0.5, 0.7 .

B

100

PrCoO3

C

50

A

D

0
100
Pr0.9Sr0.1CoO3-Ǭ

Normalized Intensity

50
0
100

Pr0.7Sr0.3CoO3-Ǭ

50
0
100

Pr0.5Sr0.5CoO3-Ǭ

50
0
100

Results
Prior to the XANES analysis, all the samples werecharacterized by the X-ray diffraction (XRD). No
extra peaks were found in the observed XRD patterns
except for those of the perovskite structured
Pr1-xSrxCoO3-G, which yield the Pr1-xSrxCoO3-G
crystallizes orthorhombic perovskite structure (Pbnm)
at ambient condition.
Observed Co-L3 XANES spectra of Pr1-xSrxCoO3-G
are shown in Fig. 1, in which fine structure of Co-L3
XANES spectra of Pr1-xSrxCoO3-G changes asincrement of the concentration of doped Sr2+ ions.
From the deconvolution of the experimental spectrausing the Gaussian function (red lines in Fig. 1), it isfound that relative intensity of peak A to B increases
as increment of Sr concentration (Fig. 2). Theseexperimental results suggest that electronic structureof Co ions in Pr1-xSrxCoO3-G changes continuously as
increment of Sr concentration.

Pr0.3Sr0.7CoO3-Ǭ

50
0
775

780
Energy (eV)Fig. 1. Observed
Pr1-xSrxCoO3-G .

Co-L3

785

XANES

spectraof

0.5

Relatuve intensity, IA/IB

Experiments
Pr1-xSrxCoO3-G samples were synthesized by theconventional solid-state reaction method. Co-L2,3
XANES spectra were collected at BL4B in UVSOR
by the total electron yield (TEY) method. All thesample powders were mounted on the first Be-Cu
dinode using the carbon adhesive tape. The incident
beam was monochromatized by the varied-line-
spacing plane grating (800 lines/mm). The energy
resolution of the incident beams, E/ǻE was set to3000 by tuning the slit height settled at the upper andlower reaches of the grating.

0.4
0.3
0.2
0.1
0

0

0.2
0.4
0.6
Sr concentration, x

Fig. 2. Relative intensity of peak A to B as a fnctionof Sr concentration, x, in Pr1-xSrxCoO3-G.
[1] S. Tsubouchi et al., Phys. Rev. B 69 (2004)
144406.
[2] H. Masuda et al., J. Phys. Soc. Jpn. 72 (2003)
873.
[3] V. Golovanov and L. Mihaly, Phys. Rev. B 53
(1996) 8207.

85

BL5U

Electronic Structure of Heusler-Type Alloys Fe2-yMyVAl (M = Co, Rh, Ir)

S. Harada1, M. Kato1, S. Yagi1, K. Soda1, Y. Nishino2 and H. Miyazaki3
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
2
Graduate School of Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan
3
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
1

Heusler-type Fe2VAl and its related alloys haveattracted much attention because of the enhancementof its thermoelectric power S by partial substitution of
the forth element M [1]. Universal dependence of S isfound on the partial substitution, which can bequalitatively explained by a rigid-band-model.
However, it is also noticed that S depends on asubstituted element M even in the same group, asshown in Fig. 1 [2]. To develop new thermoelectricmaterials, it is important to understand the origin of thedependence of their thermoelectric properties on thesubstitution. In this report, we will show the results of
the 3p-3d resonance photoemission measurement ofFe2-yMyVAl (M = Co, Rh, and Ir) to clarify the change
in their electronic structure on the substitution.
Photoemission spectra of polycrystalline samples ofFe2-yMyVAl were recorded under ~2 x 10-8 Pa at 20 K
for clean surfaces prepared by in situ fracturing.
Figure 1 summaries valence-band spectra of
Fe1.9M0.1VAl recorded at the excitation photon energy
hQ of 52 and 37 eV just below the Fe and V M2,3
thresholds, where the Fe and V 3d photoemissions aresuppressed due to so-called anti-resonance effect,
respectively. Thus the spectra at hQ = 52 eV may
represent the density of states (DOS) other than the Fe
3d states, i.e. the V and M d states, while the spectra at
hQ = 37 eV show the Fe and M d DOS. In all the
spectra, there are features recognized at the binding
energy of EB ~ 0.4, 0.8, 1.5, and 3 eV. As seen in the
spectra at hQ = 52 eV, the d DOS of V and substituted
M, is almost the same for all the studied alloys. At hQ= 37 eV, the 0.8-eV band for the Co substitution
becomes prominent, while the spectra for the Rh andIr substitution are similar to each other. Small band
shift towards the high binding energy side as a wholeis also noticed for the Ir substitution.
Observed similarity is ascribed to the common
band formation with the substituted M d states
incorporated into the main Fe-V 3d band. According
to Harisson [3], the characteristic radius rd of Co 3d
states compared with the Fe 3d ones is slightly
reduced but those of the Rh 4d and Ir 5d ones aremuch increased. Thus the interaction of the Rh and Ir
d states with the Fe 3d ones, proportional to rd3, is
expected to be larger than the Co-Fe interaction. On
the other hand, the average d state energy Hd of Co is
low and those of Rh and Ir are high, compared withthat of Fe. Therefore the Fe 3d states are expected tobe pushed towards the high and low binding energy
side for the Ir and Co substitutions, respectively; thelarge interaction of Ir with Fe suggests larger shift
than that of Co. This seems consistent with thepresent results.
Further study of the dependence on the substitution
is now in progress and will be reported elsewhere.

Fig. 1. Thermoelectric power of Heusler-type alloys
Fe2-yMyVAl (M = Co, Rh, and Ir).

Fig. 2. Valence-band photoemission spectra ofHeusler-type alloys Fe1.9M0.1VAl (M = Co, Rh, and Ir)[1] Y. Nishino, ”The Science of Complex Alloy
Phases,” (TMS, Warrendale, 2005) 325.
[2] T. Sugiura and Y. Nishino, J. Jpn. Inst. Metals 73(2009) 846.
[3] W. A. Harisson, ”Electronic Structure and theProperties of Solids,” (Dover Publications Inc., New
York, 1989).

86

BL5U

A Photon Energy-Dependent Angle-Resolved Photoemission Studyof CaCu3Ti4O12
H. J. Im1, T. Sakurada1, M. Tsunekawa2, M. Toita1, T. Watanabe1, K. Takegahara1,
H. Miyazaki3 and S. Kimura3,4
1
Department of Advanced Physics, Hirosaki Univerisy, Hirosaki 036-8561, Japan
2
Faculty of Education, Shiga University, Shiga 522-8522, Japan
3
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
4
School of Physical Sciences, The Graduate University for Advanced Studies,
Okazaki 444-8585, Japan

Recently, the discovery of the extremely highdielectric constant (as high as 104-105) of A-site
perovskite CaCu3Ti4O12 (CCTO) over a wide range oftemperature from 100 to 500 K has generated
considerable attention because of the expectation ofdeveloping a high-efficient and small capacitor [1, 2].
However, its electronic structures are not still clear
due to the absence of observation of band dispersion.
Furthermore, discrepancy between the results ofelectrical transport measurements and that of bandcalculation has stressed an importance of theobservation of band dispersion; while CCTO has ametallic phase in LDA band calculation, the electricalresistivity measurements exhibit an insulator phase.
Here, we clearly observed the band dispersions invalence band regime and found the high symmetry
points (* and H), using photon energy (hv)-dependent
angle-resolved
photoemission
spectroscopy(ARPES).
High-quality CCTO single crystal was prepared by
TSFZ method. ARPES experiments were performedat the beamline BL5U. The range of photon energies
is from 54 eV to 93 eV. The clean surface wasprepared in the (100) plane by cleaving in situ.
Measurements were carried out at room temperature(T = 300 K) in a vacuum better than 210-8 Pa. A
wide angle acceptable MBS analyzer is used withenergy resolution of 50 meV at hv = 60 eV.
Figure 1 (a) shows the energy distribution curves(EDCs) of CCTO in the valence-band region along'-direction. With increasing hv, ARPES spectra trace
the blue dashed arrow in the Brillouin zone ofbody-centered cubic structure depicted as in Fig. 1 (b).
Valence bands can be divided into three regions, 1 2.5 eV, 2.5 - 5 eV, and 5 - 8 eV. The bands in theregions of 2.5 - 5 eV and 5 - 8 eV relatively highly
disperse with intense features, while the bands in theregion of 1 - 2.5 eV are not well distinguished due to
weak intensity and broad band width. In the region of
1- 2.5 eV, we observe the intensity variation of thesmall shoulder as a function of hv, reflecting the band
dispersion. The shoulder becomes prominent closing
hv = 60 eV. This suggests that hv = 60 eV is asymmetry point. In the region of 2.5 - 5 eV, there are
two kinds of bands. One disperses from 2.9 to 3.5 eVwith a top at hv = 90 eV, while the other shows verysmall dispersion around 3.9 eV, reflecting the

localized character. In the region of 5 - 8 eV, bands
are well split with top around 5.5 eV and bottom
around 7.4 eV around hv = 90 eV. These indicate thatboth hv = 60 and 90 eV correspond to symmetry point.
According to the free electron final state model, theinner potential is estimated to be 16 eV and thesymmetry points are designated as shown in
Fig. 1 (a): the EDC at hv = 90 eV corresponds to*-point and that of hv = 60 eV to H-point. Finally, we
find the negligible intensity at the Fermi level (EF),which directly explains why CCTO shows theinsulator phase.
We performed hv-dependent ARPES measurementson A-site perovskite CCTO, which shows extremely
high-dielectric constant and has an insulator phase incontrast to band calculation results. We observed theclear band dispersions and the negligible spectral
weight at EF in good agreement with the electricalresistivity measurements.

Fig. 1. (a) The EDCs along '-direction obtained by
ARPES measurements at photon energies between hv= 54 and 93 eV in normal emission. (b) The Brillouinzone and the momentum spaces (blue dashed arrow)measured in experiments.
[1] M. A. Subramanian et al., J. Solid State Chem.
151 (2000) 323.
[2] Y. Zhu et al., Phys. Rev. Lett. 99 (2007) 037602.

87

BL5U

Angle-Resolved Photoemission Study on GdTe2㩷

R. Niwa1, S. Hirate2, T. Ito1,4, T. Hajiri1, H. Miyazaki3, B. H. Min6,
S. Kimura3,5 and Y. S. Kwon6
1
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
2
School of Engineering, Nagoya University, Nagoya 464-8603, Japan
3
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
4
Nagoya University Synchrotron radiation Research Center, Nagoya University,
Nagoya 464-8603, Japan
5
School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI),
Okazaki 444-8585, Japan
6
Department of Physics, Sungkyukwan University, Suwon 440-749, Korea
Rare-earth ditellurides (RTe2; R = rare earth) haveattracted much attention because of their anomalous
physical properties due to their two dimensionality,
especially the charge density wave (CDW) transition[1]. However, there are not many reports of CDW
formation from the electronic structure viewpoint
except for LaTe2 and CeTe2 [2].
In this study, we have performed angle-resolved
photoemission spectroscopy (ARPES) on single-crystalline GdTe2 [3] to clarify the CDW formationand its relation to the anomalous properties.
Figure 1 shows the Fermi surface (FS) of GdTe2 at
the temperature T of 10 K obtained by the intensity
plot of the ARPES spectra in the binding energy
region of EF ± 20 meV with the incident photon withthe energy of 63 eV. The perfectly symmetric FS
images are consistent with the bulk Brillouin zone.
This result implies that the observed electronic
structure corresponds to the bulk electronic structure.
From band structure calculations on RTe2 [2], FSs
dominate with Te 5px and 5py characters.
Figure 2 (a) shows the ARPES image near EF alongthe īX high-symmetry line. Two symmetric bands
with respect to the ī point are observed. These bands
are attributed to a hole pocket at around the ī point
and an electron pocket at around the X point. The
intensity of these bands are suppressed at the binding
energy (EB) of 0.25 eV. Similar suppression wasobserved in the whole Brillian zone. To check the
suppression of intensity at EB = 0.25 eV in detail, theenergy distribution curves are plotted in Fig. 2 (b).
This figure clearly indicates that the conduction bandis folded at around EB = 0.25 eV and the gap opens in
between EB = 0.1 and 0.4 eV. The band folding aswell as the energy gap is the evidence of the CDW
formation.

Fig. 1. The intensity map of the ARPES spectra of
GdTe2 in the binding energy width of EF ± 20 meVplotted in the īXM plane of the Brillouin zone. The
ARPES spectra were taken at T = 10 K using hȞ =63 eV photons. The plot can be regarded as a Fermi
surface.

(a)(b)[1] J. Chung et al., J. Kor. Phys. Soc. 38 (2001) 744.
[2] K. Y. Shin et al., Phys. Rev. B 72 (2005) 085132.
[3] Y. S. Kwon and B. H. Min, Physica B 281&282(2000) 120.

Fig. 2. (a) ARPES image along the īX symmetry line.
White dashed lines are guides for eyes. (b) ARPES
spectra near EF of GdTe2 enlarged around FS crossing
point of the electron pocket at the X point (green line
in inset). These data were taken at T = 10 K using hȞ= 63 eV photons.

88

BL6B

Terahertz Spectroscopy of Copper Ion Conducting Glasses

T. Awano
Department of Electronic Engineering, Tohoku Gakuin University, Tagajo 985-8537, Japan
An optical absorption band by a collective motionof mobile ions may appear in terahertz region, in
which motion of mobile ions changes from translationin microwave region to vibration motion in
far-infrared region. I had investigated far-infrared andmillimeter wave spectra of some silver or copper ion
conductors to study the dynamics of mobile ions. In
AM4X5 (A=alkali metal; M=Ag or Cu; X=halogen)crystal, a structure by "ionic plasmon" was observedin the spectral region below 10 cm-1 in silver ion
conductors or 30 cm-1 in copper ion conductors inenergy loss function spectra at temperatures ofsuperionic conducting phase [1].
Such collective motion may be one reason of thehigh ionic conductivity of superionic conducting
glasses even at low temperature. I had investigatedwhether such "ionic plasmon" exists in the
non-periodic structure in glass of various frameworks.
The AgI-Ag2MoO4 glass has no network structure incontrast with the AgI-AgPO3. The alkylammonium
ions (TMA, TEA) disperse randomly into AgI inorganic-inorganic glasses.
In this study, I have investigated on copper ion
conducting glasses of CuI-CuPO3, CuI-Cu2MoO4 andTMAI-TEAI-CuI glasses to compare ionic motion
with silver conducting glasses.
Figure 1 shows absorption spectra of CuI-CuPO3,
CuI-Cu2MoO4 and TMAI-TEAI-CuI glasses obtained
from reflectivity spectra by K-K analysis. They are put
in order of their CuI-contents. There observed two
broad absorption bands around 130, 60, 30 cm-1 and at
low energy tail. These absorption bands were observedin silver conducting glasses, however, peak positions
are slightly shifted toward high energy side in copper
conducting glasses. This seems to be due to thedifference of mass of conduction ion. Expected
frequency ratio of silver and copper motion is 1.3 andis in good agreement with observed ones. This meansthat these bands are due to motions of conducting ion.
Figure 2 shows absorption spectra of silver ion
conducting AgI-Ag2O-V2O5 glasses for comparison.
This glass has dispersive structure as the
AgI-Ag2MoO4 glass, therefore is able to contain much
AgI. Peak positions of broad absorption bands are 110
and 20 cm-1. These positions are the same as those inother silver conducting glasses [2].

103
298K
78K

102

(CuI)0.75(TMAI) 0.1(TEAI)0.15

1013

ABSOR PT ION COE FFICIE NT (cm -1 )

10

298K
78K

102

(CuI)0.55(Cu2MoO4)0.45

1013
10

102

(CuI)0.3(CuPO3)0.7

1013
10
102

(CuI)0.2(CuPO3)0.8

1013
10
102

(CuI)0.1(CuPO3)0.9

1013
10
102

102
WAVE NUMBER (cm )A B SO R PT IO N C O E FFIC IE N T (cm -1 )ofCuI-containing

2000
1000

x=34

0
1000

(AgI) x(0.6Ag2O࡮0.4V2O5)100-x
x=45

0
2000
1000
0
0

x=67
100
-1
WAVE NUMBER (cm )Fig. 2. Absorption spectra
superionic conducting glasses.

89

298K
78K

-1

Fig. 1. Absorption spectra
superionic conducting glasses.

[1] T. Awano, Infrared Phys. and Tech. 51 (2008) 458.
[2] T. Awano and T. Takahashi, J. Phys. Soc. Jpn. 79
Suppl. A (2010) 118.

298K
78K

298K
78K

CuPO3

101 1
10

298K
78K

of

200

AgI-containing

BL6B

1

Pressure-Dependent Reflectivity Spectra of Į-(BEDT-TTF)2I3
in the THz Region

T. Iizuka1, T. Mori2, K. Yakushi1,3 and S. Kimura1,2
School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI),
Okazaki 444-8585, Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
3
Institute for Molecular Science, Okazaki 444-8585, Japan
TO-Phonon peaks at around 123 cm-1, whichmainly appear in the polarization along the a-axis,
correspond to the antisymmetric stretching mode of I3[4]. The peaks do not shift below the M-I transitionpressure of 2.0 GPa. On the other hand, phonon peaks
at around 400 cm-1 shift to the lower wavenumber
side with elevating pressure. The latter peak comesfrom the breathing mode of ET molecules which cancouple to carriers [4]. In addition, the base line of thespectrum increases with applying pressure due to theappearance of carriers. Even in the metallic phase
above 2.0 GPa, the spectra have no Drude response,
but the R(Ȧ) intensity increases with increasing thewavenumber. The Drude response seems to appear in
the lower energy region. This result is consistent with
Dirac-cone picture which has low carrier density due
to the unique electronic structure of Fermi “point”,
not a surface, in the Fermi level [3].

Introduction
Recently, a quasi-two-dimensional organic
conductor, Į-(BEDT-TTF)2I3 [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene, here after BEDT-TTF is
abbreviated as ET] is attracting attention because of
its rich physical properties, e.g., superconductivity
under uniaxial pressure along the a-axis and a zerogap state, namely “Dirac-cone” as observed ingraphite [1]. Į-(ET)2I3 shows metal-to-insulator (M-I)
transition on cooling due to charge ordering (TCO =135 K), at ambient pressure. TCO decreases with
applying pressure, and then disappears at around 2.0
GPa. Previously reported temperature-dependent hallcoefficient and electrical resistivity data imply thatthe carrier density and mobility compensate eachother above 1.9 GPa [2]. The anomalous carrier
property is discussed to originate from the
Dirac-cone-type conduction band [3]. However, there
has been no direct observation of the electronic
structure yet. In this study, the polarized opticalreflectivity spectrum R(Ȧ) of Į-(ET)2I3 was measured
under pressures to investigate the electronic structure.

Reflectivity (arb. units)

1.0

Experimental
Single crystals of Į-(ET)2I3 were grown by anelectrochemical oxidation method. Polarized R(Ȧ) of
Į-(ET)2I3 along the a-axis was measured in theterahertz (THz) region of 70 - 450 cm-1 (2.1 - 13.6
THz), from the insulator to metallic phases byapplying pressure from 0.3 to 2.9 GPa at thetemperature of 8 K. The measurement was performedat the THz micro-spectroscopy end station of BL6B
at UVSOR-II. A diamond anvil cell (DAC) was
employed to produce high pressure, with a pressure
medium of Apiezon® N. The pressure was calibrated
by a ruby fluorescence method.
Because of the low signal intensity in the insulator
phase, the accumulation was done in form ofinterferogram. Spectra were calculated from theFouriertransformationoftheaveragedinterferograms.

D-(ET)2I3

8K

2.9 GPa
2.0
1.6

0.5
0.3
0.0

100 200 300 400-1Wavenumber (cm )ambient(50 K)

Fig. 1. Pressure-dependent reflectivity spectra [R(Ȧ,P)] of Į-(ET)2I3 at the temperature of 8 K withpolarization along the a-axis. Black line is thesimulated reflectivity spectrum at ambient pressure in
the case of a diamond window of DAC [4].
[1] A. B. Kuzmenko et al., Phys. Rev. Lett. 100(2008) 117401.
[2] N. Tajima et al., J. Phys. Soc. Jpn. 69 (2000) 543.
[3] S. Katayama et al., J. Phys. Soc. Jpn. 75 (2006)
054705.
[4] Y. Yue et al., Phys. Rev. B 82 (2010) 075134.

Results and Discussion
Obtained R(Ȧ) of Į-(ET)2I3 at T = 8 K at pressuresfrom 0.3 to 2.9 GPa are shown in Fig. 1. R(Ȧ) at
ambient pressure after the correction of using
diamond window (reflective index n = 2.4) in DAC isshown at the bottom of the figure [4].

90

BL6B

LO-Phonon Plasmon Coupled Mode of ZnO

T. Inushima1, Y. Ota1 and K. Fukui2
1
Department of Electronics, Tokai University, Hiratsuka 259-1292, Japan2
Department of Electrical and Electronics Eng., University of Fukui, Fukui 910-8507, Japan
From the first principle calculation ZnO has aspontaneous polarization of 0.05 C/m2, which iscomparable to that of the representative polar
semiconductor AlN. Hence the absence of Frölich
interaction in ZnO should be explained by taking into
account the cause of the residual carriers in the
conduction band.
In conclusion, polar semiconductor ZnO does nothave plasmon-LO phonon coupled mode. To makeclear this absence, reflectivity measurements in the
far-infrared region are necessary, which will be donein the near future.

There are several similarities in the electronic
structures between InN and ZnO; it is predicted fromthe first principle calculation that whenever there is a
defect, it produces electrons in the conduction band[1]; It is also predicted that neither the phonon
structure nor the band structure can be obtained
without taking into account the d electrons as valence
electrons. So anomaly of the plasmon-longitudinal
optical (LO) phonon coupled mode is a common
subject to be investigated [2]. In this experiment, wemeasured the plasmon-LO phonon coupled mode of
ZnO as a function of the electron density.
Using BL6B and a MCT detector we measured thereflectivity spectra of c-plane ZnO at a nearly normal
incident configuration. Figure 1 shows the electron
density dependence of the reflectivity spectra of bulk
ZnO samples which were grown by hydrothermal
method. The samples contain Li, which is considered
to produce electrons as residual carriers in the
conduction band. In this experimental configuration,
only E1 optical phonon is observed as a Reststrahren
band (TO at 409 cm-1 and LO at 587 cm-1). The inset
shows that the LO-phonon shoulder melts according
to the increase of the electron density, and that linear
plasmon-LO phonon coupling (Frölich interaction) is
not observed. Another contribution of the electron isthe decrease of reflectivity in the energy region
higher than 1100 cm-1. There is a cutoff of the second
order phonon absorption at 1100 cm-1. Then thisdecrease of the reflectivity is related to the increase of
the impurities or defects in the sample.
Usually Germanium and Silicon are non-polarmaterials and the optical phonons at k=0 are notinfrared active. Consequently they do not split at k=0
into LO and TO components, and there is no
LO-phonon plasmon coupled mode. In these
non-polar semiconductors the interaction between
free carriers and phonons is non-linear and opticalphonons are observed as a result of Fano interference.
When interference exists, the LO phonon shows
asymmetric broadening at the initial position. On theother hand, in polar semiconductors like InN,
LO-phonon-plasmon interaction is so strong that only
plasmon-LO phonon coupled modes are observed.
The results shown in Fig. 1 indicate that the LO
phonons of ZnO do not couple with the free carriers
linearly, but they show instead Fano interference.

[1] Chris G. Van der Walle and J. Newgebauer, Nature
423 (2003) 626.
[2] T. Inushima, M. Higashiwaki and T. Matsui, Phys.
Rev. B 68 (2003) 235204.
.
1.0
1.0
0.8

Reflectivity

0.8

0.6

ZnO

0.4
0.2

0.6

0.0
400
15

0.4
0.2
0.0

500

600

3000

3500

700

-3

#1 (7.2u10 cm )16
-3
#2 (2.8u10 cm )17
-3
#3 (1.1u10 cm )18
-3
#4 (1.2u10 cm )

500

1000

1500

2000

2500

4000

-1

Wavenumber (cm )Fig. 1. Electron density dependence of the reflectivity
spectra of bulk ZnO grown by a hydrothermal method.
The incident light is normal to the a-b plane. Inset isthe detail of the E1 phonon Reststrahren band of ZnO.

91

BL6B

Observation of Electronic States on Solids
Utilizing FIR Synchrotron Radiation

A. Irizawa1, T. Iizuka2 and S. Kimura2,3
1
The Institute of Science and Industrial Research, Osaka University, Ibaraki 567-0047, Japan
2
School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI),
Okazaki 444-8585, Japan
3
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
Results and Discussion
The reflectivities are obtained in the energy range
between 20 and 230 cm-1 as shown in Fig. 1. Here wecan confirm the gradual change from the metallic
spectrum at 300 K to the insulating ones withdecreasing temperature. The Drude response toward 1
clearly decreases at low temperatures. More than 3phonons are uprising in the insulating state except the
intrinsic noise coming from the Fourier-transform(FT) interferometer at around 40 cm-1. The obtained
spectra are well connected to the higher-energy
spectra measured at the laboratory using theconventional light source, i.e., black body. The
comprehensive reflectivity including this result willdevelop the reliable optical constants through the
Kramers-Kronig (KK) transform.

Introduction
The optical study is one of the most powerful
techniques for the investigation of electronic states onsolids. There can be obtained the great deal of
information about the electronic band structure, thesymmetry of crystal structure, and the dielectric
property. In the respect of experimental affinity, theoptical study is extremely compatible with themultiple conditions such as low temperature, highmagnetic field, and high pressure. In the case ofstrongly correlated electron systems, it is well known
that the temperature is one of the decisive factors forthe electronic state. The considerable types of phasetransition are induced by the change of thetemperature. Particularly, the multiferroic compounds
can be controlled of its physical properties mutuallyby the external fields such as magnetic, electronic,
and elastic ones. The other particular condition, thepressure will directly affect the electron-electroninteractions through the change of lattice constants.
The beamline BL6B in UVSOR is adjusted for theinvestigations at low temperature and high pressure in
the extremely low-energy region of IR-THz. Thereequips two type interferometers of both Michelsonand Martin-Puplett type. In this report, we have
performed the optical reflectivity measurements inthe longest wavelength region for the newly
synthesized compound having a spinel-related
structure.

㪩㪼㪽㫃㪼㪺㫋㫀㫍㫀㫋㫐

㪈㪅㪇

㪇㪅㪌
㩷㪊㪇㪇㩷㪢
㩷㪉㪌㪇
㩷㪉㪇㪇
㩷㪈㪌㪇
㩷㪈㪇㪇
㩷㪈㪊

㪇㪅㪇
㪇

㪌㪇

㪈㪇㪇
㪈㪌㪇
㫎㪸㫍㪼㫅㫌㫄㪹㪼㫉㩷㩿㪺㫄㪄㪈㪀㪉㪇㪇

Fig. 1. Temperature change of the reflectivity at the
FIR-THz region.

Experimental
The selected experimental components are theMartin-Puplett type interferometer and the
Si-bolometer detector with the liquid-He flow type
cryostat for controlling the temperature from 300 K
down to 13 K. The reflectivity was obtained from theratio of optical reflections between the sample and Au
as a standard with mirrored surfaces. They were
installed on the sample holder and were measured
through the tapered hole of 4 mm in diameter.

92

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Polarization-Dependent Angle-Resolved Photoemission Study on LiFeAs

T. Hajiri1, R. Niwa1, T. Ito1,2, M. Matsunami3,4, Y. J. Song5, S. Kimura3,4 and Y. S. Kwon5
1
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
2
Nagoya University Synchrotron radiation Research Center, Nagoya University,
Nagoya 464-8603, Japan
3
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
4
School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI),
Okazaki 444-8585, Japan
5
Department of Physics, Sungkyunkwan University, Suwon 440-749, Korea
Iron pnictide superconductor discovered recently[1]. Although they have been intensively studied fromboth experiment and theory, the mechanism of its
anomalous superconductivity has not been revealed
yet. So far, the importance of the orbital characters of
Fermi surfaces for the nesting condition has beenreported [2]. However, there are few reports about thenesting properties on the three-dimensional electronic
structure [3].
We have performed angle-resolved photoemission
spectroscopy (ARPES) on LiFeAs (Tc = 18 K), toelucidate the three-dimensional electronic structure,especially Fermi surfaces and the orbital characters,
by using S and P polarized photons at UVSOR-II
BL7U as shown in Fig. 1 (a).
Figure 1 (b) shows the photon-energy-dependent
ARPES spectra at k = 0 Å-1 of LiFeAs measured at T
= 12 K (SC state) with the P polarization. The ARPES
peak disperses from 0.4 eV (hQ = 23 eV) to 0.3 eV(hQ = 35 eV). The observed symmetry shows in good
agreement with the S/c (*Z, see Fig. 1 (c)) period.
Figures 2 (a) and (b) show the polarization-dependent ARPES image near the * and Z points,
respectively. Red and green color maps are ARPES
images for the S and P polarized photons, respectively.
In Figure 2, the three (two) hole-pockets near the *(Z) point show the clear polarization dependence
between S and P polarizations. According to thedipole selection rules [4], ARPES spectra with S
polarization are dominated by dxy and dyz orbitals(with odd symmetries), while P polarization with dxz,
dx2-y2 and dz2 (with even symmetries). It should be
noted that the dxz orbital cross section is larger thanthat of dx2-y2 at this photon energy [5]. Thus weconclude the orbital character of each FS as follows;(1) dyz for the middle hole-pocket at the * point andfor inner hole-pocket at the Z point, (2) dx2-y2 for theouter hole-pocket at the * and Z point, and (3) dxz forthe inner hole-pocket at * point, respectively. The
above characterizations are in good agreement withthe band calculation [6].

Fig. 1. (a) Experimental configuration of present
ARPES. (b) Photon-energy-dependent ARPES
spectra at k = 0 Å-1 of LiFeAs at T = 12 K with the P
polarization. (c) Brillouin zone of LiFeAs. The redline corresponds to the measured line in (b) in the*MAZ plane (yellow area).

Fig. 2. ARPES image near the * (a) and Z (b) pointsof LiFeAs at T = 12 K measured with the S (redimage) and P (green image) polarizations.
[1] Y. Kamihara et al., J. Am. Chem. Soc. 130 (2008)3296.
[2] K. Kuroki et al., Phys. Rev. B 79 224511 (2009).
[3] T. Yoshida et al., Phys. Rev. Lett. 106 (2011)
117001.
[4] A. Damascelli, Physica Scripta. T109 (2004) 61.
[5] B. Mansart et al., Phys. Rev. B 83 (2011) 064516.
[6] S.V. Borisenko et al., Phys. Rev. Lett. 105 067002(2010).

93

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Three-Dimensional ARPES Study on FeSb2
1

S. Imada , K. Terashima1, A. Yamasaki2, K. Mima1,Y. Miyata1, R. Yamaguchi1,
Y. Tachimori1, Y. Yamanoi1, H. Eto1, Y. Matsui2, J. Yamaguchi3, S. Komori3, A. Sekiyama3,
S. Suga3 and H. Nakamura4
1
Faculty of Sci. and Eng., Ritsumeikan University, Shiga 525-8577, Japan
2
Faculty of Sci. and Eng., Konan University, Kobe 658-8501, Japan
3
Grad. School of Eng. and Sci., Osaka University, Osaka 560-8531, Japan
4
Grad. School of Eng., Kyoto University, Kyoto 606-8501, Japan
calculated bands, we estimate that the spectra taken at
hQ~23 eV corresponds to *. The inner potential of
FeSb2 was calculated to be ~21 eV. With this innerpotential, X point corresponds to hQ~37 eV, and wefind that BL7U can cover the whole Brillouin Zone of
FeSb2. Further ARPES study, clarifying (i) the bulk
Fermi surfaces near the Zone edge and corner [5], (ii)the location of the surface bands, (iii) the temperaturedependence of bulk bands, is necessary to elucidate
the origin of the metal-insulator crossover in FeSb2.

FeSb2 shows a metal-insulator crossover; it ismetallic around room temperature, while it isinsulating at low temperatures with an upturn of theresistivity curve at ~50 K. A number of experimental
reports have indicated the existence of the energy gap
at low temperatures [1-3], while the mechanism forthe formation of the energy gap has not been settledyet.
So far, mainly two scenarios have been
proposed, one is that FeSb2 is a Kondo insulator [2],and the other is that FeSb2 is a narrow-gap
semiconductor [1, 3].
In our previous study on FeSb2 at UVSOR [4], wehave measured temperature dependence of the densityof states (DOS) near EF. As we reduce thetemperature, the DOS suppression is observed, but
finite intensity in the vicinity of EF remains even at 15
K where the resistivity shows insulating behavior. We
also find a free-electron like band crossing EF at 15 K,
indicative of the surface state. Thus it is importantto separate the surface contribution fromphotoemission spectra to discuss bulk property and its
mechanism.
In the present study, we have performed
angle-resolved photoemission (ARPES) on FeSb2 toreveal the mechanism of the crossover by directly
observing the temperature dependence of the energy
gap. Since the band calculation predicts
three-dimensional band structure and the existence ofa couple of small Fermi surfaces [5, 6], we measured
hQ-dependence of the normal emission spectra to
determine inner potential of the sample.
A single crystal of FeSb2 has been synthesized by
Sb-flux method. High-resolution ARPES study has
been performed at BL7U in the UVSOR facility.
Energy resolution was set at ~15 meV. We haveobtained a clean surface of the sample by fracturing
in an ultrahigh-vacuum of ~5x10-9 Pa.
Figure 1 (a) shows the normal emission spectrausing photon energy from 13 to 28 eV, where each hQ
corresponds to different k points along *-X axis. As
in Fig. 1 (a), we have observed a number ofdispersive bands of FeSb2, such as Fe 3d3z2-r2 band [5]located at ~0.3 eV, Fe 3dx2-y2 band [5] at ~1 eV, and soon. In fig. 1 (b), the calculated bands [6] aretentatively superimposed on the plot of the second
derivative of the normal emission spectra. From thecomparison between our experimental results andFig. 1. (a) ARPES spectra derived by normal
emission method using hQ = 13-28 eV at BL7U.
(b) Plot of second derivative of (a). White and brownarea corresponds to bands. Calculated bands [6] areshown as red lines. Inset shows Brillouin Zone of
FeSb2 and the measured k-direction (Red arrow).
[1] Z. Schlesinger et al., Phys. Rev. Lett. 71 (1993)
11.
[2] C. Petrovic et al., Phys. Rev. B 72 (2005) 045103.
[3] T. Koyama et al., Phys. Rev. B 76 (2007) 073203.
[4] S. Imada et al., UVSOR activity report 36 (2009)106.
[5] A. V. Lukoyanov et al., Euro. Phys. J. 53 (2006)
205.
[6] J. M. Tomczak et al., Phys. Rev. B 82 (2010)
085104.

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Angle-Resolved Photoemission Spectroscopy of YbAl2

M. Matsunami1,2, T. Hajiri3, H. Miyazaki1, M. Kosaka4 and S. Kimura1,2
1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
2
School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI),
Okazaki 444-8585, Japan
3
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
4
Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, JapanThe heavy fermion or the valence fluctuation
behavior in the strongly correlated f electron systems
can be derived from the hybridization between
conduction band and 4f state (c-f hybridization).
Therefore, to directly probe the c-f hybridized
electronic structures in the momentum space isimportant for understanding the heavy fermion
physics. Angle-resolved photoemission spectroscopy(ARPES) is a powerful tool for this purpose. Recently,
some ARPES results regarding the c-f hybridized
electronic structure were reported [1-4]. These studies
were performed on the basis of the surface Brillouinzone [1, 4] or by using the 4d-4f resonant process[2, 3]. However, ARPES for the f electron systems,
which usually have three-dimensional electronic
structure, should be done along the bulk Brillouinzone.
YbAl2, which crystallizes in the cubic Laves
MgCu2 structure, is a prototypical valence fluctuation
system. The Yb mean valence has been estimated tobe ~ +2.2 by the resonant inelastic x-ray scatteringexperiment [5]. The electronic specific-heatcoefficient as a measure of the electron-mass
enhancement exhibits a slightly high value, J~10-17
mJ/K2mol [6]. These characteristics suggest that astrong c-f hybridization effect can be realized in
YbAl2. Hence YbAl2 is well-suited system forstudying the c-f hybridized electronic structures
including the f electron dispersion.
In this work, we have performed ARPES for YbAl2.
The single crystals of YbAl2 were grown by the
Lithium flux method [6]. The ARPES experiment was
carried out at the undulator beamline BL7U"SAMRAI" in UVSOR-II [7]. The total energy
resolution was set to ~10 meV and the measurement
temperature was 12 K. The crystal orientation wasdetermined by Laue x-ray diffraction prior to the
ARPES measurements. The crystal was cleaved insitu at 12 K along the (111) plane.
Figure 1 (a) shows the bulk Brillouin zone of
YbAl2. The photon energy hQ = 20 eV were chosen so
as to obtain the electronic band dispersion in the *-Kline as shown in Fig. 1 (b). The electron pocket
centered at * point and the nearly flat bands at thebinding energy of 0.15 eV are observed, which are
attributed to Yb 5d conduction and Yb 4f7/2 bands,
respectively. These states are strongly hybridized witheach other, providing a fractional dispersion in Yb

4f7/2 bands.

Fig. 1. The Brillouin zone (a) and the ARPES
intensity map around * point at 12 K (b) of YbAl2.
[1] S. Danzenbacher et al., Phys. Rev. Lett. 96 (2006)
106402.
[2] G. A. Wigger et al., Phys. Rev. Lett. 76 (2007)
035106.
[3] H. J. Im et al., Phys. Rev. Lett. 100 (2008)
176402.
[4] D. V. Vyalikh et al., Phys. Rev. Lett. 105 (2010)
237601.
[5] C. Dallera et al., Phys. Rev. B 68 (2003) 245114.
[6] H. Nowatari et al., J. Phys. Soc. Jpn. 76 (2007)
Suppl. A, pp. 80.
[7] S. Kimura et al., Rev. Sci. Instrum. 81 (2010)053104.

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Temperature-Dependent Angle-Resolved Photoemission Spectraof SmB6 (001)
H. Miyazaki1, T. Hajiri2, T. Ito2 and S. Kimura1,3
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
2
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan3
School of Physical Sciences, The Graduate University for Advanced Studies,
Okazaki 444-8585, Japan
1

Rare-earth intermetallic compounds generally
become metals with strong electron correlations.
However, some compounds become semiconductors
with very small energy gap at the Fermi level (EF),namely “Kondo semiconductor” or “Kondo insulator”[1]. SmB6 is one of Kondo semiconductors. Theenergy gap formation of SmB6 has been debated for along time because the origin of the energy gap has
been unclear yet. To solve this problem, it isimportant to investigate the detailed electronic
structure of SmB6. Three-dimensional angle-resolved
photoemission spectroscopy (3D-ARPES) usingsynchrotron radiation is the most powerful technique
to determine the electronic band structure directly.
Then we have performed 3D-ARPES to clarify the
momentum-dependent electronic structure near EF.
A SmB6 (001) single crystal was cleaned by Ar+
ion sputtering and 1400 °C annealing cycles in theultrahigh vacuum (UHV) chamber with a basepressure of ~ 2  10-8 Pa. Clean surface of SmB6
with the 2  2 (001) patterns was confirmed by alow energy electron diffraction (LEED) method andno contamination, such as carbon, was checked by an
Auger electron spectroscopy (AES). The 3D-ARPES
measurements were performed at the beamline 7U of
UVSOR-II. The total energy and momentum
resolutions for the ARPES measurement were set to
13 meV and 0.006 Å-1 at the * point (hQ = 26 eV) and
12 meV and 0.003 Å-1 at the X point (hQ = 10.6 eV),
respectively.
Figures 1 (a1) and (b1) show the temperaturedependence of the energy distribution curves (EDCs)
at the * and X points, respectively. The Fermi edge of
the EDCs is broadened with elevating temperature
due to the thermal excitations of electrons. Withdecreasing temperature, we observed broad and sharp
peaks at around 20 meV at the * and X points,
respectively, and small gap opening at both points. To
clarify the temperature-dependent components nearEF, Figs. 1 (a2) and (b2) indicate density of states(DOS) derived from the EDCs derived by the
Fermi-Dirac function evaluated from the gold at *and X points, respectively. DOS at EF gradually
decreases with decreasing temperature at both the *and X points. However, only at the X point, a peakaround 15 meV appears below 100 K and becomeslargeroncooling.
Accordingtothetemperature-dependent X-ray adsorption spectra(XAS) at the L2, 3-edge, the mean valence of Sm ionsis dramatically changed from 2.57 at 120 K to 2.50 at
30 K [2]. The temperature dependence of the 15 meVpeak at the X point is consistent with the XAS result,i.e., the 20 meV peak is strongly related to the changeof the valence. According to the LDA band structure
calculation, the hybridization between the Sm 5d and
4f states appears and the energy gap opens at the X
point. Therefore the hybridization scenario isplausible for the semiconducting nature of SmB6.

Fig. 1. Temperature dependence of the energy
distribution curves (EDCs) and density of states(DOS) of SmB6 derived from the EDCs divided by
the Fermi-Dirac function near the * point (a1, a2) andX point (b1, b2), respectively.
[1] G. Aeppli and Z. Fisk, Comment Condens. Matter
Phys. 16 (1995) 1629.
[2] M. Mizumaki et al., J. Phys.: Conf. Ser. 176(2009) 012034.

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High-Resolution Angle-Resolved Photoemission Study of an Iron Pnictide
CaFe2P2: Observation of Strong kz Dispersion
K. Nakayama1, T. Kawahara1 P. Richard2, K. Umezawa1, T. Sato1,3 and T. Takahashi1,21Department of Physics, Tohoku University, Sendai 980-8578, Japan2
WPI Research Center, Advanced Institute for Materials Research, Tohoku University,
Sendai 980-8577, Japan
3
TRIP, Japan Science and Technology Agency (JST), Kawaguchi 332-0012, JapanThe
discoveryofsuperconductivity
in
LaFeAsO1-xFx below a transition temperature (Tc) of
26 K [1] has generated great interest. Owing tointensive researches, a lot of related compounds,
which contain iron-pnictide (FeAs or FeP) layer, havebeen discovered. An important current issue is why
physical properties show remarkable differences
between the FeAs and FeP compounds despite an
identical carrier number (Fe 3d electrons). For
example, the highest Tc is about one order ofmagnitude different (~56 K and ~7 K for FeAs and
FeP compounds, respectively). It is also known thatthe non-doped FeAs compound exhibits anantiferromagnetic (AF) order accompanied by astructural phase transition, while the FeP compound
does not show either magnetic or structural transition.
To understand the origin of these differences in termsof electronic structure, angle-resolved photoemission
spectroscopy (ARPES) is a powerful technique.
Indeed, ARPES has revealed several essential features
responsible for the superconductivity and AF order in
the FeAs compounds, particularly in AFe2As2 (A = Ca,Sr, and Ba) system, such as Fermi-surface topology,
superconducting-gap symmetry, and band folding in
the AF phase [2-5]. On the other hand, little is known
about the electronic structure of the FeP compounds.
In this study, by utilizing the low energy andvariable photon energy (hQ) characters of thebeamline BL7U, we have succeeded in directly
observing three-dimensional bulk band dispersions in
CaFe2P2, which does not show superconducting and
AF transitions.
Figure 1 (a) shows ARPES spectral intensity plot
along *(Z)-M(A) high-symmetry line measured withhQ = 23 eV. As denoted by black curves in Fig. 1 (a),we identify several highly dispersive bands such as aholelike D band and an electronlike J band whichcross Fermi level (EF) at the *(Z) and M(A) points,
respectively. To clarify the dimensionality of theelectronic structure, we have performed systematic hQ
dependence at the *(Z) point. As clearly seen from
Fig. 1 (b), the holelike bands exhibit significant
dispersions as a function of hQ. For example, the Dband is located at ~100 meV below EF for hQ = 15 eV,
while it is above EF for hQ = 23 eV [Fig. 1 (a)],clearly indicating the presence of three-dimensional
closed Fermi surface in CaFe2P2. This result is in
sharp contrast to the previous observation ofquasi-two-dimensional cylindrical Fermi surface inthe normal state of CaFe2As2 [4]. The observed
difference in the dimensionality of the electronic
structure may be a key in understanding thedifferences between the FeAs and FeP compounds. It
has been argued that the nesting between hole andelectron Fermi surfaces plays an important role forthe occurrence of the superconductivity and the AForder in iron pnictides. It is thus inferred that lower Tc
and the absence of the AF order in the FeP compoundis caused by the deterioration of the nesting condition
due to higher dimensionality, which may be
associated with the shorter c-axis length compared to
the FeAs compound.

Fig. 1. (a) ARPES intensity of CaFe2P2 plotted as afunction of wave vector and binding energy measured
at 25 K with hQ = 23 eV. (b) hQ dependence of
ARPES intensity at the *(Z) point. Black curves are
guides for the eyes to trace the band dispersion.
[1] Y. Kamihara et al., J. Am. Chem. Soc. 130 (2008)3296.
[2] H. Ding et al., Europhys. Lett. 83 (2008) 47001.
[3] K. Nakayama et al., Europhys. Lett. 85 (2009)
67002.
[4] C. Liu et al., Phys. Rev. Lett. 102 (2009) 167004.
[5] P. Richard et al., Phys. Rev. Lett. 104 (2010)
137001.

97

BL7U

Intrinsic Quasi-Particle Dynamics in Topological Metallic States

S. R. Park1, W. S. Jung1, G. R. Han1, Y. K. Kim1, Ch. Kim1, D. J. Song1, Y. Y. Koh1, C. Kim1,
S. Kimura2, K. D. Lee3, N. Hur3, J. Y. Kim4, B. K. Cho4, J. H. Kim5, Y. S. Kwon5 and J. H. Han5
1
EInstitute of Physics and Applied Physics, Yonsei University, Seoul, Korea
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan3
Department of Physics, Inha University, Incheon, Korea
4
Department of Materials Science & Engineering, GIST, Gwangju, Korea
5
Department of Physics, Sungkyunkwan University, Suwon, Korea
electron-electron interaction gives the energy
dependent width. Therefore, the constant width
indicates that the electron-electron correlation isextremely small. Other contribution to the width may
be from the electron-phonon coupling. However, noelectron-phonon kink structure was observed,
meaning electron-phonon contribution is negligible.
Therefore, the only culprit for the breadth isimpurity/inhomogeneity. These results clearly showthat the intrinsic life time of the quasi-particles insurface states is extremely long, resulting in peakwidths of less than a few meV. This value is smaller
than any other cases, including the superconducting
coherence peak in the ARPES spectrum from the(S,0) point of high temperature superconductors.

Topological metallic (TM) states, the metallic
surface states of topological insulator (TI), recently
have raised great interest in the condensed matter
community because of their novel properties. Spindegeneracy which is normally found in solids is lifted
in TM states. Electron spins in TM are locked into themomenta, forming chiral spin states. Such spin
texture should strongly suppress backscattering by
non-magnetic impurities. Due to these properties, TM
states are supposed to have high electron mobilities.
High mobilities in the surface states have not been
experimentally measured as the transport method canbe used only for bulk conductivity. On the other hand,mobility or life time of surface electrons can bemeasured by ARPES. To obtain the intrinsic
quasi-particle life time, we have performed ARPES
experiments on Bi2Se3, Bi2Te3 and Sb. Bi2Se3, Bi2Te3and Sb single crystals were grown by flux andBridgman methods. Samples were cleaved in situ and
ARPES measurements were performed at thebeamline BL7U [1, 2].
From the earlier experiments at UVSOR, we foundout that the quasi-particle dynamics is severely
affected by adsorbed atoms and molecules on thesurfaces. Therefore, we have raised samples
temperatures to anneal off the adsorbed atoms andmolecules and obtain clean surfaces. Figure 1 shows
ARPES data from Bi2Se3, Bi2Te3 and Sb surface
states taken with 8 eV photons. 8 eV is found to
suppress the bulk state spectral weight. The surface
band reaches the * point near the 0.3 eV binding
energy for Bi2Se3. This point is called the Dirac point.
The binding energies of Dirac points are 0.3, 0.11 and0.23 eV for Bi2Se3, Bi2Te3 and Sb, respectively.
Suppression of the bulk ARPES states allows us toperform reliable self energy analysis on the data. The
imaginary parts of the self energies from the data areplotted in Fig. 2. In the data, it is noted that Im6 fromtopological surface states are very much energy
independent, at around 25 meV. This casts a stark
contrast to the case of high temperature
superconductors for which Im6is very energy
dependent. In addition, the width of 25 meV is verysmall, indicating the quasi-particle life time in thesurface states is very long.
Even though 25 meV is already very small, we
believe the intrinsic width is even smaller. Generally,

Fig. 1. Near * point ARPES data from various
topological surface states : (a) Bi2Se3 (b) Bi2Te3 and(c) Sb [From ref. 2].

Fig. 2. Plot of Im6from Bi2-xSnxTe3, Sb, graphene,
Bi2Sr2CaCu2O8-G (Bi2212) and LiFeAs [From ref. 2].
[1] S. R. Park et al., Phys. Rev. B 81 (2010) 041405.
[2] S. R. Park et al., New J. Phys. 13 (2011) 013008.

98

BL7U

㩷 Angle-resolved Photoelectron Spectroscopy of HOPG
with Photon Energies of <16 eV㩷
S. Tanaka1, M. Matsunami2 and S. Kimura2
1
The Institute of Scientific and Industrial Research, Osaka University,
Ibaraki 567-0047, Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan

Graphite has been regarded as a “textbook” system
for investigations of the solid state physics for many
years because of its unique characters. It consists ofcarbon, which is one of the most basic elements, andthe network of the sp2-hybridized orbital makes thequasi-two dimensional electronic structure. The
dispersions of the valence band are schematicallyshown in Fig. 1 [1].
㪜㫅㪼㫉㪾㫐㩷㪽㫉㫆㫄㩷㪜㪝㩷㩿㪼㪭㪀

㪇
㪄㪉
㪄㪋
㪄㪍
㪄㪏
㪄㪈㪇

Fig.2 Series of the photoelectron spectra of HOPG.

㪄㪈㪉
㪄㪈㪋

One may argue that this could be ascribed to an
extrinsic origin; the defect or the adsorbed species on
the surface. The use of the ARPES, however, enables
us to deny this possibility. In Fig. 3, the result of the
ARPES at hQ=11.5 eV are shown together with thedispersion of the peak as a function of the k//. The
dispersion is similar to the calculated one at theK-point along the K-* direction of the graphite (solidlines) except a typical quasi-particle behavior.

㪄㪈㪍
㪄㪈㪏
㪄㪉㪇

.

*

0

Fig.1 Electronic structure of the graphite.
In this report, we present a photoelectron
spectroscopic study of HOPG (Highly Oriente
Pyrolytic Graphite) at the photon energy region below
16 eV for the first time.
The experiments were carried out at the BL7U at
UVSOR, which covers the photon energy of 6-43 eV,
and is equipped with the apparatus for theangle-resolved photoelectron spectroscopy (ARPES).
An HOPG sample was cleaved in UHV (<1x10-8 Pa),and its fresh surface was investigated. The reliabilityof the experiments including the sample condition
was checked with the comparison of the ARPES
spectra taken at hQ=41 eV with the previous result[2].
Figure 2 shows the series of the surface-normalphotoelectron spectra of HOPG at 11 K with photon
energies of 7.3-16 eV. Intensities are calibrated by thephoton intensities measured with the photodiode. The
most striking feature is the sharp peaks just below theFermi level, whose intensities are resonantly
enhanced at hQ=11.2 eV (the width is about 1 eV).
This is surprising since the band of the graphite
crosses the Fermi level at the K-point, and the highest
occupied state at the *-point is about 3 eV below theFermi level. We found that these peaks are only
observed below 30 K of the sample temperature. The
resonance is probably due to the excitation into theunoccupied state at 11.2 eV above the Fermi level.

Fig. 3 Dispersion of the peak near the Fermi level.
Most plausible explanation is that the K-point isbackfolded to the *-point in the two-dimensional
Brilloin zone due to the reconstruction of the HOPG
lattice which occurs below 30 K. The detailed study
to clarify this possibility is now under progress.
[1] R. Ahuja et al., Phys. Rev. B 55 (1997) 4999.
[2] K. Sugawara et al., Phys. Rev. B 73 (2006)
045124.

99

BL7B

Characterization of Indium Filter for Generationof Single Order Laser Harmonics
M. Fushitani, A. Matsuda and A. Hishikawa
Department of Chemistry, Graduate School of Science, Nagoya University,
Nagoya 464-8602, JapanThe transmittance of an indium filter with athickness of 1000 Å was measured at BL7B. The
synchrotron radiation (SR) was dispersed by the G2(600 lines/mm) and G3 (300 lines/mm) gratings in the
wavelength range of 40200 and 120800 nm,
respectively. The reference and transmitted light weredetected by a Si photodiode (AXUV100).
Figure 1 shows parts of the transmittance of theindium filter. The maximum transmittance of 10 %is observed around 80 nm with modulations due tothe interference effect [2]. The second maximum
observed in the region of 150200 nm is attributed tothe second order diffraction of the G2 grating. The
transmittance in the 320480 nm is about 0.1 %.
The expected wavelengths for the 3rd (HH3), 5th(HH5) and 7th (HH7) order harmonics driven by
ultrashort laser pulse at 400 nm (UV) are indicated byarrows in Fig. 1. The corresponding transmittances
are 0.1 % (UV), 0.07 % (HH3), 10 % (HH5), and
0.3 % (HH7). The present study shows that theindium foil is a suitable narrow-band filter for HH5 togenerate ultrashort EUV pulse at 80 nm, with a highcontrast ratio (103) to the neighboring orders (HH3and HH7).

An ultrashort laser pulse in the extreme ultraviolet(EUV) region is of great interest as a probe toinvestigate ultrafast dynamics of molecules becauseof the capabilities of ionizing most atoms andmolecules. Among others, laser high-order harmonic
generation using a gaseous nonlinear medium isproved to be a promising frequency conversion
process to obtain EUV laser pulses [1].
Due to the up-conversion process occurring every
half optical cycle of the driving laser pulse, laser
high-order harmonics is often generated as a train of
pulses, and the corresponding spectrum exhibits aseries of odd-order harmonics. For time-resolved
measurements, a single pulse is preferred rather thana pulse train. There are several methods to transforma pulse train into a single pulse. A typical method is toemploy optical components such as a grating where
the harmonics are spectrally dispersed and one of theharmonic orders is spatially selected by using a slit.
The single harmonics, however, are significantly
stretched in the time domain. An alternative method is
to use a thin metal foil as a narrow-band filter.
Because of the thickness in the order of 103 Å, lessamount of dispersion is introduced in the selected
harmonics during the propagation of a metal foil,
keeping the pulse duration still short. Here, weinvestigate the transmission property of an indium
foil to obtain ultrashort EUV laser pulse at 80 nm as
the 5th order harmonics of the UV laser pulse at
400 nm.

[1] T. Brabec and F. Krausz, Rev. Mod. Phys. 72(2000) 545.
[2] W. R. Hunter, D. W. Angel and R. Tousey, Appl.
Opt. 4 (1965) 891.

Fig. 1. Parts of the transmittance curve of an indium foil with a thickness of 1000 Å in the
wavelength region of 40 and 320480 nm.

100

BL7B

Impurity Band of Boron Doped Diamond
Observed in the Ultra-Violet Region

T. Inushima1, Y. Ota1 and K. Fukui2
Department of Electronics, Tokai University, Hiratsuka 259-1292, Japan2
Department of Electrical and Electronics Eng., University of Fukui, Fukui 910-8507, Japan
1

In order to clarify the energy dependence of the
impurity band structure, we measured refractive index
of the impurity band in the ultra-violet region using
BL7B.
Figure 2 shows the reflection spectra of 2000 ppm
boron doped diamond measured at 20 and 300 K by a
combination of S2 (6~12 eV) and S3 (3~7 eV)
detectors. The spectra have clear sinusoidal
interference fringes caused by the transmitting lightof the homo-epitaxial diamond film. Using the film
thickness of 1.6 Pm, refractive index is determined asa function of photon energy, which is shown in theinset of Fig. 2.
The observed refractive index is much smaller thanthat of the bulk diamond (n~2.4), and shows aminimum at 6 eV. The increase in the higher energy
side is due to the direct transition of the bulk diamondat 11.2 eV, and there is no reflection anomaly at theindirect gap at 5.4 eV.
Figure 2 indicates that the impurity band thatformed in the bulk diamond has energy dispersion
and that it extends up to 10 eV.

We report on the refractive index of the impurity
band which formed in a semiconductor in theultra-violet region.
Since the discovery of superconductivity inboron-doped diamond, its impurity band structure has
been a key issue for the understanding of thesuperconductivity. Recently it is reported that when
the boron concentration exceeds NB=4u1018cm-3,
impurity band forms at the first excited levels (2p) of
the impurity boron at 0.06 eV above the valence band
maximum. At higher NB, the wave functions of the 2P
states begin to overlap and the impurity band spreads
in the k-space. Simultaneously, Fermi level rises tothe impurity band and a variable range hopping isrealized [1]. This mechanism can be seen in theabsorption spectra of boron doped diamond, which isshown in Fig. 1.
When NB is low (<1000 ppm), the prominent
absorption peaks are at 0.305 and 0.347 eV, which are
the transitions from 1S to 2P0 and 2Pr1 states,
respectively.
These transitions form a sharp
Gaussian shape and are accompanied by LO-phonon
sidebands in the higher energy region. When NB
exceeds 4u1018 cm-3 (2000 ppm), the absorption
spectrum changes into a Lorentzian shape governedby the single absorption at 0.35 eV, and the sharp
structure disappears, which indicates a drastic
decrease of the relaxation time of the excited carriers,
or an appearance of the impurity band.
25
2000 ppm

2500

10000 ppm

10
5
0
0.0

2000

D

15

3

-1

D (10 cm )

20

1500
1000

5000 ppm

1000 ppm

500
0

0.2

0.4
0.6
Photon energy (eV)Fig. 2. Reflection spectra of 2000 ppm boron doped
diamond at 20 and 300 K. Inset shows the energy
dependence of the refractive index.

0.82000 ppm

0.5

1.0
1.5
Phorton Energy(eV)

2.0[1] T. Inushima, R. F. Mamin and H. Shiomi, Phys.
Rev. B 79 (2009) 045210.

2.5

Fig. 1. Boron concentration dependence of theabsorption spectra of diamond. The B/C ppm ratio
used in the crystal growth is used as the indication of
the samples and 10,000 ppm corresponds to NB of
2u1019 cm-3.

101

BL7B

Spectroscopic Analysis of Electronic States of Solids in Scope of IR-VUV
Utilizing BL7B

A. Irizawa1 and S. Kimura2,3
The Institute of Science and Industrial Research, Osaka University, Ibaraki 567-0047, Japan
2
School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI),
Okazaki 444-8585, Japan
3
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan

1

gratings and the windows still have some
controversial point, they can be connected in the
single spectrum by the proportional operation. The
temperature variation is hardly discriminated within
the limits of error in this experiment.

Introduction
The optical study is one of the most effective
approaches in terms of the tunability and thewideness of energy range for understanding ofelectronic states of solids. The accurate analysis canbe achieved from the reflectivity in the wide energy
range as far as possible where the optical conductivity
can be derived from the reflectivity throughKramers-Kronig (KK) transform. In that sense, thereflectivity data from BL7B in the wide energy range
between 1.2 and 40 eV (1000-30 nm) is the importantpart for KK transform. In this machine time, we
prepared several solid samples of strongly correlatedd-electron system. Sr2Ir1-xRhxO4 (x=0-1) is the newly
synthesized system having the perovskite-related
crystal structure, so called the Ruddlesden-Popper
phase [1]. The end members of this system are that
Sr2IrO4 is the spin-orbital Mott insulator [2, 3] andSr2RhO4 is the 2D-like metal [4, 5], respectively. The
metal-insulator transition has been reported for thesolid solution Sr2Ir1-xRhxO4 where the electronic state
will change at around x=0.8. We measured thereflectivities of all these compounds using a 3-m
normal incidence monochromator at BL7B in thisbeam time.

㩷㪞㪊㪂㫈㫌㪸㫉㫋㫑㩷㪞㪊㪂㪦㪌㪊
㩷㪞㪈㪂㪹㫃㪸㫅㫂㩷㪈㫊㫋㩷㪞㪈㪂㪹㫃㪸㫅㫂㩷㪞㪉㪂㪹㫃㪸㫅㫂㩷㪞㪉㪂㪣㫀㪝
㩷㪞㪊㪂㫈㫌㪸㫉㫋㫑㩷㪉㫅㪻

㪇㪅㪉㪇

x=0

㪩㪼㪽㫃㪼㪺㫋㫀㫍㫀㫋㫐

㪇㪅㪈㪌

㪇㪅㪈㪇

㪇㪅㪇㪌

㪇㪅㪇㪇
㪈㪇㪇

㪈㪇㪇㪇

㫅㫄
㪇㪅㪉㪇

㪇㪅㪈㪌

㪊㪇㪇㩷㪢
㩷㪞㪊㪂㪦㪌㪊
㩷㪞㪈㪂㪹㫃㪸㫅㫂㩷㪞㪉㪂㪹㫃㪸㫅㫂㩷㪞㪊㪂㫈㫌㪸㫉㫋㫑㩷㪉㫅㪻㪏㪇㩷㪢㩷㪞㪊㪂㫈㫌㪸㫉㫋㫑㩷㪞㪉㪂㪹㫃㪸㫅㫂㩷㪞㪈㪂㪹㫃㪸㫅㫂x=0

Fig. 1. Stability of reflective spectra for the twice
measurements.

Results and Discussion
As a result, the spectra can be obtained for all thecompositions of Sr2Ir1-xRhxO4 (x=0, 0.5, 0.8, 1). The
measurements were repeated twice for all thecompositions. The results are stabilized for thedifferent sequences as shown in Figs. 1 instead of the
low intensity of the reflection. Although theconnectivity among the spectra with the different[1] M. Isobe, JPS meeting (2010) 24pXP-9.
[2] S. J. Moon et al., Phys. Rev. Lett. 101 (2008)
226402.
[3] B. J. Kim et al., Science 323 (2009) 1329.
[4] F. Baumberger et al., Phys. Rev. Lett. 96 (2006)
246402.
[5] B. J. Kim et al., Phys. Rev. Lett. 97 (2006)
106401.

㪩㪼㪽㫃㪼㪺㫋㫀㫍㫀㫋㫐

Experimental
The optical reflectivities of Sr2Ir1-xRhxO4 (x=0-1)
were measured at UVSOR-II BL7B. The energy waschanged from 1.2 to 40 eV with three gratings (1200,
600, and 300 lines/mm). There are several types of
optical window for avoiding the higher degree
diffractions. Follow to the former studies in BL7B,
we selected the 3 type windows with the 2 gratings
for the main experiments. Also we checked thecontinuity of them by the G2 measurement. The
measured temperatures were selected for 300 K and
77 K as a room temperature and the lower
temperature.

㪇㪅㪈㪇

㪇㪅㪇㪌

㪇㪅㪇㪇
㪈㪇㪇

㪈㪇㪇㪇

㫅㫄

102

BL7B

Optical Study of Rattling Phonons in Clathrate Compounds

T. Mori1, M. Matsunami1,2, T. Iizuka2, S. Kimura1,2, K. Suekuni3 and T. Takabatake3,4
1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
2
School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI),
Okazaki 444-8585, Japan3
Department of Quantum Matter, ADSM and 4Institute for Advanced Material Research,
Hiroshima University, Higashi-Hiroshima 739-8530, Japan
Secondly, below about 0.1 eV, the Drude-like doped
carrier contribution is observed. Thirdly, there are two
peaks around 1.5 and 20 eV. Including higher-energy
structures, these are due to the electronic interband
transitions from valence bands to conduction bands.

Recently novel phenomena related to local
anharmonic phonons observed in cagelike materials
such as intermetallic clathrates have attracted muchinterest [1-4]. In particular, the so-called rattlingphonons of guest ions in polyhedral cages and how
they may affect such as electric and thermal
properties, have been studied extensively in light of
their potential application in thermoelectric devices.
Some of the clathrates classified in type-I structure
show not only low thermal conductivity with a highthermoelectric figure of merit, but also glasslike
temperature dependence of specific heat and thermal
conductivity [1]. Such thermal properties areuniversally observed in glasses or amorphousmaterials, though the present materials form a regularlattice. It is thus argued that such anomalous thermal
behaviors may be caused by the rattling phonons
vibrating in an off-centered anharmonic potential.
In this study we measured reflectivity spectra oftype-I Ba8Ga16Sn30 (E-BGS), type-I Ba8Ga16Ge30(BGG), and type-VIII Ba8Ga16Sn30 (D-BGS) to
investigate rattling phonon and electric structure. In
those materials, only E-BGS has off-centered rattlingphonon and shows glasslike thermal properties.
The single crystals of n- and p-type E-BGS, BGG,
and D-BGS were grown by a self-flux method [1, 2].
Reflectivity spectra of those samples were measured
in the photon energy range of 2 meV – 30 eV using
several spectroscopies. In the energy range between 2
meV and 1.5 eV, two interferometers (Martin-Puplettand Michelson type) were used. In the energy range
between 1.5 and 30 eV, a synchrotron radiation at
BL7B in UVSOR facility was used in this machine
time. The optical conductivity spectra V(Z) were
calculated from the reflectivity spectra using
Kramers-Kronig (KK) relations.
Figure 1 (a) shows obtained reflectivity spectrumof n-type E-BGS at room temperature. The spectrumof each energy region is connected smoothly based on
far-infrared spectral data. Optical conductivity
spectrum calculated from the reflectivity spectrum
using KK relation is shown in Fig. 1 (b). The V(Z)spectrum of n-type E-BGS consists of mainly three
features. Firstly, below about 10 meV, there are fourpeaks of phonons. The lowest broad peak around 3
meV is assigned as off-center rattling phonon of
Ba(2)2+ ions. This observation of the rattling phonon
is consistent with that obtained by another probe [4].

Reflectivity(a) 1.0n-type E-BGS

0.8
0.6
0.4
0.2

T=300K

0.0
-3
10

(b)

-2

10

-1

10

0

10

1

10

Photon energy (eV)4

Optical conductivity V(Z)10n-type E-BGS
3

10

T=300K

2

10

1

10 -3
10

-2

10

-1

10

0

10

1

10

Photon energy (eV)Fig. 1. (a) Observed reflectivity spectra and (b)optical conductivity spectra V(Z) of n-type E-BGS atroom temperature.
[1] M. A. Avila et al., Phys. Rev. B 74 (2006) 125109.
[2] M. A. Avila et al., Appl. Phys. Lett. 92 (2008)
041901.
[3] T. Mori et al., Phys. Rev. B 79 (2009) 212301.
[4] T. Mori et al., Phys. Rev. Lett. 106 (2011) 015501.

103

BL7B

Characterization of Wide Bandgap Oxide and Fluoride Crystal
in VUV Region

T. Nakazato1, M. Tsuboi1, K. Sakai1, K. Yamanoi1, T. Shimizu1, N. Sarukura1 and T. Fukuda2
1
Institute of Laser Engineering, Osaka University, Suita 565-0871, Japan
2
WPI-AIMR, Tohoku University, 2-1-1 Katahira, Aobaku, Sendai 980-8577, Japan
Introduction
All-solid-state lasers, optics, and scintillators in thevacuum ultraviolet (VUV) region are in demandbecause of the important roles that they play invarious fields. Fluoride and oxide crystals are key
materials, which meet these demands because of theirwide-bandgap. In our previous research, we provedthat zinc oxide (ZnO) is a promising fast scintillator[1], having achieved a luminescence lifetime of lessthan 100 ps by controlled impurity doping [2]. In thisresearch, we measure the photoluminescence (PL)spectra of two different types of ZnO crystals,
undoped and impurity-doped, at various temperaturesin order to investigate their optical properties in the
VUV region. Results provide the necessary databasefor optimizing this material for various applications.

Intensity (arb. units)1a) Undoped ZnO: 18 K: 50 K: 100 K
: 150 K
: 200 K: 300 Kb) Inpurlity doped ZnO: 22 K: 53 K: 100 K
: 150 K
: 200 K: 250 K: 300 K

0.5

0Intensity (arb. units)

1

Experiment
We have measured the PL spectra of undoped andimpurity-doped ZnO crystals at 20 K, 50 K, 100 K,
150 K, 200 K, 250 K and 300 K. The excitation
wavelength was 70 nm.
Results and Discussion
Figure 1 shows the temperature dependence of the
PL spectra. Both the undoped and impurity doped
ZnO have two emission peaks at around 380 nm and
600 nm. As shown in Fig. 1 (a), the 380 nm emission
peak, which originates from exciton recombination,
becomes narrower, more intense, and shifts to ashorter wavelength with decreasing temperature.
These indicate that the exciton changes from free
exciton to bound exciton with decreasing temperature.
On the other hand, shift in wavelength and change inpeak width of the 600 nm emission was not observed.
Intensity increases with decreasing temperature,reaching maximum at 200 K. However, further
decrease in temperature decreases the intensity. The
600-nm emission originates from transition between
defect levels in the ZnO crystal. The temperaturedependence of the broad emission in the visible
region shows a different trend in previous work [3].
Further investigation is being carried out in order tounderstand this difference. Impurity-doped ZnO also
manifests the same temperature dependence, as
shown in Fig. 1 (b), except that the intensity of the
600-nm emission reaches maximum at 150 K.
The relative intensity of exciton emission fromdefect level-transition in impurity-doped ZnO islower than that in the undoped one. This is because
the impurity prevents exciton recombination, wherein

104

0.5

x10

0
400

600
Wavelength (nm)800Fig. 1. Photoluminescence spectra of ZnO. Excitation
wavelength is 70 nm. a) Undoped ZnO. b) Impurity
doped ZnO.
the impurity works as a fluorescence quencher. On
the other hand, the wavelength of the emission
originating from exciton recombination (380 nm) andfrom transition between defect levels (600 nm) is thesame regardless of impurity doping for all
temperatures. These results show that we can control
the lifetime without changing emission wavelengthand other scintillator properties by impurity-doping.
This systematic study will help us in material survey.
[1] M. Tanaka et al., Appl. Phys. Lett. 91 (2007)
231117.
[2] T. Shimizu et al., Rev. Sci. Instrum. 81 (2010)
033102.
[3] H. Mrokoc and U. Ozgur, "Zinc Oxide:
Fundamentals, Materials and Device Technology",
Wiley-VCH, Weinheim (2009).

BL7B

Optical Constants of AlN Determined by VUV Ellipsometry

K. Ozaki1, T. Saito2, H. Iwai1, K. Fukui1 and I. Saito2
1
Dept. Elec. Elec. Engi, Univ. Fukui, Fukui 910-8507, Japan
2
National Metrology Institute of Japan, National Institute of Advanced Industrial Science andTechnology, Tsukuba 305-8563, JapanS c M a (<a , ' a )R ( E )M (<, ')R (D )S , (1)where S is the Stokes vector of the incident light tothe sample, M(Ȍ,ǻ) the Mueller matrix of a sample,
Ma(Ȍa,ǻa) the Mueller matrix of a detector, R(-Į) the
Muller matrix of a rotator (angle of rotation -Į), R(-ȕ)
the Muller matrix of a rotator (angle of rotation -ȕ).
The detector is sensitive only to the intensity
component S´0 in S´. Hence, the Eq. 1 is expanded on
S´0. At certain Es, the equation is simplified and thus,
all Stokes vector component of incident light, ǻ, Ȍand Ȍa are obtained by the fitting to the sample
chamber rotation measurement result.
Ellipsometric parameters, Ȍ and ǻ, were obtained
from measurement result of the AlN thin films on
Al2O3 substrate. Complex refractive indeces (N=n ik) of an AlN derived from Ȍ and ǻ are shown in
Figure 2. A red line is a curve calculated by KramersKronig transform and the fitting to reflectance.
In the energy range lower than 9 eV, opticalconstants obtained had large measurement errors.
This is because the optical axis does not match themechanical axis, which was brought by the change inthe position of the incident optical axis depending on
the wavelength. This is inevitable due to the spectral
design of BL7B. Another cause is the influence ofbirefringence possibly due to the strain in LiF used
for higher-order rejection. In the energy range higher
than 9 eV, optical constants of AlN obtained by theellipsometry are found to be close to those by K-K
method considering the error bars.
For better performance, mechanical improvement
and measurement automations
㩷 are planned.

Introduction
Recently, the need for quantitative measurementsof UV-VUV radiation is increasing with the
expanding applications of the radiation forsterilization, curing, etc.. However, most-widely used
Si photodiodes, have a serious problem of
degradation under UV exposure. Therefore, AlN(AlGaN) photodiodes [1] without this problem are
attractive. Accurate optical constants of materials
composing a detector are necessary for the design andthe characterization. Although commercially available
spectroscopic ellipsometers are known as a powerfultool to obtain optical constants with high accuracy,
the shortest wavelength limit is typically in the near
UV region. Among some trials to go down to the
VUV range, an AIST group designed a VUVellipsometer using a oblique incident photodiode as apolarization analyzer and proved that it worked
successfully [2]. Currently, we are tuning up the
instrument by optimizing various conditions for theuse at BL7B. In addition, we tried to obtain opticalconstants of an AlN by performing VUVspectroscopic ellipsometry measurement to AlN thinfilms, which can be used for AlN photodiodes.
Experiment
A VUV ellipsometer used this time is shown in
Figure 1. First, the polarizer is not needed in thisequipment by the reason that the polarization state ofincident light to the sample can be artificiallychanged by rotating the sample chamber. Due to thecharacteristics of synchrotron radiation, the incident
radiation is nearly linearly polarized. The beamreflected by the sample is detected by an inclined
detector, which results in polarization sensitivity. This
design eliminates the need of polarizer, which istypically realized by 3-time reflections, and greatly
simplifies the mechanism.

45㫦

Real part1.0

㩷linearlypolarized
lightsample

(a)

2.0

0.0
2.0

Imaginary partĮ

exp.
reflectance fitting result

3.0

ș

1.0
0.5
0.0

photodiode

(b)

㩷

1.5

5

10

15

20

25

Photon energy [eV]ȕFig. 2. Complex refractive indices of an AlN, (a) realpart and (b) imaginary part.

Fig. 1. Schematic layout of a VUV ellipsometer.

[1] T. Saito et al., Phys. Stat. Solid. C 6 (2009)
S658-S661.
[2] T. Saito, M. Yuri and H. Onuki, Rev. Sci. Instrum.
66 (1995) 1570.

Analysis and Discussion
The Stokes vector S’ at the detector can be
expressed by

105

BL8B

Ultraviolet Photoelectron Spectraof Mono-Metal Endohedral Fullerene Er@C82 (I)T. Miyazaki1, R. Sumii2,3, H. Umemoto4, H. Okimoto4, Y. Ito4, T. Sugai4,
H. Shinohara4 and S. Hino1
1
Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan
2
Institutes for Molecular Science, Okazaki 444-858, Japan
3
Research Center for Materials Science, Nagoya University, Nagoya 464-8602, Japan
4
Graduate School of Science, Nagoya University, Nagoya 464-8602, Japanof three Tm@C82 isomers. This observation supports
the empirical rule that the electronic structure ofmetallofullerenes is governed by the cage structure
and the oxidation state of entrapped metal atom: theelectronic structure is analogous when the cage
structure of metallofullerenes and the oxidation stateof the entrapped atom is the same, and the electronic
structure might be quite different when the cage
structure of the metallofullerenes or the oxidation
state of the entrapped atom is different. Using thisempirical rule we could deduce the cage structure ofmetallofullerenes or the oxidation state of theentrapped atom from the UPS measurements. The
upper valence band UPS of Er@C82 (I) is well
reproduced by the simulated spectrum assuming a
C2v–C823-, confirming the cage structure deduced
from NMR analysis. It suggests that Er@C82 (I) has
the C2v–C82 cage structure and the oxidation state of
the entrapped Er atom is +3.
Photoelectron counts

Erbium atom entrapped fullerenes have been
isolated, the cage symmetry of Er@C82 (I) has been
determined to be C2v (82:9) by NMR structural
analysis, and the oxidation state of Er in Er@C82 is an
issue worth to investigate. In the current work, theultraviolet photoelectron spectra (UPS) of Er@C82 (I)will be presented.
The ultraviolet photoelectron spectra (UPS)
measurement was carried out at the beamline BL8B
of Ultraviolet Synchrotron Orbital Radiation Facility(UVSOR) of Institute for Molecular Science. Thebase pressure of the measurement chamber was
4×10–8 Pa, and the pressure during the measurement
went up to about 6×10–8 Pa. Specimens for UPS
measurement were thin films vacuum deposited onto
a gold deposited molybdenum disk from a quartz
crucible under 5×10–7 Pa. The temperature of the
crucible during vacuum deposition of Er@C82 (I) was
650–700 °C. The UPS were referenced against the
Fermi level (EF) of gold and were plotted as afunction of binding energy relative to EF.
The upper valence band UPS of Er@C82 (I)obtained with hv = 20–55 eV incident photons areshown in Fig. 1. Eleven structures labeled AíK andmarked with dotted lines were clearly observed. All
spectra were normalized by the peak height of thestructure E. The intensity of these structures changed,
when the incident photon energy was tuned. For
example, the intensity ratio of B–D was almost unity
at hv = 20 eV and 40 eV but changed to about 1:2 at
hv = 30 eV. This is a typical behavior in the UPS offullerenes. The spectral onset energy (Eonset) of
Er@C82 (I) is 0.38 eV below the Fermi level and thisis much smaller than that of empty C82 (1.2eV). The
UPS of Er@C82 (I) in the deeper binding energy
region (Eb > 5 eV) is due to ó-electrons that constitutethe C82 skeletal C–C bonds. The seven peaks in thisregion were at 5.4 eV (E), 6.8 eV (F), 7.7 eV (G), 8.4
eV (H), 10.1 eV (I), 10.9 eV (J) and 12.9 eV (K). The
UPS of the upper binding energy region (Eb < 5 eV)
are derived from S-electrons and four structures areobserved at 0.7 eV (A), 1.6 eV (B), 2.1 eV (C) and3.3 eV (D).
The UPS of Er@C82 is substantially identical withthose of La@C82 and Tb@C82, which suggests theiranalogous electronic structure. The upper valenceband UPS of Er@C82 differs significantly from those

Er@C82(I)
D

C

B
A

hQ =
55eV
50eV
45eV
40eV

K

35eV
J I
H

G

30eV
25eV
21.2eVF
E

20eV
14 12 10

8

6

4

2

0

Binding Energy / eV

Fig. 1. The incident photon energy dependent UPS of
Er@C82 (I) at hȞ = 20 ~ 55 eV. Eleven structures areobserved and their intensity changes upon the tuningof the incident photon energy.

106