BL7U

ARPES Studies of Quasi-Particle Dynamics in Topological Insulators
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, J. H. Han5
1
Institute 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
Topological insulators (TIs) are a new class ofmaterials which always have metallic surface states.
These metallic surfaces states are protected by thetime reversal symmetry and have their spins locked
into the momentum. The latter fact brings about
suppression of back scattering and quasi-particles are
expected to have a longer life time. An experimental
verification of this fact is essential to understand thenovel properties of the metallic states, and therefore
we have performed temperature and photon energy
dependent angle resolved photoemission (ARPES)
experiments on various TIs.
Bi2Se3, Bi2Te3 and Sb single crystals were grownby flux and Bridgman methods. Samples werecleaved in situ and ARPES measurements were
performed at the beam line 7U [1][2]. Samples weresometimes left in the vacuum for a few days to age
the surface.
Figure 1(a) shows ARPES data from a freshly
cleaved surface of Bi2Se3 taken with 8 eV photons. At
8 eV, the photoemission matrix element for the bulk
states is suppressed that the bulk signal is very weak(intensity at the * point near the Fermi energy). The
cone shape and fast dispersing band is the surface
metallic states. The surface band reaches the * point
near the 0.3 eV binding energy. This point is called
the Dirac point. After exposing the surface to a
relatively poor vacuum for 4 days ages the surface,
ARPES data was retaken and is shown in figure 1(b).
Note that the Dirac point has shifted to a higher
binding energy side by about 0.1 eV. This is due tothe electron doping effect by the adsorbed atoms andmolecules on the surface.
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 figure 2(a). In the data, it is noted that Im6
initially increases but then decreases near the bulk
band bottom energy (dashed lines). This is true forboth fresh and aged surfaces and thus strongly
indicates that the kinks are related to the bulk states.
The scattering of the quasi-particles in the metallic
surface states can be analyzed in terms ofelectron-electron, electron-phonon and impurity
scatterings. The analysis results in figure 2(b) showthat the quasi-particles in the surface states mostly
decay to the bulk states but not to other surface states.

Even though the above results provide us valuable
information, we still do not have the intrinsic
properties because the quasi-particle dynamics isdominated by the coupling to the bulk states. To
overcome this, we performed ARPES on higher
quality TIs, e.g., Bi2Te3 shown in figure 1(a). The
result plotted in figure 2(a) along with the result from
Sb show that the scattering rate is more or lessbinding energy independent. This constant term is
attributed to the contribution from uneven surfaces,
thus an extrinsic effect. These results strongly suggestthat the intrinsic life time of the quasi-particles insurface states is extremely long. This is an important
property for TIs as the candidate materials for the
future spintronic applications.

Fig. 1. ARPES data near the * point from TIs undervarious conditions. Bi2Se3 data from (a) fresh and (b)aged surfaces. (c) Data from fresh Bi2Te3 surface.

Fig. 2. (a) Imaginary parts of the self energy fromvarious TIs. (b) Contributions from various scatteringchannels.
[1] S. R. Park et al., Phys. Rev. B 81 (2010) 041405.
[2] S. R. Park et al., in preparation.

85

BL1A

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

Normalized Intensity

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. It is not apparent why
the huntite which is a rare mineral was used as apigment of the white. It is reported that the huntite is
produced in Nagasaki. For various light element
compounds, we have studied the electronic structureby X-ray absorption spectroscopy [1]. These results
suggested that there is a correlation between XANES
spectra and the local structures.
In this work, we performed the XANES spectra
measurement about various calcium salts and mineralssuch as huntite and dolomite. The obtained
experimental XANES spectra are analyzed using the
calculated theoretical spectra from DV-X
calculations. The X-ray absorption spectra were
measured at BL1A of the UVSOR in the Institute of
Molecular Science, Okazaki [4]. The ring energy of
the UVSOR storage ring was 750MeV and the stored
current was 110-230 mA. P K-edge absorption spectra
were recorded in the regions of 2125-2270eV by use of
two InSb crystals. The absorption was monitored by
the total electron yield using a photomultiplier. The
samples were spread into the carbon tape on the first
photodynode made of CuBe of the photomultiplier.
Figure 1 shows the observed Ca K-edge XANES
spectra for the Huntite and Dolomite. The Ca-K
XANES spectra of Huntite and Dolomite show
different peak profiles. This result shows that thecalcium in these minerals has different electronic
states. We are going to try to calculate the spectra by
DV-X molecular orbital calculations. The calculated
XANES spectra and the observed XANES spectra of
the Huntite and the Dolomite are shown in Fig. 2.
The observed XANES spectra and the calculated
XANES spectra showed good agreement. The peaks
A and B are estimated to the electron transition(mainly Ca 1s to unoccupied mixed orbital consistingof O 3d, O 3p, O 3s and O 4s).

4050

4100

4150
4200
Energy / eV

4250

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

4150
4200
Energy / eV

A

obs.
calc.

B

Intensity / a.u.

Intensity / a.u.

B

4100

Dolomite

obs.
calc.

Huntite
A

4100

4150
4200
Energy / eV

Fig. 2. The comparison of observed XANES
spectra with calvulated XANES spectra for Ca
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. Matsudo and M. Watatabe, Rev. Sci. Instrum. 63(1992) 1309.

86

BL1B

Excitonic Structures of PbMoO4 and PbWO4 Crystals
M. Fujita1, M. Itoh2, H. Mitani2, Sangeeta3 and M. Tyagi3
1
Japan Coast Guard Academy, Kure 737-8512, Japan
2
Dept. of Electrical and Electronic Engineering, Shinshu University, Nagano 380-8553,
Japan
3
Crystal Technology Laboratory, Bhabha Atomic Research Center, Munbai 400 085, India
PbWO4, and vice versa in PbMoO4. In PbWO4, peaks
c1 and c2 are observed clearly, while no appreciable
structure is seen on the weak plateau in the 3.4–4.0
eV region for E//c in PbMoO4. According to thetheoretical calculation [2], the magnitudes of thecontribution of Pb state to the top of the valence bandand the bottom of the conduction band of PbMoO4
are about 0.7 times as small as those of PbWO4. It is
supposed that the difference in magnitudes of thecontribution of Pb state is responsible for the different
features of the exciton band of PbMoO4 and PbWO4[3].

Metal tungstates and molybdates are widely used
for scintillation detectors. Therefore information on
the exciton states is deeply desired to understand thede-excitation processes after high-energy electronic
excitation in these materials. PbWO4 and PbMoO4
crystallize in the scheelite structure. We found thatPbWO4 exhibits a pronounced exciton band with
distinct dichroism, and proposed that the exciton
transition is explained in terms of the cationic Pb 6s
 6p excitation [1]. In the present study, reflectivity
spectra of PbMoO4 crystals are measured in order to
get more detailed information on the excitonic
structures in Pb compounds.
Figure 1 shows reflectivity spectra of PbMoO4 up
to 20 eV measured with the light polarized parallel tothe a-axis (E//a) and c-axis (E//c). Remarkabledichroism is seen in the region below 6 eV. In Fig. 2
are presented the imaginary-part 2 spectra of thedielectric function calculated from the spectra in Fig.
1 using a Kramers-Kronig analysis. The lowest
exciton band 1 for E//a has two peaks a1 and a2. On
the other hand, the spectrum for E//c exhibits a weakplateau in this region. The 2 spectra of PbWO4 [1]are also shown in Fig. 2 for the sake of comparison.
Common features are observed between the structuresin PbMoO4 and PbWO4; i. e. the lowest band 1 shows
doublet structure for E//a and the band 2 is observedfor E//c.
Theoretical calculation of the electronic structureof PbMoO4 (PbWO4) by the DV-X method indicatesthat the valence band and the conduction band aremainly composed of O 2p and Mo 4d (W 5d) states,
respectively, and the Pb state contributes appreciably
to the top of the valence band and the bottom of the
conduction band [2]. Similarity of the optical spectrain Fig. 2 suggests that the exciton transition of
PbMoO4 is also attributed to the cationic excitation as
in the case of PbWO4. In the scheelite structure, aPb2+ ion is at the site of S4 symmetry surrounded byeight O2 ions. The Pb 6p state splits into the 3,4
level and the 2 level due to the uniaxial crystal-fieldalong the c-axis. The band 1 observed for E//a andthe band 2 for E//c are ascribed to the transitions fromthe top of the valence band of the 1 level to the 3,4
level and the 2 level, respectively. The splitting of
the peaks a1 and a2 of the band 1 originates from thespin-orbit splitting of the Pb 6p state.
On the other hand, some differences are also
observed between the spectra of both materials. The
intensity of the peak a1 is stronger than that of a2 in

REFLECTIVITY

0.4
E//a
E//c

0.2

0.2
0
0

5

10
15
PHOTON ENERGY (eV)20

Fig. 1. Reflectivity spectra of PbMoO4 at 6 K for E//aand E//c.

E//a
E//c

15

a1
a2

ǭ2

1
10

PbMoO 4
a1

5

0

a2
1

3

c1

10

PbWO 4
2

5

c2
0

2

4
5
PHOTON ENERGY (eV)6Fig. 2. Imaginary part of the dielectric function of
PbMoO4 and PbWO4 for E//a and E//c in the region
below 6 eV.
[1] M. Fujita et al., Phys. Rev. B 65 (2002) 195105.
[2] M. Itoh et al., J. Phys. Soc. Jpn. 75 (2006)
084705.
[3] M. Fujita et al., Phys. Status Solidi B 247 (2010)
405.

87

BL1B

Identification of Cr3+ Impurity in LaAlO3 by Photoluminescence

E. Hirata and Y. Ohki
Dept. Electrical Engineering and Bioscience, Waseda University, Shinjuku 169-8555, Japan
Lanthanum aluminate (LaAlO3) has been attracting
much attention as a promising candidate for a gateinsulator in advanced metal-oxide-semiconductor
devices. By analyzing photoluminescence (PL)properties of LaAlO3, we have been examining
presence of localized states in the band gap, which
may induce leakage current.
Experimental
The samples examined are LaAlO3 (100) single
crystals grown by the Czochralski method and LaAlO3
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.
Results and Discussion
Figure 1(a) shows PL spectra induced by 6.4-eVphotons, obtained at 10 K for the crystal and the films
deposited and annealed on Si monocrystal substrates.
The films annealed at 600 and 700 ºC have no PL
peaks, while those annealed at 800, 900, and 1000 ºC
and the crystal sample have three sharp PL peaks at
about 1.60, 1.64, and 1.68 eV. Figure 1(b) shows PL
spectra similarly obtained for the films deposited on
CaF2 monocrystal substrates. When the films wereannealed at 800 ºC or higher, three PL peaks appear
similarly to the films on the Si substrates, but at muchhigher energies, namely at 1.86, 2.00, and 2.14 eV.
The XRD patterns shown in Fig. 2 indicate that thefilms annealed at 800 °C or higher are polycrystalline,
while those annealed at 600 and 700 °C are amorphous.
Comparison of XRD spectra between the two annealed
polycrystalline films on the Si and CaF2 substrates
indicates that the lattice constant is smaller if substrateis CaF2.
It is known that the luminescence due to Cr3+ in
Al2O3 appears only when the sample is crystalline [1].
This is due to energy state splitting of d orbitals in Cr3+
by the crystal field [2]. Furthermore, the PL spectral
shapes observed in the single crystal samples andpolycrystalline films on the Si substrate in the present
study and those of R lines of Cr3+ in LaAlO3 aresimilar [3]. Therefore, there is a possibility that thepresent LaAlO3 samples contain Cr3+ ions as animpurity. Since the energy state of Cr3+ ions is
significantly influenced by the crystal field [2], thedifference in PL peak energy among the samples
deposited on different substrates should be due to thelattice distortion.
The authors bought the single crystals andchemicals for making thin films from different
companies. Furthermore, it is known that Al2O3 often
contains Cr3+ ions as an impurity [1,4]. Therefore,
there is a high possibility that the raw material or oreof Al contains chromium as an impurity. Detection of
Cr3+ ions by inductively coupled plasma opticalemission spectrometry (ICP-OES) failed, indicatingthat the Cr3+ content in the present samples is below 1
ppm.
Single Crystal (×1/3)

Single Crystal (×3)

Films on CaF2 o
Annealed at 1000 C

PL Intensity [a.u.]

PL Intensity [a.u.]

Films on SioAnnealed at 1000 C

o

900 C

o

800 C

900 oC

o

800 C

o

700 C

o

700 C

o

o

600 C

600 C

2
2.2 2.32
2.2 2.3 1.5 1.6 1.8
1.5 1.6 1.8
Detected Photon Energy [eV] Detected Photon Energy [eV]

(128)

(312)

(208)

(300)

(122)

(024)

(202)

Film on CaF 2
Annealed at 1000 °C

Intensity [a.u.]

(110)

(a)
(b)
Fig. 1. Comparison of PL spectra induced by 6.4-eVphotons, obtained at 10 K in film samples depositedon Si substrates (a) and on CaF2 ones (b). The PL
spectrum obtained in the crystal sample is also shownin each figure (a, b).

Films on Si
Annealed at 1000 °C
900 °C
800 °C
700 °C
600 °C

30

40

50
60
2T [deg]7080Fig. 2. In-plane XRD patterns of LaAlO3 film
samples.
[1] J. Kakoš et al., J. Sol̄Gel Sci. Technol. 21 (2001)
167.
[2] S. Xia et al., Phys. Rev. B 35 (1987) 7671.
[3] J. Heber et al., Z. Phys. 246 (1971) 261.
[4] T.-P. Lee et al., J. Vac. Sci. Technol. B 22 (2004)
2295.

88

BL1B

Photoluminescence from a Bundle of Single-Walled Carbon Nanotubes
M. Itoh, T. Kajitani and H. Mitani
Department of Electrical and Electronic Engineering, Shinshu University, Nagano 380-8553,
Japan
The observation of photoluminescence (PL) fromsingle-walled carbon nanotubes (SWNTs) underlines
their enormous and promising potential for photonic
and optoelectronic applications. The PL is ascribed to
the radiative decay of free excitons (FEs).
In practical growth condition, about one-third of all
possible SWNTs exhibit metallic properties and theremaining two-third act as semiconductors. Therefore,
there is a broad consensus [1, 2] that, if one wants toobserve the PL from SWNTs, a semiconducting
SWNT should be spatially isolated, because thebundling leads to rapid energy transfer to metallic
SWNTs where the FEs decay nonradiatively. Thisconsensus, however, neglects an inviting possibility
that the FEs can emit PL before the energy transfer
takes place, because they have finite lifetimes. In thepresent study, we have examined whether thispossibility is ruled out or not.

chiral index [1, 2]. Of course, only a small part of FEs
could decay in semiconducting SWNTs by emitting
PL. From the present observation, however, it is
supposed that the coherent length of energy transfer is
not so long as one has predicted so far.
The emission intensity of the 950 nm peak wasnearly temperature-independent in the range T =5300 K, which suggests that the coherent length isnot so sensitive to the temperature.
The authors would like to thank Mr. Y. Aihara ofKeyence Co. for his help in the observation of
microscope images of our sample. This work was
supported by the CLUSTER (the second stage) of the
Ministry of Education of Japan.

Experiment
HiPco-grown SWNTs (CNI; USA) were dispersed
in ethanol with the help of a homogenizer. By
spraying the nanotube suspension onto a fleshly
cleaved BaF2 crystal, homogeneous SWNT film witha thickness of about 105 cm was prepared. PL wasobserved
by
an
Acton
SpectraPro300i
monochromator equipped with a liquid-nitrogencooled ICCD camera. This detection system hassuitable sensitivity in the wavelength range Om =2001000 nm, with a spectral resolution of 10 nm.
The excitation wavelength Oex was changed in 2 nm
steps with a resolution of 0.5 nm, and for each step
the emission spectrum was integrated over 300 sec.
Although as-made and aged films were examined, noclear difference was found between both results.
Morphological information of the new film was
obtained by using a digital microscope VHX-1000(Keyence).

m
10PPm
5
Fig. 1. Microscope image (u 5000) of an as-made
SWNT film investigated in the present study.

Results and Discussion
Figure 1 shows a microscope image (u 5000) of the
as-made film investigated in this study. From thefigure, it is obvious that our sample consists of abundle of well-dispersed SWNTs.
In Fig. 2 is presented the contour plot of thethree-dimensional emission-excitation spectrum of
our SWNTs measured at a temperature T = 5 K. An
emission band peaking at Om | 950 nm is clearlyobserved. This emission is excited in the range of Oex! 590 nm. Judging from these spectral characteristics,
we likely attribute the 950 nm emission peak to theradiative recombination of FEs in STWNs with (8, 3)

89

Fig. 2. Contour plot of the three-dimensionalemission-excitation spectrum of a bundle of SWNTs
dispersed on a BaF2 substrate measured at T = 5 K.
[1] S. M. Bachilo, et al., Science 298 (2002) 2361.
[2] P. H. Tan, et al., Phys. Rev. Lett. 99 (2007)
137402.

BL1B

Optical Study of the Insulator-Metal Transition in R2-xSrxNiO4
M. Uchida1, J. Fujioka2, Y. Kaneko2, Y. Onose1, 2 and Y. Tokura1, 2, 3
1
Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
2
Multiferroics Project, ERATO, Japan Science and Technology Agency (JST),Tokyo 113-8656, Japan
3
Cross-Correlated Materials Research Group (CMRG), ASI, RIKEN, Wako,
Saitama 351-0198, Japan

Insulator-metal transition in strongly correlated
electron systems has attracted great attention in thefield of condensed matter physics [1]. It results in thereconstruction of the electronic structure on a large
energy scale of several eV accompanying the changesof charge/spin/orbital states. Optical spectroscopy is
one of the most powerful probes for investigating thechange of high-energy electronic states systematicallyand provides the essential information forunderstanding the multi-degree-of-freedom coupledphenomena.
During this beam time, we measured the in-planereflectivity spectra for layered perovskite nickelates
R2-xSrxNiO4 (R : rare earth element) in an energy
region between 4 and 42eV at room temperatureusing the beam line BL1B. We obtained the opticalconductivity spectra by the Kramers-Kronig analysisof the measured spectra connected to the lowerenergy ones. R2-xSrxNiO4 is a typical two-dimensional
insulator-metal transition system. While diagonalstripe charge-spin ordering and checkerboard chargeordering are discerned around x~1/3 and x~1/2,
respectively, the insulator-metal transition occurs at
x~1 accompanying the melting of the charge orderedstate [2, 3].
First, we discuss the overall features of thehigh-energy electronic states. Figure 1 shows the
optical conductivity of Nd2-xSrxNiO4 (x=1.0) in theenergy range 0-35eV at room temperature. It is notedthat several peaks are observed in the spectrum.
Previous optical study reported similar spectra forother layered perovskites La2-xSrxMO4 (M = Cr, Mn,
Fe, and Co, x=1.0). The authors could successfully
assign the origin of the peaks based on the systematic
M-dependent energy shift [4]. Comparing our data
Nd2-xSrxNiO4 x=1.0

3

A
B

ED
4

[1] M. Imada et al., Rev. Mod. Phys. 70 (1998) 1039.
[2] S. Shinomori et al., J. Phys. Soc. Jpn. 71 (2002)
705.
[3] K. Ishizaka et al., Phys. Rev. B 67 (2003) 184418.
[4] Y. Moritomo et al., J. Phys. Soc. Jpn. 64 (1995)
4117.

Vab (103:-1cm-1)

Vab (103:-1cm-1)

6with them, three peaks above 10 eV (A, B, and C) canbe assigned to Nd4pNd5d, Sr4pSr4d, andO2sNi3d excitations, respectively. We also assign
two peaks D and E to the transitions from the O2pband to some higher-lying conduction bands. The
broad peak indicated by the solid triangle is remnantof the charge transfer (CT) gap excitation. The small
CT gap compared with the other layered perovskites
implies the instability of the insulator-metal transition
for M=Ni (except the superconducting transition for
M=Cu).
Next, we focus on the doping variation of theelectronic structure in the course of the insulatormetal transition. In Fig. 2, we show the low-energy
optical conductivity spectra of Nd2-xSrxNiO4 (x=0.51.3) at 10K. The magnitude of the gap rapidly
decreases with hole doping from x=0.5, reflecting themelting of the checkerboard-type charge order. While
the gap is almost closed around x=1.0, a conspicuous
peak structure is observed around 0.2 eV, indicating
the incoherent transport caused by the strong charge
correlation. In the higher doping range (x=1.3), thepeak shifts to the lower energy and is merged toDrude response. Thus, we show in terms of optical
spectra that the checkerboard type charge correlation
dominates the charge dynamics near the insulatormetal transition in this system.

C2

Nd2-xSrxNiO4

1.3
1.1

2

1.0
0.7

1
x=0.5

0
0

10
20
Photon Energy (eV)

0
0

30

0.2

0.4
0.6
0.8
Photon Energy (eV)1Fig. 2. Doping variation of the in-plane
optical conductivity of Nd2-xSrxNiO4 (x=0.51.3) at the lowest temperature, 10K.

Fig. 1. Optical conductivity spectrum of
Nd2-xSrxNiO4 (x=1.0) over wide energy
region at room temperature.

90

BL3U

Soft X-Ray Spectroscopy of DNA Duplexes
1

2

H. Yamane , W. Hua , H. S. Kato3, T. Hatsui4, M. Kawai3, 5, Y. Luo2, H. Ågren2 and N. Kosugi1
1
Institute for Molecular Science, Okazaki 444-8585, Japan
2
Dept of Theoretical Chemistry, Royal Institute of Technology, S-106 91 Stockholm, Sweden
3
The Institute of Physical and Chemical Research (RIKEN), Wako 351-0198, Japan
4
XFEL Project Head Office, RIKEN & JST-PREST, Hyogo 679-5148, Japan
5
Dept of Advanced Materials Science, University of Tokyo, Kashiwa 277-8501, Japandifferent photon energy (h = 394 eV) with weakintensity as compared to that appears at the imine site.
From the theoretical calculation [1], we found that theeffect of stacking between pairs is of little
consequence for the experimental XES spectra andthat these with a good approximation can be
represented just by one of its constituting base pairs
with the hydrogen-bonding interaction.
Furthermore, one can find that the elastic peak in
the XES spectra shows an asymmetric lineshape witha low-energy tail structure. The intensity of this tailstructure depends on h; in particular at the imine site,
the tail structure is more intense. This tail structure
may originate from the recombination emission [3]due to the fast motion of hydrogen bonded protons
between the pairs, which depends on the degree of
localization of the excited state. The observation of
the tail structure indicates that the excited * state at
the imine site is strongly localized, which agrees wellwith the scenario of a charge hopping transport in
DNA duplexes proposed by the previous resonant
photoemission work [2].

Introduction
The electronic structure and its properties in DNA
polymers have received great attention for possible
molecular nanoelectronics since the discovery of the
double helix structure of DNA. In the present work,
we have undertaken experimental and theoretical
study of soft X-ray spectroscopies for DNA base pairs,that is, fluorescence yield X-ray absorption (FY-XAS)
and X-ray emission (XES) spectroscopies applied topoly(dA)-poly(dT) (AT) and poly(dG)-poly(dC) (GC)DNA duplexes. We have found that the stacking ofpairs has very little influence on the spectra. The
X-ray spectra of a DNA composed by mixed WatsonCrick base pairs are well reproduced by linear
combinations of base pairs involved, and the amine
and imine nitrogens show noticeable differences asbuilding blocks in the FY-XAS and XES spectra [1].
Experiment
We prepared thick films of AT- and GC-DNA.
These duplexes were diluted with deionized water(17.8 M·cm) to a concentration of 1.25 mg/ml. The
solutions were casted on the SiO2/p-Si(111) substrates.
The film thickness is about 100–200 nm. Thesesamples are almost the same as those measured at theprevious resonant photoemission experiments [2].
The FY-XAS and XES experiments were performed
at BL3U. The FY-XAS spectra were measured by thehighly efficient fluorescence-yield method using an
MCP assembly with the center hole (F2223-21SH,
Hamamatsu Photonics K.K.). The XES spectra were
measured using a transmission-grating spectrometer.
All measurements were performed at 300 K. In order
to avoid the radiation damage of the sample during
the measurements, the X-ray beam spot on the sample
was continuously moved by scanning the sample.

HN H O

N
N

N H N
N

N

Intensity / normalized to photon flux

AdenineResults and Discussion
Figure 1 shows a set of the N K-edge FY-XAS
spectrum and the excitation energy (h) dependenceof the N K XES spectra for the AT-DNA film. One
finds in the XES spectra that the spectral difference
between the imine (–N=) and amine (–NH–) groups
can be distinguished; the resonance photoabsorption
for the imine nitrogens appears at h = 399.4 eV,
while that for the amine nitrogens appears at a higherh = 401.6 eV. At the imine site, the resonant XES
spectrum has two prominent features; one shoulder ath = 390 eV and one peak at h = 393 eV. At theamine site, in contrast, the peak structure appears at

CH3

ThymineOpoly(dA)-poly(dT)FY-XASN KD XES hv /eV
410.0
Elastic

1+

407.0
405.0
403.5
401.6
400.9
399.7

1
380

390

400

399.4
398.8

410

Photon Energy / eV

Fig. 1. Set of the N K-edge FY-XAS spectrum and the
N K XES spectra for the AT-DNA film. The
molecular structure of adenine and thymine, includedin the AT-DNA, at the configuration of the hydrogenbonding interaction are also shown.
[1] W. Hua et al., J. Phys. Chem. C, submitted.
[2] H.S. Kato et al., Phys. Rev. Lett. 93 (2004) 086403.
[3] Y. Ma et al., Phys. Rev. Lett. 71 (1993) 3725.

91

BL3U

Local Intermolecular Interaction in -Crystalline Films of
Zinc-Phthalocyanine Studied by Soft X-Ray Emission Spectroscopy
H. Yamane1, T. Hatsui2 and N. Kosugi1
Institute for Molecular Science, Okazaki 444-8585, Japan
2
XFEL Project Head Office, RIKEN & JST-PREST, Hyogo 679-5148, Japan1

substrate temperature, form an ordered -crystalline
structure where the molecular plane is tilted of about
70° with respect to the substrate surface, while themolecules deposited at 84 K form an amorphous
structure.
The normal XES spectra, which reflect theoccupied pDOS, do not show the clear difference
between the crystalline and amorphous film (notshown). On the other hand, as shown in Fig. 1, thedistinct crystalline structure dependence wasobserved in the RXES spectra. The elastic peak in the
C K RXES shows a symmetric lineshape for boththe crystalline and amorphous films, but the elastic
peak in the N K RXES shows an asymmetriclineshape for the crystalline film, while theamorphous film gives the symmetric elastic peak,which is the indication of the presence of the Raman
active intermolecular interaction at the N site in the
crystalline film. As the origin of the asymmetricelastic peak, the vibronic excitation that accompanies
direct recombination emission [2] can be considered.
In this context, the broad peak around 0~1 eV in therecombination emission suggests the higher degree of
localized character of the * states as compared to thecase of graphite [2] and DNA molecules [3].

Introduction
The photon-in/photon-out technique of X-ray
emission spectroscopy (XES) measures X-rays
emitted as a resonant Raman scattering or decay
process of the inner-shell excitation, which gives
information about element-specific excitations andoccupied partial density of states (pDOS) of
unionized and ionized materials. This technique has
no restriction about samples and its atmospheres, andmoreover, it is known as bulk sensitive methods(typically, hundreds of nm for soft X-rays).
In this work, we investigated the local electronic
structure of Zn-phthalocyanine (ZnPc), which is oneof the promising materials for organic photovoltaics.
In order to study the local intermolecular interactionin ZnPc solids, we prepared the crystalline andamorphous films of ZnPc. We found a distinct
crystalline structure dependence in the resonant XES(RXES) spectra, which can be originated from thesite-specific intermolecular interaction.
Experiment
The ZnPc sample was prepared by the vacuum
evaporation onto the naturally oxidized p-Si(111)
surface. The deposition amounts of 400 nm anddeposition rate of 1 nm/min were measured with aquartz crystal microbalance. The crystalline ZnPc
film was obtained at the substrate temperature during
the deposition of 340 K, while the amorphous ZnPc
film was obtained at 84 K. The crystallinity of these
ZnPc films at the room temperature of 300 K wasconfirmed by X-ray absorption spectroscopy (XAS)
and by X-ray diffraction (XRD).
The XES measurements were performed by using atransmission grating spectrometer at BL3U of the
UVSOR facility [1]. The incident soft X-ray beam
was focused to 40 m (horizontal) × 20 m (vertical)
at the sample position. All XES measurements were
performed at 300 K. For the XES spectra, we need asmall photon spot size and long accumulation timesof 60–90 min per 1 spectrum. Even in the photon fluxof 1011 photons/sec range, the XES spectra of ZnPc
films show the evidence for the radiation damage
within 10–15 min X-ray irradiation. In order to avoid
the radiation damage, the X-ray irradiated spot wascontinuously moved by scanning the sample at therate of 20 m/min during the measurements.

Intensity / arb. units

Resonant XES
Cryst.ZnPc
Amorp.ZnPc

N
N

N
Zn

N
N

×0.2

N
N

N

Fluorescence

hv = 284.1 eVC KD
hv = 398.1 eV
N KD
20

10

0

Energy (vs. Eelastic) / eV

Fig. 1. The C K and N K resonant XES spectra of
crystalline and amorphous films of ZnPc. The C Kand N K excitation energies h were set to the firstC K and N K resonance peaks in XAS.
[1] T. Hatsui et al., J. Electron Spectrosc. Relat.
Phenom. 144-147 (2005) 1059.
[2] Y. Ma et al., Phys. Rev. Lett. 71 (1993) 3725.
[3] H. Yamane et al., a separate page in this volume.

Results and Discussion
Judging from the XRD and XAS results (notshown), the ZnPc molecules, deposited at the 340 K

92

BL4B

Dilute Multi Ferroic Semiconductor GaCrN-II
S. Emura1, H. Tambo1, K. Higashi1 and K. Fukui2
ISIR, Osaka University, Ibaraki 567-0047, Japan
2
Faculty of Engineering, University of Fukui, Fukui 910-8507, Japan1

induced around the doped Cr ion. Here, it should benoticed that there are four nearly equivalent legs
because the original site symmetry is nearly Td, andwe cannot distinguish these four legs from the XAFS
analysis. The parallel shifts may randomly be
distributed in the matrix or ordered along somedirection of the matrix.
The GaN:Cr is one of the dilute magnetic
semiconductors. From our observation, it behave as aferro-electric and elastic material. The coupling among
those may lead to substances of a new category inmulti-ferroic research field.

Semiconductor systems with a wurtzite structure
carrying a small amount of transition metals sometimes
show the various physical properties. GaN:Cr is one ofthose. This substance exhibits magnetism andferro-electrics. Therefore, we can expect to observe theanisotropic phenomena. The specimens studied here
are single crystals in the thin layer form on a MgO andan Al2O3 substrate. On the MgO substrate, the crystal
constructs the cubic structure (zinc-blende structure)and grows with a wurtzite structure on the Al2O3
substrate.
Figure 1 exhibits the incident angle dependence of
the absorption spectra of the crystal with the wurtzite
structure and CrN with rock salt structure for thereference in VUV region observed at BL4B at UVSOR.
At zero angles, the electric field of incident photon isvertical to the crystal c-axis of GaN:Cr. The absorption
band around 28 nm (44.3 eV), which is tentatively
assigned to the transition from 3p of the Cr atom to thes-like conduction band of matrix because of the wide
half width, strongly depends on the direction of theelectric polarization of the incident photon. In theparallel electric field to the c-axis, the band vanishes,
indicating anisotropy along the c-axis. As the XAFS
analysis of these specimens predicts the spontaneous
elastic deformation (quasi Jahn – Teller effect)
involving the local electric polarization, this finding is anevidence of the ordering of the deformation andconsequent electric polarization.
Figure 2 present the linear dichroism spectra for the
cubic GaN:Cr in X-ray energy region. Here, theintensity of the spectra is normalized at the end (6030eV) of the spectra shown in the figure. A clear doublet
peak in the pre-edge region, which is assigned to thetransition from 1s to 3d, and splits owing to the ligand
field, can be found. The lower peak of the doublet
exhibits the explicit linear dichroism. The observation
indicates the ordering of the deformation in the cubic
GaN:Cr also. In conclusion, the both hexagonal andcubic GaN:Cr crystals show the Jahn – Teller
deformation around the Cr ions and the deformation isin ordering.
The detailed VUV and X-ray excitation spectra areobserved. The strong angular dependence of the peakaround 28 nm and linear dichroism at 5988 eV are
found. This observation of the dichroism in GaN:Cr
with the cubic structure indicates that the CrN4
tetrahedron in the local coordination at the Cr ion
suffers deformation. The Cr ions may shift slightly
from the lattice center. it is apparent from the figuresthat the local spontaneous electric polarization isFig. 1. Angular dependence of the absorption spectra.
.

Fig. 2. Linear dichroism spectra in X-ray energy region.

93

BL4B

Charge State Analysis of Mn Ions in Pr1-xCaxMnO3-G 㩷
by Mn-L2,3 XANES Spectra
H. Kanamori, T. Yoshioka and T. Yamamoto
Faculty of Science and Engineering, Waseda University, Shinjuku 169-8555, Japan

Pr1-xCaxMnO3-G has been extensively studied,because it has unique electronic and magnetic
properties such as colossal magnetoresistance andmetal-insulator transition [1]. In order to understandsuch properties, it is essential to know the charge
state, i.e., valence, of Mn ions in these materials.
There are two scenarios to compensate the system; i)valence of Mn ions control the system, i.e., from
Mn3+ to Mn4+, ii) oxygen vacancy is created, when Ca
is incorporated into PrMnO3. However, thesemechanisms have not yet been understood. Then we
have performed the charge state analysis of Mn ionsin Pr1-xCaxMnO3-G by the Mn-L2,3 X-ray absorption
near-edge structure (XANES) measurements.
Pr1-xCaxMnO3-G samples are synthesized by theconventional solid-state reaction method. Mn-L2,3
XANES spectra were observed at BL4B in UVSOR by
the total electron yield (TEY) method. The incident
beam was monochromatized with agrating (800 lines/mm) double crystal monochromator.
Prior to XANES analysis, all the samples werecharacterized by the X-ray diffraction (XRD)
Pr1-xCaxMnO3-G crystallizes perovskite structure atambient condition. No extra peaks can be found in
the observed XRD patterns except for those of theperovskite structured Pr1-xCaxMnO3-G.
Observed Mn-L3 XANES spectra of the samples
here synthesized are shown in Fig. 1. Valences of Mn
ions in PrMnO3 and CaMnO3 are 3+ and 4+,
respectively, when it is assumed that there is nooxygen vacancy in these materials. Spectral profilesof Mn-L3 XANES spectra of these two materials
show significant difference. Those of Pr1-xCaxMnO3-G
with x=0.3, 0.4, 0.5 changes from the shape of Mn3+
to Mn4+ as increment of Ca concentration. Theseexperimental results suggest that valence of Mn ionsin Pr1-xCaxMnO3-G changes continuously as incrementof Ca concentration.

100

CaMnO 350

Normalized Intensity

0100

Pr0.5Ca0.5MnO 3-㱐

50
0
100

Pr0.6Ca0.4MnO 3-㱐

50
0
100

Pr0.7Ca0.3MnO 3-㱐

50
0
100

PrMnO 3

50
0
630

635

640

Energy (eV)Fig. 1.
Observed Mn-L3 XANES spectra ofPr1-xCaxMnO3-G.
[1] Y. Tokura and N. Nagaosa, Science 288 (2000)
462.

94

BL5U

Three-Dimensional Angle-Resolved Photoemission Study on EuFe2As2
T. Hajiri1, R. Niwa2, S. Murakami2, H. Miyazaki3, J. B. Hong4, Y. R. Jang4, S. Kimura3, 5,
Y. S. Kwon4 and T. Ito1, 6
1Graduated 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
Department of Physics, Sungkyukwan University, Suwon 440-749, Korea
5
School of Physical Sciences, the Graduate University for Advanced Studies (SOKENDAI),
Okazaki 444-8585, Japan
6
Nagoya University Synchrotron Radiation Research Center, Nagoya University, Nagoya
464-8603, Japan
Iron pnictides superconductor discovered recently
has been intensively studied both from experiment andtheory, however an essential mechanism of its
anomalous superconductivity has not been clarified
yet.
To
understandthemechanismofsuperconductivity,
thesuperconducting
gap
information, i.e., the origin of superconducting
pairing, is one of the most important issues.
To clarify the fundamental electronic structure ofIron pnictides, especially of ‘122’ series, we have
performed
angle-resolved
photoemission
spectroscopy (ARPES) on EuFe2As2 in the heavily
underdoped regime where the spin density wave(SDW) formation at TSDW=190 K as well as an
antiferromagnetic transition at TN=20 K has been
expected [2].
Figure 1 (a) and (b) shows momentum distribution
curves (MDC) at EF±50 meV together with theexperimental band structure along X and ZR,
respectively,
obtainedbyplottingthesecond-derivative intensity of ARPES spectra of
EuFe2As2 at 30 K. We have found two and one
hole-like FS’s around * and Z points, respectively,
while the band structure calculation on the ‘122’ series
BaFe2As2 expects one and two hole-like FS’s aroundthe * and Z points [3] in against to the present ARPES
result.
Furthermore, from the inter-plane FS alongkz-direction (Fig. 2) obtained by utilizing the photon
energy dependent ARPES on the same sample
EuFe2As2 at 30K, we found that the hole-like FS
observed around the * point has been separated to thetwo hole-like FS as approaching to the Z point together
with the reduction of FS sizes. This suggests that asingle hole-like FS pocket around * point originatingfrom the combination of two hole-like FS
degenerating within our experimental momentum
resolution ('T~0.17°~'k~0.011 Å-1 at hQ=38 eV).

Fig. 1. Band structure along X (a) and ZR (b) axis
obtained by plotting the second-derivative intensity of
ARPES spectra of EuFe2As2 with hQ=24 eV (a) and
38 eV (b), respectively, at T=30 K. Upper panel
shows MDC spectrum at EF±50 meV.

Fig. 2. Fermi surface image on *XRZ plane of
EuFe2As2. Open circles and squares are the peak
positions of MDC spectra at EF. Thin lines are guidefor the eyes.
[1] Y. Kamihara et al., J. Am. Chem. Soc. 130 (2008)3296.
[2] H. S. Jeevan et al., Phys. Rev. B 78 (2008)
020503(R).
[3] Walid MALAEB et al., arXiv: 0906 (2009)1846v1.

95

BL5U

Ce 4d-4f Resonant Angle-Resolved Photoemission Studies on Quantum
Criticality in Heavy-Fermion Ce112 Systems
H. J. Im1, H. Miyazaki2, T. Ito3, S. Kimura2, 4, K. E. Lee5 and Y. S. Kwon5
Department of Advanced Physics, Hirosaki Univerisy, Hirosaki 036-8561, Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
3
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
4
School of Physical Sciences, The Graduate University for Advanced Studies, Okazaki444-8585, Japan
5
Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea
1

We have performed Ce 4d-4f resonant
angle-resolved
photoemission
spectroscopy(rARPES) on CeNiGe2 and CeNi0.7Co0.3Ge2, whose
grounds states are antiferromagnetism (AFM) andquantum critical point (QCP), respectively, in thebeamline BL5U of UVSOR. In this weakly
hybridized regime, Ce 4f-electrons are considered tobe highly localized due to the large repulsive
Coulomb interaction between f-electrons and theweak hybridization strength between f- andconduction electrons. In addition, the localized
character of f-electrons has been a key point tounderstand QCP. Therefore, the systematicinvestigation of Ce 4f-character in weakly hybridized
regime is inevitable.
Figure (a) shows the high-resolution on-rARPES
spectral image of CeNiGe2. In order to investigate themomentum-dependence of Kondo resonance (KR)peaks near the Fermi level (EF), the spectra (energy
distribution curves, EDCs) are normalized to theintensity of Ce 4f17/2 peak around -0.3 eV: it is wellknown that the intensity ratio of Ce 4f15/2 peak (thetail of KR peak) to Ce 4f17/2 peak empirically
represents the strength of Kondo coupling inangle-integrated photoemission (AIPES) spectra.
Near EF, the momentum-dependence of Ce 4f15/2 stateis clearly observed. Besides the above Ce 4f-bands,
there is another linearly dispersive band from -0.6 eV
to EF with weak intensity. This band is composed of
non-f states and corresponds to conduction band. We
find that the intensity of KR peak is largely enhanced
where the conduction band crosses EF as in
CeCoGe0.8Si1.2, whose ground state is non-magnetism
due to large hybridization strength [1]. Suchbehaviors are also observed in CeNi0.7Co0.3Ge2 (notshown here). As a result, we conclude that thecharacter of f-electrons is itinerant across QCP inagreement with the results of AIPES studies ofCeNi1-xCoxGe2 [2].

Fig. (a) Ce 4d-4f on-rARPES image of CeNiGe2(hv = 122 eV, T = 10 K). (b) The energy distribution
curves at k = 0 Έ-1 (red line) and k = 0.6 Έ-1 (blackline).
[1] H. J. Im et al., Phys. Rev. Lett. 100 (2008)
176402.
[2] H. J. Im et al., Phys. Rev. B 72 (2005) 220405.

96

BL5U

Temperature-Dependent Photoemission Spectra of
Alkanethiolate-Passivated Au Nanoparticles
M. Imamura, T. Fujino, S. Fujimasa and H. Yasuda
Department of Mechanical Engineering, Kobe University, Kobe 657-8501, Japan

Metallic nanoparticles attract much attention due totheir characteristic properties such as Coulomb
blockade, nonlinear optical response, and catalytic
activity. In order to elucidate their detailed intriguing
properties and to develop the future devices, it isindispensable to understand the electronic structures.
In this work, we have carried out thesynchrotron-radiation photoemission study of
dodecanethiolate-passivatedAu
nanoparticles
supported on the HOPG substrate at various
temperatures.
The synthesis procedure of DT-passivated Au
nanoparticle is described elsewhere [1]. The
synchrotron photoemission study was carried out with
the incident photon energy of 190 eV at BL-5U of
UVSOR facility.
Figure 1 shows the synchrotron-radiation
photoemission spectra of DT-passivated Au
nanoparticles with the mean diameter of 2.5 nm in thevicinity of Fermi-level measured at room temperature,
160, and 100 K. The leading edges of the spectra in
the vicinity of Fermi-level of all the Au nanoparticles
at various temperatures moved away from Fermi-level.
Furthermore, the photoemission intensities of theleading edges decrease with decreasing themeasurement temperature, and change to thebackground intensity of HOPG. In our previous work,
we have reported that the photoemission spectra in thevicinity of Fermi level of DT-passivated Ag
nanoparticles show the deviation from the metallic
Fermi-edge, and concluded that these spectra reflect
the final-state effect [1, 2]. We have explained theexperimental spectra using a theoretical model thattakes into account the influence of the photohole
remaining on the nanoparticle during thephotoemission process and the nanoparticle-substrateinteraction. The shift of Fermi edge in the present
photoemission spectrum measured at room
temperature is well explained by the same picture.
Since the final-state effects do not depend on themeasurement
temperature,thetemperaturedependence of present photoemission spectra isconsidered to originate from the change of intrinsic
electronic structure of Au nanoparticles. Kubo
criterion predicted that the discrete energy states are
exhibited when the mean energy-level spacing near
Fermi-energy exceeds the characteristic thermalenergy. In the present case, Au nanoparticles at room
temperature remain bulk-like electronic structure,since the energy level spacing is narrower than thethermal broadening. With decreasing temperature, themean energy-level spacing of Au nanoparticles
exceedsthecharacteristic
thermalenergy.
Consequently, Au nanoparticles is expected to havediscrete electronic structures. The discrete and low
densities of states of Au nanoparticle will lead todecrease the photoemission intensities. From theseresults it is concluded that the temperature-dependent
photoemission spectra in the vicinity of Fermi-level is
attributed to temperature-dependent metal-insulatortransition that is theoretically predicted.

Fig. 1. Synchrotron-radiation photoemission spectraof DT-passivated Au nanparticles supported on the
HOPG substrates in the vicinity of Fermi-levelmeasured with the photon energy of 190 eV at room
temperature, 160K, and 100K.
[1] A. Tanaka, Y. Takeda, M. Imamura and S. Sato,Appl. Surf. Sci. 237 (2004) 537-542.
[2] M. Imamura and A. Tanaka, Phys. Rev. B 73
(2006) 125409-1-125409-5.

97

BL5U

Detection of Spin-Filtered Surface-States of a Topological Insulator
by Polarization Dependent Angle-Resolved Photoemission Spectroscopy
A. Nishide, F. Nakamura and I. Matsuda
Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, JapanThe quantum spin Hall (QSH) effect, whichmakes it possible to produce topologicallyprotected spin currents without magnet, has now
attracted interests of researchers in condensed
matterphysics.
Recently,
photoemission
spectroscopy (PES) studies [1] have mapped band
dispersion curves of the surface states of thesemiconductor alloy, Bi1-xSbx (x~0.1), andhigh-resolution spin-resolved PES measurements[2] have determined their spin orientations between
the two time-reversal invariant momenta. Theseexperimental results have clearly indicated a traceof the predicted topological band structure, giving
direct evidence of the topological insulator. Theseprevious works have also demonstrated that PES is
the most powerful tool for such researches. On theother hand, it has been known that, during the PE
process, photoexcitation of electrons with different
spin orientations depend on circular polarizationsof incident light [3]. Thus, we conductedpolarization dependent angle-resolved PES
measurements to obtain direct evidence ofspin-filtered surface-states of the QSH phase. The
experiment was performed at UVSOR BL-5U with
the Apple-II type undulator, generating right andleft helical light. As shown in Fig., we observed
clear variations of photoemission intensity with thecircular polarizations. Moreover, we succeeded in
detecting two surface-state bands [2], 62 and 61’,

Fig. Schematic drawing of the energy contour mapof Bi1-xSbx (x=0.15) around Fermi level with aseries of (a,b) experimental photoemission spectra
taken at 20K.
Acknowledgements
Alexey A. Taskin and Yoichi Ando in Osaka
University are gratefully acknowledged for sample
crystals and valuable comments

㨋of the three-dimensional QSH phase at the M
point, Fig.(b), confirming the non-trivial
topological nature of Bi1-xSbx (x=0.15).

[1] D. Hsieh et al., Nature 452 (2008) 970.
[2] A. Nishide et al., Phys. Rev. B 81 (2010)
041309 (R).
[3] M. Mulazzi et al., Phys. Rev. B 74 (2006)
035118.

98

BL5U

Resonance Photoemission Study of Pseudo-One Dimensional Cobalt Oxide
Ba3Co2O6(CO3)0.7
K. Soda1, M. Kikuchi2, H. Miyazaki1, 3, M. Inukai1, 4, M. Kato1, S. Yagi1,
K. Iwasaki1, 5 and T. Matsui1, 6
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
Venture Business Laboratory, Nagoya University, Nagoya 464-8603, Japan
5
Toyota Boshoku Coporation, Kariya 448-8651, Japan
6
EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan
Recently it has been found that a pseudo-one
dimensional Ba3Co2O6(CO3)0.7 shows a fairly large
thermoelectric power factor of 0.9 mWm-1K-2 at 300
K with the thermoelectric power of about +120 PV
K-1 and metallic behavior of its electric conductivity
above 300 K [1]. In order to understand its physicalproperties, we have investigated its valence-band
electronic structure by photoelectron spectroscopy.
Photoelectron measurements were carried out withuse of single-crystalline specimens prepared in size of
5 x 0.5 x 0.5 mm3 by a flux method [1]. A Pt wire was
attached to one end of the specimen to avoid thecharging up at low temperatures, and the cleansurface was obtained by in situ fracturing another endof the specimen in perpendicular to the c axis.
Figure 1 shows typical valence-band photoelectron
spectra recorded at 20 K with the excitation photon
energies hQ of 60 and 75 eV and their difference
spectrum. Each spectrum is normalized with the
integrated intensity and subtracted the background by
an iteration method [2]. Observed features A to C areascribed to the hybridized bands of the Co 3d and O
2p states, and the Co 3p-3d resonance photoemission
shows the relatively large Co 3d contribution to thefeatures A and B, as seen in the difference spectrum.
The other features D, E, F, G and H are assigned to
the CO3-derived states, the surface components, Ba
5p spin-orbit doublets and O 2s state, respectively.
Figure 2 shows temperature dependence of thevalence-band spectra near the Fermi level EF together
with reference Au spectra. Ba3Co2O6(CO3)0.7 reveals
large reduction in intensity towards EF but clear finite
intensity at EF, which suggests the electron doping
into the low-spin Co4+(t2g 3d5) bands and the positive
thermoelectric power of 81 or 141 PV K-1 at hightemperatures for the Co4+ concentration x of 0.7 [3].
Although a 1/8-power-law dependence of theintensity on the binding energy EB might be expected
in one-dimensional fermion system [4], theanomalous exponents from 0.5 at 20 K to 0.8 at 200
K are obtained for EB = 0.01~0.1 eV, which suggests
the larger short-range interaction at the lower
temperature. A small hump at EB ~ 0.02 eV observed
at T = 20 K might also indicate the opening of apseudogap or a magnetic ordering, which may cause
the recently observed reduction in the electric
conductivity at low temperatures [5].
2

INTENSITY / arb. units

Ba3Co2O6(CO3)0.7
hQ / eV7560
difference

1

A

C
B

D
F
G

H

E

0
20

15
10
5
BINDING ENERGY / eV0

Fig.1. Valence-band spectra and difference spectrumof Ba3Co2O6(CO3)0.7.

T/K
200
100
20

INTENSITY / arb. units

0.05

1

EB1/8

Au

0

0
0

Ba Co O (CO )

0
0.15

3

2

0.1

6

3 0.7

0.05
0
-0.05 -0.1
BINDING ENERGY / eV

-0.15

Fig.2. Temperature dependence of valence-band
spectra near the Fermi level of Ba3Co2O6(CO3)0.7.
[1] T. Yamamoto et al., Proc. Int. Symp. on EcoTopia
Science 2007, p.145.
[2] K. Soda et al., J. Phys. Soc.Jpn. 60 (1991) 3059.
[3] W. Koshibae et al., Phys. Rev. B 62 (2000) 6869.
[4] J. Voit, Pep. Prog. Phys. 57 (1994) 977.
[5] K. Iwasaki, private communication.

99

BL6B

Terahertz Spectroscopy of Doped Superionic Conductors
T. Awano
Department of Electronic Engineering, Tohoku Gakuin University, Tagajo 985-8537, Japan
RbA4X5 (A=Ag or Cu, X=halogen) crystal has highionic conductivity as liquid even at room temperature.
Silver (copper) ions move fast in conductive channels
which are composed of silver (copper) sites
surrounded by iodide tetrahedra. Sixteen ions in a unit
cell are able to occupy 56 such sites. These available
sites for the conducting ion are not enough to avoid
Coulomb repulsion by each other. This suggests thatcollective movement of conducting ions should occur.
Such correlative motion should be enhanced largely
by doping excess conduction ions. In this study,
terahertz spectra of excess silver (copper) doped
RbA4X5 and silver (copper) vacancy doped one were
investigated to study such effect by the collective
motion.
Excess silver was doped by evaporation of silver
film on the crystal and diffusion by heating. Vacancy
of silver was introduced by iodine vapor addition atroom temperature.
Figure 1 shows reflectivity spectra of RbAg4I5:Ag
and RbAg4I5:I2. The latter corresponds to introducing
silver vacancy. Figures 2 and 3 show dielectric
constants obtained by Kramers-Kronig analysis. The
dielectric constants decreased by silver addition andincreased by iodine addition in the spectral region
below 20cm-1. This spectral region corresponds to"attempt mode" which is a movement of a silver ion
going out from surrounding iodide tetrahedral cage.
Therefore spectral change of this region seems toconcern dynamics of the ion conduction strongly.
In the study of Raman scattering spectra of doped
RbAg4I5, drastic spectral changes were reported
previously1, 2. The scattering intensity in terahertz
region (under 50cm-1) increased by silver additionand decreased by iodine addition. Moreover finestructures at low temperature disappeared in the caseof iodine addition2.
The FIR results of this study are contrary to thoseof the Raman spectroscopic study. The intensitychange of low wave number region was exactly
reverse between them. The fine structure did notdisappear in the FIR spectra. These differences
between them seem to be due to difference of opticalprocess between FIR and Raman spectroscopy.

0.4
0.3
0.2

RbAg4I5
RbAg4I5 :Ag
RbAg4I5 :I2

298K

RbAg4I5
RbAg4I5 :Ag
RbAg4I5 :I2

78K

RbAg4I5
RbAg4I5 :Ag
RbAg4I5 :I2

8K

REFLECTIVITY

0.1
0
0.4
0.3
0.2
0.1
0
0.6
0.4
0.2
0
10

50
WAVE NUMBER (cm -1)

100

Fig. 1. Reflectivity spectra of doped-RbAg4I5.
10
8
6
4
2

RbAg4I5
RbAg4I5 :Ag
RbAg4I5 :I2

298K

10
78K

ǭ1 5

RbAg4I5
RbAg4I5 :Ag
RbAg4I5 :I2

0

8K

10
RbAg4I5
RbAg4I5 :Ag
RbAg4I5 :I2
50
100
WAVE NUMBER (cm -1)

0
-10
10

Fig. 2. Real part
doped-RbAg4I5.

ofdielectric

constant

of

8
6
4

RbAg4I5
RbAg4I5 :Ag
RbAg4I5 :I 2

298K

RbAg4I5
RbAg4I5 :Ag
RbAg4I5 :I 2

78K

RbAg4I5
RbAg4I5 :Ag
RbAg4I5 :I 2

8K

2
0
10

ǭ2

5
0

[1] S. Bredikhin, T. Hattori and M. Ishigame, Solid
State Ionics 67 (1994) 311.
[2] S. Bredikhin, N. Kovaleva, T. Hattori and M.
Ishigame, Solid State Ionics 74 (1994) 149.

15
10
5
0
10

50
WAVE NUMBER (cm -1)

100

Fig. 3. Imaginary part of dielectric constant of
doped-RbAg4I5.

100

BL6B

Evolution of c-f Hybridization State of CeIn3

T. Iizuka1, T. Mizuno1, C. I. Lee2, Y. S. Kwon2, S. Kimura3, 1
1School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI),
Okazaki 444-8585, Japan2
Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea
3
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan

Experimental
The single phase CeIn3 was synthesized by anarc-melting method under argon atmosphere, and thenwas annealed at 600 qC for three weeks inside anevacuated quartz tube. The THz reflection
spectroscopy under pressure was performed at theTHz micro-spectroscopy end station of BL6B,UVSOR-II. A diamond anvil cell was employed to
produce high pressure. KBr was used as a pressure
medium and gold as a reference. The pressure wascalibrated by a ruby fluorescence method.

17meV. The peak corresponds to the optical transition
between the bonding and antibonding state of the c-f
hybridization state. The peak shifts to higher photon
energy side with increasing pressure. The peak shift isconsistent with the increase of the c-f hybridization
intensity behavior predicted by PAM.
The c-f hybridization state is evolved not only inthe nonmagnetic state but also in the AFM state. This
result supports the spin-fluctuation (SDW) model [1,3], which has a phase boundary dividing the
local/itinerant regime in the AFM phase.

T*

b.
a.

TN

10%

AFM
0.2 GPa
0.9

Reflectivity

Introduction“Heavy fermion” attracts attention because of theirvarious physical properties, which originate from thehybridization between the localized 4f and conduction
electrons, namely c-f hybridization [1]. According to
the periodic Anderson model (PAM), which
theoretically describes the change of electronic
structure with the c-f hybridization, the bonding andantibonding bands due to the c-f hybridization are
created. The size of the energy gap increases withincreasing c-f hybridization intensity.
The purpose of this study is to investigate theevolution of the c-f hybridization state. The
hybridization intensity can be controlled by applying
pressure. The target material of this study, CeIn3,
which has a localized 4f-electron, is theantiferromagnetic (AFM) ground state (the Néel
temperature TN ~ 10 K). With applying pressure, TN ismonotonically suppressed and is disappeared at PC =2.6 GPa. Then a superconducting phase appears. In
addition, from a NQR measurement on CeIn3 underpressure, there is T*-line which divides the local
regime (T* > T) and the itinerant regime (T* < T), at
around 2 GPa in the nonmagnetic state [2]. Therefore
this material is suitable to investigate the evolution of
the c-f hybridization state by applying pressure.

1.7
2.1
2.7

CeIn3
5.6 K

15

20

25

Photon Energy (meV)Fig. 1. (a) Pressure-dependent reflectivity spectra[R-(Z,-P)] of CeIn3 at 5.6 K. Triangles suggest peak
positions. (b) Temperature-pressure phase diagram
previously obtained from a NQR measurement (TNéel
and T*) [2]. T* divide local (blue mesh) and itinerant(red mesh) regime. The solid circles indicate thepositions where the optical spectra were taken.

Results and Discussion
We obtained pressure-dependent reflectivity
spectra [R (Z, P)] of CeIn3 at 5.6 K in the pressure
region from 0.2 to 2.7 GPa as shown in Fig. 1a.At thelowest pressure (P = 0.2 GPa), the spectrum intensity
monotonically decreases with increasing photon
energy indicating the normal metallic character. At
0.9 GPa in the AFM phase, a peak appears at around[1] P. Gegenwart et. al., Nat. Phys. 4 (2008) 186.
[2] S. Kawasaki et. al., J. Phys. Soc. Jpn. 73 (2004)
1647.
[3] S. Watanabe and K. Miyake, J. Phys. Soc. Jpn. 79(2010) 033707.

101

BL6B

Magneto-Plasma Reflection Study of Superconducting InN
T. Inushima1, T. Iizuka2, S. Kimura2, 3 and K. Fukui4
Department of Electronics, Tokai University, Hiratsuka 259-1292, 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
4
Department of Electrical and Electronics Engineering, University of Fukui, Fukui, 910-8507,
Japan
1

Among the III-nitride semiconductors, InN is a key
material for optical and high temperature device applications. Since it was reported that InN had asmall band gap energy (0.64 eV) and a superconducting character [1], it has become a big issue to determine the electron effective mass. Hence we measured the electron effective mass of superconducting
InN from the reflection spectra with the cyclotron resonance.
The InN film (GS1949) investigated here wasgrown by the Cornell University group and had a carrier density of ~4.5u1017cm-3 and a mobility of ~2000
cm2(Vs)-1. The sample showed Meissner effect below 1.8 K. It was set in an Oxford spectromag 6T andthe reflectivity was measured from 50 to 660 cm-1 at
6 K by Jasco FTIR610 at Faraday configuration.
The optical resolution was 0.5 cm-1.
Figure 1 shows the reflectivity spectrum at 6 K.
It is analyzed using the following dielectric function(n1),
n12 (Z) H f (1 

Zc

eB
,
m*

face and a unique effective mass m*. The best fittings
are obtained when m*=0.08 m0, Zp1=0.016 eV with
J1=15 meV and Zp2=0.028 eV with J2=3.3 meV. This
result indicates that the conduction band of superconducting InN has at least two electrons with different mobilities. This will explain many of thecontroversial problems reported for InN[2].
1.0
Zp1=0.016 eV, J1=0.015eV,

2
LO

Reflectivity

m*=0.08 m0

0.6

0.4

0.2

0.0

Z
(Z  Z )), 2
Z(Z r Zc  iJ ) ZTO
 Z 2  i*Z2
p

Zp2=0.028 eV, J=0.0033eV

0.8

GS1949
6.0 K

100

200

2
TO

(1)

where Zp and Zc are plasma and cyclotron frequencies,
respectively. Reflectivity R is calculated using thefollowing equation,
E0r
E0i(n1  n2 )(n1  n0 )eikn1d  (n2  n1 )(n0  n1 )e ikn1d;(n2  n1 )(n1  n0 )eikn1d  (n2  n1 )(n0  n1 )e ikn1d

R

E0r
,
E0i

where d(~5 Pm) is the film thickness, n0 and n2 are
the refractive indices of air and sapphire, respectively.
Figure 2 shows the change of the reflectivity('R(B)=R(B)-R(0)) caused by the magnetic field. In
this experiment, unpolarized light was used. Hence,
when there is a plasma oscillation, 'R(B) has a peaknear Zp. The two peaks (~230 cm-1 and 100 cm-1) inthe spectrum indicate that there are two plasma oscillations in the sample investigated. Therefore we add
another plasma oscillation in the dielectric function as
follows,
Z p21
Z p2 2n12 (Z ) H f (1 
Z (Z r Zc  iJ 1 ) Z (Z r Zc  iJ 2 )
2
2
(Z LO
 ZTO
)
eB
 2
), Zc
.
(3)
2
ZTO  Z  i*Z
m*

600

0.10

'R(B)/R(0T)

(2)

500

Fig. 1. Reflectivity spectrum of GS1949 at 6 K at zero magnetic field. Fitting is done using Eqs. (2) and(3) with parameters given in the figure.

6T
5T
4T
3T
2T

0.05

2

300
400
-1
Wavenumber(cm )GS19496K
BAa-b plane

0.00

Zp1=0.016 eV, J1=0.015eV,-0.05

Zp2=0.028 eV, J=0.0033eV

m*=0.08 m0
-0.10

100

200
300
-1
Wavenumber(cm )

400

500

Fig. 2. Change of the reflectivity 'R(B) as a functionof the applied magnetic field at 6K. Fittings aredone using Eqs. (2) and (3) with parameters given in
the figure.
[1] T. Inushima et al., J. Crystal Growth 227-228(2001) 481.
[2] C. H. Swartz et al., J. Crystal Growth 269(2004)
29.

Here we assume that InN has a spherical Fermi sur-

102

BL6B

Optical Study of Metal-Insulator Change in LiV2O4 under High Pressure
A. Irizawa1, T. Iizuka2 and S. Kimura2, 3
1
Graduate School of Science and Technology, Kobe University, Kobe 657-8501, 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, Japanbond length is confirmed by a x-ray analysis and an
EXAFS experiment. We have investigated theelectronic states of LiV2O4 under pressures and low
temperatures analyzed by optical study using an
infrared microscope in SPring-8 [2-4]. There are threedifferent phases showing the characteristic optical
spectra. In the middle phase between the metallic andinsulating ones, the spectra show a gradual changewith pressure and temperature. The purpose ofexperiment in this beam time, we will observe thisgradual change carefully inhibiting the pressure-shiftas far as possible during temperature change.

Introduction
The optical study under multi-extreme condition
such as high-pressure and low-temperature is one of
the most attractive techniques for the investigation ofelectronic states of solids. In case of strongly
correlated electron systems, the pressure will directly
affect the electron-electron interactions through thechange of lattice constants. Optical reflectivity
experiment under high pressure can be carried out
using a diamond anvil cell (DAC) because of not only
the highest hardness on earth but also the hightransparency in the wide energy region from terahertz(THz) to ultraviolet (UV) of the diamond. In addition
the highest thermal conductivity on earth of the
diamond also enables to get lower temperature in thesample space. On the other hand, there are somedifficulties for this experiment. The sample space isas small as the culet of diamond which is less thanseveral hundred m in usual. At the same time, themaximum pressure is controlled by the size of culet.
For example, DAC with 600 mI culet can achieve
the maximum pressure up to 20 GPa, but it is riskythat we go beyond 12 GPa with 800 mI one withoutspare. Here we should consider the diffraction limit at
far-infrared (FIR) region. For example the photon
energy of 10 meV is equal to the wavelength of 125
m. Therefore the lowest photon energy and thehighest pressure are exclusive for each other. A
serious problem at the scene of experiment is that thepressure and temperature are hardly controlledindividually because of the hysteresis procedure ofthem caused by finite friction of the moving part ofapparatus at low temperatures. The sample position
should be also adjusted at every pressure andtemperature in the case of microscopy. On the basisof these experimental restrictions, we tried thereflectivity measurement at FIR region for
metal-insulator change compound LiV2O4 underpressure at low temperature. Spinel oxide LiV2O4 has
been found for the first time to show a heavy fermion
state among 3d metal oxides. LiV2O4 has vanadiumwith half integer valence number of +3.5 (d1.5) in a
metallic state. In an insulating state it should be
exhibit the alternated valence numbers of
V3+:V4+=1:1 or V3+:V5+=3:1 together with theconstant states of Li1+ and O2-. Under pressures, thelow temperature metallic phase drastically changes toan insulating one [1]. The formation of chargeordering has been predicted in this insulating phase. A
structural distortion involving the distribution of V-V

Experimental
The optical reflectivities of single crystal LiV2O4
were measured at the microscope end station of
UVSOR-II BL6B. The procedure of high pressure
experiment was similar to the former studies. The
observed wavenumber was between 80 and 700 cm-1
with the fixed pressure of 8 GPa. The temperature
was changed between 20 and 300 K.
Results and Discussion
The tentative reflectivities obtained from thespectra of sample and Au film in DAC are shown in
Fig. 1. Here we can confirm the gradual change frommetallic spectrum at 300 K to the other ones of
reduced intensities with decreasing temperature. The
measurements were succeeded both in decay modeand Top-up mode.
1

Reflectivity

LiV 2O4

300 K
100 K
20 K

0.5

0
0

200

400

600

Wavenumber (cm-1)

Fig. 1. Temperature change of reflectivity at FIR
region under 8 GPa.
[1] C. Urano, PhD thesis, University of Tokyo (2000).
[2] K. Shimai, A. Irizawa et al., Infrared Physics &Technology 51 (2008) 468.
[3] A. Irizawa et al., Physica B, 403 (2008) 948.
[4] A. Irizawa et al., Journal of Phys: Conf. Ser. 150(2009) 042070.

103

BL6B

Infrared Reflection Spectra of InxAl1-xN
K. Ozaki1, R. Ueda1, K. Fukui1, A. Yamamoto1
1
Dept. Elec. Elec. Engi, Univ. Fukui, Fukui 910-8507, JapanThe ternary InxAl1-xN has great potential for
applications in the solar cell with a wide variablebandgap range from 0.7 eV to 6.2 eV. However, manyof its basic properties are still unknown, because it isdifficult to obtain high quality thin films due to thehuge differences of the growth conditions between
InN and AlN. Our purpose in this report is to present
the vibrational properties of the InxAl1-xN over a wideIn fraction range using the selected thin films whose
infrared reflection spectra can be explained by thefitting calculations based on the dielectric function
written by both LO and TO phonon contributions [1]

㩷
㪈㪅㪇

2

㪇㪅㪍

㩷

Z  Z  iǌ jL Z(1)
2
2
 iǌ jT Z
j Z jT  Z

㪇㪅㪋

㪇㪅㪉

where Hf , ZjL , ZjT , J jL , J jT are high-frequency

104

㪇㪅㪇

㪇

㪉㪇㪇

㪋㪇㪇

㪍㪇㪇

㪏㪇㪇

㪈㪇㪇㪇

㪈㪉㪇㪇

㪮㪸㫍㪼㫅㫌㫄㪹㪼㫉㩷㪲㪺㫄㪄㪈㪴

Fig. 1. Reflection spectrum of InxAl1-xN (x = 0.6) andthe fitting result.
㩷
㪐㪇㪇
㪏㪌㪇
㪏㪇㪇

㪮㪸㫍㪼㫅㫌㫄㪹㪼㫉㩷㪲㪺㫄㪄㪈㪴dielectric constant, j-th branch longitudinal andtransverse optical phonon (LO, TO) frequencies andj-th branch LO and TO phonon damping.
Each InxAl1-xN thin film was grown on the sapphire
substrate with a GaN buffer layer by MOVPE at
University of Fukui. The In fraction of the thin films
were x = 0.90, 0.70, 0.68, 0.60, 0.35, and 0.30. The
film thickness and the carrier density are 0.4 ~ 1.1 Pm
and 1018 ~ 1019 cm-3, respectively.
By using Eq. 1 and the transfer matrix method, thetheoretical total reflectance with the consideration of
the multiple reflection based on InAlN-sapphire twolayers model can be described. Then, the fitting
calculations between experimental and theoreticalreflectance spectra have been carried out. The best
fitting results suggest that the TO phonons consist ofboth InN-like and AlN-like TO phonons (two-mode
behavior), and the LO phonon consists of one phonon(one-mode behavior). Similar one-mode LO –two-mode TO result was found in the ternary AlGaN[2], and suggest that the TO phonon vibration is so
localized and investigated theoretically [3].
The infrared reflection spectrum of InxAl1-xN (x =0.6) and the fitting result are shown in Figure 1. The
dip at 480 cm-1 is attributed to the InN-like TO
phonon. According to the fitting calculation, it isfound that the shoulder like spectral feature at thelower wavenumber side of the 580 cm-1 peak is
attributed to the AlN-like TO phonon, though it is notclearly seen in the spectrum. The rising edge ataround 900 cm-1 from higher wavenumber side tolower is due to the beginning of the Reststrahlen band.
Figure 2 shows the In fraction dependence of thefitted TO phonon and LO phonon wavenumber. It isclear that the In fraction dependences of both the
InN-like TO phonon wavenumber and AlN-like TO

㪎㪌㪇
㪎㪇㪇

㩷

Hf

2
jL

㩷㫆㪹㫊㪅
㩷㪺㪸㫃㪺㪅

㪇㪅㪏㪩㪼㪽㫃㪼㪺㫋㪸㫅㪺㪼

H (Z )

phonon wavenumber are relatively weak. On theother hand, the LO phonon wavenumber shows theclear In fraction dependence, and obvious softening
above 0.7 In fraction. Such a strong softening is notpredicted by Ref. 3 and further investigation isunderway.

㪍㪌㪇
㪍㪇㪇
㪌㪌㪇
㪌㪇㪇
㪋㪌㪇

㪇㪅㪇

㪇㪅㪈

㪇㪅㪉

㪇㪅㪊

㪇㪅㪋

㪇㪅㪌

㪇㪅㪍

㪇㪅㪎

㪇㪅㪏

㪇㪅㪐

㪈㪅㪇

㪠㫅㩷㪽㫉㪸㪺㫋㫀㫆㫅

Fig. 2. Indium fraction dependence of the AlN-like
TO phonon (circle), InN-like TO phonon (square),and LO phonon (triangle) wavenumber for InxAl1-xN.

[1] F. Gervais and B. Piriou, J. Phys. C 7 (1974)
2374.
[2] A. Cros, H. Angerer, R. Handschuh, O. Ambacher
and M. Stutzmann, Solid State Commun. 104 (1997)35.
[3] H. Grille, Ch. Schnittler and F. Bechstedt, Phys.
Rev. B 61 (2000) 6091.

BL7U

Temperature Dependent Angle-Resolved Photoemission Study on
BaFe1.8Co0.2As2

T. Hajiri1, R. Niwa2, S. Murakami2, H. Miyazaki3, J. B. Hong4, Y. R. Jang4, S. Kimura3, 5,
Y. S. Kwon4 and T. Ito1, 6
1Graduated 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
Department of Physics, Sungkyukwan University, Suwon 440-749, Korea
5
School of Physical Sciences, the Graduate University for Advanced Studies (SOKENDAI),
Okazaki 444-8585, Japan
6
Nagoya University Synchrotron Radiation Research Center, Nagoya University, Nagoya
464-8603, Japan
Figures 3(a) and (b) show the temperaturedependence of ARPES spectra and their spectral DOS
across Tc obtained at the hole-like FS (white circle inFig. 1), respectively. We have observed
superconducting gap opening below T=22.5 K. Withdecreasing temperature, DOS at EF gradually
decreases and the gap size increases. We haveestimated the gap size is about 5~6 meV.

Iron pnictides superconductor discovered recently
has been intensively studied both from experiment andtheory, however an essential mechanism of its
anomalous superconductivity has not been clarified
yet.
To
understandthemechanismofsuperconductivity,
thesuperconducting
gap
information, i.e., the origin of superconducting
pairing is one of the most important issues. For
example, multiple isotropic gaps suggesting s-wave
pairing have been reported on the optimallyhole-doped Ba0.6K0.4Fe2As2 [1].
We have performed angle-resolved photoemission
spectroscopy (ARPES) on electron-doped ironpnictides superconductor BaFe1.8Co0.2As2 in theslightly underdoped regime (Tc=25 K), to elucidate
the essential role of the doped carriers to theanomalous superconductivity [2].
Figure 1 shows the Fermi surface image of
BaFe1.8Co0.2As2 at 35 K (normal state) obtained byplotting the intensity of ARPES spectrum at EF±10
meV. Figures 2(a) and (b) show the ARPES spectra
around the * and M point, respectively. From thepresent ARPES results, we have clearly found thehole- and the electron-pocket around the * and M
point, respectively, which can be qualitatively
reproduced by the band calculation on the ‘122’ series[3]. It should be noted that the large electron-pocket
expected in the calculation along cut2 disappears
because of the matrix element effect with horizontal
polarization of the present light source [4].

(a)Around *(b)Around M
T=35 K
hQ=22.2eV

 
  
Fig. 2. ARPES spectra of BaFe1.8Co0.2As2 at 35 K
along cut 1 (a) and cut 2 (b) in Fig. 1.

Fig. 3. (a) Temperature dependence of ARPES spectraat the hole-like Fermi surface (white circle in Fig. 1)of BaFe1.8Co0.2As2. (b) Spectral DOS of Fig. 3(a)obtained by the symmetrization method [5]. Thin redlines correspond to one at 35 K.

EuFe2As2
T=35 K
hQ=22.2eV[1] H. Ding et al., Euro Phys. Lett. 83 (2008) 47001.
[2] Y. Kamihara et al., J. Am. Chem. Soc. 130 (2008)3296.
[3] D. J. Singh et al., Phys. Rev. B 78 (2008) 094511.
[4] S. de Jong et al., arXiv: 0912 (2009) 3434v1.
[5] M. R. Norman et al., Nature 392 (1998) 157-160.

   Fig. 1. Fermi surface image of BaFe1.8Co0.2As2
compared with the expected Fermi surface topology
from the band calculation on BaFe2As2[3].

105

BL7U

Angle-Resolved Photoemission Study on Possible Kondo Insulator FeSb2
S. Imada1, K. Terashima1, A. Sekiyama2, S. Suga2, A. Yamasaki3,
K. Mima1,Y. Miyata1, R. Yamaguchi1, Y. Tachimori1, Y. Yamanoi1,
Y. Matsui3, J. Yamaguchi2, S. Komori2 and H. Nakamura4
1
Faculty of Sci. and Eng., Ritsumeikan Univ., Shiga 525-8577, Japan
2
Grad. School of Eng. and Sci., Osaka Univ., Osaka 560-8531, Japan
3
Faculty of Sci. and Eng., Konan Univ., Kobe 658-8501, Japan
4
Grad. School of Eng., Kyoto Univ., Kyoto 606-8501, Japan
Magnetic impurities in a metal trigger an upturn in
the resistivity curve, so-called Kondo effect. Thisphenomenon has been reported in rare-earth
compounds that contain localized f-electrons, as wellas in transition metals with partial substitution of
atoms.
In a class of materials called Kondo
insulators, Kondo effect induces a insulator-to-metal
crossover under heating. Interestingly, even in someof the iron-based compounds, FeSi and FeSb2, thetemperature-dependence of the resistivity resembles
that of Kondo insulator. A number of experimental
techniques have indicated the existence of the energy
gap at the low temperature in FeSb2[1-3], while themechanism for the formation of the energy gap
remains highly controversial; whether FeSb2 is a
Kondo insulator[2] or a narrow-gap semiconductor[1,3, 4]. To resolve this issue, a high-resolution
angle-resolved photoemission (ARPES) study has
been highly anticipated, since ARPES can directly
observe the band structure and possible formation of
the energy gap.
A single crystal of FeSb2 has been synthesized by
Sb-flux method.
High-resolution photoemission
study has been performed at BL7U in the UVSOR
facility. Energy resolution was set at ~6 and ~15
meV for angle-integrated and angle-resolved
measurement, respectively. We have obtained aclean surface of the sample by fracturing in an
ultrahigh-vacuum of ~2x10-8 Pa.
Figure 1 shows a typical ARPES intensity plot of
FeSb2 at T = 15 K using 17-eV photons. Several
dispersive bands are clearly seen in the figure,showing that we have succeeded in the ARPES
measurement of FeSb2. What is interesting is that
there is a hole-like band crossing EF (shown by arrowin the figure) although the bulk is insulating at thistemperature[1]. Such an inconsistency between PES
results and bulk property is also reported in FeSi[5],where the contribution of the surface state is
discussed. In fig. 1, the hole-like band lies in a
k-region where other bands are absent. Thus weattribute the origin of metallic band to the surface.
In Fig. 2, we show a temperature-dependence ofangle-integrated photoemission spectra in FeSb2
taken at hQ = 6.5 eV.
As we decrease thetemperature, the spectral weight near EF is gradually
suppressed. However, a Fermi-edge cutoff remains
in the spectra at T = 20 K, indicative of metallic state.

This metallic state may originate from the surface
states shown in Fig. 1. Further study including thetemperature dependence of bulk bands is necessary to
clarify the origin of the energy gap in FeSb2.

Fig. 1. ARPES intensity plot of FeSb2 at T = 15 K
(insulating phase). Green arrow points a hole-likeband crossing EF.

Fig. 2. Temperature-dependence of angle-integrated
photoemission spectra in FeSb2.
[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] A. V. Lukoyanov et al., Euro. Phys. J. 53 (2006)
205.
[5] K. Ishizaka et al., Phys. Rev. 72 (2005) 233202.

106

BL7U

High-Resolution Angle-Resolved Photoemission Study ofIron Pnictide Superconductor Ba1-xKxFe2As2

K. Nakayama1, T. Kawahara1, T. Arakane1, T. Qian1, P. Richard2, S. Souma2, T. Sato1 and
T. Takahashi1, 2
1
Department of Physics, Tohoku University, Sendai 980-8578, Japan
2WPI Research Center, Advanced Institute for Material Research,
Tohoku University, Sendai 980-8577, JapanThe
discoveryofsuperconductivity
in
LaFeAsO1-xFx [1] has triggered intensive researcheson the high-temperature (Tc) superconductivity of iron(Fe) pnictides. The Tc value has already exceeded 55
K by replacing La atom with other rare-earth atoms orby introducing oxygen vacancies, opening a new
avenue for high-Tc material research beside cuprates.
RemarkableaspectsoftheFeAs-based
superconductors are (i) electrons in the Fe orbitals,
generally believed to be the foe, indeed play anessential role in superconductivity, (ii) non-doped
parent compounds commonly exhibit a collinear
antiferromagnetic (AF) spin density wave (SDW), and(iii) superconductivity emerges either by the hole orelectron doping into the parent compounds.
Angle-resolved photoemission spectroscopy (ARPES)
is a key experimental technique to elucidate electronic
states responsible for the occurrence ofsuperconductivity and the AF transition. Indeed,
ARPES has revealed several essential features incuprate superconductors, like the large Fermi surface,d-wave superconducting gap, and the pseudogap
above Tc.
In this study, we have performed high-resolution
ARPES study of iron-based superconductor
Ba1-xKxFe2As2 as a function of potassium
concentration x. Figure 1 shows the comparison of
ARPES intensity as a function of wave vector andbindingenergyfor(a)
optimally-doped
superconducting Ba0.6K0.4Fe2As2 (Tc = 37 K) and (b)non-superconducting parent compound BaFe2As2(TSDW = 138 K) measured at T = 17 K and 25 K,
respectively.
Corresponding energy distribution
curves (EDCs) are also displayed in (c) and (d),respectively. As clearly seen in Figs. 1(a) and (b),we find a highly dispersive holelike band centered atthe * point. Leading-edge midpoint of this bandaround the Fermi vector is clearly shifted towardhigher binding energy as shown in Fig. 1(c),indicating an opening of the superconducting gap, asin previous studies [2, 3]. In contrast, in the parent
compound, the band structure seems to be apparently
different. We clearly find two holelike branches
centered at the * point. In addition, we also observea weaker electronlike band showing an opposite
dispersion from these two holelike bands. This
dramatic difference of band structure between the
superconducting and the AF compounds are ascribedto the absence (appearance) of magnetic order in the

superconducting (parent) compound.
The
complicated band structure in the AF sample would
originate in the band folding which basically follows
the periodicity of new magnetic Brillouin zone. All
these experimental results indicate strong influence ofmagnetic order / fluctuations to the fine electronic
structure in the vicinity of EF of iron-based
superconductors.

Fig. 1. ARPES intensity of (a) optimally-doped
Ba0.6K0.4Fe2As2 (Tc = 37 K) at T = 17 K and (b) parent
compound BaFe2As2 at T = 25 K plotted as a functionof wave vector and binding energy, measured with the
21-eV photons. Corresponding energy distribution
curves are plotted in (c) and (d), respectively.
Dashed lines are guides for eyes to highlight 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.

107

BL7U

Anisotropy of Electron Scattering and its Effect on Hall Coefficient of (Bi, Pb)2(Sr,La)2CuO6+d High-Tc Superconductor

T. Takeuchi1, 2, Y. Hamaya2, K. Ogawa2, T. Ito3 and S. Kimura4
1
EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan
2
Depaetment of Crystalline Materials Science, Nagoya University, Nagoya 464-8603, Japan
3
Depaetment of Materials Science, Nagoya University, Nagoya 464-8603, Japan
4
UVSOR, Institute for Molecular Science, Okazaki 444-8585, Japan
Hall coefficient RH of metals is generally
accounted for with the well-known formula ofR H 1/(ne) , where n and e represent carrier density
and unit charge of carriers. This formula is valid only
when the scattering of the electron near the Fermi
level is isotopic. If we measure the Hall coefficient of
the material that is characterized by the anisotropy inelectron scattering, the measured Hall coefficient
exceeds the value of R H 1/(ne) .
High-Tc superconductors are typical example thatpossesses anisotropy of electron scattering due to thestrong electron correlation. The anisotropy of electron
scattering is caused by the fluctuation of theanti-ferromagnetic spin ordering, and therefore
electrons near the Fermi level are scattered only in thevicinity of the anti-ferromagnetic zone boundary(AFZB). These materials indeed possess a muchlarger magnitude of RH than that expected fromR H 1/(ne) . Since the strong scattering at AFZB is
getting weakened with increasing temperature or
increasing carrier concentration, the Hall coefficientincreases with decreasing temperature or decreasing
carrier concentration.
In this study, we try to quantitatively reproduce thebehaviors of Hall coefficient observed for the high-Tc
superconductors by using the information about bothelectronic structure and electron scatterings obtainedby the angle resolved photoemission spectroscopy(ARPES). For this purpose, we employed the ARPES
system located in BL7U of UVSOR, because it
possesses an extremely good energy resolution. By
using 9 eV photons as the incident beam of ARPES,
we obtain the energy resolution better than a few
meV.
We used the single crystal (Bi,Pb)2(Sr,La)2CuO6+dsuperconductors, in which Pb and La are partiallysubstituted for Bi and Sr, respectively. The former
substitution effectively weakens the superstructure in
Bi-O layers, which generally prevents us from easily
analyzing the ARPES spectra by creating theunfavorable umklapp-bands. The latter substitution isused to decrease the hole-concentration in the sample.
By employing subsequent heat-treatment, we succeed
in preparing variety of samples from heavily
overdoped to underdoped ones.
In this study, we selected three samples, which are
one overdoped sample possessing Tc = 25 K and two
underdoped ones of Tc = 31 K and 28 K. The ARPES

measurement was performed at at 50 K, 100 K, and
150 K with incident energy of 9 eV.
The energy distribution curves (EDC’s) weresuccessfully extracted at each Fermi wave vector. By
symmetrizing the EDC spectra at the Fermi level (HF),
we confirmed that the consequently obtained spectra
obviously possessed a peak at HF. The shape of thepeak is well accounted for with the Lorentzianfunction and its FWHM is distributed over 30 ~ 50
meV. This fact indicates that the energy variation ofimaginary part of self-energy, which represents the
lifetime of coherent states, is roughly approximated tobe constant over the narrow energy range of a few
tenth of meV in width in the vicinity of HF.
By using the experimentally determined H-kdispersion, the group velocity, and the lifetime ofcoherent states, we calculated RH of the samples using
the Bloch-Boltzmann equation. The resulting value isplotted in Fig.1 together with the measured ones as afunction of temperature. Although the temperaturedependence and hole concentration dependence of RH
were qualitatively reproduced by the calculation, theabsolute value was not completely reproduced. This
fact unambiguously suggests that we have to consider
some other special mechanism, such as the back flow
current in vertex correction.
We are now in progress for quantitatively
evaluating the effect of this back flow current on RH
of present samples using the present data.
UD25K meas.
UD25K calc.
UD25K 1/(ne)UD31K meas.
UD31K calc.
UD31K 1/(ne)OD28K meas.
OD28K calc.
OD28K 1/(ne)

-9

3 -1

RH (10 m C )

8
6
4
2
0

0

100

200

300

Temperature (K)Fig. 1.
The calculated and measured Hall
coefficient (RH) of (Bi,Pb)2(Sr,La)2CuO6+d. The
absolute value of RH was not reproduced by thepresent calculation.

108

BL7B

Study of VUV Reflection Spectroscopy of ZnO Thin Films
Irradiated By Ar Plasma
N. Fujita, T. Yabumoto, H. Ito, T. Mizunuma, Y. Iwai,
S. Matsumoto and H. Matsumoto
School of Science & Technology, Meiji University, Kawasaki 214-8571, Japan
Transparent conductive oxide (TCO) films havebeen widely used in the field of optoelectronic
devices, such as transparent electrodes for various
kinds of flat panel displays (FPDs) or photovoltaic
cell [1,2].
Recently, the demands for the large-area andhigh-quality FPDs, transparent conductive films
including tin-doped In2O3 (ITO) films have attracted
significant attention. However, due to rapid demandin recent years, the price of indium composing of ITO
may raise even higher the future for a stable supply isconcerned.
So, Instead of Indium as materials, ZnO is focused.
ZnO is cheaper than ITO and, has the nature of thetransparent conductive film by donor level is formedby oxygen vacancies or interstitial zinc in the film. Asa means to lower resistance of ZnO, there is a way toinclude the addition of impurities and hydrogenplasma irradiation [3,4]. For Ar plasma irradiation
method, no report of electrical characteristics has
been made in details, and also optical properties are
not reported [5].
In this study, ZnO thin films prepared by RFmagnetron sputtering and fabricating by Ar
plasma-irradiation. We report the optical properties of
ZnO thin films for various irradiation times.

B

C

Reflectivity (a.u.)

A

20min
12min
8min

15.5eV
12.8eV
8.6eV
5

10

15

4min
2min
0.75min
0min

20

Photon Energy (eV)25

Fig. 1. VUV Reflection Spectra ZnO film by Ar
plasma irradiation up to 20 min.

Experiment
ZnO thin films were deposited by RF magnetron
sputtering on fused quartz substrate, and at room
temperature. Sintered object of zinc oxide plate wereused as the target, and the sputtering was carried out
in argon gas with a base pressure 2.0 u 10-2 Pa. The
film thickness is about 390 nm.
Thin films were irradiated with Ar plasma on
various times up to 20 minutes. The RF power and Ar
flow rate were 20 W and 3.0 sccm. Volume
resistivities were measured for ZnO thin films
irradiated Ar plasma.
Reflection spectra of the films were measured in
the vacuum ultraviolet region up to 25 eV with the
3-m normal incident monochromator (grating: G1 and
G2) at BL-7B of UVSOR-II. A silicon photodiodesensor was used as a detector of the reflection light.

intensity. Three peaks A(8.6eV), B(12.8eV),C(15.5eV) appeared in all spectra. It was clearlyobserved that the intensity of peak increased with
irradiation times of Ar plasma. Intensity of peak C isincreased. However, A and B are not increased. These
results are considered that the reflectivity increased
with change of crystalline state with decreased
irradiation times.
[1] H. Koh, K. Sawada, M. Ohgawara, T. Kuwata, M.
Akatsuka and M. Matsuhiro, SID Dig. Tech. Pap. 19(1988) 53.
[2] Y-M. LU et al., Thin Solid Films 447- 448 (2004)
56.
[3] P. F. Cai et al., J Appl. Phys. 105 (2009) 083713.
[4] H. Akazawa, Appl. Phys. Express 2 (2009)081601.

Results and Discussion
XRD analysis showed that as deposited films werecomposed of polycrystalline of ZnO. Figure 1 shows
the VUV reflection spectra between 5 and 25 eV of
ZnO films by Ar plasma Irradiated at room
temperature. The vertical axis indicates reflected

109

BL7B

Measurement of Reflectivity of Strongly Correlated Electron Systems
between Infrared and VUV Regions at BL7B
A. Irizawa1 and S. Kimura2, 3
1
Graduate School of Science and Technology, Kobe University, Kobe 657-8501, 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

110

Experimental
The optical reflectivities of (Ca1-xNax)Co2O4 andCeT2Al10 were measured at UVSOR-II BL7B. Theenergy was changed from 1.2 to 40 eV with threegratings (1200, 600, and 300 lines/mm). There areseveral type of optical windows for avoiding thehigher degree diffractions. In the former studies, theobtained spectra using 4 type optical windows with 3
gratings were hardly connected without complex
process. This time we select 3 type windows with 2
gratings for simplifying.
Results and Discussion
As a result, the spectra can be connected without
any compensation of raw datas even in the case ofsmall polycrystal samples. Figure 1 is one of theobtained results. The wide energy scan using G3seems to be good agreement with the other resultswith 3 different pairs of optical windows and gratings.
We are using these results of both series ofcompounds for getting optical conductivities with theexperiments in the lower energy reflectivity
measurements.
㪇㪅㪊㪌
㪇㪅㪊㪇
㫀㫅㫋㪼㫅㫊㫀㫋㫐㩷㩿㪸㪅㫌㪅㪀

Introduction
Studying optical conductivity is one of the mosteffective methods for understanding the electronic
states of solids. In usual we measure the reflectivity
between far-infrared (FIR) and near-infrared (NIR)
regions using Fourier transfer type (FT)interferometer and at visible (VIS) region using the
traditional grating type spectroscope. The opticalconductivity can be derived from the reflectivity
through Kramers-Kronig (KK) transform where theaccurate analysis can be achieved from thereflectivity in the wide energy range as far as possible.
In that sense the reflectivity data from BL7B in thewide energy range between 1.2 and 40 eV (1000 nm –30 nm) is one of the most important part for KK
transform. Otherwise the reflectivity in the higher
energy range should be extrapolated with a simple -4function. In this machine time, we prepared several
solid samples of strongly correlated electron systems
for both d electron and f electrons. One of them(Ca1-xNax)Co2O4 (x=0-0.5) is the newly synthesized
system which shows metal-insulator change withincreasing Na content x [1]. CaCo2O4 has been
known to show a temperature-dependent large
thermoelectric power with semiconductor-likeresistivity [2-4]. The crystal structure is
Ca-ferrite-type which belongs to one of the highpressure phases of spinel structure. It includes Co2O4
double chain component along b axis which consists
t2g orbital connection of Co 3d electrons. In that sense
the success of carrier doping to this compound isinterested in view of 1D conductor. Similar orbital
network is also seen for the NaxCoO2 which has a2-dimensional t2g orbital connection. For substituting
Ca2+ to Na+, doping of hole carriers can be expected
into Co3+ t2g low spin state. Actually the resistivity
decreases drastically with increasing x and theinsulating behavior transforms to a metallic one at
x=0.5. The other samples are selected from the also
newly synthesized compounds. CeT2Al10 (T=Fe,Ru,Os) is the series having cage-structure of T-Al in
which Ce 4f electron acts an important role ofelectronic properties such as Kondo insulator andheavy fermion behavior [5]. In the beam time, wemeasured the reflectivities of all these compounds
using 3-m normal incidence monochrometor at
BL7B.

㪇㪅㪉㪌
㪇㪅㪉㪇

㩷㪞㪊㪂㫈㫌㪸㫉㫋㫑㩷㪞㪈㩷㪞㪊㪂㪦㪌㪊
㩷㪞㪊㪂㪧㫐㫉㪼㫏

㪇㪅㪈㪌
㪇㪅㪈㪇
㪇㪅㪇㪌
㪇㪅㪇㪇

㪇

㪉㪇㪇

㪋㪇㪇

㪍㪇㪇

㪏㪇㪇

㪈㪇㪇㪇

㫎㪸㫍㪼㩷㫃㪼㫅㪾㫋㪿㩷㩿㫅㫄㪀

Fig. 1. Comparison of reflectivities with different
optical windows and gratings.
[1] M. Isobe et al., the oral presentation in theconference of physical society of Japan (2009)26aRG-5.
[2] M. Shizuya, M. Isobe et al., J. Solid State Chem.,
180 (2007) 2550.
[3] M. Isobe et al., Physica C, 469 (2009) 948.
[4] M. Isobe et al., J. ELECTRONIC MATERIALS,
38 (2009) 1166.
[5] Y. Muro et al., JPSJ 78 (2009) 083707; ICM 09
proceedings.

BL7B

Photoluminescence and Photoluminescence Excitation Spectra of AlN
T. Ito, R. Ikematsu, S. Sawai and K. Fukui
Dept. Elec. Elec. Engi., Univ. Fukui, Fukui 910-8507, Japan

111

Normalized Intensity

PL intensity ratio of Main and 1LO

is excited at the lower excitation photon energy.
Comparison between these PLE spectra and the OR
spectra, the peaks at 6.24 eV and 6.28 eV of the IEAc
spectra in Fig. 2 are attributed to the B and C
transition, respectively, and a 6.07 eV peak of the IEcspectrum is to the A transition. These assignments arein good agreement with other work [1]. However,other peaks at 6.20 eV and 6.40 eV are also observed.
The 6.20 eV transition is allowed under the E // ccondition, and the 6.40 eV transition is under both the
E // c and the EAc conditions. It seems that thepolarization characters of these two unknown
transitions correspond to A and B (or C) transitions,
respectively.

50
40
30

22K

PL Intensity ratio
Exciton - 1LO
Incident Angle
0 deg
60 deg20
100
6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8
Photone Energy (eV)Exci. 6.28eV
8 deg
60 deg

5.7

5.8

5.9

6.0

6.1

Photon Energy (eV)Fig 1. Incident angle dependence of PL spectra, and
the PL intensity ratio of main and 1-LO bands as afunction of the photon energy (inset).

22K

8deg(EAc)
60deg
E//c

EA c
E//c

Normalized Intensity

Intensity (arb.units)

Aluminum nitride (AlN) has the widest direct
band-gap in III-V nitride compound semiconductors.
Therefore, the AlN is expected to utilize for deep UVlight emitting diodes (LED) etc. It is known that theuppermost valence band (VB) of AlN splits into threebands (CH, HH and LH bands from bottom to top of
the uppermost VB), and the transition between eachlevel and the conduction band (CB) has the different
polarization character, and knowledge of thesetransitions is important for fabricating the
opt-electronic devices. These three transitions areusually labeled as A (from CH to CB), B (HH to CB),and C transition (LH to CB). Our purpose in thiswork is to present the uppermost VB structure ofwurtzite AlN including A to C transition energies andtheir polarization characters. For this purpose, notonly optical reflectance (OR) measurements but also
photoluminescence (PL) and PL excitation (PLE)measurements are performed.
The AlN films were grown on the Al2O3 substrates
by the MOVPE. The c-axis of the AlN isperpendicular to the sample surface, and the thickness
is about 1 Pm. Highly linear polarized incident lightof BL7B were used to excite the samples with T = 8°
and 60° in respect to the sample surface in the pconfiguration, where T is an incident angle.
Figure 1 shows the PL spectra at T = 8° and 60°.
The measurement temperature is 20K. There is no
incidence angle dependence between two PL spectra.
The emission bands at 5.94 eV and 5.84 eV are firstand second LO phonon replicas, and the main
emission band is at 6.05 eV. The inset of Fig. 1 shows
the ratio between the 1-LO phonon replica band andthe main band as a function of the photon energy.
Figure 2 shows that the PLE spectra at T = 8° and
60°. The PLE spectra are mainly monitored at thepeak photon energy of main band (6.05 eV). However,the PLE spectra at the lower photon energy side aremonitored at the peak photon energy of 1-LO band
(5.94 eV). The measurement temperature is 20K.
Since the c-axis of the sample is perpendicular to thesample surface, the PLE Intensity at T = 8° mostly
consists of the E A c component (IEAc), where E is theelectric field of the incident light. On the other hand,since that at T = 60° consists of both the E A c andthe E // c components (IEc), tthe comparison between
the spectra at T = 8° and 60° shows the contributionof the components. Therefore, using an expression,
I(T) = IEAccos2T+ IE//csin2T
we also show calculated IEc in Fig. 2. The inset of
Fig. 2 shows the normalized IEAc and IEc forconvenience. It is clear that only the IEc component

6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7
Photon Energy (eV)

6.0

6.1

6.2

6.3

6.4

6.5

6.6

6.7

Photon Energy (eV)Fig 2. Incident angle dependence of PLE
spectra , and normalized E//c and EAc
components of PLE spectra (inset).

[1] T. Onuma. T. Shibata. K. Kosaka, K. Asai, S.
Sumiya, M. Tanaka, T. Sota, A. Uedono and S. F.
Chichibu, J. Appl. Phys. 105 (2009) 023529.

BL7B

Pressure Tuning of an Ionic Insulator into a Heavy Electron Metal:
An Infrared Study of YbS
H. Okamura1, M. Matsunami2, A. Ochiai3 and T. Nanba1
Graduate School of Science, Kobe University, Kobe 657-8501, Japan
2
Institute of Solid State Physics, The University of Tokyo, Kashiwa 277-8581,Japan
3
Center for Low Temperature Science, Tohoku University, Sendai 980-8578, Japan1

YbS at ambient pressure is an ionic insulator(Yb2+S2-) with the NaCl crystal structure, and has aband gap of about 1.4 eV [1, 2]. A previous optical
study [1] showed that the band gap of YbS decreased
under increasing external pressure. The extrapolationof the shifts of the fundamental absorption edge withpressure suggested a gap closing at ~10 GPa. In
addition, volume compression data and X-ray
absorption data suggested that the Yb mean valencein YbS increased from 2 above 10 GPa, and reached~2.4 at 20 GPa [2]. Namely, YbS above 10 GPa wasin an intermediate-valence (IV) state, where thevalence of each Yb ion is fluctuating between 2 and 3.
However, since the previous optical study was
performed at photon energies above 0.5 eV only, it
was unclear whether or not the gap actually closed at~ 10 GPa, and whether or not the IV state above 10
GPa was metallic. To answer these questions and toclarify the low-energy electronic structures of YbS
under high pressure, we have carried out IR
reflectivity studies of YbS [3]. The reflectivity
spectra R(Z) under high pressure were measured at
the IR beam line of SPring-8, while those at ambientpressure were measured using the apparatus at Kobe
University below 2 eV, and using BL7B above 2 eV
up to 30 eV. High pressure was generated with adiamond anvil cell (DAC). Optical conductivity
spectra V Z were derived from the measured R(Z)using Drude-Lorentz fitting technique.
Figure 1 shows the measured V Z under pressure,
together with that measured in the vacuum. The
narrow peak below 0.1 eV, which are seen above 8
GPa, is a Drude peak due to free carriers. This
result explicitly demonstrates that the high pressure
phase in YbS has a metallic ground state. Theenergy gap is closed at 8 GPa, rather than at 10 GPa
as previously conjectured [2]. In addition, two
pronounced absorption peaks are seen to grow withpressure above 10 GPa. Namely, the high pressure
phase above 10 GPa is not a simple metal, but it has
some peculiar features in the electronic structuresnear the Fermi edge. The detailed analyses of thedata show that the carriers giving rise to the Drude
peak has a large effective mass of the order of 10 m0,
where m0 is the rest electron mass. [3] It can beshown that the observation of the Drude peak due tosuch heavy electrons, and that of the pronounced
absorption peaks, can be understood together using amodel based on the hybridization of the f state with aconduction band, which is also consistent with the IVpreviously observed above 10 GPa.

Fig. 1. Optical conductivity spectra V Z of YbS atroom temperature at different pressures, obtainedwith Drude-Lorentz fitting to the measured
reflectivity spectra. The spectra are offset for clarity.
Note the logarithmic scale below 0.1 eV.

[1] K. Syassen, H. Winsen, H. G. Zimmer, H. Tups,and J. M. Leger, Phys. Rev. B 32 (1985) 8246.
[2] K. Syassen, Physica B+C 139-140 (1985) 277.
[3] M. Matsunami, H. Okamura, A. Ochiai and T.
Nanba, Phys. Rev. Lett. 103 (2009) 237202.

112

BL7B

Angular Dependence of Spectral Responsivity and Reflectance for UVPhotodetectors
T. Saito1, K. Ozaki2, S. Sawai2, K. Fukui2 and I. Saito1
1National Metrology Institute of Japan, National Institute of Advanced Industrial Science andTechnology, Tsukuba 305-8563, Japan,
2
Faculty of Engineering, Univ. of Fukui, Fukui 910-8507, Japanbut also important polarization property and otheroptical property like a overlayer thickness and its
absorption.

Introduction
Needs for quantitative measurements in the
UV-VUV region are increasing in various fields like
UV curing, disinfection, surface cleaning etc.. Inthese applications, UV irradiance meters are often
used quite near the source, which means that theirradiance meters receive wide divergent beams. It is,therefore, very important to check if the meterssatisfy the so-called cosine law, which is required as
an ideal photodetector. In addition, internal quantum
efficiency, Kint, that is a fundamental measure toevaluate detector performance, can be obtained by thetwo measurement results of the external quantum
efficiency, Kext, and the reflectance, R, from theequation,
K ext .
K int

㪇㪅㪈㪋

15 deg30 deg45 deg60 deg0.2*Calc.(0 deg)

Diamond (#2) bias +20V㪩㪼㪽㫃㪼㪺㫋㪸㫅㪺㪼

㪇㪅㪈㪉
㪇㪅㪈㪇
㪇㪅㪇㪏
㪇㪅㪇㪍
㪇㪅㪇㪋
㪇㪅㪇㪉
㪇㪅㪇㪇

1 R

㪌㪇

㪈㪇㪇

㪈㪌㪇

㫎㪸㫍㪼㫃㪼㫅㪾㫋㪿㩷㪆㩷㫅㫄

㪉㪇㪇

㪉㪌㪇

Fig. 1. Measured spectral reflectance spectra for a
diamond detector (Calculated spectrum at normal
incidence is also shown in dashed line).

㪩㪼㪽㫃㪼㪺㫋㪸㫅㪺㪼

Experimental
We have characterized various kinds of UV
detectors (photoconductive diamond detectors, AlN,
AlGaN & GaAsP Shottky photodiodes, Si
photodiodes), especially about their dependence onangle of incidence of spectral responsivity and ofspectral reflectance by using p-polarization dominantradiation from the McPherson type monochromator
beamline at BL7B.
Results and Summary
Figure 1 shows measurement results of spectralreflectance for a photoconductive HOD (Highly
Oriented Diamond) detector at the angles of incidentof 15, 30, 45 and 60 degrees. Also shown by a dashedline is a calculated spectrum at normal incidence.
More detailed calculation results includings-polarization components are shown in Fig. 2. It has
been confirmed that relative spectra agree well
between the measurements and the calculations.
Lower reflectance for the measurements can be
attributed by its diffusive surface, which is easily
visible.
The results also show that reflectance loss forp-polarization decreases as the angle of incidence
increases at most of the wavelengths and hence it
results in increase in detector responsivity, which was
also experimentally confirmed.
In conclusion, angular measurements forreflectance and responsivity are powerful techniqueto characterize not only internal quantum efficiency

㪇㪅㪐
㪇㪅㪏
㪇㪅㪎
㪇㪅㪍
㪇㪅㪌
㪇㪅㪋
㪇㪅㪊
㪇㪅㪉
㪇㪅㪈
㪇

㪩㪇
㪩㫊㪄㪈㪌
㪩㫊㪄㪊㪇
㪩㫊㪄㪋㪌
㪩㫊㪄㪍㪇

㪇

㪈㪇㪇
㪉㪇㪇
㪮㪸㫍㪼㫃㪼㫅㪾㫋㪿㩷㪆㫅㫄

㪩㫇㪄㪈㪌
㪩㫇㪄㪊㪇
㪩㫇㪄㪋㪌
㪩㫇㪄㪍㪇

㪊㪇㪇

Fig. 2. Calculated spectral reflectance spectra for a
diamond detector for both p- and s-components.
Acknowledgement
Authors are grateful to Dr. Kazushi Hayashi at Kobe
Steel Ltd. for providing the diamond photodetectors.
[1] T. Saito and K. Hayashi, Appl. Phys. Lett. 86(2005) 122113.
[2] T. Saito et al., Diamond & Related Materials 14(2005) 1984.
[3] T. Saito et al., Metrologia 43 (2006) S51.
[4] T. Saito et al., Phys Status Solidi C 6 (2009) S658.
[5] T. Saito et al., Metrologia 46 (2009) S272.

113

BL7B

Photoluminescence and Photoluminescence Excitation Spectrafrom AlN Doped with Gd3+
S. Sawai1, T. Ito1, K. Fukui1, S. Emura2, T. Araki3 and A. Suzuki4
1
Dept. Elec. Elec. Engi., Univ. Fukui, Fukui 910-8507, Japan
2
Inst. Sci. Indust. Res., Osaka Univ., Osaka 567-0047, Japan
3
Dept. Photonics, Ritsumeikan Univ., Shiga 525-8577, Japan
4
Res. Organi. Sci. Engi., Ritsumeikan Univ., Shiga 525-8577, Japan
III-V nitride compound semiconductor doped withrare elements are attractive systems because of thepossibility of magnetic ordering coupled with asemiconducting nature and luminescence properties.
In the ultraviolet region, a wide bandgap matrix
semiconductor in combination with a Gd element is
one of the target systems. In the case of Al1-xGdxN
there are several cathodeluminescence (CL) [1-4],
electroluminescence [5], and photoluminescence (PL)[6] properties, in this case every result shows a sharp
line at 3.90㨪3.94eV which is clearly observed andwell assigned to the 6P7/28S7/2 4f intra transition of
Gd3+. However, the electronic structure of Al1-xGdxN
is still not clear. In consequence, we have performed
not only PL measurements but also PL excitation
and
(PLE)
measurementsof
Al0.98Gd0.02N
Al0.87Gd0.13N.
Al1-xGdxN thin films were grown in a MBE systemwith a RF-plasma assisted radical cell on the Si-faceof (0001)-oriented 6H-SiC substrates. The
thicknesses of the samples are about 120 nm.
Figure 1 shows the PL spectra of (a) Al0.98Gd0.02N
and (b) Al0.87Gd0.13N at 8K. The excitation photon
energy is 6.27eV. The PL spectra mainly consist of
two parts. The first part is the sharp lines seen ataround 3.95 eV. These sharp lines are in agreementwith previous research [1] and other results [2-6], asmentioned above. The second part is the broad
emission bands observed in the lower energy sidewhich may have been weakly observed in previous
CL work [1]. Since the band width of the broad
emission band in (b) is much wider than in (a), it is
supposed that this band is correlated to the Gdconcentration. The PLE spectra and the OR spectrafrom Al0.98Gd0.02N at 23 K are shown in Figs. 2(a) (T= 8q) and 2(b) (T = 60q). The emission photon
energies are 3.95 eV and 2.76 eV which correspond to
the peak energy positions of the sharp line and thebroad band, respectively. Notable features in Fig. 2
are as follows: (i) PLE spectra monitored at 2.76 eVclearly show the band edge. (ii) PLE spectra
monitored at 3.95 eV also show the band edge with apeak structure at 6.3 eV. (iii) Additional peaks andshoulders at 6.11 eV are obviously found in the PLE
spectra at T = 60q. These (i) ~ (iii) features suggestthat the crystal structure of Al0.98Gd0.02N is basicallysimilar to that of AlN, and the uniaxial anisotropy
character of AlN is still present.
㩷Al1-xGdxN㩷
PL Intensity㩷 (arb.units)

0.8

8K
exci. 6.27eV

0.6

PL Intensity (arb.units)

0.4

0.2

0.4
x=0.02(a)00.2
x=0.13

(b)
0

2.0

2.5

3.0

3.5

4.0

Photon Energy (eV)Fig. 1. Photoluminescence spectra from (a)Al0.98Gd0.02N and (b) Al0.87Gd0.13N at 8K.
Al1-xGdxN (x=0.02) 23K

㩷(b) T = 60ٟ

Reflectance (arb.units)

PLE Intensity (arb. units)emi. 3.95eVemi. 2.76eV(a) T = 8ٟemi. 3.95eVemi. 2.76eV

5.0

5.5

6.0

6.5

7.0

Photon Energy (eV)Fig. 2. Photoluminescence excitation spectra andoptical reflectance spectra from Al0.98Gd0.02N at 23
K. The angles of incidenceT are (a) 8q and (b)60q.
[1] S. W. Choi, S. Emura, S. Kimura, M. S. Kim, Y. K.
Zhou, N. Teraguchi, A. Suzuki, A. Yanase and H. Asahi, J
Alloys Compounds 408-412 (2006) 717.
[2] U. Vetter, J. Zenneck and H. Hofsäss, Appl. Phys. Lett.
83 (2003) 2145.
[3] J. B. Gruber, U. Vetter, H. Hofsäss, Phys. Rev. B 69
(2004) 195202.
[4] T. Kita, S. Kitayama, M. Kawamura, O. Wada, Y. Chigi,
Y. Kasai, T. Nishimoto, H. Tanaka and M. Kobayashi,Appl. Phys. Lett. 93 (2008) 211901.
[5] T. Toyama, J. Ota, D. Adachi, Y. Niioka, D. H. Lee andH. Okamoto, J. Appl. Phys. 105 (2009) 084512.
[6] J. M. Zavada, N. Nepal, J. Y. Lin, H. X. Jiang, E.
Brown, U. Hömmerich, J. Hite, G. T. Thaler, C. R.
Abemathy, S. J. Pearton and R. Gwilliam, Appl. Phys. Lett.
89 (2006) 152107.

114

BL8B2

Electronic Structure of Yttrium and Carbon Atoms Encapsulated
Metallofullerenes, Y2C2@C82: Ultraviolet Photoelectron Spectroscopy andTheoretical Calculation

S. Hino1, T. Miyazaki1, Y. Aoki1, N. Wanita2, M. Kato2, R. Sumii3, T. Akachi4, T. Inoue4,
Y. Ito4, T. Sugai4 and H. Shinohara4
1
Graduate School of Science & Technology, Ehime University, Matsuyama 790-8577 Japan
2
Graduate School of Science & Technology, Chiba University, Chiba 263-8522 Japan3
Photon Factory, Institute do Materials Structure Science, Tsukuba 305-0801 Japan
4
Graduate School of Science, Nagoya University, Nagoya 464-8602 Japan
Extensive UPS study of metallofullerenes
established an empirical rule that the cage structure isprincipally responsible for the electronic structure ofmono-metal atom encapsulated metallofullerenes.
That is, metallofullerenes of the same cage symmetry
with an incorporated metal atom of the same oxidation
state give essentially the same UPS. This empiricalrule also holds in Y2C2@C82 (III) and Y2@C82 (III),
both of which have the same C2v cage structure.
However, it does not hold in Ti2C2@C78 and La2@C78
although these endohedral fullerenes have the same
D3h (78:5) structure [11]. The case of Ti2C2@C78 and
La2@C78 is considered rather exceptional, since Ti
atoms are fond of forming carbide and hybridization oforbitals derived from entrapped Ti and caged C atoms
deforms the electronic structure of the fullerene cageso much. It is worthwhile to examine whether theempirical rule holds in three Y2C2@C82 isomers.

Figures 1 and 2 show the valence band UPS of
Y2C2@C82 (I) and Y2C2@C82 (II) obtained with hQ =20 – 55 eV photon energy. Their spectral onsets were0.88 eV and 0.67 eV, respectively, and the previously
reported value of Y2C2@C82 (III) was 0.8eV [7]. The
absorption spectral edge of these three isomers is 1400
nm (0.89 eV), longer than 2000 nm (0.62 eV) and 1100
nm (1.13 eV), respectively.
The UPS of Y2C2@C82 (I) and Y2C2@C82 (II) in 0 –15 eV energy region show 10 explicit structureslabeled by A – J in the figure. Their positions deviateslightly and their intensity changes when the incident
photon energy is tuned, as has been observe in the UPSof other fullerenes [7-11, 15].
Theoretical calculation on Y2C2@C82 (II) assuming
D3h cage structure re produced The UPS very well.
Optimized structure of entrapped Y2C2 was fond to
be distorted tetragonal form.

Fig. 1. UPS of Y2C2@C82 (I)Fig. 2. UPS of Y2C2@C82 (II)[1] S. Hino et al., Phys. Rev. B 75 (2005) 125418.

115