Solid State Communications 217 (2015) 17–20

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Solid State Communications
journal homepage: www.elsevier.com/locate/ssc

Fast-track Communication

Electronic structure of Mott-insulator CaCu3Ti4O12: Photoemission
and inverse photoemission study
H.J. Im a,n, M. Iwataki a, S. Yamazaki a, T. Usui a, S. Adachi a, M. Tsunekawa b, T. Watanabe a,
K. Takegahara a, S. Kimura c,d, M. Matsunami d,e, H. Sato f, H. Namatame f, M. Taniguchi faGraduate School of Science and Technology, Hirosaki University, Hirosaki 036-8224, Japan
Faculty of Education, Shiga University, Otsu 520-0862, Japanc
Graduate School of Frontier Biosciences, Osaka University, Suita 565-0871, JapandUVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japane
School of Physical Sciences, The Graduate University for Advanced Studies, Okazaki 444-8585, Japanf
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japanb

art ic l e i nf o

a b s t r a c t

Article history:
Received 31 March 2015
Received in revised form
8 May 2015
Accepted 11 May 2015
Communicated by A. Pinczuk
Available online 16 May 2015

We have performed the photoemission and inverse photoemission experiments to elucidate the origin ofMott insulating states in A-site ordered perovskite CaCu3Ti4O12 (CCTO). Experimental results haverevealed that Cu 3d–O 2p hybridized bands, which are located around the Fermi level in the prediction of
the local-density approximation (LDA) band calculations, are actually separated into the upper Hubbard
band at  1.5 eV and the lower Hubbard band at   1.7 eV with a band gap of 1.5–1.8 eV. We also
observed that Cu 3d peak at   3.8 eV and Ti 3d peak at  3.8 eV are further away from each other thanas indicated in the LDA calculations. In addition, it is found that the multiplet structure around  9 eV
includes a considerable number of O 2p states. These observations indicate that the Cu 3d and Ti 3delectrons hybridized with the O 2p states are strongly correlated, which originates in the Mott-insulatingstates of CCTO.
& 2015 Elsevier Ltd. All rights reserved.

Keywords:
A. A-site ordered perovskite CaCu3Ti4O12
D. Strong correlation effects
D. Mott-insulating state
E. Photoemission and inverse
photoemission

1. Introduction
Physics of Mott-insulator has been an underlying concept tounderstand the intrigue phenomena in transition metal oxides
such as perovskite-type materials [1,2]. One of the most wellknown systems is the high-temperature superconductor with thelayered perovskite structure, e.g. La2  x Srx CuO4 and La2  x Bax CuO4[3]. The perovskites LaVO3 and YTiO3 are also typical Mottinsulators with the antiferromagnetic and ferromagnetic ordering,
respectively [4,5]. Generally, Mott-insulating states can be understood within the framework of the Mott–Hubbard model as its
name says. When the on-site repulsive Coulomb energy between
the d-electrons (U) is larger than the bandwidth of the d-states(W), the Mott-insulating phase appears in contrast to the band
insulator where the d-shell is fully occupied [1,2]. However, there
exists many sophisticated situations rather than an ideal situation
to be explained simply. For instance, the layered perovskite
Ca2RuO4 has shown the orbital selective Metal-insulator transitionnCorresponding author.
E-mail address: hojun@hirosaki-u.ac.jp (H.J. Im).

http://dx.doi.org/10.1016/j.ssc.2015.05.007
0038-1098/& 2015 Elsevier Ltd. All rights reserved.

[6–8]. In Sr2IrO4, it has been reported that Mott insulator develops
due to the strong spin–orbit interaction of Ir 5d-electrons [9]. In
addition, it has been well known that the physics of the Mott
insulating states plays an important role to understand themetallic peroskite CaVO3 and SrVO3 [10,11].
Mott-insulating A-site ordered perovskite CaCu3Ti4O12 (CCTO)
has attracted much attention in both fundamental science andtechnological application, since the discovery of the extremely
high dielectric constant (ε) in 2000 [12]. However, the mechanismof the high ε has not been fully understood yet, and it is still
debating whether the high ε is intrinsic or extrinsic [13,14]. Thisrequires a study to clarify the fundamental properties such as theelectronic structure. Recently, we have successfully observed theband dispersion of CCTO in the angle-resolved photoemission(ARPES) experiments and revealed that CCTO is a Mott-type
insulator caused by the strong correlation effects: the Cu 3d–O
2p hybridized bands are shifted toward the lower energy side by 1.7 eV and show band narrowing around  2 eV in comparison
with the local-density approximation (LDA) band calculations [15].
However, in order to fully understand the mechanism of the Mott
insulating states in CCTO, the electronic structures in both theoccupied and unoccupied regions should be investigated. In this18

H.J. Im et al. / Solid State Communications 217 (2015) 17–20

paper, we report the electronic structures of CCTO in the occupied
and unoccupied regions revealed by photoemission (PES) andinverse photoemission spectroscopy (IPES).

2. Experimental methods
The polycrystalline sample of CCTO has been synthesized by
the conventional solid state reaction technique using CaCO3, CuO,
and TiO2 powders. The well mixed powders with the stoichiometric weights were calcined at 700 1C for 12 h. The pellets
pressed by 40 MPa were sintered at 1050 1C for 24 h. These
processes have been repeated several times to obtain the highquality sample. Single phase of CCTO was conﬁrmed by the X-ray
diffraction pattern.
The PES experiments have been performed at the synchrotron
radiation beam line BL5U of UVSOR. The used photon energies ðhνÞare 30, 65, and 90 eV. The total energy resolution is about 75 meV
at hν ¼ 65 eV. The IPES experiments have been carried out at
HiSOR, using a stand-alone apparatus with the tunable photon
energy mode [16]. The used kinetic energies of incident electrons(Ek) are 40 and 46 eV. The energy resolution was set to about
650 meV at Ek ¼ 46 eV. All measurements have been performed at
the room-temperature (T  300 K). The clean surface of sample
was prepared by in situ fracturing in the ultra-high vacuum of2  10  8 Pa. The Fermi-level (EF) in PES and IPES measurements
was referred to that of Au at 300 K and 10 K, respectively.

3. Results and discussion
Fig. 1 shows the PES spectra of CCTO at hν ¼ 30, 65, and 90 eV.
In the region from 0 to  8 eV, there are three features in good
agreement with the previous ARPES results; the intense peakaround  4 eV (mainly Cu 3d states), the shoulder structurearound  2 eV (mainly Cu 3d and O 2p states) and the hump
structure around  6 eV (mainly O 2p states) [15]. It is found that
the Cu 3d peak position around  4 eV is located at  0.2 eV lower

CaCu 3 Ti4 O12
T = 300 K

hv = 65 eV

Intensity (arb. units)

▼
-2

-1

B

A

0

90 eV

65 eV

30 eV

-12

-8

-4

0

Energy Relative to E F (eV)Fig. 1. (Color online) PES spectra of CCTO at hν ¼30, 65, 90 eV and T ¼ 300 K. The
inset shows the enlarged PES spectrum near EF at hν ¼ 65 eV. The solid triangle andarrow indicate the peak position (   1.7 eV) and the tail (   1.2 eV) of the lower
Hubbard band, respectively. (In the inset, the EF of the enlarged PES spectrum wascalibrated by shifting toward the higher energy side by 0.2 eV, referring to theprevious ARPES results [15].)

energy than that of the previous ARPES experiments [15]. This may
come from the charging effects of an insulating sample during themeasurement process. In spite of that, the good consistency of thespectral shape with the ARPES results indicates that the observed
PES spectra are revealing the intrinsic electronic structures of
CCTO. The inset shows the enlarged spectrum of the shoulder
structure around 2 eV, which was plotted by shifting toward thehigher energy side by 0.2 eV to correct EF. The peak position can be
estimated to be   1.7 eV, and the tail of the band is also
observed at   1.2 eV. In the region from  8 to  13 eV, there
are two peaks, A around  9 eV and B around  12 eV. These canbe considered as multiplet structures of Cu 3d states hybridized
with O 2p states as in CuO which is representative of strongly
correlated electrons systems [15,17,18]. Actually, the valence bandof CCTO is very similar to that of CuO except for the position of theintense Cu 3d peak which is located around 3 eV in CuO [17]. (Itshould be noted that most of the Ti 3d states exist in theunoccupied region and that most of the Ca sp-states thinly spreadover the whole valence band.) This means that the electronic
structure of Cu 3d states of CCTO is close to that of CuO. Therefore,
the two peaks A and B in CCTO can be understood on the basis of
the interpretation of CuO. The peak A around  9 eV is the ﬁnal
10
states of d8 and d L 2 (L is a hole of ligands), while the peak B
around  12 eV comes mainly from the d8 ﬁnal states [17]. It is
recognized that the peak B drastically changes with hν. In thephotoemission process, it is well known that the photoionization
cross section (σ) depends on the kinetic energy of photoelectrons,
which can be changed with hν [19,20]. In the hν of 30–90 eV, the σof the O 2p orbital rapidly increases with decreasing hν comparedto that of the Cu 3d orbital [19]. The intensity of peak B decreases
with decreasing hν and seems to disappear at hν ¼ 30 eV, while
the intensity of peak A does not change with hν, relatively. In otherwords, the relative intensity of peak A to peak B increases withdecreasing hν. From the above argument, we can conclude that thepeak A includes a considerable number of O 2p states while peak B
consists mainly of Cu 3d states. This indicates that the Cu 3d–O 2p
hybridized bands compose the multiplet structures, which areusually considered to come from atomic states.
Fig. 2 shows the IPES spectra around the Ti 3p–3d edge. The
cross section of the Ti 3d states can be largely enhanced aroundthe Ti 3p-3d edge via the Fano-resonance between the standard
IPES process and the Coster–Kronig Auger process [2,21]. Additionally, it has been reported that the energies of on- and offresonance IPES are about 46 eV and 39 eV, respectively [21]. In theoff-resonant IPES spectrum (Ek ¼40 eV), we observe three features; a broad shoulder structure in 0–3 eV, a relatively intense
peak at  3.8 eV, and a broad hump structure in 8–12 eV. The inset
of Fig. 2 shows the enlarged spectrum around the broad shoulder
structure. The spectral weight is negligible around EF, indicating an
insulating character. It then begins to increase just above EF,
forming a broad shoulder from 0.5 to 2.5 eV. The peak position
can be evaluated from the midpoint of the broad shoulder( 1.5 eV). When we take into account the energy resolution of
0.65 eV, the tail of the band should exist between 0 and 0.6 eV andits midpoint (  0.3 eV) is indicated by the arrow. In the onresonant IPES spectrum (Ek ¼46 eV), we ﬁnd that the peak at 3.8 eV is slightly sharper than that of the off-resonant IPES
spectrum. However, it is difﬁcult to extract the Ti 3d spectrum
from the difference between the on- and off-resonant spectra dueto the small resonant effects. In order to discover the reason why
the resonance is not effective in CCTO, further study is necessary.
For our purpose here, we identify elements of the spectral weightby comparing the IPES spectra with the band calculations. In thelower part of Fig. 2, we plot the partial density of states (DOS) of Ti
3d, Ca 3d, Cu 3d, and O 2p states obtained from the LDA
calculations, shifting it toward the higher energy side by about

H.J. Im et al. / Solid State Communications 217 (2015) 17–20

19

Intensity (arb. units)

T = 300 K E = 40 eV
▼

0

1

2

3

Ca 3d

Ti 3d

46 eV

40 eV

Intensity (arb. units)

CaCu3 Ti4 O12

-8

-6

-4

-2

0

LDA
0

4

2

4

6

8

Energy Relative to EF (eV)

Ti 3d
Ca 3d
Cu 3d
O 2p

IPES

8

4
0
12
8
Energy Relative to EF (eV)Fig. 2. (Color online) IPES spectra of CCTO at Ek ¼ 40, 46 eV and T ¼300 K. The
partial DOS obtained from the LDA calculations is also plotted below the IPES
spectra. The inset shows the enlarged IPES spectrum near EF at Ek ¼ 40 eV. The solid
triangle and arrow indicate the peak position (  1.5 eV) and the tail (  0.3 eV) of
the upper Hubbard band, respectively.

1.5 eV. We can recognize that the shifted partial DOS reproduces
the IPES spectra. The spectral weight of peak at  3.8 eV comesmainly from Ti 3d states, and the broad hump structure in 8–12 eV
can be attributed mainly to Ca 3d states. Moreover, the shoulder
structure at  1.5 eV consists of a large number of Cu 3d, O 2p
states and a small number of Ti 3d states, which are said to belocated around EF in the prediction of the LDA calculations.
Finally, we would like to discuss the mechanism of the Mottinsulating states in CCTO, considering the strong correlation
effects. In Fig. 3(a), the PES spectrum at hν ¼65 eV and the IPES
spectrum at Ek ¼ 40 eV are plotted together. For the sake ofcomparison, the partial DOS of the Cu 3d and Ti 3d states obtained
from the LDA calculations has been also presented in the lower
part of Fig. 3(a). It should be noted that the strong correlation
effects cannot be fully understood within the framework of
the LDA calculations in general and have often been discussed
by comparing experimental results and LDA calculations [2,15].
Fig. 3(b) is a schematic diagram, which explains how the electronic
structures of CCTO are affected by the strong correlation effects.
First, let us discuss the spectra near EF. The spectral weights are
negligible in both PES and IPES experiments showing the insulating phase, while there exists the DOS of the Cu 3d and small Ti 3dstates, which are hybridized with the O 2p states, in the LDA
calculations. It is also observed that the shoulder structures at  1.7 eV and  1.5 eV are symmetric with respect to EF as in theother Mott-insulators [2]. In other words, the DOS near EF of the
LDA calculation is separated into the upper Hubbard band (UHB) at
1.5 eV and the lower Hubbard band (LHB) at   1.7 eV [2]. Theenergy gap (Eg) can be estimated to be 1.57 0.3 eV from theenergy difference between the tails of LHB and UHB. Moreover,when we take into account the results of the optical experiments,
where Eg is larger than 1.5 eV [22], Eg should be  1.5–1.8 eV. From
the above results, U is estimated to be  3.2 eV. The W of LHB is1 eV in the PES spectrum, and the W of UHB can be estimated tobe  1.5–2 eV considering the experimental resolution in the IPES
spectrum. These values are qualitatively consistent with thesimple Mott–Hubbard model, where Eg ¼ U  W.

~ 1.4 eV
StrongCorrelation~ 2.3 eVPES
Fig. 3. (Color online) (a) The PES and IPES spectra of CCTO obtained at hν ¼65 eV
and Ek ¼ 40 eV, respectively. (The EF of the PES spectrum was calibrated referring to
the previous ARPES results [15].) The Cu 3d and Ti 3d partial DOS of the LDA
calculations are also given in the lower part. The solid triangles and arrows on the
spectra indicate the peak position and the tail of the Hubbard bands, respectively(b) Schematic diagram of the strong correlation effects in CCTO.

Secondly, let us discuss the spectra in the region from  3 to
 6 eV and from 3 to 6 eV. In comparison with the LDA calculations, the Cu 3d peak is shifted from 1.5 to  3.8 eV in theoccupied region, and the Ti 3d peak is shifted from 2.4 to 3.8 eV in
the unoccupied region. This seems similar to the behavior of theincoherent states in the simple Mott–Hubbard picture. However,
CCTO looks more complicated. The observed peak shifts occur far
away from EF and are not directly relevant to the insulating phase.
In addition, the Cu– and Ti–ions are located in different sites (the A0– and B–sites, respectively) and that the O-ions are in between
them; A–site ordered perovskites have a crystal structure of theformula AA 0 3B4O12 [12,15]. Nevertheless, we can recognize thatthe shifts of the Cu 3d and Ti 3d peaks appear in pairs and areaccompanied by the separation of the Hubbard bands. For theoccupied region, it has been reported in the previous ARPES
studies that the strong correlation effects in the Cu 3d–O 2p
hybridized bands cause the shift of the Cu 3d peak toward thelower energy side [15]. And, the UHB and LHB come from not only
the Cu 3d–O 2p hybridized bands but also small Ti 3d–O 2p
hybridized bands as discussed in Fig. 2. These indicate that the Ti
3d electrons hybridized with O 2p states are also strongly
correlated, which causes the shift of the Ti 3d peak toward thehigher energy side, as in the case of the Cu 3d electrons. As aresult, the Cu 3d and Ti 3d electrons hybridized with the O 2p
states are strongly correlated, which is responsible for the Mottinsulating states and the band shifting in CCTO.
We believe that these results will contribute to the understanding of new-type Mott-insulators and the duality of thestrongly correlated d-electrons.

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H.J. Im et al. / Solid State Communications 217 (2015) 17–20

4. Summary
The mechanism of Mott-insulating behavior of CCTO has been
investigated by the PES and IPES experiments. In the region from
2 to 2 eV, UHB and LHB, which consist mainly of Cu 3d–O 2p
hybridized bands, have been observed at  1.5 and   1.7 eV,
respectively. The Cu 3d peak at  3.8 eV and Ti 3d peak at 3.8 eV
are further away from each other than as predicted in the LDA
calculations. In addition, the strong hν dependence of the PES
spectrum in the region from  8 to  13 eV indicates that themultiplet structure A around 9 eV includes a considerable
number of O 2p states. These results reveal that the strong
correlation effects of the Cu 3d and Ti 3d electrons, which are
hybridized with the O 2p states, give rise to the Mott-insulatingstates of CCTO.
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