Hard x-ray photoemission study of LaAlO3/LaVO3 multilayers
Hard x-ray photoemission study of LaAlO3/LaVO3 multilayers
H. Wadati,1, ∗ Y. Hotta,2 A. Fujimori,1 T. Susaki,2 H. Y. Hwang,2, 3 Y. Takata,4 K. Horiba,4
M. Matsunami,4 S. Shin,4, 5 M. Yabashi,6, 7 K. Tamasaku,6 Y. Nishino,6 and T. Ishikawa6, 7
1Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
2Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561, Japan
3Japan Science and Technology Agency, Kawaguchi 332-0012, Japan
4Soft X-ray Spectroscopy Laboratory, RIKEN/SPring-8,
1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
5Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
6Coherent X-ray Optics Laboratory, RIKEN/SPring-8,
1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
7JASRI/SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan
(Dated: November 28, 2018)
We have studied the electronic structure of multilayers composed of a band insulator LaAlO3
(LAO) and a Mott insulator LaVO3 (LVO) by means of hard x-ray photoemission spectroscopy,
which has a probing depth as large as ∼ 60 Å. The Mott-Hubbard gap of LVO remained open at
the interface, indicating that the interface is insulating unlike the LaTiO3/SrTiO3 multilayers. We
found that the valence of V in LVO were partially converted from V3+ to V4+ only at the interface
on the top side of the LVO layer and that the amount of V4+ increased with LVO layer thickness. We
suggest that the electronic reconstruction to eliminate the polarity catastrophe inherent in the polar
heterostructure is the origin of the highly asymmetric valence change at the LVO/LAO interfaces.
PACS numbers: 71.28.+d, 73.20.-r, 79.60.Dp, 71.30.+h
I. INTRODUCTION
The interfaces of hetero-junctions composed of
transition-metal oxides have recently attracted great in-
terest. For example, it has been suggested that the inter-
face between a band insulator SrTiO3 (STO) and a Mott
insulator LaTiO3 (LTO) shows metallic conductivity.
1,2,3
Recently, Takizawa et al.4 measured photoemission spec-
tra of this interface and observed a clear Fermi cut-off,
indicating that an electronic reconstruction indeed oc-
curs at this interface. In the case of STO/LTO, elec-
trons penetrate from the layers of the Mott insulator to
the layers of the band insulator, resulting in the inter-
mediate band filling and hence the metallic conductivity
of the interfaces. It is therefore interesting to investi-
gate how electrons behave if we confine electrons in the
layers of the Mott insulator. In this paper, we inves-
tigate the electronic structure of multilayers consisting
of a band insulator LaAlO3 (LAO) and a Mott insula-
tor LaVO3 (LVO). LAO is a band insulator with a large
band gap of about 5.6 eV. LVO is a Mott-Hubbard in-
sulator with a band gap of about 1.0 eV.5 This material
shows G-type orbital ordering and C-type spin ordering
below the transition temperature TOO = TSO = 143 K.
From the previous results of photoemission and inverse
photoemission spectroscopy, it was revealed that in the
valence band there are O 2p bands at 4 − 8 eV and V
3d bands (lower Hubbard bands; LHB) at 0 − 3 eV and
that above EF there are upper Hubbard bands (UHB) of
V 3d origin separated by a band gap of about 1 eV from
the LHB.7 Since the bottom of the conduction band of
LAO has predominantly La 5d character and its energy
position is well above that of the LHB of LVO,8 the V
3d electrons are expected to be confined within the LVO
layers as a “quantum well” and not to penetrate into
the LAO layers, making this interface insulating unlike
the LTO/STO case.1,2,3,4 Recently, Hotta et al.9 investi-
gated the electronic structure of 1-5 unit cell thick layers
of LVO embedded in LAO by means of soft x-ray (SX)
photoemission spectroscopy. They found that the V 2p
core-level spectra had both V3+ and V4+ components
and that the V4+ was localized in the topmost layer.
However, due to the surface sensitivity of SX photoemis-
sion, the information about deeply buried interfaces in
the multilayers is still lacking. Also, they used an un-
monochromatized x-ray source, whose energy resolution
was not sufficient for detailed studies of the valence band.
In the present work, we have investigated the electronic
structure of the LAO/LVO interfaces by means of hard
x-ray (HX) photoemission spectroscopy (hν = 7937 eV)
at SPring-8 BL29XU. HX photoemission spectroscopy is
a bulk-sensitive experimental technique compared with
ultraviolet and SX photoemission spectroscopy, and is
very powerful for investigating buried interfaces in mul-
tilayers. From the valence-band spectra, we found that
a Mott-Hubbard gap of LVO remained open at the in-
terface, indicating the insulating nature of this interface.
From the V 1s and 2p core-level spectra, the valence of V
in LVO was found to be partially converted from V3+ to
V4+ at the interface, confirming the previous study.9 Fur-
thermore, the amount of V3+ was found to increase with
LVO layer thickness. We attribute this valence change to
the electronic reconstruction due to polarity of the layers.
http://arxiv.org/abs/0704.1837v1
SampleB: 
LAO (3ML)/LVO (50ML)
LaAlO3 (3ML)
LaVO3 (50ML)
SrTiO3 (5ML)
SrTiO3 (100)
Sample A: 
LAO (3ML)/LVO (3ML)/LAO (30ML)
LaAlO3 (3ML)
LaVO3 (3ML)
SrTiO3 (5ML)
SrTiO3 (100)
LaAlO3 (30ML)
Sample C: 
LVO (50ML)
LaVO3 (50ML)
SrTiO3 (100)
FIG. 1: Schematic view of the LaAlO3/LaVO3 mul-
tilayer samples. Sample A: LaAlO3 (3ML)/LaVO3
(50ML)/SrTiO3. Sample B: LaAlO3 (3ML)/LaVO3
(50ML)/LaAlO3 (30ML)/SrTiO3. Sample C: LaVO3
(50ML)/SrTiO3.
II. EXPERIMENT
The LAO/LVOmultilayer thin films were fabricated on
TiO2-terminated STO(001) substrates
10 using the pulsed
laser deposition (PLD) technique. An infrared heat-
ing system was used for heating the substrates. The
films were grown on the substrates at an oxygen pres-
sure of 10−6 Torr using a KrF excimer laser (λ = 248
nm) operating at 4 Hz. The laser fluency to ablate
LaVO4 polycrystalline and LAO single crystal targets
was ∼ 2.5 J/cm2. The film growth was monitored us-
ing real-time reflection high-energy electron diffraction
(RHEED). Schematic views of the fabricated thin films
are shown in Fig. 1. Sample A consisted of 3ML LVO
capped with 3ML LAO. Below the 3ML LVO, 30ML
LAO was grown, making LVO sandwiched by LAO. Sam-
ple B consisted of 50ML LVO capped with 3ML LAO.
Sample C was 50ML LVO without LAO capping lay-
ers. Details of the fabrication and characterization of the
films were described elsewhere.11 The characterization of
the electronic structure of uncapped LaVO3 thin films
by x-ray photoemission spectroscopy will be described
elsewhere.12 HX photoemission experiments were per-
formed at an undulator beamline, BL29XU, of SPring-8.
The experimental details are described in Refs. 13,14,15.
The total energy resolution was set to about 180 meV.
All the spectra were measured at room temperature. The
Fermi level (EF ) position was determined by measuring
gold spectra.
III. RESULTS AND DISCUSSION
Figure 2 shows the valence-band photoemission spectra
of the LAO/LVO multilayer samples. Figure 2 (a) shows
the entire valence-band region. Compared with the pre-
vious photoemission results,7 structures from 9 to 3 eV
are assigned to the O 2p dominant bands, and emission
from 3 eV to EF to the V 3d bands. The energy posi-
tions of the O 2p bands were almost the same in these
three samples, indicating that the band bending effect
at the interface of LAO and LVO was negligible. Fig-
3.0 2.0 1.0 0
Binding Energy (eV)
hn = 7937 eV
A: LAO (3ML)/LVO (3ML)
/LAO (30ML)
B: LAO (3ML)/LVO (50ML)
C: LVO (50ML)
10 8 6 4 2 0 -2 -4
Binding Energy (eV)
hn = 7937 eV
O 2p bands
V 3d bands
A: LAO (3ML)/LVO (3ML)
/LAO (30ML)
C: LVO (50ML)
B: LAO (3ML)/LVO (50ML)
FIG. 2: Valence-band photoemission spectra of the
LaAlO3/LaVO3 multilayer samples. (a) Valence-band spec-
tra over a wide energy range. (b) V 3d band region.
ure 2 (b) shows an enlarged plot of the spectra in the V
3d-band region. A Mott-Hubbard gap of LVO remained
open at the interface between LAO and LVO, indicat-
ing that this interface is insulating unlike the STO/LTO
interfaces.1,2,3,4 The line shapes of the V 3d bands were
almost the same in these three samples, except for the
energy shift in sample A. We estimated the value of the
band gap from the linear extrapolation of the rising part
of the peak as shown in Fig. 2 (b). The gap size of sample
B was almost the same (∼ 100 meV) as that of sample
C, while that of sample A was much larger (∼ 400 meV)
due to the energy shift of the V 3d bands. The origin
of the enhanced energy gap is unclear at present, but
an increase of the on-site Coulomb repulsion U in the
thin LVO layers compared to the thick LVO layers or
bulk LVO due to a decrease of dielectric screening may
explain the experimental observation.
Figure 3 shows the V 1s core-level photoemission spec-
tra of the LAO/LVO multilayer samples. The V 1s spec-
tra had a main peak at 5467 eV and a satellite structure
at 5478 eV. The main peaks were not simple symmet-
ric peaks but exhibited complex line shapes. We there-
fore consider that the main peaks consisted of V3+ and
V4+ components. In sample C, there is a considerable
amount of V4+ probably due to the oxidation of the sur-
face of the uncapped LVO. A satellite structure has also
been observed in the V 1s spectrum of V2O3
16 and inter-
preted as a charge transfer (CT) satellites arising from
the 1s13d3L final state, where L denotes a hole in the O
2p band. Screening-derived peaks at the lower-binding-
energy side of V 1s, which have been observed in the
metallic phase of V2−xCrxO3,
16,17 were not observed in
the present samples, again indicating the insulating na-
ture of these interfaces.
Figure 4 shows the O 1s and V 2p core-level photoemis-
sion spectra of the LAO/LVO multilayer samples. The
O 1s spectra consisted of single peaks without surface
contamination signal on the higher-binding-energy side,
indicating the bulk sensitivity of HX photoemission spec-
troscopy. The energy position of the O 1s peak of sample
5490 5480 5470 5460 5450
Binding Energy (eV)
La 2p3/2
CT satellite
hn = 7937 eV
B: LAO (3ML)/LVO (50ML)
A: LAO (3ML)/LVO (3ML)
/LAO (30ML)
C: LVO (50ML)
FIG. 3: V 1s core-level photoemission spectra of the
LaAlO3/LaVO3 multilayer samples.
520 518 516 514 512 510 508
Binding Energy (eV)
hn = 7937 eV
V 2p3/2
B: LAO (3ML)/LVO (50ML)
A: LAO (3ML)/LVO (3ML)
/LAO (30ML)
C: LVO (50ML)
535 530 525 520 515 510
Binding Energy (eV)
V 2p1/2
V 2p3/2
hn = 7937 eV(a)
B: LAO (3ML)/LVO (50ML)
A: LAO (3ML)/LVO (3ML)
/LAO (30ML)
C: LVO (50ML)
FIG. 4: O 1s and V 2p core-level photoemission spectra of the
LaAlO3/LaVO3 multilayer samples. (a) shows wide energy
region and (b) is an enlarged plot of the V 2p3/2 spectra.
A, whose LVO layer thickness was only 3 ML, was differ-
ent from those of the rest because LAO and LVO have
different energy positions of the O 1s core levels. Fig-
ure 4 (b) shows an enlarged plot of the V 2p3/2 spectra.
Here again, the V 2p3/2 photoemission spectra showed
complex line shapes consisting of V3+ and V4+ com-
ponents, and no screening-derived peaks on the lower-
binding-energy side of V 2p3/2 were observed. The line
shapes of the V 2p3/2 spectra were very similar for sam-
ples A and B. The amount of V4+ was larger in sample
C, consistent with the case of V 1s and again shows the
effect of the oxidation of the uncapped LVO.
We have fitted the core-level spectra of samples A and
B to a Gaussian convoluted with a Lorentzian to esti-
mate the amount of V3+, V4+ and V5+ at the interface
following the procedure of Ref. 9. Figure 5 shows the
fitting results of the V 1s and V 2p3/2 core-level spec-
tra. Here, the spectra were decomposed into the V3+
and V4+ components, and the V5+ component was not
necessary. The full width at half maximum (FWHM) of
the Lorentzian has been fixed to 1.01 eV for V 1s and to
0.24 eV for V 2p3/2 according to Ref. 18. The FWHM of
the Gaussian has been chosen 0.90 eV for V 1s and 1.87
eV for V 2p3/2, reflecting the larger multiplet splitting
for V 2p than for V 1s. In Fig. 6, we summarize the ratio
of the V3+ component thus estimated, together with the
results of the emission angle (θe) dependence of the V
 experiment
A: LAO (3ML)
/LVO (3ML)
/LAO (30ML)
hn = 7937 eV
5472 5470 5468 5466 5464
Binding Energy (eV)
 experiment
hn = 7937 eV
B: LAO (3ML)
/LVO (50ML)
 experiment
A: LAO (3ML)
/LVO (3ML)
/LAO (30ML)
hn = 7937 eV
V 2p3/2
520 518 516 514 512
Binding Energy (eV)
 experiment
hn = 7937 eV
B: LAO (3ML)
/LVO (50ML)
V 2p3/2
FIG. 5: Fitting results for the V 1s and 2p3/2 core-level spec-
tra. (a) V 1s core level of sample A (LaVO3 3ML), (b) V
2p3/2 core level of sample A (LaVO3 3ML), (c) V 1s core
level of sample B (LaVO3 50ML), (d) V 2p3/2 core level of
sample B (LaVO3 50ML).
3 4 5 6 7 8 9
2 3 4 5 6 7
Mean free path (Å)
(V 2p)
(V 1s)(30
 SX SX SX
 surface  bulk
B: LAO (3ML)/LVO (50ML)
 Experiment
 Model (B)
3 4 5 6 7 8 9
2 3 4 5 6 7
Mean free path (Å)
(V 2p)
(V 1s)
 SX SX  SX
 surface  bulk
A: LAO (3ML)/LVO (3ML)
/LAO (30ML)
 Experiment
 Asymetric model (A-1)
 Symetric model (A-2)
FIG. 6: Ratio of V4+ or V4++ V5+ determined under various
experimental conditions using hard x-rays and soft x-rays.9
(a) Sample A (3ML LaVO3), (b) Sample B (50ML LaVO3).
Here, SX is a result of soft x-ray photoemission, and HX is
of hard x-ray photoemission. In the case of SX, the values in
the parenthesis denote the values of θe.
2p core-level SX photoemission spectra measured using a
laboratory SX source.9
In order to interpret those results qualitatively, first we
have to know the probing depth of photoemission spec-
troscopy under various measurement conditions. From
the kinetic energies of photoelectrons, the mean free
paths of the respective measurements are obtained as de-
scribed in Ref. 19.20 When we measure V 2p3/2 spectra
with the Mg Kα line (hν = 1253.6 eV), the kinetic en-
ergy of photoelectrons is about 700 eV, and the mean
free path is estimated to be about 10 Å. Likewise, we
also estimate the mean free path in the HX case. The
values are summarized in Table I. In the SX case, these
values are 10 cos θe Å. One can obtain the most surface-
sensitive spectra under the condition of SX with θe = 70
[denoted by SX(70o)] and the most bulk-sensitive spectra
for HX measurements of the V 2p3/2 core level [denoted
TABLE I: Mean free path of photoelectrons (in units of Å)
SX SX SX HX HX
(70◦) (55◦) (30◦) (V 1s) (V 2p)
3.4 5.7 8.7 30 60
A-2: Symmetric model
:50%V
B: LAO (3ML)/LVO (50ML)
:85%V
A: LAO (3ML)/LVO (3ML)/LAO (30ML)
:70%V
A-1: Asymmetric model
FIG. 7: Models for the V valence distributions in the
LaAlO3/LaVO3 multilayer samples. A: LaVO3 3ML. A-1 is
an asymmetric model, whereas A-2 is a symmetric model. B:
LaVO3 50ML.
by HX(V 2p)]. From Fig. 6 and Table I, one observes a
larger amount of V4+ components under more surface-
sensitive conditions. These results demonstrate that the
valence of V in LVO is partially converted from V3+ to
V4+ at the interface.
In order to reproduce the present experimental result
and the result reported in Ref. 9 (shown in Fig. 6), we
propose a model of the V valence distribution at the in-
terface as shown in Fig. 7. For sample A, we consider two
models, that is, an asymmetric model and a symmetric
model. In the asymmetric model (A-1), no symmetry is
assumed between the first and the third layers. As shown
in Fig. 6, the best fit result was obtained for the valence
distribution that 70 % of the first layer is V4+ and there
are no V4+ in the second and third layers, assuming the
above-mentioned mean free paths in Table I and expo-
nential decrease of the number of photoelectrons. In the
symmetric model (A-2), it is assumed that the electronic
structures are symmetric between the first and the third
layers. The best fit was obtained when 50 % of the first
and third layers are V4+. In sample B, a model (B) where
85 % of the first layer and 50 % of the second layer are
V4+ best reproduced the experimental result. As shown
in Fig. 6, for the 3ML case, the model (A-2) did not
reproduce the experimental results well compared to (A-
1), which demonstrates that the valence distribution of
V was highly asymmetric at these interfaces.
The origin of this highly asymmetric valence change
from V3+ to V4+ at the interfaces can be interpreted
in two ways. One possible scenario is a simple chemical
effect during the fabrication process of the PLD tech-
nique. The topmost LVO layer spends a longer time be-
fore the next deposition of LAO than the rest LVO lay-
ers, and therefore, oxidation process may easily proceed
at the topmost layer. In this scenario, if we make sam-
ples under different experimental conditions, especially
under different oxygen pressures, the amount of V4+ at
the interface may change greatly. In the other scenario,
we consider that the polarity of the LAO/LVO multilay-
ers plays an essential role. In the present samples, both
the LAO and LVO layers are polar, and do not consist of
charge neutral layers, that is, they consist of alternating
stack of LaO+ and AlO−2 or VO
2 layers. As recently
discussed by Nakagawa et al.,23 electronic reconstruction
occurs during the fabrication of the polar layers in or-
der to prevent the divergence of Madelung potential, i.e.,
so-called polar catastrophe.24 We consider that the elec-
tronic reconstruction occurs in the present samples, and
that the valence change of V at the interface is a result of
this reconstruction. This effect explains 0.5 ML of V4+,
but we cannot explain the total amount of V4+ exceed-
ing 0.5 ML, and we must also consider some chemical
effects that V atoms are relatively easily oxidized at the
topmost layer. Similar studies on samples with different
termination layers will be necessary to test this scenario.
Recently, Huijben et al.25 studied STO/LAO multilayers
and found a critical thickness of LAO and STO, below
which a decrease of the interface conductivity and car-
rier density occurs. Therefore, changing the numbers of
LAO capping layers may also change the valence of V at
the interface. Further systematic studies, including other
systems like LTO/STO1,2,3,4 and LAO/STO23,25,26, will
reveal the origin of the valence change at the interface.
IV. CONCLUSION
We have investigated the electronic structure of the
multilayers composed of a band insulator LaAlO3 and a
Mott insulator LaVO3 (LVO) by means of HX photoe-
mission spectroscopy. The Mott-Hubbard gap of LVO
remained open at the interface, indicating that the inter-
face is insulating and the delocalization of 3d electrons
does not occur unlike the LaTiO3/SrTiO3 multilayers.
From the V 1s and 2p core-level photoemission intensi-
ties, we found that the valence of V in LVO was partially
converted from V3+ to V4+ at the interface only on the
top side of the LVO layer and that the amount of V4+
increased with LVO layer thickness. We constructed a
model for the V valence redistribution in order to ex-
plain the experimental result and found that the V4+ is
preferentially distributed on the top of the LVO layers.
We suggest that the electronic reconstruction to elimi-
nate polar catastrophe may be the origin of the highly
asymmetric valence change at the interfaces.
V. ACKNOWLEDGMENTS
The HX photoemission experiments reported here have
benefited tremendously from the efforts of Dr. D. Miwa
of the coherent x-ray optics laboratory RIKEN/SPring-
8, Japan and we dedicate this work to him. This work
was supported by a Grant-in-Aid for Scientific Research
(A16204024) from the Japan Society for the Promotion
of Science (JSPS) and a Grant-in-Aid for Scientific Re-
search in Priority Areas “Invention of Anomalous Quan-
tum Materials” from the Ministry of Education, Culture,
Sports, Science and Technology. H. W. acknowledges fi-
nancial support from JSPS. Y. H. acknowledges support
from QPEC, Graduate School of Engineering, University
of Tokyo.
∗ Electronic address: wadati@wyvern.phys.s.u-tokyo.ac.jp;
URL: http://www.geocities.jp/qxbqd097/index2.htm;
Present address: Department of Physics and Astron-
omy, University of British Columbia, Vancouver, British
Columbia V6T-1Z1, Canada
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