arXiv:0708.1387v2 [cond-mat.str-el] 17 Aug 2007
In-situ photoemission study of Pr1−xCaxMnO3epitaxial thin films with suppressed
H. Wadati,1, ∗A. Maniwa,2A. Chikamatsu,2I. Ohkubo,2H. Kumigashira,2
M. Oshima,2A. Fujimori,1M. Lippmaa,3M. Kawasaki,4and H. Koinuma5
1Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
2Department of Applied Chemistry, University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
3Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
4Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan
5National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047
(Dated: February 1, 2008)
We have performed an in-situ photoemission study of Pr1−xCaxMnO3 (PCMO) thin films grown
on LaAlO3 (001) substrates and observed the effect of epitaxial strain on the electronic structure.
We found that the chemical potential shifted monotonically with doping, unlike bulk PCMO, imply-
ing the disappearance of incommensurate charge fluctuations of bulk PCMO. In the valence-band
spectra, we found a doping-induced energy shift toward the Fermi level (EF) but there was no
spectral weight transfer, which was observed in bulk PCMO. The gap at EF was clearly seen in the
experimental band dispersions determined by angle-resolved photoemission spectroscopy and could
not be explained by the metallic band structure of the C-type antiferromagnetic state, probably
due to localization of electrons along the ferromagnetic chain direction or due to another type of
Strongly correlated systems have attracted great in-
terest because of their various interesting physical prop-
erties, such as metal-insulator transition, colossal mag-
netoresistance (CMR), and the ordering of spin, charge,
and orbitals . One of the peculiar features of these
systems is their high sensitivity to external stimuli. Pres-
sure as well as carrier concentration are among the most
important parameters.The effects of carrier doping
and chemical pressure on their electronic structures have
been extensively studied by transport measurements and
various spectroscopic methods, including photoemission
spectroscopy, although it has been impossible to directly
observe the electronic structures under physical hydro-
static pressure by photoemission spectroscopy because of
the fundamental limitation of the technique. However, if
one grows thin films epitaxially on single crystalline sub-
strates, one can effectively perform photoemission mea-
surements under (anisotropic) high pressure. As for the
high-Tccuprates, Abrecht et al. [2, 3] performed an in-
situ angle-resolved photoemission spectroscopy (ARPES)
study of La2−xSrxCuO4 (LSCO) thin films grown on
SrLaAlO4 (001) substrates (under in-plane compressive
strain), and found that the topology of the Fermi surface
changed from that of unstrained bulk LSCO concomi-
tantly with an increase of Tcfrom 38 K to 40 K.
Effects of pressure should be particularly striking for
charge-orbital-related phenomena since the charge and
orbital degrees of freedom are strongly coupled to lat-
tice distortion. In this sense, direct observation of pres-
sure effects on the electronic structure of hole-doped per-
ovskite manganites is highly attractive. The hole-doped
perovskite manganites R1−xAxMnO3, where R is a rare-
earth (R = La, Nd, Pr) and A is an alkaline-earth atom
(A = Sr, Ba, Ca), exhibit CMR and spin, charge, and
orbital ordering [4, 5, 6, 7, 8]. Most of the half-doped
manganites (x ≃ 0.5) with a small bandwidth W exhibit
the so-called “CE-type” antiferromagnetic (AF) charge
ordering (CO) with alternating Mn3+and Mn4+states
within the (001) plane in the form of stripes . The
inter-stripe distance increases with further hole doping
and such charge modulations persist at high tempera-
tures as fluctuations well above the CO temperature TCO
[10, 11]. Pr1−xCaxMnO3(PCMO), where W is the small-
est, has a particularly stable CO state at low tempera-
tures in a wide hole concentration range 0.3 ≤ x ≤ 0.75
. It is known that the magnetic and electronic phases
of Mn oxides can be controlled in thin films grown on
substrates with various lattice parameters. For example,
La0.5Sr0.5MnO3thin films remain ferromagnetic (FM) on
(LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7(LSAT) substrates, but
becomes A-type AF on SrTiO3 (STO) substrates, and
C-type AF on LAO substrates . The CO states of
(Nd1−xPrx)0.5Sr0.5MnO3 thin films were also found to
be controlled by the strain effects from the substrates
. Recently, it was reported that PCMO thin films
grown on STO (001) substrates have higher CO temper-
atures than bulk samples, indicating that in-plane tensile
strain leads to the stabilization of the CO state .
In this work, we have studied the electronic structure
of PCMO thin films grown on LaAlO3(LAO) substrates
by photoemission spectroscopy. This work is the first ex-
perimental observation of the electronic structure of the
strained manganites. The fabricated PCMO thin films
were under compressive strain from the LAO substrates,
which is considered to suppress CO or charge modulation
. From core-level photoemission studies, we found
that the chemical potential shift as a function of hole
doping was not suppressed in the doping region where
incommensurate charge fluctuations are observed in the
bulk samples. From the valence-band spectra, we found
that no new states appeared near the Fermi level (EF)
with hole doping. These results are in striking contrast
to the recent results of PCMO bulk samples reported
by Ebata et al. , who concluded that the chemical
potential pinning and the spectral weight transfer do oc-
cur, and are considered to be spectroscopic evidence for
the suppression of incommensurate charge modulation in
PCMO thin films grown on LAO.
The experiments were performed at beamlines 1C and
2C of the Photon Factory, High Energy Accelerators Re-
search Organization (KEK), using a combined laser MBE
photoemission spectrometer system . Epitaxial thin
films of PCMO with a thickness of about 400˚ A were
fabricated by the pulsed laser deposition method from
ceramic targets of desired chemical compositions. Single
crystals of LAO (001) were used as the substrates. Atom-
ically flat step-and-terrace structures were observed by
atomic force microscopy. The crystal structure was char-
acterized by four circle x-ray diffraction measurements.
The in-plane lattice constants of the PCMO thin films
were the same as that of LAO (a = 3.792˚ A), confirming
the epitaxial and coherent growth of the thin films on the
substrates. For all the compositions, the out-of-plane lat-
tice constants were longer than the in-plane lattice con-
stants, indicating that the present PCMO thin films were
under compressive strain [see Fig. 1 (c)]. In the LEED
patterns, sharp 1×1 spots were observed without surface-
reconstruction-derived spots. The electrical resistivities
were high and showed no jump as a function of tempera-
ture, indicative of a suppression of CO and the associated
lattice distortion due to the compressive strain imposed
by the LAO substrates. Details of the sample growth and
characterization can be found in Ref. . The photoe-
mission spectra were taken using a Gammadata Scienta
SES-100 spectrometer. All the spectra were measured at
room temperature, except for ARPES measurements (20
K). The total energy resolution was about 150−400 meV
depending on photon energies. The Fermi level (EF) po-
sition was determined by measuring gold spectra.
From the core-level photoemission spectra, we found
that all the spectra were shifted toward lower binding en-
ergies with x, as plotted in Fig. 1 (a). Here, the “relative
energies” are referenced to the core levels of the x = 0.4
sample. The chemical potential shift ∆µ can be obtained
from the average of the energy shifts of the O 1s, Ca 2p,
and Pr 4d core levels as in the case of bulk PCMO .
Figure 1 (b) shows ∆µ thus determined plotted as a func-
tion of x. The shift of the chemical potential is monotonic
without any sign of suppression at least up to x = 0.5,
which is similar to bulk and thin film LSMO [19, 20] but
is quite different from bulk PCMO . The suppres-
sion of the chemical potential shift has been observed in
the region of incommensurate charge fluctuations [10, 11]
in bulk PCMO (x > 0.3) at room temperature ,
??(film) - ??(bulk)
Relative Energy (eV)
x, Ca concentration
x, Ca concentration
x, Ca concentration
0.6 0.5 0.40.3
x, Ca concentration
chemical-potential shifts in PCMO. (a) Shift of each core level
of PCMO thin films grown on LAO substrates. (b) Chem-
ical potential shift (∆µ(film)) deduced from the core-level
shifts and the difference of the chemical potential (∆µ(film)
− ∆µ(bulk)) between the film and the bulk. The insets to (a)
and (b) show the results of bulk samples taken from Ref. .
(c) c/a of PCMO thin films as a function of x taken from
Ref. . (d) ∆µ as a function of c/a and x. PI and AFI (C)
denote the paramagnetic insulating and C-type antiferromag-
netic insulating states, respectively.
(Color online) Core-level binding-energy and
La2−xSrxNiO4, and underdoped La2−xSrxCuO4.
Their origin has been attributed to dynamical stripe-type
charge fluctuations, a kind of “microscopic phase sepa-
ration” between hole-rich and hole-poor regions. That
is, the suppression of the chemical potential shift occurs
when the distance between stripes changes as a function
of hole concentration. Such a “microscopic phase separa-
tion” may be absent in the PCMO thin films considering
the suppression of charge modulation caused by the com-
pressive strain effects from the LAO substrates. Figure
1 (b) shows ∆µ(film) − ∆µ(bulk), plotted as a function
of x. Note that c/a extrapolates to 1 at x ∼ 0.7 in
PCMO thin films on LAO substrates  [see Fig. 1 (c)].
Chemical potential shifts to higher energy by applying
compressive strain. Figure 1 (d) shows interpolated ∆µ
as a function of c/a and x. This panel clearly shows the
pinning of ∆µ in the paramagnetic insulating region near
c/a ∼ 1 for x ≥ 0.3, whereas the pinning disappears for
smaller x and larger c/a.
Figure 2 (a) shows the doping dependence of the
valence-band photoemission spectra. Following Ref. ,
structures A, B, C, and D are assigned to Mn 3d - O
2p bonding, non-bonding O 2p, Mn 3d t2g plus Pr 4f,
and Mn 3d egstates, respectively. A gap (absence of fi-
Intensity (arb. units)
Energy relative to ?? (eV)
?? = 600 eV
x = 0.2
-1.5 -1.0 -0.5
Energy relative to ?? (eV)
?? = 600 eV
x = 0.2
x = 0.3
x = 0.4
x = 0.5
x = 0.6
FIG. 2: (Color online) Doping dependence of the valence-
band photoemission spectra of PCMO thin films. (a) Valence-
band photoemission spectra in a wide energy range.
Valence-band spectra near EF. The inset shows the result of
bulk samples taken from Ref. . (c) Valence-band spectra
near EF with energy positions shifted considering the chem-
ical potential shift. Arrows indicate systematic spectral fea-
ture shifts with increasing doping.
nite density of states at EF) was seen for all values of x,
which is considered to be a natural consequence of the
insulating nature of the PCMO thin films. Structures
A-D moved toward EF upon hole doping. This is con-
trasted with the results of bulk PCMO, where spectral
weight transfer occurs near EF from high to low ener-
gies with hole doping as shown in the inset to Fig. 2 (b)
. Sekiyama et al.  also reported finite intensity
at EF in the insulating state of Nd1−xSrxMnO3. The
main panel of Fig. 2 (b) shows the valence-band spectra
of the PCMO thin films near EF. The overall shift of the
spectra is attributed to the effect of the chemical poten-
tial shift, and no new states appeared near EF with hole
doping. From the present spectra near EF, we conclude
that our PCMO thin films were good insulators without
any dynamical “phase separation”.
The absence of chemical potential “pinning” and spec-
tral weight transfer toward EF in the thin films, which
were observed in bulk PCMO , are attributed to
the suppression of incommensurate charge fluctuations
due to the compressive strain effects from the LAO sub-
strates, and are considered as the spectroscopic evidence
for the change of the electronic structures due to the epi-
taxial strain effects from the substrates.
Figure 3 (a) shows the ARPES spectra of a PCMO
(x = 0.4) thin film taken with a photon energy of 88 eV.
Here, the second derivatives of the energy distribution
curves are plotted as a false-color image, where bright
parts correspond to peaks or shoulders in energy distri-
bution curves. Hereafter, k?denotes the in-plane mo-
mentum expressed in units of π/a. The structures at
−(1.5−3) eV show weak dispersions and are assigned to
FIG. 3: (Color online) ARPES spectra of PCMO (a) and
LSMO (taken from Ref. ) (b) taken at 88 eV. Bright parts
correspond to energy bands. The insets show the traces in k
the Mn 3d eg↑and t2g↑bands. The structures at −(3−7)
eV show strong dispersions and are assigned to the O 2p
bands. There is no intensity at EF, consistent with the
insulating behavior of this film. The same plot for an
LSMO thin film (x = 0.4) grown on an STO substrate is
shown for comparison in Fig. 3 (b) . The dispersions
of the O 2p bands were similar between these two spec-
tra, while those of the Mn 3d bands were very different.
The Mn 3d eg↑bands show a clear dispersion and cross
EF in LSMO, but are weak and show only very weak
dispersions in PCMO.
The magnetic ground state of the PCMO (x = 0.4)
thin film may be inferred from the lattice constants and
the phase diagram of LSMO proposed by Konishi et al.
. The c/a of the PCMO (x = 0.4) films was 1.03 as
shown in Fig. 1 (c). From the phase diagram in Ref. ,
c/a = 1.03 and x = 0.4 is just at the boundary of the FM
and C-type AF states. We confirmed that this film was
not FM by magnetization measurements, and considered
it to be in the C-type AF state. In the C-type AF state,
FM metallic chains are formed along the c-axis. There-
fore, we have performed normal emission ARPES mea-
surements to study the out-of-plane band dispersions as
shown in Fig. 4 (a). For comparison, Fig. 4 (b) shows the
normal-emission ARPES spectra of an LSMO (x = 0.4)
thin film taken from Ref. . Here again, the dispersions
of the O 2p bands are similar between these two spectra.
On the other hand, the dispersions of the Mn 3d eg↑
bands are clear in LSMO (b), but very weak in PCMO
(a). In order to interpret the experimental band dis-
persions, we have performed a tight-binding (TB) band-
structure calculation with empirical parameters. Here,
we have performed the calculation by assuming the C-
type AF state for PCMO and the FM state for LSMO
as shown in Figs. 4 (c) and (d). In the case of LSMO,
agreement between experiment and calculation is good
FIG. 4: (Color online) Comparison of the ARPES spectra
measured in normal emission geometry with tight-binding
band-structure calculation (obtained with the parameter set
of ǫd − ǫp = 2.0 eV, (pdσ) = −1.9 eV, and the exchange
splitting ∆E = 4.8 eV.) Bright parts correspond to energy
bands. (a) Experimental band structure of PCMO (x = 0.4).
(b) Experimental band structure of LSMO (x = 0.4) taken
from Ref. . (c) Tight-binding calculation of the C-type
AF state. (d) Tight-binding calculation of the FM state. The
arrow indicates the effect of mass renormalization. Both simu-
lations have taken into account the finite photoelectron mean-
free path . In (a) and (c) we plotted the photoemission
spectral weight calculated by projecting the obtained C-type
AF bands to their original PM bands .
except for the narrowing of the conduction band due to
strong electron correlation. In the case of PCMO, the
strong dispersion of the Mn 3d eg↑ bands predicted by
the calculation of Fig. 4 (c) was not observed in the ex-
periment of Fig. 4 (a). The egelectrons appear to be lo-
calized along the ferromagnetic chain direction, probably
due to disorder and/or electron correlation. This result
is also consistent with the fact that in C-type AF materi-
als the expected one-dimensional metalicity has not been
observed so far.
In summary, we have performed an in-situ photoemis-
sion study of Pr1−xCaxMnO3(PCMO) thin films grown
on LaAlO3(LAO) substrates. From the core-level pho-
toemission study, we found that unlike bulk PCMO, in
strained films the chemical potential shifted monoton-
ically with doping, implying the disappearance of in-
commensurate charge fluctuations. In the valence-band
spectra, we found no spectral weight at the Fermi level
(EF) nor doping-induced spectral weight transfer toward
EF, also unlike bulk PCMO. The gap at EF was clearly
seen in the experimental band dispersions determined by
angle-resolvedphotoemission spectroscopy (ARPES) and
could not be explained by the metallic band structure of
the C-type AF state as previously proposed, probably
due to the localization of electrons along the ferro-chain
direction caused by disorder and electron correlation.
Informative discussion with K. Ebata and experimen-
tal support by M. Takizawa are gratefully acknowledged.
This work was supported by a Grant-in-Aid for Scien-
tific Research (A16204024) from the Japan Society for
the Promotion of Science (JSPS) and a Grant-in-Aid
for Scientific Research in Priority Areas “Invention of
Anomalous Quantum Materials” from the Ministry of
Education, Culture, Sports, Science and Technology. H.
W. acknowledges financial support from JSPS. The work
was done under the approval of the Photon Factory Pro-
gram Advisory Committee (Proposal Nos. 2005G101 and
2005S2-002) at the Institute of Material Structure Sci-
Present address: Department of Physics and Astronomy,
University of British Columbia,
Columbia V6T-1Z1, Canada
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