PHYSICAL REVIEW B 72, 224403 共2005兲

Interfacial charge transfer excitation with large optical nonlinearity in manganite heterostructure
Takuya Satoh,1 Kenjiro Miyano,1 Yasushi Ogimoto,1,2 Hiroharu Tamaru,1,3 and Sumio Ishihara41

Research Center for Advanced Science and Technology, The University of Tokyo, Tokyo 153-8904, Japan
2Devices Technology Research Laboratories, SHARP Corporation, 2613-1 Tenri, Nara 632-8567, Japan
3PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
4Department of Physics, Tohoku University, Sendai 980-8578, Japan
共Received 20 July 2005; published 5 December 2005兲

Interfacial electronic states between LaMnO3 and SrMnO3 have been studied by second-harmonic 共SH兲
spectroscopy. The SH contributions from bulk, surface, and interface were separated by measuring four distinct
types of manganite thin films. In the SH spectra, we found a broad peak at 2 eV unique to the
LaMnO3 / SrMnO3 interface, which was assigned to interfacial charge transfer excitation based on a HartreeFock calculation. No metallic phase showed up at the interface. Experimental and theoretical second-ordersusceptibilities ␹共2兲 at the interface were estimated to be ⬃10−6 esu, which is ten times as large as the highest␹共2兲 value of BaTiO3.
DOI: 10.1103/PhysRevB.72.224403

PACS number共s兲: 75.47.Lx, 73.20.⫺r, 42.65.Ky, 78.20.⫺e

The interface between two dissimilar materials has beenof fundamental importance in basic research and applications. For example, manipulation of electrons in a field-effecttransistor is the basis for modern day electronics and provides an arena for the study of two-dimensional electronic
states. In addition to these well characterized systems, muchattention has recently been paid to the interfaces that involve
strongly correlated electron systems, in which one may hopethat the strong correlation leads to a novel electronic statebound to the interface.1 In this context, manganites arequite attractive since their electronic state is particularly sensitive to carrier doping, lattice constant, and external stimuli
because of a subtle balance of spin, charge, orbital, andlattice degrees of freedom.2 For example, photoinduced
antiferromagnetic-ferromagnetic and insulator-metal transitions have been reported.3,4 Due to the sensitivity, it is natural to expect that the electronic state at the interface between
two different bulk manganites can be substantially different
from the electronic states of the bulk phases. Appearance of
unique phases, spin canting, and spin frustration have beensuggested, which are driven by charge transfer, exchange interaction, and strain effect at the interface.5,6 Making use of
the half-metallic behavior of La1−xSrxMnO3, a tunnelingmagnetoresistance 共TMR兲 device has been fabricated. The
importance of the dead layer at the interface that prevents the
TMR near the Curie temperature has been welldemonstrated.7
The characterization of the interfaces is not straightforward, however, because many surface sensitive approaches,e.g., scanning probe microscopy and photoemission spectroscopy, are not applicable to the buried interfaces in general. A
different approach to the interface problem is to study thecross section. By combination of high resolution transmission electron microscopy and electron energy-loss spectroscopy, the valence of the atoms at the interface has been identified with nanometer-scale resolution.1,8,9 However, sample
treatment by ion milling or mechanical polish may cause an
artificial effect on the interface properties.
In contrast to the techniques mentioned above, opticalsecond-harmonic generation 共SHG兲 is a convenient probe to
study buried interfaces.10–13 In the electric-dipole 共ED兲 ap1098-0121/2005/72共22兲/224403共4兲/$23.00

proximation, the nonlinear polarization P as the source of
SHG is given by
Pi共2␻兲 = ␹共2兲
共1兲ijk E j共␻兲Ek共␻兲,where E共␻兲 is the fundamental electric field with frequency
␻ and ␹共2兲ijk is the second-order susceptibility. Equation 共1兲
indicates that SHG is forbidden in centrosymmetricmaterials.10 It is allowed only at surfaces and interfaces thatbreak the centrosymmetry. Since ␹共2兲 depends on the site
symmetry, the SHG is drastically changed by subtle local
modulations. Without modifying the sample, the buried interfaces are thus accessible by the SHG technique provided
that it lies within the penetration depth of light 共⬃100 nm兲.
Magnetic SHG from the interfaces of perovskite manganite
superstructures has been reported recently.14,15
In this paper, we report on nonlinear spectroscopic investigation and theoretical calculation of a manganite interface.
The interface we study here is formed between LaMnO3
共LMO兲 and SrMnO3 共SMO兲. The nominal valences of theMn ions of the respective manganites in the bulk are 3+ and
4+. They are insulators, but in solid solution, they exhibit avery complex phase diagram because of the aforementioned
competition.2 A natural question is whether the interfacial
layer is equivalent to a bulk La0.5Sr0.5MnO3 crystal or not.
Because SHG is sensitive to the asymmetry of the crystal
field of the Mn ion at the interface, we are able to single out
the spectral feature reflecting the interfacial electronic state.
By comparing with a Hartree-Fock calculation, we assigned
it to an interface-bound charge transfer excitation, which is
not expected from bulk LMO, SMO, and La0.5Sr0.5MnO3.
The calculation also suggests that there is no metallic
La0.5Sr0.5MnO3 at the interface.
Four distinct types of thin films were grown on
共LaAlO3兲0.3-共SrAl0.5Ta0.5O3兲0.7 共LSAT兲 substrates by pulsed
laser deposition: LMO/ LSAT, SMO/ LSAT, LMO/ SMO/
LSAT, and SMO/ LMO/ LSAT. Perovskite type LSAT共001兲
was chosen as the substrate because its lattice constant
共3.87 Å兲 is close to the ones of bulk LMO 共3.95 Å兲 and bulk
SMO 共3.81 Å兲 so that we can expect epitaxial growth. The
substrate temperature was kept at 760 ° C for

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PHYSICAL REVIEW B 72, 224403 共2005兲

SATOH et al.

FIG. 1. Angular dependence of the SH intensities of manganite
thin films at 2.74 eV as SH photon energy. The solid line is obtained from a model calculation based on the Maxwell equations fora heterostructure with an effective nonlinear medium localized at
the interface. The angle of incidence is given with respect to thesample normal. The inset shows a schematic of the manganite heterostructure 共SrMnO3 / LaMnO3 / LSAT兲.

LMO/ SMO/ LSAT and SMO/ LMO/ LSAT, 790 ° C for
SMO/ LSAT, and 800 ° C for LMO/ LSAT, and the oxygenpressure was 0.1 mTorr during the deposition. The films
were grown in the two-dimensional layer-by-layer mode,
which is confirmed by the oscillation of the specular spot
intensity in the reflection high-energy electron diffraction.
The thickness of each layer was regulated to be 4–5 unit
cells. Terraces with about 100 nm width and the step heightof one unit cell were observed in atomic force microscopy
images.16
The SHG measurements were performed at room temperature in air. An optical parametric amplifier with 150 fs
light pulses pumped by an amplified Ti: sapphire laser system at 1 kHz repetition rate was used to produce the fundamental light. The fundamental photon energy ranged between 0.82 and 1.57 eV. The angle of incidence was varied
from −90° to 90° by means of Maker fringe technique.17 The
fundamental light was p polarized and the SH light was detected as p polarized. The fundamental light is incident on
the back of the substrate in order to avoid absorption of SH
light in the yellow-colored LSAT substrate. Since the intensity and the width of the incident light pulses fluctuate shotby shot, the signal was normalized by SHG from urea powder. Typically 5000 pulses were averaged. The calibration of
the manganite thin-film SH spectra was done relative to thatof ␣-quartz. The Maker fringe of a Y-cut ␣-quartz slab with
1 mm thickness was measured and fitted by a model
calculation.18 The finite spectral width of the 150 fs incident
light was taken into account for fitting the Maker fringe pattern of the ␣-quartz. For the ␹共2兲 dispersion of ␣-quartz, Miller’s rule was applied.19
Figure 1 shows the angular dependence of the SH intensities of the four samples at an SH photon energy of 2.74 eV.
The signal from the substrate alone was negligible. We confirmed the square dependence of the SH intensity on the

input power in the range of 5–50 mJ/ cm2. On the basis ofsymmetry consideration of the input/output polarization,
SHG from electric-quadrupole and magnetic-dipole was excluded. Other films twice and four times as thick did notshow an increase of SH intensity. Therefore, bulk contributions are ignored as well. Thus, it is reasonable to consider
that the ED-type SH source is located in almost one-unit cell
layer. This is also confirmed by the theoretical calculation as
discussed later. There are three types of interfaces; airmanganite, manganite-manganite, and manganite-substrate.
The interfaces of air-LMO, air-SMO, LMO-SMO, SMOLMO, LMO-LSAT, and SMO-LSAT are denoted as aL,
aS, LS, SL, Lᐉ, and Sᐉ, respectively. The peak SH intensities
for four samples 共I LMO/LSAT = 0.20, ISMO/LSAT = 0.15,
ILMO/SMO/LSAT = 0.02, ISMO/LMO/LSAT = 0.86兲 shown in Fig. 1
already indicate that SHG is sensitive to the crystal fieldasymmetry at the interface and the interference of SHG fromdifferent interfaces is important. Note that linear optical
spectra cannot distinguish LMO/ SMO/ LSAT and
SMO/ LMO/ LSAT samples.
The ␹共2兲s from interfaces all contribute coherently to
SHG. By combining SMO/ LSAT and LMO/ LSAT films
face to face, we found that ␹aL and ␹aS have the same signand similar magnitude. Furthermore, the contribution from␹Sᐉ and ␹Lᐉ is small. This conclusion is also confirmed bymeasuring the SHG of films grown on a different substrate
共LaSrAlO4兲. Thus the SH intensities from various films canbe written in terms of ␹共2兲 as
ILMO/LSAT ⬀ 兩␹aL兩2 ,共2兲

ISMO/LSAT ⬀ 兩␹aS兩2 ,共3兲

ILMO/SMO/LSAT ⬀ 兩␹aL + ␹LS兩2 ,共4兲

ISMO/LMO/LSAT ⬀ 兩␹aS + ␹SL兩2 .

共5兲

The polarity of the SL interface in the sample leads to ␹LS= −␹SL in Eqs. 共4兲 and 共5兲. There are four equations 关Eqs.
共2兲–共5兲兴 for three variables. However, one set of ␹共2兲s cansatisfy all the equations self-consistently, confirming our assertion that ␹Sᐉ and ␹Lᐉ are negligible.
The peak at around 60° in Fig. 1 is consistent with theassumed tetragonal symmetry 共4mm兲. In this symmetry, the
共2兲
共2兲
共2兲
= ␹共2兲
following ␹共2兲 tensors are allowed: ␹xzx
yzy = ␹xxz = ␹ yyz,
共2兲
共2兲
共2兲
␹zxx = ␹zyy, and ␹zzz, where z is normal to the interface. As
shown in Fig. 1 we fit the angular dependence of the SH
intensity to a model based on the Maxwell equations for a
multilayer structure with an effective nonlinear medium with
one unit cell thickness at the interface. Thus we estimate the␹共2兲 value of SMO/ LMO/ LSAT as ⬃10−6 esu. The ␹共2兲value are corroborated by the theoretical calculation.
In order to investigate the contributing electronic processes, SH spectroscopy was performed in the range 1.64–
3.14 eV. Figure 2 illustrates the calibrated SH intensity. The
spectra is monotonously increasing as a function of photon
energy, except for a resonance peak at around 2.0 eV in

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INTERFACIAL CHARGE TRANSFER EXCITATION WITH…

共2兲

FIG. 2. Calibrated SH spectra of the manganite thin films as afunction of SH photon energy. The spectra are normalized to thepeak SH intensity of SrMnO3 / LaMnO3 / LSAT at about 2 eV.

FIG. 3. The 兩␹zzz兩2 spectra based on the p-d model 共see Ref. 24兲.
The spectra are normalized so that the peak 兩␹aS + ␹SL兩 is unity,
which corresponds to 1.184⫻ 10−6 esu.

SMO/ LMO/ LSAT, SMO/ LSAT, and LMO/ SMO/ LSAT.
The broad peak is not observed in the optical spectra of bulk
LMO, SMO, and La0.5Sr0.5MnO3.20
Now that the contributing ␹共2兲s are identified, we cancompare the results with a theoretical calculation. The calculation was performed for a model, in which a number oflayers are stacked along the z-axis in a sequence of 关MnO2-LaO兴n-MnO2-关SrO-MnO2兴n. We adopt a tight-binding pd-type Hamiltonian, H = Hd + H p + Hdp, where the Mn 3d andO 2p electrons are taken into account explicitly and the La
and Sr ions are assumed to be point charges. The stronglyinteracting Mn eg electrons which couple ferromagneticallywith the localized t2g spins are described by Hd. The O 2p
degree of freedom introduced in H p and the electron hopping
between the Mn eg and O 2p orbitals in Hdp are essential for
the SH spectra. The Jahn-Teller coupling between the eg orbital and the lattice distortion is included in Hd. Details of H
for the bulk manganites are presented in Ref. 21.
The electronic structures are calculated by the HartreeFock 共HF兲 method21 where spin, charge, and orbital orderparameters are introduced in each layer designated by m andthe in-plane momentum kជ 储. Unrenormalized energy level␧d共m兲关␧ p共m兲兴 for d共p兲 orbital is given in the ionic model
where the Madelung potential22 is obtained self-consistently
with the calculated electron densities. The SHG spectra formulated in the HF scheme23 is given as
Ne3
共2兲
␹zzz
=− 2
共6兲
兺 XmnXnlXlm共F1 + F2兲,q 兩m,n共⫽m兲,l,kជ

Xnm共⬅具n 兩 X 兩 m典兲 is the matrix elements of the current
operator

储

and
F1 =

f nl
f ml
+ 3
,
3
␧lm共2␧lm − ␧nm兲共␻ − ␧lm兲 ␧nl共2␧nl − ␧nm兲共␻ − ␧nl兲
F2 =

再

冎

16
f ml
f nl
+
,
3
␧mn
共2␻ − ␧mn兲 ␧nm − 2␧lm ␧nm − 2␧nl

共7兲with the difference of the Fermi distribution functions f ij
= f F共␧i兲 − f F共␧ j兲 and that of the HF energies ␧ij = ␧i − ␧ j. Here,X=−t兺 ␦兵d3z −r ␴共i兲†pz␴共i + ␦兲 − H.c.其,2i,␦=±aẑ,␴

2

共8兲with the transfer integral t and the Mn-O bond length a. The
operator d3z2−r2␴共i兲 关pz␴共i兲兴 is introduced for the d3z2−r2 共pz兲electron with spin ␴ at site i. The film thickness is chosen to
be n = 4, reflecting the experimental condition. Larger n results in similar SH characteristics, which indicates the SHG
comes only from the interface. Thus both the experiment andcalculation capture the essential physics at the interface.
By restricting the summation of site i in Eq. 共8兲, we can
共2兲
from different parts of thecalculate the contributions to ␹zzz
film separately. This is possible because the electronic states
are sufficiently localized along the film normal. Note that thebulk LMO and SMO are insulators. As is verified experimen共2兲
is found from layers away fromtally, no contribution to ␹zzz
the interfaces and single MnO2 layers at the interfaces carry
共2兲 2most of the SH contribution. The calculated 兩␹zzz
兩 s areshown in Fig. 3 in the form of SH intensities that correspondto the experiment 关see Eqs. 共2兲–共5兲兴. The SH photon energy
2␻ in the horizontal axis is normalized by the exchange coupling J at a Mn site being around 1 eV. Two features areworth noting; 共i兲 ␹SL, ␹aS, and ␹aL have the same sign, and共ii兲 ␹SL is the largest among them. As a result, ISMO/LMO/LSAT
determined by 兩␹aS + ␹SL兩 is the largest. These features are ingood agreement with the experimental observations. The absolute value of 兩␹aS + ␹SL兩 at the peak in Fig. 3 is 1.184⫻ 10−6 esu. The calculated results also explain the experimentally observed ␹共2兲 value 共⬃10−6 esu兲 for the
SMO/ LMO/ LSAT sample. In Fig. 3, negligible ␹aS meansthat interfacial ␹SL alone brings almost all ␹共2兲 value, which
is about ten times as high as the highest ␹共2兲 component
共2兲
共␹xzx
兲 of BaTiO3 with the 4mm point group.10 The large ␹共2兲value in the manganite heterostructure mainly comes fromthe large electric field gradient of the static electric field at
the interface which is particular to the strongly correlated

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SATOH et al.

electron systems. It should be added, however, that the optical transition of the SHG process under consideration isclose to resonance, which enhances the ␹共2兲 value.
The SH spectra are dominated by the two-photon absorption processes where the spectra enhance resonantly when2␻ is close to the charge-transfer excitation energies in Eq.
共7兲. In bulk LMO and SMO, the charge-transfer excitations
are 共eg兲1共2p兲6 → 共eg兲2共2p兲5 and 共eg兲0共2p兲6 → 共eg兲1共2p兲5, respectively. At the interface between LMO and SMO, aunique electronic state emerges, in which transitions fromthe O 2p state in LMO to the empty eg state in SMO, andfrom the O 2p state in SMO to the upper Hubbard band in
LMO are dominant. This leads to the enhancement of2␻ / J ⬃ 2.5, corresponding to the peak around 2.0 eV in Fig.
2.
However, the calculated results are not consistent with apeak at 2.0 eV in SMO/ LSAT and no peak in LMO/ LSAT
in experiments. This may be due to the choice of the calculation parameter values which are determined by linear optical spectra reflecting 100 nm thick bulk properties of LMO
and SMO.
The Mn valency of the interfacial layer sandwiched between LaO layer and SrO layer is 3.5+ but no metallicity
shows up. This is in stark contrast to the case of Ti oxide1
and demonstrates the extreme confinement of the two-

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dimensional electronic state in manganites. In fact, it has
been known that, while the bulk La0.6Sr0.4MnO3 is a metal in
the ground state, La0.6Sr0.4MnO3 in the form of 5 unit cell
layers is insulating when separated by insulating layers
thicker than 3 unit cells.5
In conclusion, we found an interface-bound charge transfer excitation between LaMnO3 and SrMnO3 by opticalsecond-harmonic spectroscopy and a theoretical calculation.
The interface second-order susceptibility ␹共2兲 is ⬃10−6 esu,
which is ten times higher than the highest ␹共2兲 component of
BaTiO3. The large nonlinearity mainly comes from the steep
gradient of the static electric field at the interface. The
calculation indicates no metallic La0.5Sr0.5MnO3 phase at
the interface, reflecting the strong electron correlations inmanganites.
We are appreciative to M. Izumi for the use of his samples
in the initial feasibility study. T.S. would like to thank M.
Fiebig and B.B. Van Aken for valuable discussion. This work
was supported by JSPS KAKENHI 共15104006 and
16540305兲, MEXT TOKUTEI 共16076207兲, NAREGI andCREST. Part of the numerical calculation has been performed by supercomputers in IMR, Tohoku University, and
ISSP, University of Tokyo.

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24
Parameter values used for the numerical calculation are the intraorbital Coulomb interaction for the eg electrons U / J = 6.75, thesuperexchange interaction between the nearest-neighboring t2g
spins JAF / J = 0.1, the d–p transfer integral t / J = 0.8, and the dielectric constant at infinite frequency ␧⬁ = 2. The exchange interaction at a Mn site J is around 1 eV. These values are chosen on
the basis of the previous estimations and the linear-optical spectra in bulk manganites.
13

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