Multiple-order Raman scattering from rare-earth manganites: Oxygen isotope and rare-earthsubstitution effects

M. N. Iliev, V. G. Hadjiev, A. P. Litvinchuk, F. Yen, Y.-Q. Wang, and Y. Y. SunTexas Center for Superconductivity and Department of Physics, University of Houston, Houston, Texas 77204-5002, USA

S. Jandl and J. LaverdièreRegroupement Québecois sur les Matériaux de Pointe, Département de Physique, Université de Sherbrooke, Sherbrooke,

Canada J1K 2R1

V. N. PopovFaculty of Physics, University of Sofia, 1164 Sofia, Bulgaria

M. M. GospodinovInstitute of Solid State Physics, Bulgarian Academy of Sciences, 1184 Sofia, Bulgaria

�Received 17 October 2006; published 9 February 2007�

The paper presents results of a detail study of polarized multiple-order Raman spectra of several, isostruc-tural to LaMnO3, RMnO3 compounds �R=Pr,Eu,Dy,Ho,Y� and oxygen isotope substitution effects. A com-parison of the spectra of EuMn 16O3 and isotopically substituted EuMn 18O3 provides strong evidence that themultiple-order Raman spectra are of pure phonon rather than of orbiton or mixed orbiton-phonon origin. Basedon polarization properties and in close comparison with the results of lattice-dynamics calculations, we arguethat the prominent structure at �650 cm−1, replicated in the multiple-order scattering, is of phonon density-of-states origin. We also present a group-theoretical analysis of the selection rules for the two-phonon Ramanspectra of orthorhombic manganites. The polarized two-phonon Raman spectra of RMnO3 are consistent witha second-order scattering process involving only Brillouin-zone boundary phonons.

DOI: 10.1103/PhysRevB.75.064303 PACS number�s�: 78.30.�j, 63.20.Dj, 75.47.Lx

I. INTRODUCTION

The effect of orbital ordering in the Raman spectra of therare-earth manganites, in particular LaMnO3, has beeninvestigated intensively, both theoretically1–4 andexperimentally.3,5,7,8 Allen and Perebeinos1,2 have predictedthat an incident photon creates a vibrationally excited self-trapped “orbiton” state from the orbitally ordered Jahn-Teller�JT� ground state of LaMnO3. Then the Franck-Condonmechanism activates relatively strong multiphonon Ramanscattering, observed experimentally in the 900–1400 cm−1

range. Using a different approach, Saitoh et al.3 argue thatthese spectral features are in fact the �-point modes of or-bital waves �delocalized orbitons�. In a third model, whichincorporates both superexchange and electron-phonon cou-pling, van den Brink4 has proposed that the elementary ex-citations of the system are mixed modes with both orbitaland phonon character, and the structures in the multiple-order Raman spectra are actually orbiton-derived satellites inthe phonon spectral function.

The resonance Raman scattering experiments of Krügeret al.5 are consistent with the model of Allen and Perebeinosin observation of a strong resonance close to the JT gap of2 eV for the Raman phonon peaks at 496 �JT antistretching�,611 �JT stretching�, and 655 cm−1 modes, the last one as-signed to an oxygen breathing vibration. The same resonancehas been observed for the main peaks between 1000 and1400 cm−1, thus suggesting a phononic origin of high-frequency Raman lines. Independently, Martin-Carrón and deAndrés6 have also come to the conclusion that the high-

frequency Raman peaks correspond to second- and third-order phonons, which couple to some degree to the orbitalorder through the Franck-Condon-type resonance, althoughthe cross sections have been found smaller than expected.The study of temperature-dependent polarized Raman spec-tra of La1−xSrxMnO3 �Refs. 7 and 8� has also led to a con-clusion for the dominant role of the Franck-Condon mecha-nism, but from the doping dependence of the second-orderscattering along with the frequency shift of several higher-frequency structures, a conclusion is made that the multiple-order scattering rather has mixed orbiton-phonon character.All the above conclusions are heavily based on the assump-tion that the phonon peak of higher energy in the first-ordersegment �641 cm−1�, strongly enhanced in the second-ordersegment, is a �-point mode and that in general the FC-mechanism-related multiphonon scattering is a replica of thefirst-order Raman spectrum. It was recently shown, however,that if high-energy Raman-active excitations are consideredas �-point phonon combinations in various rare-earth man-ganites with A-type antiferromagnetism, they appear to beblueshifted in large size rare-earth ion compounds.9 Theyalso reflect subtleties that suggest an orbiton-phonon cou-pling scheme in contrast with a description that relies exclu-sively on the Franck-Condon mechanism.

In this paper, we report the effect of rare-earth and oxygenisotope substitution on the polarized multiple-order Ramanspectra of RMnO3 compounds �R=La,Pr,Eu,Dy,Ho,Y�.We compare the spectra of carefully prepared EuMn 16O3and isotopically substituted EuMn 18O3 with the expected�within the main models2–4� Raman lines isotope shift. The

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experimental results provide strong evidence that themultiple-order spectra are of pure phononic origin andorbiton-phonon mixing is negligible. We also measured theRaman structure at �650 cm−1 in exact scattering configu-rations. The structure is associated with the phonon densityof states �PDOS� based on its polarization properties and inclose comparison with predictions of lattice-dynamical cal-culations �LDC�. At the end, we present the group-theoryderived two-phonon Raman scattering selection rules fororthorhombic manganites and use them to analyze thesecond-order polarized Raman spectra of DyMnO3.

II. SAMPLES AND EXPERIMENT

The preparation of RMnO3 samples used in our experi-ments is described elsewhere.10 Except for DyMnO3, all thesamples were polycrystalline containing microcrystals largerthan the laser spot. This allowed us to select microcrystalswith crystallographic orientation such that the spectra in par-allel and crossed scattering configurations were dominatedby Ag and B2g contributions, respectively. The DyMnO3 crys-tals displayed ac and ab surfaces with edges parallel to

�101��x�, �101̄��z�, and �010��y directions �in Pnma no-tation, y is the long axis�, which allowed measurements ofRaman spectra with exact Ag �xx, zz, or yy�, B2g �xz or zx�,and B1g+B3g �yx� or yz�� scattering configurations.

For the oxygen isotope substitution experiment, twoEuMnO3 samples of similar mass were enclosed in oxygenenvironments of 1.5 atm at room temperature, one in 18O andthe other one, which served as reference, in 16O. The 18O gasused has 99% purity. Both samples simultaneously wentthrough the same temperature treatment. The temperaturewas first increased at a rate of 4.5 K/min from23 to 1095 °C and kept at that temperature for 96 h. In thefinal step, the temperature was decreased at the same rate of4.5 K/min to room temperature. All the first-order oxygenRaman modes in EuMn 18O3 were downshifted by 5.0% withrespect to those in EuMn 16O3, which corresponded to�80% oxygen isotope substitution.

The Raman spectra were measured in backscattering con-figuration using two different spectrometers: Horiba JY La-bRam HR800 and Jobin Yvon HR640, both equipped withmicroscope, liquid-nitrogen-cooled CCD detectors, andmicro-helium cryostats. The spectra obtained with 488.0 nm,514.5 nm �Ar+�, and 632.8 nm �He-Ne� were practicallyidentical in the position and shape of the Raman lines.

III. RESULTS AND DISCUSSION

A. Isotopic substitution

The oxygen isotope substitution in RMnO3 provides aunique opportunity for distinguishing between spectral struc-tures of phonon, orbiton, and mixed orbiton-phonon origin.In the case of full substitution, the frequency of a phononmode involving mainly oxygen vibrations is proportional tomO

−1/2, and it is expected to decrease by �6% as��O18� /��O16�= �16/18�1/2=0.943. The substitution willpractically have no effect on the orbiton frequencies calcu-lated within the model given in Ref. 3 as they are pure elec-

tron excitations of Mn sublattice. For mixed orbiton-phononmodes,4 one expects about a four times smaller change infrequency of the orbital peak compared to that of thephonons. Indeed, assuming a weak electron-phonon couplingand localized limit �static JT distortions�, the orbiton peakof a mixed orbiton-phonon mode should appear at �orb=3J+4g2 /�0, where J is the superexchange energy, g is theelectron-phonon coupling constant, and �0 is the JT phononfrequency. For weak electron-phonon coupling g=0.35�0 �gis expressed in units of energy, �0�, �0=645 cm−1 �80 meV�,and J=�0 /2 as suggested in Ref. 4, one obtains�orb�O18� /�orb�O16�=1.015, provided g is the same for bothO18 and O16 vibrations. Given the high spectral resolution ofRaman spectroscopy, the frequency changes of 6% and 1.5%expected for pure phononic and mixed orbiton-phononmodes, respectively, would be readily distinguishable.

Figure 1 compares the Raman spectra of EuMn 16O3 andEuMn 18O3 at 6 K. For the 18O-substituted sample, all thefirst-order Raman lines between 200 and 600 cm−1, assignedto oxygen vibrations, are shifted by 5% toward lower wave

FIG. 1. �Color online� Ag and B2g Raman spectra of EuMn 16O3and EuMn 18O3 as obtained at 6 K with 632.8 nm �1.96 eV� exci-tation. The spectrum between 16O and 18O ones is that ofEuMn 18O3 with wave-number scale multiplied by a factor of 1.05.The peak at 920 cm−1 originates from Eu3+ crystal-field excitation�Ref. 14�.

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numbers, which corresponds to a substitution rate of 80%.The fact that same relative shift is observed for all oxygenmodes indicates that the orbiton-phonon coupling is negli-gible as the Raman line renormalization due to orbiton-phonon mixing should be different for the stretching �JT�,bending, and rotational modes. The spectral structures athigher wave numbers, where the model of van den Brink4

predicts orbiton satellites of the phonon peaks, exhibit thesame relative shift. This is obvious from Fig. 1, where the Agand B2g spectra of EuMn 18O3 are also plotted against anenergy scale expanded by 5%. The practical identity of thesespectra to the corresponding EuMn 16O3’s ones gives strongevidence that all high-frequency structures in the Ramanspectra of RMnO3 are of pure phononic origin.

B. Phonon density-of-states bands and multiphonon scattering

In Fig. 2, we compare the Ag and B2g spectra at 6 K ofseveral RMnO3 compounds, characterized by different mag-netic ordering: A-type AFM �PrMnO3�, incommensurateAFM �EuMnO3, DyMnO3, YMnO3�, and E-type AFM�HoMnO3�. Except for small hardening with decreasing ionicradius of R, the spectra are nearly the same, which impliesthat the type of magnetic ordering has no significant effect onthe spectral profiles. In the first-order segment, except for thewell known �-point modes, an additional relatively broadstructure centered at �650 cm−1 is always present, althoughthe intensity may vary for samples from different sources. Itsnature is discussed below. The structure in the second-ordersegment is well reproducible, being practically the same for

all RMnO3 compounds, which justifies the generalization ofconclusions based on a more detailed study of one of thesecompounds to all RMnO3.

The assignment of the first-order structure at640–665 cm−1 is of primary importance as its multiples aredominating in the second- and third-order spectral segments.This peak is apparently not a �-point Raman mode as it ismost strongly pronounced in the Ag spectra, where all�-point modes are already accounted for at lower wave num-bers. In Fig. 3, we compare the first-order Raman spectra ofDyMnO3 single crystal in exact xx�Ag�, yy�Ag�, xz�B2g�, andyx��B1g+B3g� scattering configurations excited with 488 nm�2.54 eV�, 514.5 nm �2.41 eV�, and 632.8 nm �1.96 eV� la-

FIG. 2. Ag and B2g Raman spectra of RMnO3 as obtained at 6 Kwith 632.8 nm �1.96 eV� excitation. The Ag spectra are normalizedto the integrated intensity of antistretching �Ag�3�� and bending�Ag�1�� modes, whereas the B2g spectra are normalized to the inten-sity of the stretching �B2g�1�� mode �Ref. 10�.

FIG. 3. �Color online� First-order Raman spectra of DyMnO3

single crystal in exact xx�Ag�, yy�Ag�, xz�B2g�, and yx��B1g+B3g�scattering configurations with 488 nm �2.54 eV�, 514.5 nm�2.41 eV�, and 632.8 nm �1.96 eV� excitation. In the same figureare shown the partial contributions of apex oxygens �O1� and planeoxygens �O2� to the smeared phonon density of states��=30 cm−1� as predicted by lattice dynamical calculations using ashell model. �Ref. 11�.

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ser lines. In the same figure are shown the partial contribu-tions of apex oxygens �O1� and plane oxygens �O2� to thesmeared phonon density of states ��=30 cm−1� as predictedby lattice-dynamical calculations using a shell model.11 Al-

though these calculations were done for LaMnO3, similarPDOS profiles were obtained for YMnO3, which is at theother end of the RMnO3 series. This justifies that in a roughapproximation the experimental spectra of all intermediateRMnO3, including DyMnO3, could be compared with thesePDOS profiles.

The relatively narrow peak at 645 cm−1 in theyx��B1g+B3g� spectra of Fig. 3 can reasonably be assigned tothe stretching B1g�1� and/or breathing B3g�1� modes, pre-dicted to have frequencies slightly higher than that of theB2g�1� mode.12 The much broader structure at 640–670 cm−1

resembles to a great extent the partial PDOS of oxygen vi-brations. In the nonmetallic doped manganites,11 PDOS ap-pears in the Raman spectra due to the coexistence of Mn3+

and Mn4+ ions, which drives Jahn-Teller dynamical disorder.In the present case of nominally undoped, orbitally orderedRMnO3, the appearance of PDOS structures could be relatedto disorder induced by some lattice imperfections as cationvacancies that, although in a small amount, inevitably existin these materials.

Figure 4 shows the Raman spectra of DyMnO3 in one-,two-, and three-phonon segments, obtained at 6 K withyy�Ag�, xx�Ag�, and xz�B2g� exact scattering configurations�the second- and third-order yx��B1g+B3g� spectrum was ofnegligible intensity�. It is evident that the second-order spec-tral profiles are strongly polarization-dependent and the in-tensity is much higher for xx and xz than yy scattered polar-izations. It is also evident that there is no correspondencebetween the position of the strongest second-order peaks andthe expected position of the �-point Raman phonon over-tones and combinations. This implies only small, if not neg-ligible, contribution of the zone-center phonons to thesecond-order scattering.

The comparable intensity and spectral range of thesecond-order xz�B2g� and xx�Ag� structures and some sym-metry considerations provide additional arguments for thedominating contribution of zone-boundary phonons to the

TABLE I. Group-theoretical selection rules for two-phonon scattering processes in orthorhombicRMnO3.

Special points, their position,and irreducible representations Overtones Combinations

��0,0 ,0��1 ,�2 ,�3 ,�4 ,�5 ,�6 ��1�2, ��2�2, ��3�2, ��4�2=Ag �1��3, �2��4, �5��7, �6��8=B1g

�7 ,�8 ��5�2, ��6�2, ��7�2, ��8�2=Ag �1��5, �2��6, �3��7, �4��8=B2g

�1��7, �2��8, �3��5, �4��6=B3g

X �1/2 ,0 ,0�: X1 ,X2 �X1�2,�X2�2�Ag, B3g X1�X2�B1g, B2g

Z �0,0 ,1 /2�: Z1 ,Z2 �Z1�2, �Z2�2�Ag, B3g Z1�Z2�B1g, B2g

Y �0,1 /2 ,0�: Y1 ,Y2 �Y1�2, �Y2�2�Ag, B1g Y1�Y2�B2g, B3g

S �1/2 ,1 /2 ,0�: S1 ,S2 �S1�2, �S2�2�B1g, B3g S1�S2�Ag, B2g

R �1/2 ,1 /2 ,1 /2�: R1 ,R2 �R1�2, �R2�2�B2g, B3g R1�R2�Ag, B1g

T �0,1 /2 ,1 /2�: T1 ,T2 �T1�2, �T2�2�Ag, B2g T1�T2�B1g, B3g

U �1/2 ,0 ,1 /2�U1 ,U2 ,U3 ,U4 ,U5 ,U6 �U1�2, �U2�2, �U3�2, �U4�2=B3g U1�U2, U3�U4, U5�U6, U7�U8=B2g

U7 ,U8 �U5�2, �U6�2, �U7�2, �U8�2=B3g U1�U3, U2�U4, U5�U7, U6�U8=B1g

U1�U4, U2�U3, U5�U8, U6�U7=Ag

FIG. 4. Multiphonon Raman spectra of DyMnO3 single crystalat 6 K in exact xx�Ag�, yy�Ag�, and xz�B2g� scattering configura-tions. Only the lowest part of the main first-order lines is seen.

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multiphonon scattering in RMnO3. Indeed, in Table I we givethe group-theoretical selection rules for the two-phonon scat-tering with participation of phonons from several generalpoints in the Brillouin zone of the Pmna structure �Fig. 5�.13

It follows from these selection rules that second-order over-tones of the �-point phonons are allowed in Ag, but not inB2g scattering configuration, whereas the overtones of zone-boundary phonons are allowed in both Ag and B2g spectra.Therefore, the fact that the Ag and B2g spectra are of compa-

rable intensity strongly supports a conclusion that thesecond-order scattering is determined mainly by zone-boundary phonon overtones and combinations.

In summary, using the oxygen isotope effect we havedemonstrated that multiple-order Raman scattering in theorthorhombic manganites is of purely phononic origin. TheRaman spectra also show a lack of any signature of phononrenormalization due to interaction with orbitons. The spectralstructure at �650 cm−1 that replicates and combines withother phonons in the multiphonon spectra is shown to reflectthe oxygen PDOS. Based on a group-theory reasoning, wehave given arguments that the two-phonon Raman scatteringfrom orthorhombic manganites involves only zone-boundaryphonons.

ACKNOWLEDGMENTS

This work is supported in part by the State of Texasthrough the Texas Center for Superconductivity, the NationalScience and Engineering Research Council of Canada, andthe Fonds Québécois de la Recherche sur la Nature et lesTechnologies. The work of M.M.G. is supported by the Bul-garian National Research Fund �Project F-1207�. J. Birmanis greatly acknowledged for useful discussions on grouptheory issues.

1 P. B. Allen and V. Perebeinos, Phys. Rev. Lett. 83, 4828 �1999�.2 V. Perebeinos and P. B. Allen, Phys. Rev. B 64, 085118 �2001�.3 E. Saitoh, S. Okamoto, K. T. Takahashi, K. Tobe, K. Yamamoto,

T. Kimura, S. Ishihara, S. Maekawa, and Y. Tokura, Nature�London� 410, 180 �2001�.

4 J. van den Brink, Phys. Rev. Lett. 87, 217202 �2001�.5 R. Krüger, B. Schulz, S. Naler, R. Rauer, D. Budelmann, J. Bäck-

ström, K. H. Kim, S.-W. Cheong, V. Perebeinos, and M. Rüb-hausen, Phys. Rev. Lett. 92, 097203 �2004�.

6 L. Martin-Carrón and A. de Andrés, Phys. Rev. Lett. 92, 175501�2004�.

7 K.-Y. Choi, P. Lemmens, T. Sahaouri, G. Güntherodt, Yu. G. Pa-shkevich, V. P. Gnezdilov, P. Reutler, L. Pinsard-Gaudart, B.Büchner, and A. Revcolevschi, Phys. Rev. B 71, 174402 �2005�.

8 K.-Y. Choi, P. Lemmens, G. Güntherodt, Yu. G. Pashkevich, V. P.Gnezdilov, P. Reutler, L. Pinsard-Gaudart, B. Büchner, and A.

Revcolevschi, Phys. Rev. B 72, 024301 �2005�.9 J. Laverdiere, S. Jandl, A. A. Mukhin, and V. Yu. Ivanov Eur.

Phys. J. B 54, 67 �2006�.10 M. N. Iliev, M. V. Abrashev, J. Laverdière, S. Jandl, M. M. Gos-

podinov, Y.-Q. Wang, and Y.-Y. Sun, Phys. Rev. B 73, 064302�2006�.

11 M. N. Iliev, M. V. Abrashev, V. N. Popov, and V. G. Hadjiev,Phys. Rev. B 67, 212301 �2003�.

12 M. N. Iliev, M. V. Abrashev, H.-G. Lee, V. N. Popov, Y. Y. Sun,C. Thomsen, R. L. Meng, and C. W. Chu, Phys. Rev. B 57, 2872�1998�. �Note that some phonon assignments have been cor-rected in Ref. 10.�

13 A. P. Cracknell, and B. L. Davis, Kronecker Product Tables �Ple-num, New York, 1979�, Vol. 2.

14 S. Taboada, A. de Andrés, and J. E. Muñoz-Santiuste, J. Phys.:Condens. Matter 10, 8983 �1998�.

FIG. 5. Brillouin zone of RMnO3 �Pnma structure�.

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