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JAHN-TELLER PHONON ANOMALY AND PHASEINHOMOGENEITIES IN La0.7Ca0.3MnO3

1

J. Zhang,2 P. Dai, 3 J. A. Fernandez-Baca, E. W. Plummer,4 Y. Tomioka,5 and Y. Tokura5,6

It is known that the richness of the novel prop-erties of colossal magnetoresistance (CMR) manga-nites originates from the strong interplay betweenthe charge, lattice, orbital, and spin degrees of free-dom found in these systems. Although severalmechanisms [involving double-exchange (DE), elec-tron-phonon interactions, and electron-electron cor-relations] have been proposed to explain the ob-served CMR effect in this class of materials, a clearunderstanding of its microscopic origin is still lack-ing. In particular, if dynamic Jahn-Teller (JT) dis-tortions of the MnO6 octahedra are crucial to themetal-to-insulator transition (MIT) and the CMReffect, the lattice vibrations associated with suchdistortions (i.e., the so-called JT phonons) would beexpected to exhibit unusual behavior across theMIT/magnetic transition temperature (Tc). The T-dependence of the optical phonons in La1-xCaxMnO3

and in La1 xSrxMnO3 has been studied with infraredreflectivity and optical conductivity, and the ob-served energy shifts across Tc were attributed to astrong electron-phonon interaction. In this work,inelastic neutron scattering is used to demonstratethat the JT phonons in La0.7Ca0.3MnO3 do indeedexhibit an abnormal behavior across the MIT/Tc. As in the other CMR manganites, the optimallydoped La0.7Ca0.3MnO3 exhibits a ferromagnetic (FM)to paramagnetic (PM) transition and a simultaneousMIT at Tc ≅ 240 K. While its structure maintains theslightly distorted cubic perovskite (orthorhombic-pnma) symmetry, there are small incoherent dy-namic distortions across Tc. In general, the electron-phonon coupling across Tc should renormalize thephonon energies without drastically changing theirintensities. This should be especially true for thehigh-energy optical phonons because the change intheir population factors is negligible across Tc. Sur-

prisingly, it has been discovered that one of Mn-Ostretching JT phonon modes shows anomalousdamping with increasing temperature in the FMphase and vanishes around Tc, in concurrence withFM ordering and metallicity of the system (Fig. 1).Such drastic T-dependent damping cannot be ex-plained with simple arguments based on the dy-namic JT distortions at individual J-T sites. It is ar-gued that this behavior may be directly related todecoherence effects because of the growing dynamic

phase fluctuations of the short-range polaron orcharge/orbital ordering associated with JT-distor-tion in the FM metallic phase matrix as T → Tc.

Fig. 1. (a) Relative integrated intensities of theJT breathing phonon mode as a function of tempera-ture at the middle of the zone (Q = {3.7, 1, 1}) andthe zone boundary (Q = {3.5, 1, 1}) along the [1,0,0]direction. (b) Schematic illustration of the evolu-tion of dynamic phase segregation with increasingtemperature.

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1. Summary of paper: Phys. Rev. Lett. 86,3823 (2001).

2. ORNL/UT joint faculty participant.3. UT/ORNL Distinguished Scientist Pro-

gram.4. Joint Research Center for Atomic Technol-

ogy (JRCAT), Tsukuba, Japan.5. University of Tokyo, Tokyo, Japan.

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sity

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q = 0.3

TC

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