Molecular orbital cluster calculation study of electron correlation and local lattice instability in La{sub 2-2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7}

Journal Article · Tue Feb 15 00:00:00 EST 2005 · Physical Review. B, Condensed Matter and Materials Physics

DOI:https://doi.org/10.1103/PhysRevB.71.085117· OSTI ID:20665139

Miyaki, Satoru; Uzuhara, Shinji; Terada, Kazuto; Makoshi, Kenji; Koizumi, Hiroyasu ^[1]

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1. Graduate School of Material Science, University of Hyogo, Kamigori, Ako-gun, Hyogo 678-1297 (Japan)

In order to examine covalence and polaron effects in the bilayer manganite La{sub 2-2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7}, we have performed molecular orbital cluster calculations. Two types of (Mn{sub 2}O{sub 11}){sup 15-} clusters, one with the manganese aligned in the a direction and the other in the c direction, were embedded in a point charge environment that mimicked the crystal environment of the bilayer manganite at x=0.40, and their electronic states were calculated by the unrestricted Hartree-Fock (UHF) and the complete active-space self-consistent field (CASSCF) methods. The CASSCF result for the cluster along the a direction exhibits double-well potential energy surfaces for symmetry-breaking deformations. This indicates small polaron formation in this system. On the other hand, the UHF calculation did not give double-well potential surfaces, showing the importance of the electron correlation for the polaron formation. Significantly large wells are obtained for the in-plane antiphase breathing and in-plane antiphase O-Mn-O stretching deformations. The double-well barrier for the former is 68 meV and that for the latter is 92 meV, where the former is close to the experimentally obtained polaron hopping activation energy above T{sub c}. A similar calculation for the cluster along the c direction exhibits a negligibly small double well, indicating that the polaron effect is very small in the carrier hopping in the c direction within a bilayer. Electronic structures have been investigated using natural orbitals. At a double-well minimum, a localized polaron orbital is seen. In the ground state, a small but significant hole population is found in p orbitals of the bridging oxygen, and a slight electron population is found in the e{sub g} orbital above the localized polaron orbital. For the cluster along the a direction and without deformation, the first excited state is an electron-transfer state where an electron is moved from the bridging oxygen p{sub z} to a manganese e{sub g} orbital. This excited state couples with the ground state by the pseudo-Jahn-Teller effect, thus, the polaron is the 'pseudo-Jahn-Teller polaron'. Using the natural orbitals, we have calculated magnetic Compton profiles and compared with experiment. Comparison between the experimental and theoretical results suggests the presence of polarons below T{sub c}. We briefly discuss the implication of this result in relation to the colossal magnetoresistance effect.

• OSTI ID: 20665139
• Journal Information: Physical Review. B, Condensed Matter and Materials Physics, Vol. 71, Issue 8; Other Information: DOI: 10.1103/PhysRevB.71.085117; (c) 2005 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); ISSN 1098-0121
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
36 MATERIALS SCIENCE
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
ACTIVATION ENERGY
COMPARATIVE EVALUATIONS
COMPTON EFFECT
DEFORMATION
ELECTRON CORRELATION
ELECTRON DENSITY
EXCITED STATES
GROUND STATES
HARTREE-FOCK METHOD
JAHN-TELLER EFFECT
LANTHANUM COMPOUNDS
MAGNETORESISTANCE
MANGANESE COMPOUNDS
MILLI EV RANGE
OXIDES
POINT CHARGE
POLARONS
STRONTIUM COMPOUNDS
SURFACES
THIN FILMS