Condensed Matter Physics Seminar

Thursday, April 6, 2000, 2 p.m.
Plant Sciences Building, Room 1130

Self-localized holes and electron-hole pairs in LaMnO3

Philip Allen

(SUNY, Stony Brook)

Abstract:  Orbital ordering (a cooperative Jahn-Teller state) sets in at 800K while magnetic ordering (ferromagnetic layers coupled antiferromagnetically) sets in at 170K in the S=2 ionic insulator LaMnO3.  There is a very natural Hamiltonian which successfully models these states, where only the partly filled (Eg) subshell of Mn d-electrons is explicitly included.  The filled S=3/2 (T2g) subshell is treated as an inert core which (because of Hund's rule) can accommodate only a parallel spin Eg electron.  The onsite Coulomb interaction U is large and included by using only singly occupied Eg shells (infinite U approximation).  Then the important terms in the Hamiltonian are hopping to nearest neighbors if spin orientation allows (the "double-exchange" Hamiltonian), and coupling of Eg electrons to oxygen displacements (the Jahn-Teller term).  This Hamiltonian successfully explains both the orbital ordering and the magnetic ordering.

Without further approximation or adjustment, the Hamiltonian also makes many interesting predictions about the nature of doped-in holes and about the low-lying electronic excitations of the system.  Specifically, holes self-localize, forming small polaron defects in the Jahn-Teller order parameter, which accounts for the insulating nature of La(1-x)CaxMnO3 for x<0.2, and provides a description of the metal-to-insulator transition.  The lowest electronic excitation is a flipped orbital which traps by oxygen un-distortion, forming a non-propagating, self-trapped exciton.  Existing optical measurements are given a natural reinterpretion.  New optical effects are predicted.  Signatures of these phenomena are predicted to appear in angle-resolved photoemission spectra.

This work is a collaboration with Vasili Perebeinos.  Parts have been published:

Host:  Jaroslav Fabian

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