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
(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
Host: Jaroslav Fabian
P. B. Allen and V. Perebeinos, "The Anti-Jahn-Teller Polaron in LaMnO3",
Phys. Rev. B 60, 10747-53 (1999).
P. B. Allen and V. Perebeinos, "Self-Trapped Exciton and Franck-Condon
Spectra Predicted in LaMnO3", Phys. Rev. Letters 83,
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