The ground state of the Colossal Magnetoresistance (CMR) parent compound
LaMnO3 has cooperative Jahn-Teller order
(orbital ordering below T=750 K) and magnetic order
(ferromagnetic layers
coupled antiferromagnetically below T=140 K).
The model Hamiltonian explicitly includes only the partly filled Eg
subshell of the Mn d-electrons.
The onsite Coulomb interaction U is large and handled by using only
singly occupied Eg subshells. 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 and magnetic ordering.
The Hamiltonian makes many interesting predictions about the nature
of doped-in holes and about the low-lying electronic excitations of the
system. The lowest electronic excitation is a flipped orbital which traps by
oxygen un-distortion, forming a self-trapped exciton. The optical spectrum
should show a Franck-Condon series, a Gaussian envelope of vibrational
sidebands. Existing optical measurements seem to agree with this.
When an electron is removed from the Jahn-Teller-ordered ground state
the electron-phonon coupling causes the hole to self-localize in an
``anti-Jahn-Teller'' small polaron state.
By exact diagonalization of the Hamiltonian in a truncated Hilbert space,
which consist of the hole at each site and vibrational quanta of six nearest
oxygens, we predict a photoemission spectrum for each wavevector k,
which consist
of a central delta-function at the energy of the frozen lattice
(dispersive) band E(k), plus multiple vibrational
side-bands at energy
E(k) hbar w, with an overall Gaussian envelope
whose width is approximately the polaron binding energy.
Due to the Jahn-Teller distorted ground state a multi phonon resonant Raman
peaks are predicted.
The absolute cross sections of the first and second Raman Resonant peaks
have been obtained in a Frank-Condon approximation for the Jahn-Teller
active phonon mode in LaMnO3.
The absolute intensity of the first Raman peak
depends on the frequency of the incident photon
and is estimated at
10-26cm2sterad-1 for Ar+ laser frequency.
[1] P. B. Allen and V. Perebeinos, Phys. Rev. Lett. 83, 4828 (1999).
[2] P. B. Allen and V. Perebeinos, Phys. Rev. B 60, 10747 (1999).
[3] V. Perebeinos and P. B. Allen, cond-mat/0005146 (submitted to PRL).
[4] V. Perebeinos and P. B. Allen, cond-mat/0007301.