Orbital-spin-phonon coupling in Jahn-Teller-distorted LaMnO3: Softening of the 490 and 610 cm(-1) Raman-active modes

Abstract

The spin-phonon coupling model that excludes a possible orbital-spin-phonon (OSP) coupling does not adequately explain the softening behavior of the two most intense Raman bands of LaMnO3, the parent compound of "colossal" magnetoresistivity. To quantitatively interpret the softening behavior of these two Raman bands at 490 and 610 cm(-1), we have derived theoretical expressions for the phonon softening (Delta omega(N)) that take account of a simultaneous OSP coupling. The present analysis successfully explains the softening behavior of these two Raman modes and indicates that a simultaneous OSP coupling is present below the Neel temperature, T-N. The estimated effective force constant (k(osp)) of the OSP coupling is approximately 70 dynes/cm, which is an order of magnitude smaller than that of the spin-phonon coupling (k(s-ph)).