Coherent lattice vibrations are excited and probed with pulses of 10 fs duration in ${\mathrm{LaMnO}}_3$. The measured frequencies correspond to those of Jahn-Teller stretching and of out-of phase rotations of the oxygen octahedra. Surprisingly, the amplitude and damping rate of both modes exhibit a sharp discontinuity at the Néel temperature, highlighting nontrivial coupling between light, lattice, and magnetic structure. We explain this effect by applying the Goodenough-Kanamori rules to the excited state of ${\mathrm{LaMnO}}_3$, and note that charge transfer can invert the sign of the semicovalent exchange interaction, which in turn perturbs the equilibrium bond lengths.

• Received 15 March 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.097402