X-ray and neutron scattering

Abstract:

Polarized- and unpolarized-neutron scattering measurements of the spin excitation spectrum in the stripe-ordered phase of La(2)NiO4+delta(delta similar or equal to 0.11) are presented. At low energies, the magnetic spectral weight is found to shift anomalously toward the two-dimensional antiferromagnetic wave vector, similar to the low-energy dispersions observed in cuprate superconductors. While spin-wave spectra in stripe phases can exhibit an apparent inward dispersion, we find that the peak shifts measured here cannot be accounted for by this effect. Possible extensions of the model are discussed.

Abstract:

Coherent lattice vibrations are excited and probed with pulses of 10 fs duration in 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 LaMnO(3), and note that charge transfer can invert the sign of the semicovalent exchange interaction, which in turn perturbs the equilibrium bond lengths.