Raman scattering in a HeisenbergS=12antiferromagnet on the triangular lattice

Abstract

We investigate the two-magnon Raman scattering from the $S=\frac{1}{2}$ Heisenberg antiferromagnet on the triangular lattice by considering both the effect of the renormalization of the one-magnon spectrum by $1/S$ corrections and the final-state magnon-magnon interactions. The bare Raman intensity displays two peaks related to one-magnon Van Hove singularities. We find that $1/S$ self-energy corrections to the one-magnon spectrum strongly modify this intensity profile. The central Raman peak is significantly enhanced due to plateaus in the magnon dispersion, the high frequency peak is suppressed due to magnon damping, and the overall spectral support narrows considerably. Additionally, we investigate final-state interactions by solving the Bethe-Salpeter equation to $O(1/S)$. In contrast to collinear antiferromagnets, the noncollinear nature of the magnetic ground state leads to an irreducible magnon scattering that is retarded and nonseparable already for the lowest order. We show that final-state interactions lead to a rather broad Raman continuum centered on approximately twice the ``roton'' energy. We also discuss the dependence on the scattering geometry.