Signatures of indirect K-edge resonant inelastic x-ray scattering on magnetic excitations in triangular lattice antiferromagnet

TL;DR

This paper calculates the K-edge RIXS spectrum of a triangular lattice antiferromagnet, revealing how magnon decay, bimagnon interactions, and anisotropy influence spectral features, providing insights into magnetic excitations and potential experimental signatures.

Contribution

It introduces a detailed theoretical analysis of K-edge RIXS spectra in triangular antiferromagnets, including effects of magnon decay, bimagnon interactions, and anisotropy, highlighting unique spectral signatures.

Findings

RIXS spectra show multipeak structures influenced by magnon damping.

Stable single peaks occur at specific wavevectors where magnon decay is zero.

RIXS intensity remains nonvanishing at key points, contrasting square lattice behavior.

Abstract

We compute the K-edge indirect resonant inelastic x-ray scattering (RIXS) spectrum of a triangular lattice antiferromagnet in its ordered coplanar 3- sublattice 120 degree magnetic state. By considering the first order self$-$energy corrections to the spin wave spectrum, magnon decay rate, bimagnon interactions within the ladder approximation Bethe-Salpeter scheme, and the effect of three-magnon contributions up to 1/S- order we find that the RIXS spectra is non-trivially affected. For a purely isotropic triangular lattice model, the peak splitting mechanism and the appearance of a multipeak RIXS structure is primarily dictated by the damping of magnon modes. At a scattering wavevector corresponding to the zone center \Gamma point and at the roton point q=M, where the magnon decay rate is zero, a stable single peak forms. At the $\Gamma$ point, the contribution is purely trimagnon at the 1/S level and occurs approximately at the trimagnon energy of 6JS. The roton peak occurs at a lower energy of 4JS. The K-edge single peak RIXS spectra at the roton momentum can be utilized as an experimental signature to detect the presence of roton excitations. A unique feature of the triangular lattice K-edge RIXS spectra is the nonvanishing RIXS intensity at both the zone center $\Gamma$ point and the antiferromagnetic wavevector K point. This result is in sharp contrast to the vanishing K-edge RIXS intensity of the collinear magnetic phases on the square lattice. We find that including XXZ anisotropy leads to additional peak splitting, including at the roton scattering wavevector where the single peak destabilizes towards a two-peak structure. The observed splitting is consistent with our earlier theoretical prediction of the effects of spatial anisotropy on the RIXS spectra of a frustrated quantum magnet [Luo, Datta, and Yao, Phys. Rev. B 89, 165103 (2014)].