Multi-$\vec{k}$ Configurations

TL;DR

This study reports the discovery of novel $<kkk>$ reflections in UAs$_{0.8}$Se$_{0.2}$ using resonant x-ray scattering, suggesting complex multi-$oldsymbol{k}$ magnetic structures involving electric dipole transitions, with their intensity linked to temperature and phase transitions.

Contribution

The paper provides the first experimental characterization of $<kkk>$ reflections in UAs$_{0.8}$Se$_{0.2}$ and discusses their possible origin from multi-$oldsymbol{k}$ magnetic order involving electric dipole operators.

Findings

$<kkk>$ reflections are observed with very low intensity.

The $<kkk>$ intensity decreases below 50 K, indicating a phase change.

Reflections likely originate from electric dipole transitions involving 3d and 5f states.

Abstract

Using resonant x-ray scattering to perform diffraction experiments at the U M$_{4}$ edge novel reflections of the generic form $<kkk>$ have been observed in UAs$_{0.8}$Se$_{0.2}$ where $\vec{k} = < k00>$, with $k = {1/2}$ reciprocal lattice units, is the wave vector of the primary (magnetic) order parameter. The $<kkk>$ reflections, with $10^{-4}$ of the $<k00>$ magnetic intensities, cannot be explained on the basis of the primary order parameter within standard scattering theory. A full experimental characterisation of these reflections is presented including their energy, azimuthal and temperature dependencies. On this basis we establish that the reflections most likely arise from the electric dipole operator involving transitions between the core 3d and partially filled $5f$ states. The temperature dependence couples the $<kkk>$ peak to the triple-$\vec{k}$ region of the phase diagram: Below $\sim 50$ K, where previous studies have suggested a transition to a double-$\vec{k}$ state, the intensity of the $<kkk>$ is dramatically reduced. Whilst we are unable to give a definite explanation of how these novel reflections appear, this paper concludes with a discussion of possible ideas for these reflections in terms of the coherent superposition of the 3 primary (magnetic) order parameters.