# Crystalline Electric Field Randomness in the Triangular Lattice Spin-Liquid YbMgGaO$_4$

## TL;DR

This study investigates the crystalline electric field levels in the spin-liquid candidate YbMgGaO$_4$ using inelastic neutron scattering, revealing broadening effects due to site disorder that influence magnetic excitations.

## Contribution

It demonstrates that CEF level broadening in YbMgGaO$_4$ arises from Mg/Ga site mixing, affecting magnetic properties and excitation spectra.

## Key findings

- CEF excitations at 39, 61, and 97 meV with broadening
- Site disorder causes distribution of Yb--O distances and CEF parameters
- Broadening explains low-energy excitation features in the polarized phase

## Abstract

We apply moderate-high-energy inelastic neutron scattering (INS) measurements to investigate Yb$^{3+}$ crystalline electric field (CEF) levels in the triangular spin-liquid candidate YbMgGaO$_4$. Three CEF excitations from the ground-state Kramers doublet are centered at the energies $\hbar \omega$ = 39, 61, and 97\,meV in agreement with the effective \mbox{spin-1/2} $g$-factors and experimental heat capacity, but reveal sizable broadening. We argue that this broadening originates from the site mixing between Mg$^{2+}$ and Ga$^{3+}$ giving rise to a distribution of Yb--O distances and orientations and, thus, of CEF parameters that account for the peculiar energy profile of the CEF excitations. The CEF randomness gives rise to a distribution of the effective spin-1/2 $g$-factors and explains the unprecedented broadening of low-energy magnetic excitations in the fully polarized ferromagnetic phase of YbMgGaO$_4$, although a distribution of magnetic couplings due to the Mg/Ga disorder may be important as well.