We apply moderate-high-energy inelastic neutron scattering (INS) measurements to investigate ${\mathrm{Yb}}^{3+}$ crystalline electric field (CEF) levels in the triangular spin-liquid candidate ${\mathrm{YbMgGaO}}_4$. Three CEF excitations from the ground-state Kramers doublet are centered at the energies $\hslash \omega =39$, 61, and 97 meV in agreement with the effective spin-$1/2$ $g$ factors and experimental heat capacity, but reveal sizable broadening. We argue that this broadening originates from the site mixing between ${\mathrm{Mg}}^{2+}$ and ${\mathrm{Ga}}^{3+}$ 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 ${\mathrm{YbMgGaO}}_4$, although a distribution of magnetic couplings due to the $\mathrm{Mg}/\mathrm{Ga}$ disorder may be important as well.

• Received 4 October 2016

DOI:https://doi.org/10.1103/PhysRevLett.118.107202