Crystalline Electric-Field Randomness in the Triangular Lattice Spin-Liquid YbMgGaO4

We apply moderate-high-energy inelastic neutron scattering (INS) measurements to investigate Yb3+ crystalline electric field (CEF) levels in the triangular spin-liquid candidate YbMgGaO4. Three CEF excitations from the ground-state Kramers doublet are centered at the energies ω=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 Mg2+ and Ga3+ 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 YbMgGaO4, although a distribution of magnetic couplings due to the Mg/Ga disorder may be important as well.