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

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 $\hslash \omega$ = 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.

Accepted in Phys. Rev. Lett

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