Title: Hierarchy of exchange interactions in the triangular-lattice spin-liquid YbMgGaO$_{4}$,
Abstract: The spin-1/2 triangular lattice antiferromagnet YbMgGaO$_{4}$ has attracted
recent attention as a quantum spin-liquid candidate with the possible presence
of off-diagonal anisotropic exchange interactions induced by spin-orbit
coupling. Whether a quantum spin-liquid is stabilized or not depends on the
interplay of various exchange interactions with chemical disorder that is
inherent to the layered structure of the compound. We combine time-domain
terahertz spectroscopy and inelastic neutron scattering measurements in the
field polarized state of YbMgGaO$_{4}$ to obtain better microscopic insights on
its exchange interactions. Terahertz spectroscopy in this fashion functions as
high-field electron spin resonance and probes the spin-wave excitations at the
Brillouin zone center, ideally complementing neutron scattering. A global
spin-wave fit to all our spectroscopic data at fields over 4T, informed by the
analysis of the terahertz spectroscopy linewidths, yields stringent constraints
on $g$-factors and exchange interactions. Our results paint YbMgGaO$_{4}$ as an
easy-plane XXZ antiferromagnet with the combined and necessary presence of
sub-leading next-nearest neighbor and weak anisotropic off-diagonal
nearest-neighbor interactions. Moreover, the obtained $g$-factors are
substantially different from previous reports. This works establishes the
hierarchy of exchange interactions in YbMgGaO$_{4}$ from high-field data alone
and thus strongly constrains possible mechanisms responsible for the observed
spin-liquid phenomenology.