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.
Highlights
Quantum spin liquids (QSLs) are exotic states of matter in which spins are highly correlated but remain dynamic down to zero temperature due to strong quantum fluctuations [1,2]
We turn to our central result, which is to refine the parameters of YbMgGaO4 by combining all the data at hand
As spin-space anisotropy is primarily a property of the effective spin-1=2 doublet of Yb3þ, we adopt the same overall XXZ exchange anisotropy for both nearest neighbor and next-nearest neighbor interactions; i.e., Jz2z=JÆ2 1⁄4 Jz1z=JÆ1. This reasonable assumption helps in reducing the size of the parameter space for the global fit
Summary
Quantum spin liquids (QSLs) are exotic states of matter in which spins are highly correlated but remain dynamic down to zero temperature due to strong quantum fluctuations [1,2]. Many distinct QSL states have been proposed theoretically [3,4] and classified according to their nonlocal (topological) properties [5] Their detection, remains a central challenge for condensed matter physics [6], and relies on the presence of quantum entanglement in their ground state and fractional quasiparticles in their excitation spectra.
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