Frustrated magnetic materials with competing interactions are a rich source of novel emergent phenomena, as frustration inhibits long-range order and may give rise to a quantum spin liquid (QSL) behavior with strong quantum entanglement, topological order, and often fractionalized excitations. Here, we report an in-depth Raman spectroscopic analysis to probe a single crystal of a rare-earth triangular-lattice antiferromagnet, $\mathrm{YbMgGa}{\mathrm{O}}_4$, along the $ab(/ac)$ crystallographic planes. $\mathrm{YbMgGa}{\mathrm{O}}_4$ has recently attracted significant interest as a promising gapless U(1) QSL candidate with a spinon Fermi surface. We observe an underlying broad continuum of excitations, with a possible origin in disorder arising from $\mathrm{M}{\mathrm{g}}^{2+}/\mathrm{G}{\mathrm{a}}^{3+}$ site mixing and a broad distribution of crystal field excitation (CFE) parameters, rather than a single set of well-defined levels. The development of the Fano asymmetry in the low-frequency phonon mode indicates the coupling with the underlying continuum. These underlying excitations are also reflected in the dynamic spin susceptibility estimated from the Raman response. The apparent renormalization of the phonon self-energy parameters at $\sim 50$ K, which are ascribed to the phonon decaying into itinerant underlying continuum excitation from structural disorder, arises with $\mathrm{M}{\mathrm{g}}^{2+}/\mathrm{G}{\mathrm{a}}^{3+}$ site mixing. Additionally, we analyze the angle-resolved polarization measurements for both $ab(/ac)$ crystallographic planes to decipher the symmetry of the phonons, CFEs, and underlying excitations. The observed variations of the phonon mode behavior as a function of incident photon polarization provide evidence of anisotropic light-matter interactions along with the anisotropy in the spin susceptibility, spin-spin correlation length, and phonon renormalization.

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• Crystal-field excitations
• Quantum spin liquid
• Quasiparticles & collective excitations
• Rare-earth magnetic materials
• Raman spectroscopy