Crystalline electric field excitations in the quantum spin liquid candidate NaYbSe 2

By employing inelastic neutron scattering (INS) and Raman scattering (RS) experiments, we comprehensively investigate crystalline electric field (CEF) excitations in ${\\mathrm{NaYbSe}}_{2}$, a new quantum spin liquid candidate that belongs to a large family of triangular-lattice rare-earth chalcogenides with a high-symmetry structure and negligible structural, spin, and charge disordering effects. We can identify CEF excitations at 15.8, 24.3, and 30.5 meV at 5 K. The selected cuts of the INS spectra are well reproduced with a large anisotropy of ${g}_{ab}$ = 2.9 and ${g}_{c}$ = 1. The CEF excitations are further confirmed by our calculations based on the point charge model. Interestingly, ${\\mathrm{NaYbSe}}_{2}$ exhibits an unusual shift of CEF levels to higher energies with increasing temperatures. Further, the Raman mode close to the first CEF excitation shows an anomalously large softening with decreasing temperatures. The absence of these anomalies in the nonmagnetic isostructural material ${\\mathrm{NaLuSe}}_{2}$ allows us to argue that ${\\mathrm{NaYbSe}}_{2}$ incorporates an unusually strong CEF-phonon resonancelike coupling not reported in any of the triangular-lattice rare-earth chalcogenides. The determination of the CEF excitations suggests the validity of the picture of an effective spin 1/2 at low temperatures.