Crystal electric field excitation and vibrational properties of the quantum spin liquid candidate ${\mathrm{LiYbSe}}_2$

${\mathrm{LiYbSe}}_2$ was recently discovered and assumed to be a new member of the quantum spin liquid candidates. In the paper, polarization-resolved Raman scattering and temperature-dependent Raman scattering experiments were utilized to study the elementary excitations in ${\mathrm{LiYbSe}}_2$. Four phonon modes with $E_g$, $T_{2g}^1$, $T_{2g}^2$, and $A_{1g}$ symmetry, respectively, and three crystal electric field (CEF) excitations, including CEF1, CEF2, and CEF3 at 116.2, 200.9, and $232.1\mathrm{c}{\mathrm{m}}^{-1}$, respectively, were conclusively identified. With 633 and 532 nm excitation, resonant conditions were satisfied and caused the breakdown of selection rules for $T_{2g}$ and also prompted the observation of higher-order modes. Helicity-resolved CEF modes were studied based on circularly polarized light and the conservation law of angular momentum. Moreover, CEF modes showed abnormal redshift with decreasing temperature which can be attributed to the reduction of electronegativity of Se atoms due to the lattice contraction. Contributing to the close energy between $E_g$ and CEF1, weak CEF-phonon coupling was induced in this system and caused abnormal softening of $E_g$ at low temperature. The CEF excitations and vibration properties studied here provide information about the electronic elementary excitations, which help to further study or control the frustrated magnetism and novel quantum spin liquid state in ${\mathrm{LiYbSe}}_2$.

Figure 1

Structure and characterizations of the ${\mathrm{LiYbSe}}_2$ single crystal. (a) Unit cell of ${\mathrm{LiYbSe}}_2$. (b) ${\mathrm{YbSe}}_6$ and ${\mathrm{LiSe}}_6$ octahedra arrange along the [111] direction. (c) The EDS and (d) XRD pattern of ${\mathrm{LiYbSe}}_2$ single crystal. The inset in (d) is the optical image of one typical piece of the ${\mathrm{LiYbSe}}_2$ sample.

Figure 2

(a) Polarization-resolved Raman spectra in ${\mathrm{LiYbSe}}_2$ single crystal at room temperature. (b) Top view along the [111] axis of the eigenvector displacements of Raman active phonon modes in ${\mathrm{LiYbSe}}_2$.

Figure 3

(a) Temperature-dependent Raman spectra for ${\mathrm{LiYbSe}}_2$ single crystal at selected temperature. The asterisks mark the broad peaks rising at low temperature. All plots are evenly spaced in a linear scale for clarity. (b) Contour plots of the temperature-dependent Raman intensity from 6 to 300 K. The black dashed lines are the guidelines for the Raman frequencies shift.

Figure 4

(a) Temperature-dependent Raman spectra for the nonmagnetic reference of ${\mathrm{LiLuSe}}_2$ single crystal. All plots are evenly spaced in a linear scale for clarity. (b), (c) The FWHM and peak position of Raman modes vs temperature in ${\mathrm{LiLuSe}}_2$.

Figure 5

Experimental results for the temperature dependence of the peak area, FWHM, and position of the main Raman modes. The red corresponds to the bare phonon modes, and the blue corresponds to the CEF modes.

Figure 6

Polarization-resolved Raman spectra of ${\mathrm{LiYbSe}}_2$ with (a) linearly polarized and (b) circularly polarized light at 6 K. The inset in (b) is the enlarged view for the combination modes at high wave numbers.

Figure 7

Raman spectra of ${\mathrm{LiYbSe}}_2$ and ${\mathrm{LiLuSe}}_2$ with different excitation wavelengths. The Raman intensities are normalized by that of the $A_{1g}$ modes.