Crystal-field excitations and spin-phonon interactions in ${\text{DyNi}}_2{\text{B}}_2\text{C}$: Raman scattering study

We report polarized Raman results of magnetic superconductor ${\text{DyNi}}_2{\text{B}}_2\text{C}$ $\left(T_c=6.2\text{K},T_N=10.3\text{K}\right)$ to explore crystal-field (CF) excitations and spin-phonon interactions. In addition to the $\text{Ni-}{B}_{1g}$ phonon mode at $199{\text{cm}}^{-1}$, we observed additional Raman modes at 124, 151, and $221{\text{cm}}^{-1}$. By careful analysis of the temperature evolution of these modes, we attribute the $124{\text{cm}}^{-1}$ excitation to a CF transition. The 151 and $221{\text{cm}}^{-1}$ modes correspond to zone-folded phonons because they are completely quenched for $T>T_N$. With increasing temperature across $T_N$, the $124{\text{cm}}^{-1}$ excitation diminishes rapidly in intensity and, interestingly, an additional mode appears at $119{\text{cm}}^{-1}$. This excitation grows in intensity with increasing temperature toward 50 K and gradually decreases with increasing temperature further. We attribute the $119{\text{cm}}^{-1}$ excitation to an excited CF transition from a low-lying CF level at $5{\text{cm}}^{-1}$ to the higher CF level at $124{\text{cm}}^{-1}$. Anomalous temperature-dependent behavior of the $\text{Ni-}{B}_{1g}$ phonon mode was observed in peak energy and in spectral width, i.e., both the phonon energy and the linewidth are enhanced in the vicinity of $T_N$, suggesting the presence of strong spin-phonon interactions near the antiferromagnetic ordering temperature in ${\text{DyNi}}_2{\text{B}}_2\text{C}$.

Figure 1

(Color online) Polarized Raman spectra of ${\text{DyNi}}_2{\text{B}}_2\text{C}$ at (a) $T=4\text{K}$ and (b) $T=15\text{K}$. From top to bottom, $\left({\mathbf{E}}_i,{\mathbf{E}}_s\right)=\left(x^{\prime },y^{\prime }\right)$, $B_{1g}+A_{2g}$; $\left(x,x\right)$, $B_{1g}+A_{1g}$; $\left(L,R\right)$, $B_{1g}+B_{2g}$; $\left(L,L\right)$, $A_{1g}+A_{2g}$; $\left(x,y\right)$, $B_{2g}+A_{2g}$; and $\left(x^{\prime },x^{\prime }\right)$, $B_{2g}+A_{1g}$, respectively.

Figure 2

(Color online) Temperature-dependent Raman spectra of ${\text{DyNi}}_2{\text{B}}_2\text{C}$ in $\left({\mathbf{E}}_i,{\mathbf{E}}_s\right)=\left(x^{\prime },y^{\prime }\right)$ scattering configuration.

Figure 3

(Color online) (a) A representative Raman spectrum (open squares) at $T=8\text{K}$ with individual fitting results (dashed lines) for the 119 and the $124{\text{cm}}^{-1}$ modes. A solid line indicates a fitted result to a Lorentzian line shape. (b) Intensity and (c) spectral width changes as a function of temperature for the 119 and the $124{\text{cm}}^{-1}$ modes.

Figure 4

(Color online) Temperature evolution of the Raman peak energies of the $\text{Ni-}{B}_{1g}$ phonons for ${\text{DyNi}}_2{\text{B}}_2\text{C}$ (asterisks), ${\text{ErNi}}_2{\text{B}}_2\text{C}$ (solid squares), and ${\text{LuNi}}_2{\text{B}}_2\text{C}$ (solid triangles). The inset shows spectral width changes as a function of temperature of the ${\text{Ni-B}}_{1g}$ phonons for ${\text{DyNi}}_2{\text{B}}_2\text{C}$ (asterisks), ${\text{ErNi}}_2{\text{B}}_2\text{C}$ (solid squares), and ${\text{LuNi}}_2{\text{B}}_2\text{C}$ (solid triangles).