Polarized Raman scattering studies of crystal-field excitations in ErNi₂B₂C

Raman-scattering measurements have been carried out to study magnetic properties of the quaternary magnetic superconductor ErNi₂B₂C (T_c=10 K, T_N=6 K). The crystal field (CF) of Er³⁺ splits the 16-fold degenerate ⁴I_15/2 ground-state multiplet of 4f¹¹ electrons into eight Kramers doublets 4Γ₆+4Γ₇ within the D_4h site symmetry. Therefore, one expects seven ground-state CF transitions from the lowest Γ₆ state to the higher states. Excitation symmetries and transition characteristics between these levels can be identified systematically by a proper use of the Raman polarization selection rules. As a result, temperature-dependent and polarized Raman-scattering spectra reveal six ground-state CF transitions: 7 cm^-1 (Γ₆→Γ₇), 48 cm^-1 (Γ₆→Γ₆), 56 cm^-1 (Γ₆→Γ₇), 145 cm^-1 (Γ₆→Γ₇), 149 cm^-1 (Γ₆→Γ₆), and 153 cm^-1 (Γ₆→Γ₇). Using the CF transition characteristics of the obtained data, we expect that one unobserved CF level transition (Γ₆→Γ₆) lies between 56 cm^-1 and 145 cm^-1.