Crystal electric field in YbNi2B2C and YbNiBC observed by 172Yb perturbed angular correlations

Abstract We have measured the thermal variation of the quadrupole hyperfine interaction in the non-superconducting tetragonal borocarbides YbNi 2 B 2 C and YbNiBC over the temperature range from 20 to 1000 K using the 172 Yb perturbed angular correlations technique. The observed variations are due to the changes in the 4f shell contribution to the electric field gradient at the 172 Yb nucleus which are related to the thermal dependent populations of the crystal electric field levels of the Yb 3+ 2 F 7/2 ground state. In YbNiBC, we determine the complete Yb 3+ crystal field level scheme and we confirm that any hybridisation between the 4f and band electrons is weak. In YbNi 2 B 2 C, the electric field gradient does show a thermal dependence, thus evidencing the presence of crystal-field levels, but the total variation is relatively small. We consider this result in terms of the combined influence of the crystal field and 4f-conduction band hybridisation.