Magnetic pair breaking in superconducting HoNi2B2C studied on a single crystal by thermal conductivity in magnetic fields

The magnetic field dependence of the thermal conductivity kappa of a HoNi2B2C single crystal (parallel to [110]; 2 < T/K < 10 ; mu H-0 < 0.4 T) is reported. It exhibits a characteristic change in the slope of kappa(T) at significantly lower temperatures (or lower fields) than the resistively measured critical temperature T-c(H) (or the upper critical field H-c2(T)), thus pointing to an enhanced fraction of quasiparticles due to strong magnetic pair breaking. This demands a careful interpretation of kappa(T,H) data for distinct superconductors. Different scenarios for the occurrence of small energy gaps below and above the magnetic-ordering temperatures are discussed including multiband aspects of the superconductivity. In particular, we suggest an improved multiband approach for the commensurate antiferromagnetic phase: whereas a nearly isotropic band unaffected by the rare-earth magnetism shows a BCS-like gap, a second band is strongly affected by the competition between superconductivity and magnetism leading to a second smaller gap. The latter band with relatively large Fermi velocities dominates the thermal conductivity.