Magnetic pair breaking in superconductingHoNi2B2Cstudied on a single crystal by thermal conductivity in magnetic fields

Abstract

The magnetic field dependence of the thermal conductivity $\ensuremath{\kappa}$ of a ${\text{HoNi}}_{2}{\text{B}}_{2}\text{C}$ single crystal (parallel to [110]; $2<T/\text{K}<10\text{ }$; ${\ensuremath{\mu}}_{0}H\ensuremath{\le}0.4\text{ }\text{T}$) is reported. It exhibits a characteristic change in the slope of $\ensuremath{\kappa}(T)$ at significantly lower temperatures (or lower fields) than the resistively measured critical temperature ${T}_{\text{c}}(H)$ (or the upper critical field ${H}_{\text{c}2}(T)$), thus pointing to an enhanced fraction of quasiparticles due to strong magnetic pair breaking. This demands a careful interpretation of $\ensuremath{\kappa}(T,H)$ 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.