Electronic thermal conductivity of multigap superconductors: Application to ${\mathrm{MgB}}_2$

The remarkable field dependence of the electronic thermal conductivity observed in ${\mathrm{MgB}}_2$ can be explained as a consequence of multigap superconductivity. A key point is that for moderately clean samples, the mean free path becomes comparable to the coherence length of the smaller gap over its entire Fermi surface. In this case, quasiparticle excitations bound in vortex cores can easily be delocalized, causing a rapid rise in the thermal conductivity at low magnetic fields. This feature is in marked contrast to that for anisotropic or nodal gaps, where delocalization occurs only on part of the Fermi surface.