Heat Conduction in the Vortex State of NbSe_2: Evidence for Multi-Band   Superconductivity
  The thermal conductivity kappa of the layered s-wave superconductor NbSe_2 was measured down to T_c/100 throughout the vortex state. With increasing field, we identify two regimes: one with localized states at fields very near H_c1 and one with highly delocalized quasiparticle excitations at higher fields. The two associated length scales are naturally explained as multi-band superconductivity, with distinct small and large superconducting gaps on different sheets of the Fermi surface. This behavior is compared to that of the multi-band superconductor MgB_2 and the conventional superconductor V_3Si. 
Etienne Boaknin, M.A. Tanatar, Johnpierre Paglione, D. Hawthorn, F.   Ronning, R.W. Hill, M. Sutherland, Louis Taillefer, Je...