Low temperature limit of the vortex core radius and the Kramer-Pesch effect in NbSe2

Muon spin rotation (mu SR) has been used to measure the magnetic field distribution in the vortex state of the type-II superconductor NbSe2 (T-c = 7.0 K) below T = 2 K. The distribution is consistent with a highly ordered hexagonal vortex lattice with a well resolved high-field cutoff associated with the finite size of the vortex cores. The temperature dependence of the core radius is much weaker than the temperature dependence predicted from the Bogoliubov-de Gennes theory. Furthermore, the vortex radius measured by mu SR near the low temperature quantum limit is about an order of magnitude larger than predicted.