Low Temperature Limit of the Vortex Core Radius and the Kramer-Pesch Effect in ${\mathrm{NbSe}}_2$

Muon spin rotation ( $\mu \mathrm{SR}$) has been used to measure the magnetic field distribution in the vortex state of the type-II superconductor ${\mathrm{NbSe}}_2$ ( $T_c=7.0\mathrm{}\mathrm{K}$) below $T=2\mathrm{}\mathrm{K}$. 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 $\mu \mathrm{SR}$ near the low temperature quantum limit is about an order of magnitude larger than predicted.