Low-Field Anomaly in the Specific Heat of s-wave Superconductors due to the Expansion of the Vortex Cores

The magnetic field dependence of the electronic specific heat C(H) in the s-wave superconductor NbSe_2 shows curvature at low fields, resembling the near H^1/2 term in C(H) which has been reported in high-T_c superconductors and attributed to a d-wave pairing state. In NbSe_2 we find that the low field behavior in C(H) is described quantitatively by the expansion of vortex cores and the field dependence of the magnetic induction B above H_c1. The associated change in the density of quasiparticle states localized in the vortex cores provides a simple explanation for the "low-field anomaly" in C(H) observed in s-wave superconductors.