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 NbSe₂ shows curvature at low fields, resembling the near vH term in C(H) which has been reported in high- T_c superconductors and attributed to a d_x²-y²-wave pairing state. In NbSe₂ 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.