Low Field Anomaly in the Specific Heat ofs-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 √H term in $C(H)$ which has been reported in high- Tc 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 Hc1. 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.