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 ne ar root H 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-c 1. The associated change in the density of quasiparticle states localized i n the vortex cores provides a simple explanation for the "low field anomaly " in C(H) observed in s-wave superconductors. [S0031-9007(99)09363-1].