  The magnetic field (B) dependence of the electronic specific heat for a
simple BCS type-II superconductor has been determined from measurements on pure
niobium (Nb). Contrary to expectations, the electronic specific heat
coefficient gamma(T,B) is observed to be a sublinear function of B at fields
above the lower critical field H_{c1}. This behavior is attributed to the
delocalization of quasiparticles bound to the vortex cores. The results
underscore the ambiguity of interpretation that arises in specific heat studies
of this kind on newly discovered type-II superconductors, and also emphasize
the need to such measurements under field-cooled conditions.
