Surface effects at the superconducting phase transition in tantalum and niobium

Abstract

A sharp diamagnetic drop is observed in the 14-MHz transition of niobium and tantalum very close to ${T}_{c}$, in small parallel dc magnetic fields. The drop occurs in the regime where $\ensuremath{\lambda}>{\ensuremath{\delta}}_{n}$ and occurs over a temperature interval that is smaller than the one corresponding to the BCS dependence of $\ensuremath{\lambda}$ with temperature. Because of the sharpness of the drop it is analyzed in terms of an increased conductivity in the normal state very close to ${T}_{c}$ due to enhanced fluctuations of superconducting regions at the surface. The possible role of these fluctuations for the nucleation of the superconducting state is discussed.