Vortex overlapping in a BCS type-II superconductor revealed by Andreev reflection spectroscopy

Abstract

The point-contact Andreev reflection spectroscopy for a simple BCS type-II superconductor was investigated in pure niobium (Nb). The small broadening factor of $\ensuremath{\Gamma}∕\ensuremath{\Delta}(0)<0.09$ was achieved by careful preparation of the junctions. Then we attempted to use the recently proposed two-channel model [Phys. Rev. B 72, 012502 (2005)] to explain the spectra measured in various magnetic fields for such low-$\ensuremath{\Gamma}$ case. It was found that this rigid vortex core model is appropriate below a crossover field ${H}^{*}$, above which it is not adequate due to the onset of substantial vortex overlapping effect.