Reduction of core size of vortices in clean superconducting lead films at T<<Tsub(c)

Kramer and Pesch have recently predicted that in clean superconductors at T << Tsub(c), the characteristic radius over which the order parameter varies near a vortex centre, is much less than the superconducting coherence length. The density of vortex core states is correspondingly reduced. In the paper tunnelling conductance measurements on lead-magnesium junctions with the lead films in the superconducting vortex state are reported which confirm this prediction. (Auth.)