Densities of states in isolated flux vortices in superconductors

The densities of states of quasiparticles in isolated flux vortices in clean limit superconductors have been calculated for various kappa, using an extension of calculations by Bardeen et al. (Phys. Rev.; 187:556 (1969)). Results similar to the dirty limit calculations of Watts-Tobin et al. (J. Low Temp. Phys.; 17:71 (1974)) were found, with the density per unit length at zero energy varying from 4πxi2n(0) at low kappa to 7 eta xi2n(0) at kappa =30, where xi is the coherence length and n(0) the one-electron density of states per unit volume at the Fermi surface. Experimental studies of the density N(E) as a function of energy were performed by tunnelling on thin films of Pb, Sn and Al at less than 0.1 K. Good agreement was found between the experimental and theoretical reduced densities N(E)/N(0). The absolute magnitude of N(0) was, however, too small by a factor of about 5. This may be evidence for the contribution of the tunnelling anisotropy factor first discussed by Band and Donaldson (J. Phys. F. Metal Phys.; 4:2017 (1974)). (author)