A possible hypo-critical point in the phase diagram of a moderately small superconductor in a magnetic field

Abstract

An expression for the difference in Gibbs free energy between the superconducting and normal states of a moderately small superconductor, in terms of jr, the wave function of the superconducting electrons, has been applied to experimental observations on the transitions of tin whiskers (Bibby, Nabarro, McLachlan & Stephen, 1974 preceding paper) and fails if the coefficient of the term in rjr6 is negative at the Landau critical point. It is suggested that a term of order ifr6 be incorporated, with a positive coefficient. It is shown that this gives rise to a transition from the normal state to a superconducting state with a finite value of ^ at a point (H", T"), denoted the hypo-critical point since it occurs at a lower field than does the Landau critical point. The phase boundary describing this transition branches at a finite angle from that describing the second order transition at lower fields, the ratio of these slopes, for a cylinder in parallel field, being less than or equal to 1.32.