Interface potential for nucleation of a superconducting layer

The effective interface potential V for nucleation of a superconducting surface layer in superconductors is derived as a function of the expelled magnetic moment M. We show that the associated variational problem with integral constraint is soluble, in contrast with the case of adsorbed fluids, where a similar constraint poses serious difficulties. Minimalization of the effective potential V(M) gives the location of the nucleation phase transition. Depending on the temperature or other parameters this transition can be of second order and can go over in a first-order transition through a tricritical point on the nucleation line, in agreement with previous numerical work on the same phenomenon.