The Upper Critical Field for Unconventional Superconducting Film: a Kink due to the Boundary Conditions

Abstract

Boundary conditions for unconventional superconducting order parameter are obtained on the basis of a microscopic theory. The upper critical field in a superconducting film is examined for unconventional superconductors with two-component order parameter and is compared with the case of accidental degeneracy. It is shown, that for both cases temperature dependences of the upper critical field in a superconducting film may depend substantially on the quality of boundaries, and under certain conditions have a kink due to the influence of film boundaries. The location of a kink point appears to be dependent on a film thickness. If for a massive sample there is another reason for the existance of a kink, the interplay of the reasons may lead to a specific behaviour of a kink point location as a function of a temperature or a film thickness. A new test is suggested permitting to distinguish between $E_{1}$ and $E_{2}$ types of pairing in a hexagonal superconductor near $T_c$.