Boundary conditions for an 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 a two-component order parameter and is compared with the case of accidental degeneracy. It is shown that for both cases the 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 film thickness. If for a massive sample there is another reason for the existence of a kink, the interplay of the reasons may lead to a specific behavior of a kink point location as a function of temperature or film thickness. A test is suggested permitting to distinguish between ${\mathit{E}}_1$ and ${\mathit{E}}_2$ types of pairing in a hexagonal superconductor near ${\mathit{T}}_{\mathit{c}}$.

• Received 22 February 1995

DOI:https://doi.org/10.1103/PhysRevB.52.10344