Upper critical field in the model with finite-range interaction between electrons

We develop a theory of the upper critical field in a BCS superconductor with a nonlocal interaction between electrons. We have shown that the nonlocal interaction is characterized by the parameter k_Fρ₀ , where k_F is the Fermi momentum and ρ₀ is the radius of electron-electron interaction. The presence of the external magnetic field leads to the generation of additional components of the order parameter with different angular momenta. This effect leads to the enhancement of the upper critical field above the orbital limiting field. In addition the upward curvature in the temperature dependence of H_{c 2}(T ) in the clean limit is predicted. The impurity scattering suppresses the effect in the dirty limit.