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 $k_F{\rho }_0$, where $k_F$ is the Fermi momentum and ${\rho }_0$ 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 $H_{c2}\left(T\right)$ in the clean limit is predicted. The impurity scattering suppresses the effect in the dirty limit.

• Received 2 August 2016
• Revised 10 October 2016

DOI:https://doi.org/10.1103/PhysRevB.94.174506