Upper critical field in noncentrosymmetric superconductors

We calculate the upper critical field in superconductors without inversion symmetry at arbitrary temperatures in the presence of scalar impurities. Both orbital and spin (paramagnetic) mechanisms of pair breaking are considered. The superconducting phase transition at nonzero field occurs into a helical vortex state, in which the order parameter is modulated along the applied field (in a cubic crystal). The helical state is stable with respect to disorder. However, if the difference between the densities of states in the electron bands that are split by spin-orbit coupling is neglected, then the order-parameter modulation is present only at low temperatures, and only if the system is sufficiently clean and paramagnetically limited. This state resembles the Larkin-Ovchinnikov-Fulde-Ferrell state in centrosymmetric superconductors.