ORDER-PARAMETER MIXING EFFECT IN THE FULDE-FERRELL STATE

It is shown that when the triplet pairing interaction is finite in the singlet pairing superconductor, the Fulde-Ferrell (FF) state is significantly enhanced by the mixing effect which is inherent to nonuniform superconducting states in a strong magnetic field. The mixed FF state can exist at higher temperatures than the pure singlet FF state. This may explain the behavior of the recently observed high-field phase of the heavy fermion superconductor UPd2Al3. The phase diagram of the second-order phase transition is given for a simple model of s-wave-p-wave mixing state.