Enhancement of broken-time-reversal-symmetry pairing state in d-wave superconductors with nonmagnetic impurities

The phase diagrams of a superconductor with dominant d-wave and minor s-wave pairing interactions are obtained with the self-consistent gap equations. A possible broken-time-reversal-symmetry s+id state in an appropriate interaction/temperature range is shown. We point out that a minor s-wave pairing interaction normally cannot compete with the dominant d-wave pairing so that the bulk state usually has a pure d-wave symmetry. Moreover, we show that the possibility to observe the s+id state in a bulk system can be enhanced greatly if nonmagnetic impurity scatterings are introduced. Also interestingly, the amplitude of d-wave order parameter in the s+id phase is shown to decrease with lowering temperature.