GAUGE-INVARIANT RESPONSE OF A SUPERCONDUCTOR WITH DX(2)-Y(2) SYMMETRY- APPLICATION TO ELECTRONIC RAMAN-SCATTERING

A gauge-invariant theory for response functions in a superconductor wi th d(x2-y2) symmetry is presented. Using both a diagrammatic and a kin etic equation approach, the gauge invariance of the theory is explicit ly shown and the positions of the collective modes are identified. Par ticular attention is paid to Raman scattering as a probe of the anisot ropic superconducting state. It is shown that the Raman response is st rongly polarization dependent which is reflected in the peak position and the low frequency and temperature behavior of the response. Surpri sing agreement is reflected in the experimental results for the cuprat e superconductors.