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 with d(x2-y2) symmetry is presented. Using both a diagrammatic and a kinetic equation approach, the gauge invariance of the theory is explicitly shown and the positions of the collective modes are identified. Particular attention is paid to Raman scattering as a probe of the anisotropic superconducting state. It is shown that the Raman response is strongly polarization dependent which is reflected in the peak position and the low frequency and temperature behavior of the response. Surprising agreement is reflected in the experimental results for the cuprate superconductors.