Universal zero-frequency Raman slope in a-wave superconductor

It is known that for a superconductor of a nodal gap, the in-plane microwave conductivity saturates at low temperatures to a universal value independent of the impurity concentration. We demonstrate that a similar feature can be accessed using channel-dependent Raman scattering. For a (Formula presented)-wave superconductor, the slope of low-temperature Raman intensity at zero frequency is found to be universal in the (Formula presented) and (Formula presented) channels, but not in the (Formula presented) channel. Moreover, universal Raman slopes are sensitive not only to the existence of a node, but also to different pairing states and should allow one to distinguish between such pairing states.