Universal zero-frequency Raman slope in a d-wave superconductor

TL;DR

This paper reveals that the low-temperature Raman intensity slope at zero frequency exhibits channel-dependent universality in d-wave superconductors, enabling differentiation of pairing states beyond microwave conductivity measurements.

Contribution

It introduces the concept of universal zero-frequency Raman slopes in d-wave superconductors and shows their sensitivity to pairing states, unlike microwave conductivity.

Findings

Universal Raman slopes are found in the $A_{1g}$ and $B_{2g}$ channels.

These slopes are sensitive to different pairing states.

The $B_{1g}$ channel does not exhibit universality.

Abstract

It is known that for an unconventional superconductor with nodes in the gap, the in-plane microwave or dc 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. It is found that, for a $d_{x^2-y^2}$-wave superconductor, the slope of low-temperature Raman intensity at zero frequency is universal in the $A_{1g}$ and $B_{2g}$ channels, but not in the $B_{1g}$ channel. Moreover, as opposed to the microwave conductivity, 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.