Electromagnetic power absorption by collective modes in unconventional superconductors

Abstract

We calculate the gauge-invariant response of an unconventional ${\mathit{E}}_{1}$ state superconductor to electromagnetic fields in the collisionless limit using a matrix kinetic equation approach. We show that, despite broadening by nodal quasiparticles and screening currents, excitonic modes with collective frequencies of order the superconducting gap frequency should be observable as large, well-defined peaks in the power absorption. Such an observation would be a definitive signal for unconventional superconductivity. We further argue that the effect of collision broadening in real samples should be small at low temperatures.