Current-enabled optical conductivity of superconductors

In most superconductors, optical excitations require impurity scattering or the presence of multiple bands. This is because in clean single-band superconductors, the combination of particle-hole and inversion symmetries prevents momentum-conserving transitions. In this work we show how the flow of supercurrent can lead to new contributions to optical conductivity. As supercurrent breaks inversion symmetry, transitions across the superconducting gap become allowed even in clean superconductors and dominate over impurity-induced contributions for energies comparable to the gap width. The response is dependent on the nature of the underlying normal state as well as on the type of superconducting order. Through use of the screening supercurrent with controllable magnitude and direction, that arises from an external magnetic field, this enables a detailed investigation of the superconducting state and possible gap symmetry determination in unconventional superconductors for which other techniques have not been practicable.