Linear optical response from the odd-parity Bardasis-Schrieffer mode in locally non-centrosymmetric superconductors

On the recent report of a field-induced first order transition in the superconducting state of CeRh2As2, which is a possible indication of a parity-switching transition of the superconductor, the microscopic physics is still under investigation. However, if two competing paring channels of opposite parities do exist, a particle-particle collective mode referred to as the Bardasis-Schrieffer (BS) mode should generically exist below the pair-breaking continuum. The BS mode of the CeRh2As2 superconductor can couple to the light, as it arises from a pairing channel with the parity opposite to that of the superconducting condensate. Here, by using a generic model Hamiltonian we carry out a qualitative investigation on the excitation energy of the BS mode with respect to the out-of-plane magnetic fields and its contribution to the optical conductivity. Our findings indicate that the distinct coupling between the BS mode and the light can serve as evidence for the competing odd-parity channels of CeRh2As2 and other locally non-centrosymmetric superconductors. Superconductors with odd-parity Cooper pairs are rare and their experimental confirmation is significantly challenging. In the CeRh2As2 superconductor, the authors' investigation reveals that the presence of competing pairings with opposite parities gives rise to a unique collective mode, which can be observed through the optical response.