Bardasis-Schrieffer polaritons in excitonic insulators

Bardasis-Schrieffer (BaSh) modes in superconductors are fluctuations in subdominant pairing channels, e.g., d-wave fluctuations in an s-wave superconductor. We generalize the notion to excitonic insulators and show that these modes generically occur. In s-wave excitonic insulators, a p-wave BaSh mode exists below the gap energy and has a non-vanishing optical matrix element with light. This mode hybridizes strongly with photons to form Bardasis-Schrieffer polaritons, which are observable in both far-field and near-field optical experiments. We also show that a strong photon pulse induces order parameter rotation on the $s+ip$ plane which results in Thouless charge pumping through the system.

Reference: Z. Sun, A. J. Millis, arxiv:2003.02997.