Signatures of Bardasis-Schrieffer mode excitation in third-harmonic generated currents

Müller, M.A. and Eremin, I.M.

Volume: 104 Pages:
DOI: 10.1103/PhysRevB.104.144508
Published: 2021

We theoretically analyze collective modes in unconventional superconductors focusing on the Bardasis-Schrieffer (BS) mode and its contribution to the third-harmonic generation currents. Starting from a model with competing superconducting pairing instabilities we add fluctuations of the fields beyond the saddle point approximation and calculate their response to a driving electric field. To model phase fluctuations appropriately we take into account the effect of the long-range Coulomb interaction. While the phase mode is pushed into a plasmon frequency, as known from the literature, we show that the BS mode remains unaffected. Furthermore, it has a characteristic polarization dependence and, unlike the Higgs mode, generates a current in the perpendicular direction to the applied field. We find that the Bardasis-Schrieffer excitations contribute a sizable signal to the third-harmonic generated current, which is clearly distinguishable from the charge density fluctuations due to Cooper-pair-breaking effects and can be straightforwardly detected in experiment. © 2021 American Physical Society.

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