In-phase and out-of-phase Bardasis–Schrieffer modes from the electronic Raman response of multiband superconductors

It is known that non-resonant electronic Raman spectroscopy can be used to probe collective modes of a superconductor in different angular momentum symmetry channels by tracking the location of spectral resonances in distinct scattering geometries. What is less appreciated is the information contained in the details of spectral line profiles themselves. To demonstrate this point, we discuss the Raman spectrum for a two-band 2D superconductor. Due to the multiband nature of the system, sub-leading interactions could lead to multiband Bardasis–Schrieffer collective modes resulting from in-phase and out-of-phase fluctuations, analogous to the Nambu–Goldstone and Leggett modes in the leading channel. We show that the Raman probe couples to these modes in a characteristically distinguishable manner and that these differences manifest themselves through the suppression or enhancement of 2Δ features relative to the collective mode features in the spectrum. Since the formation of collective modes can be directly related to the sign of the components of the pairing interaction in the sub-leading channel, one is able to deduce phase-sensitive information about the pairing interaction from the Raman responsez