If a superconductor has a Raman-active phonon mode of low frequency (larger but of the order of magnitude of the superconducting energy gap $2\Delta$, we show that electron-phonon coupling leads to a complex bound excitation, also Raman active, with a discrete frequency lower than $2\Delta$ and with intensity which can be appreciable. We propose that these are the lines found by Sooryakumar and Klein at approximately the energy-gap frequency in super-conducting $2H$-Nb${\mathrm{Se}}_2$.

• Received 24 March 1980

DOI:https://doi.org/10.1103/PhysRevLett.45.662