The long-wavelength amplitude mode of the charge-density-wave state modulates the average density of states at the Fermi surface leading to a coupling between it and the order parameter for superconductivity. This is shown, in a gauge-invariant theory, to lead to a bound state below twice the BCS gap. Our results are discussed in the context of recent experiments on $2H$-Nb${\mathrm{Se}}_2$.

• Received 15 June 1981

DOI:https://doi.org/10.1103/PhysRevLett.47.811