Energy gaps and Kohn anomalies in elemental superconductors

The momentum and temperature dependence of the lifetimes of acoustic phonons in the elemental superconductors lead and niobium were determined by resonant spin- echo spectroscopy with neutrons. In both elements, the superconducting energy gap extracted from these measurements was found to converge with sharp anomalies originating from Fermi- surface nesting ( Kohn anomalies) at low temperatures. The results indicate electron many- body correlations beyond the standard theoretical framework for conventional superconductivity. A possible mechanism is the interplay between superconductivity and spin- or charge- density- wave fluctuations, which may induce dynamical nesting of the Fermi surface.