Microscopic interpretation of the Dynes formula for the tunneling density of states

Abstract

Excellent fits of the tunneling density of states in disordered superconductors can be often achieved making use of the phenomenological Dynes formula. However, no consistent derivation of this formula has been available so far. The Dynes formula can be interpreted by the simplest causal frequency-dependent gap function $\Delta(\omega)$ with a vanishing gap at the Fermi level. Here we show, within the coherent potential approximation, that precisely such gap function describes superconductors with a Lorentzian distribution of pair-breaking fields and arbitrary potential disorder. We predict spectral and thermodynamic properties of such superconductors.