Effect of layer thickness on the superconducting properties in ultrathin Pb films

Abstract

Recently, superconductivity was found in one atomic layer of Pb film, promising a new field of research where superconductors can be studied on the atomic level. In the presented paper, we report a theoretical study of the superconductivity in ultrathin Pb films consisting of five to ten monolayers. Using the strong coupling Eliashberg formalism we reproduced the experimental values of critical temperature (TC) and we estimated the superconducting energy gap (), thermodynamic critical field (HC) and the specific heat jump at critical temperature () for a wide range of film thicknesses. In these systems, we found an oscillatory behaviour of the above thermodynamic properties modulated by quantum size effects. Moreover, the large values of and , and the small values of prove that the thermodynamic properties of Pb films cannot be correctly described using the Bardeen–Cooper–Schrieffer (BCS) theory of superconductivity due to the strong coupling and retardation effects.