Universal and nonuniversal features of Bardeen-Cooper-Schrieffer theory with finite-range interactions

Abstract: We study analytic solutions to the Bardeen-Cooper-Schrieffer (BCS) gap equation for isotropic superconductors with finite-range interaction potentials over the full range of temperatures from absolute zero to the superconducting critical temperature, $0\leq T\leq T_c$. Using these solutions, $\Delta(\epsilon,T)$, we provide a proof of the universality of the temperature dependence of the BCS gap ratio at the Fermi level, $\Delta(\epsilon=0,T)/T_c$. Moreover, by examining the behavior of this ratio as a function of energy, $\epsilon$, we find that non-universal features emerge away from the Fermi level, and these features take the form of a temperature independent multiplicative factor, $F(\epsilon)$, which is equal to $\Delta(\epsilon,T)/\Delta(\epsilon=0,T)$ up to exponentially small corrections, i.e., the error terms vanish like $\rm{e}^{-1/\lambda}$ in the weak-coupling limit $\lambda\rightarrow 0$. We discuss the model-dependent features of both $F(\epsilon)$ and $T_c$, and we illustrate their behavior focusing on several concrete examples of physically-relevant finite-range potentials. Comparing these cases for fixed coupling constants, we highlight the importance of the functional form of the interaction potential in determining the size of the critical temperature and provide guidelines for choosing potentials which lead to higher values of $T_c$. We also propose experimental signatures which could be used to probe the energy-dependence of the gap and potentially shed light on the underlying mechanisms giving rise to superconductivity.