Homes scaling and BCS

It is argued on the basis of the BCS theory that the zero-$T$ penetration depth satisfies $\lambda^{-2}(0)\propto\sigma T_c$ ($\sigma$ is the normal state dc conductivity) not only in the extreme dirty limit $\xi_0/\ell \gg 1$, but in a broad range of scattering parameters down to $\xi_0/\ell \sim 1$ ($\xi_0$ is the zero-$T$ BCS coherence length and $\ell$ is the mean-free path). Hence, the scaling $\lambda^{-2}(0)\propto\sigma T_c$, suggested as a new universal property of superconductors,\cite{Homes} finds a natural explanation within the BCS theory.