Scaling relation for the superfluid density of cuprate superconductors: Origins and limits

A universal scaling relation, rho(s)proportional to sigma(T-c)xT(c) has been reported by Homes [Nature (London) 430, 539 (2004)] where rho(s) is the superfluid density and sigma(T) is the dc conductivity. The relation was shown to apply to both c-axis and in-plane dynamics for high-T-c superconductors as well as to the more conventional superconductors Nb and Pb, suggesting common physics in these systems. We show quantitatively that the scaling behavior has several possible origins, including marginal Fermi-liquid behavior, Josephson coupling, dirty-limit superconductivity, and unitary impurity scattering for a d-wave order parameter. However, the relation breaks down seriously in overdoped cuprates, and possibly even at lower doping.