Finite-temperature superfluid density in very underdoped cuprates

The combination of a large gap, low transition temperature, and quasi-two-dimensionality in strongly underdoped high-temperature superconductors severely constrains the behavior of the ab-plane superfluid density rho with temperature T. In particular, we argue that the contribution of nodal quasiparticles to rho(T) is essential to account both for the amplitude of, and the recently observed deviations from, the Uemura scaling. A relation between T-c and rho(0) which combines the effects of quasiparticle excitations at low temperatures and of vortex fluctuations near critical temperature is proposed and discussed in light of recent experiments.