Theory of strongly phase fluctuating d-wave superconductors and the spin response in underdoped cuprates

General theory of d-wave quasiparticles coupled to phase fluctuations of superconducting order parameter is discussed. In the charge sector the superfluid density is found to conform to Uemura scaling, and the charge renormalization factor to decrease with underdoping. The spin susceptibilty exhibits four distinct regimes with increasing frequency and scales with the superconducting Tc, as observed.