D charge-density-wave versus superconducting phase fluctuations in underdoped high-Tc superconductors

The underdoped region in high-T-c superconductors is characterized by an increase of the d-wave gap and a decrease of the superconducting transition temperature T with decreasing doping, Two effects may cause such a behavior in the t - J model. On the one hand, superconductivity competes and may also coexist with a d charge-density-wave state. We discuss this effect by studying the phase diagram of the t - J model at large N (N is the number of internal degrees of freedom), the gap structure in the coexisting region and the resulting c-axis tunneling spectra. On the other hand. we give rather general arguments in favor of a strong reduction of the phase stiffness near a Mott transition and calculate the resulting drop in T-c at low dopings due to thermal phase fluctuations.