D charge‐density‐wave versus superconducting phase fluctuations in underdoped high‐ T c 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 c 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. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)