Phase fluctuations and the pseudogap in YBa2Cu3Ox

The thermodynamics of the superconducting transition is studied as a functi on of doping using high-resolution expansivity data of YBa2Cu3Ox single cry stals and Monte Carlo simulations of the anisotropic 3D-XY model. We direct ly show that T-c of underjoped YBa2Cu3Ox is strongly suppressed from its me an-field value (T-C(MF)) by phase fluctuations of the superconducting order parameter. For overdoped YBa2Cu3Ox fluctuation effects are greatly reduced and T-c approximate to T-c(MF). We find that T-c(MF) exhibits a similar do ping dependence as the pseudogap energy, naturally suggesting that the pseu dogap arises from phase-incoherent Cooper pairing.