Phase fluctuations and the pseudogap in YBa2Cu3Ox

The thermodynamics of the superconducting transition is studied as a function of doping using high-resolution expansivity data of YBa2Cu3Ox single crystals and Monte-Carlo simulations of the anisotropic 3D-XY model. We directly show that Tc of underdoped YBa2Cu3Ox is strongly suppressed from its mean-field value (Tc-MF) by phase fluctuations of the superconducting order parameter. For overdoped YBa2Cu3Ox fluctuation effects are greatly reduced and Tc ~ Tc-MF . We find that Tc-MF exhibits a similar doping dependence as the pseudogap energy, naturally suggesting that the pseudogap arises from phase-incoherent Cooper pairing.Comment: 9 pages, 3 Figure

Two Pseudogaps in the Cuprates: Meingast et al. Reply

In classical superconductors Cooper-pair formation and phase coherence occur simultaneously as the temperature is lowered below Tc. In high-temperature superconductors (HTSC), on the other hand, the small superfluid density and low associated phase stiffness of the superconducting condensate are expected to lead to a separation of the Cooper-pair formation and the phase-coherence temperatures, especially in underdoped materials [3]. The only real phase transition in this scenario is the 3d-XY phase-ordering transition at Tc [3,4]. In our Letter [2] we showed that Tc in underdoped and optimally doped YBCO is just such a phase-ordering temperature, and then the question naturally arises - where do the Cooper pairs form? The observed 3d-XY scaling of our thermal expansion data over a wide temperature range [2] suggests that pairing occurs at temperatures considerably above Tc, and it thus appeared quite natural for us to associate the opening of the pseudogap at T*.Comment: 3 pages, 1 Figure (Reply to Comment by R. S. Markiewicz, Phys. Rev. Lett. 89, 229703 (2002

Anomalously large oxygen-ordering contribution to the thermal expansion of untwinned YBa2Cu3O6.95 single crystals: a glass-like transition near room temperature

We present high-resolution capacitance dilatometry studies from 5 - 500 K of untwinned YBa2Cu3Ox (Y123) single crystals for x ~ 6.95 and x = 7.0. Large contributions to the thermal expansivities due to O-ordering are found for x ~ 6.95, which disappear below a kinetic glass-like transition near room temperature. The kinetics at this glass transition is governed by an energy barrier of 0.98 +- 0.07 eV, in very good agreement with other O-ordering studies. Using thermodynamic arguments, we show that O-ordering in the Y123 system is particularly sensitive to uniaxial pressure (stress) along the chain axis and that the lack of well-ordered chains in Nd123 and La123 is most likely a consequence of a chemical-pressure effect.Comment: 4 pages, 3 figures, submitted to PR