Factors affecting the Optimal Design of High-Tc Superconductors - the Pseudogap and Critical Currents

The impact of the normal-state pseudogap, present in all optimal and underdoped HTS cuprates, on critical currents and critical temperature is surveyed. With the opening of the pseudogap around a doping state of p=0.19 the condensation energy and superfluid density are rapidly suppressed due to reduction in the normal-state spectral weight. Even by optimal doping (p=0.16) these measures of the 'strength' of superconductivity are diminished by up to 40%. This results in a sharp reduction in critical currents and irreversibility field, respectively. The optimal doping state where these properties are maximised is therefore not at maximum Tc but in the lightly overdoped region where the pseudogap energy falls to zero at p=0.19. The presence of impurities and grain boundaries further heightens these effects.