Doping dependence of superconducting parameters in HgBa₂CuO_4+δ single crystals

Torque magnetization measurements on HgBa2CuO4+δ microcrystals with different oxygen doping δ were performed in order to extract the doping dependence of the effective mass anisotropy γ = √mc*/mab*, of the in-plane penetration depth λab, and of the in-plane coherence length ξab. The anisotropy γ increases monotonically with decreasing δ, suggesting a smooth 3D - 2D crossover in the superconducting properties on reducing the oxygen content. Within the framework of a bipolaronic model this increase can be explained by an increase of the polaron binding energy with reduced doping. © 1999 Plenum Publishing Corporation.