Doping dependence of superconducting parameters in HgBa₂CuO_4+δ single crystals

Magnetization measurements on HgBa2CuO4+δ microcrystals with different oxygen doping δ were performed using a miniaturized torque sensor. From angular dependent torque measurements 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 was extracted. The anisotropy y increases monotonically with decreasing δ, suggesting a smooth 3D-2D cross-over in the superconducting properties on reducing the oxygen content. The increase of γ may be due to an increase of the effective mass of the charge carriers, which in the framework of a bipolaronic model can be explained by an increase of the polaron binding energy with reduced doping. © 1998 Elsevier Science B.V.