Doping dependence of superconducting parameters in HgBa2CuO4+delta single crystals

Torque magnetization measurements on HgBa2CuO4+delta microcrystals with dif ferent oxygen doping delta were performed in order to extract the doping de pendence of the effective mass anisotropy gamma = root m(c)*/m(ab)*, of the in-plane penetration depth lambda(ab), and of the in-plane coherence lengt h xi(ab). The anisotropy gamma increases monotonically with decreasing delt a, 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 energ y with reduced doping.