Torque magnetometry on single-crystal high-temperature superconductors near the critical temperature: A scaling approach

Angular-dependent magnetic torque measurements performed near the critical temperature on single crystals of HgBa2CuO4+y, La2-xSrxCuO4, and YBa2Cu3O6.93 are scaled, following the three-dimensional (3D) XY model, in order to determine the scaling function dG(+/-)(z)/dz which describes the universal critical properties of high-T-c cuprates near T-c. A systematic shift of the scaling function with increasing effective mass anisotropy gamma = root m(c)*/m(a)* is observed, which may be understood in terms of a 3D-2D crossover. Further evidence for a 3D-2D crossover is found from temperature-dependent torque measurements carried out in different magnetic fields at different field orientations delta, which show a quasi-2D crossing region (M*,T*). The occurrence of this crossing phenomenon is explained in a phenomenological way from the weak zi dependence of the scaling function around a value z=z*. The crossing temperature T* is found to be angular dependent. Torque measurements above T-c reveal that fluctuations are strongly enhanced in the underdoped regime where the anisotropy is large, whereas they are less important in the overdoped regime.