Synthesis from separate oxide targets of high quality La2−xSrxCuO4 thin films and dependence with doping of their superconducting transition width

A series of superconducting La2−xSrxCuO4 thin films, with 0.09 x 0.22, is grown over (100)SrTiO3 substrates by means of a novel pulsed laser deposition method devised to increase the homogeneity and control of doping. We employ two separate parent oxide targets that receive ablation shots at arbitrary computer-controlled relative rates, instead of the conventional procedure that uses a single target whose doping determines the one of the film. We characterize the films both through conventional techniques (XRD, SEM, AFM and EDX) and by measuring their superconducting transition with a high-sensitivity SQUID magnetometer. The latter allows one to determine not only their average critical temperatures but also their dispersions due to inhomogeneities, ΔTc(x). For we obtain the conventional parabolic law centered at x = 0.16, plus a Gaussian depression near x = 1/8 with a -height of about 5 K and x-width about 0.03. For ΔTc(x) we obtain, for all the dopings, values among the lowest reported up to now for La2−xSrxCuO4. The ΔTc(x) dependence can be explained in terms of the unavoidable randomness of the positioning of the Sr ions (the so-called intrinsic chemical inhomogeneity) and a separate residual Tc-inhomogeneity contribution of the order of 0.5 K, this last associated with the samples' structural inhomogeneities and films' substrate.