COMPOSITIONAL DEPENDENCE OF TRANSPORT ANISOTROPY IN LARGE (LA, SR)2CUO4 SINGLE-CRYSTALS AND 2ND PEAK IN MAGNETIZATION CURVES

Large single crystals of (La1-xSrx)2CuO4, up to 5 mm along the crystal c-axis, have been grown over a composition range of x = 0.03 to 0.15, covering the whole superconducting region in this system. A complete set of electrical resistivity data along both the c-axis (rho(c)) and ab-plane (rho(ab)) have been obtained via the DC four-probe method for the first time. As x was varied, both resistivities changed monotonically from semiconductive to metallic in character and the critical temperature, T(c), defined by the mid-point of the resistive transition curves, passed through a maximum value of 38.8 K at x = 0.072. The anisotropy ratio rho(c)/rho(ab) at 50 K decreased drastically with increasing x, from nearly 4000 to 160, while it was approximately 350 at x = 0.072. Magnetization hysteresis curves (H parallel-to c, +/- 9 T) revealed the presence of an additional pinning mechanism in the over-doped region (x greater-than-or-equal-to 0.072) giving rise to a second peak at moderately high fields. This was attributed to enhanced flux pinning by oxygen vacancies nominally present in this composition range.