Anisotropic nature of superconductivity in hexagonal rubidium tungsten bronze Rb0.23WO3+δ

Low-field M(H), transport, and magnetization measurements have been performed on hexagonal rubidium tungsten bronze Rb0.23WO3+(delta). Various thermodynamic parameters, such as H-c1(0), H-c2(0), lambda(ab)(0), lambda(c) (0), (0), anisotropy parameter gamma, and Ginzburg-Landau parameter kappa are reported. The irreversibility line H-irr(T) follows (1 - T/T-c)(n) with exponent n = 1.51 for sample with delta similar to -0.10 and 2.11 for sample with delta similar to 0.02, indicating that the vortex dimensionality of Rb0.23WO2.90 is anisotropic three-dimensional (31)) and that of Rb0.23WO3.02 is quasi two-dimensional (21)). In addition, the magnetization curves in high-field regime near T, collapse onto a universal curve 4 pi M/(TH)(beta) =F[A((T-T-c(H))/(TH)(beta))] with exponent beta = 2/3 for sample with delta similar to -0.10 and 1/2 for sample with delta similar to 0.02, indicative of an anisotropic 3D and a quasi-2D scaling behavior for Rb0.23WO2.90 and Rb0.23WO3.02, respectively. The observed results strongly suggest that oxygen content has a significant influence on the nature of the superconducting properties of Rb0.23WO3+delta.