Superconductivity at 23 K and low anisotropy in Rb-substituted BaFe2As2 single crystals

Single crystals of Ba1-xRbxFe2As2 with x=0.05-0.1 have been grown from Sn flux and are bulk superconductors with T-c up to 23 K. The crystal structure was determined by x-ray diffraction analysis, and Sn is found to be incorporated for similar to 9% Ba, shifted by similar to 1.1 A away from the Ba site toward the (Fe2As2) layers. The upper critical field deduced from resistance measurements is anisotropic with slopes of 7.1(3) T/K (H parallel to ab plane) and 4.2(2) T/K (H parallel to c axis), sufficiently far below T-c. The extracted upper critical field anisotropy gamma(H)similar to 3 close to T-c is in good agreement with the estimate from magnetic torque measurements. This indicates that the electronic properties in the doped BaFe2As2 compound are significantly more isotropic than those in the LnFeAsO family. The in-plane critical current density at 5 K exceeds 1x10(6) A/cm(2), making Ba1-xRbxFe2As2 a promising candidate for technical applications.