Anisotropic thermodynamic and transport properties of single-crystalline Ba1-xKxFe2As2 (x=0 and 0.45)

Single crystals of BaFe2As2 and Ba0.55K0.45Fe2As2 have been grown out of excess Sn with 1% or less incorporation of solvent. The crystals are exceptionally micaceous, are easily exfoliated, and can have dimensions as large as 3x3x0.2 mm(3). The BaFe2As2 single crystals manifest a structural phase transition from a high-temperature tetragonal phase to a low-temperature orthorhombic phase near 85 K and do not show any sign of superconductivity down to 1.8 K. This transition can be detected in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In the Ba0.55K0.45Fe2As2 single crystals this transition is suppressed and instead superconductivity occurs with a transition temperature near 30 K. Whereas the superconducting transition is easily detected in resistivity and magnetization measurements, the change in specific heat near T-c is small, but resolvable, giving Delta C-p/gamma T-c approximate to 1. The application of a 140 kOe magnetic field suppresses T-c by only similar to 4 K when applied along the c axis and by similar to 2 K when applied perpendicular to the c axis. The ratio of the anisotropic upper critical fields, gamma=H-c2(perpendicular to c)/H-c2(parallel to c), varies between 2.5 and 3.5 for temperatures down to similar to 2 K below T-c.