Anisotropic thermodynamic and transport properties of single-crystalline Ba_1−xK_xFe₂As₂ (x=0 and 0.45)

Single crystals of BaFe₂As₂ and Ba_0.55K_0.45Fe₂As₂ 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 3×3×0.2 mm³. 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 Ba_0.55K_0.45Fe₂As₂ 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 ΔC_p/γT_c≈1. The application of a 140 kOe magnetic field suppresses T_c by only ∼4 K when applied along the c axis and by ∼2 K when applied perpendicular to the c axis. The ratio of the anisotropic upper critical fields, γ=H_c2^⊥c/H_c2^∥c, varies between 2.5 and 3.5 for temperatures down to ∼2 K below T_c.