Anisotropic thermodynamic and transport properties of single crystalline (Ba1-xKx)Fe2As2 (x = 0 and 0.45)
Abstract
Single crystals of BaFe2As2 and (Ba0.55K0.45)Fe2As2 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 x 3 x 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 (Ba0.55K0.45)Fe2As2 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 Tc is small, but resolvable, giving Cp/γ Tc ≈ 1. The application of a 140 kOe magnetic field suppresses Tc 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 = Hc2 c / Hc2\| c, varies between 2.5 and 3.5 for temperatures down to 2 K below Tc.