Anisotropic Superconducting Properties and Pressure Effects in 1144-Structure-Type Pnictides*

New Fe-based CaK-1144 superconductors (Fe-SC’s) with T_c ∼ 35 K were recently discovered. We grew phase-pure 1144 superconducting single crystals out of a quaternary molten solution of FeAs, self-flux. The crystal structure is determined by x-ray diffraction. The effects of pressure on T_c and the collapsed tetragonal phase transition are studied. In the superconducting state, temparature dependent magnetization and resistivity were measured under magnetic fields at different angles with respect to the basal plane. Lower critical field (H_c1), superfluid density (ρ_s), critical current densities, and the London penetration depth are extracted from magnetization data. Upper critical field (H_c2) and Irreversible field (H_irr) are determined by resistive transitions along c and ab directions. Various pairing gap functions were fit to the normalized p_s(T) and specific heat data, and are compared with the nodeless two gap s^± wave pairing symmetry reported by µSR and ARPES measurements. Normal-state magnetic susceptibility with characteristic T-dependence are used to discuss the possibility of antiferromagnetic spin fluctuations as the pairing mechanism in 1144 Fe-SC's will be presented.