Effects of Co substitution on thermodynamic and transport properties and anisotropic H_c2 in Ba (Fe_1-xCo_x) ₂As₂ single crystals

Single crystalline samples of Ba (Fe_1-xCo_x) ₂As₂ with x<0.12 have been grown and characterized via microscopic, thermodynamic, and transport measurements. With increasing Co substitution, the thermodynamic and transport signatures of the structural (high-temperature tetragonal to low-temperature orthorhombic) and magnetic (high-temperature nonmagnetic to low-temperature antiferromagnetic) transitions are suppressed at a rate of roughly 15 K/% Co. In addition, for x≥0.038 superconductivity is stabilized, rising to a maximum T_c of approximately 23 K for x≈0.07 and decreasing for higher x values. The T-x phase diagram for Ba (Fe_1-xCo_x)₂As₂ indicates that either superconductivity can exist in both low-temperature crystallographic phases or that there is a structural phase separation. Anisotropic superconducting upper critical-field data [H_c2(T)] show a significant and clear change in anisotropy between samples that have higher temperature structural phase transitions and those that do not. These data show that the superconductivity is sensitive to the suppression of the higher temperature phase transition.