Thermal annealing and pressure effects on BaFe_2−xCo_xAs₂ single crystals

We investigate the pressure and thermal annealing effects on BaFe_2−xCo_xAs₂ (Co-Ba122) single crystals with x  =  0.1 and 0.17 via electrical transport measurements. The thermal annealing treatment not only enhances the superconducting transition temperature (T _c) from 9.6 to 12.7 K for x  =  0.1 and from 18.1 to 21.0 K for x  =  0.17, but also increases the antiferromagnetic transition temperature (T _N). Simultaneous enhancement of T _c and T _N by the thermal annealing treatment indicates that thermal annealing could substantially improve the quality of the Co-doped Ba122 samples. Interestingly, T _c of the Co-Ba122 compounds shows a scaling behavior with a linear dependence on the resistivity value at 290 K, irrespective of tuning parameters such as chemical doping, pressure, and thermal annealing. These results not only provide an effective way to access the intrinsic properties of the BaFe₂As₂ system, but may also shed a light on designing new materials with higher superconducting transition temperature. (paper)