Pressure effects on the electron-doped high T_c superconductor BaFe_2-xCo_xAs₂

Application of pressure or electron doping through Co substitution into Fe sites transforms the itinerant antiferromagnet BaFe₂As₂ into a superconductor with T_c exceeding 20 K. We carried out systematic transport measurements of BaFe_2-xCo_xAs ₂ superconductors in pressures up to 2.5 GPa, and elucidated the interplay between the effects of electron doping and pressure. For the underdoped sample with nominal composition x = 0.08, application of pressure strongly suppresses a magnetic instability while enhancing T_c by nearly a factor of two from 11 to 21 K. In contrast, the optimally doped x = 0.20 sample shows very little enhancement of T_c = 22 K under applied pressure. Our results strongly suggest that the proximity to a magnetic instability is the key to the mechanism of superconductivity in iron-pnictides.