Thermal expansion and anisotropic pressure derivatives of T_c in Ba(Fe_1−xCo_x)₂As₂ single crystals

Heat capacity and anisotropic thermal expansion were measured for Ba(Fe_1−xCo_x)₂As₂ (x=0,0.038,0.074) single crystals. Thermal expansion is anisotropic and, in tetragonal phase, is significantly higher along the c axis. Previously reported phase transitions, including possibly split structural and magnetic for x=0.038, are clearly seen in both measurements. Uniaxial pressure derivatives of the superconducting transition temperature inferred from the Ehrenfest relation have opposite signs for in-plane and c-axis pressures for both Ba(Fe_0.962Co_0.038)₂As₂ and Ba(Fe_0.926Co_0.074)₂As₂, with the opposite sign of this anisotropy.