Thermal expansion and anisotropic pressure derivatives of Tc in Ba(Fe1-xCox)2As2 single crystals

Heat capacity and anisotropic thermal expansion were measured for Ba(Fe1-xCox)(2)As-2 (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(Fe0.962Co0.038)(2)As-2 and Ba(Fe0.926Co0.074)(2)As-2, with the opposite sign of this anisotropy.