Tuning the superconducting dome of Ba(Fe_1-xCo_x)₂As₂ with symmetric and antisymmetric strain

We present comprehensive doping dependence in Ba(Fe_1-xCo_x)₂As₂ of superconducting T_c versus strain for uniaxial stress along both [100] and [110] crystallographic axes.  The response for stress along [100] is dominated by a linear response to the symmetric A_1g strain, demonstrating a systematic doping dependence suggesting that A_1g strain and composition act similarly as tuning parameters across the whole superconducting dome.  Moreover, this linear response becomes very large in the heavily overdoped region of the phase diagram; for x = 0.124, 1% of strain modulates T_c by 30%.  In contrast, the behavior for stress along [110] demonstrates a nonlinear response to the antisymmetric B_2g strain in addition to the linear A_1g contribution.  This nonlinear contribution diverges towards optimal doping where superconductivity is insensitive to A_1g strain but extremely sensitive to the anisotropic B_2g strain enhancing nematic order.  Additionally, we identify a qualitative difference between the two types of strain-tuned transitions; B_2g strain significantly broadens the transition while A_1g strain does not.