Symmetric And Antisymmetric Strain As Continuous Tuning Parameters For Electronic Nematic Order

We report the separate response of the critical temperature of the nematic phase transition T-S to symmetric and antisymmetric strains for the prototypical underdoped iron pnictide Ba(Fe0.975Co0.025)(2)As-2. This decomposition is achieved by comparing the response of T-S to in-plane uniaxial stress and hydrostatic pressure. In addition to quantifying the two distinct linear responses to symmetric strains, we find a quadratic variation of T-S as a response to antisymmetric strains epsilon(B1g) = 1/2(epsilon(xx) - epsilon(yy)), exceeding the nonlinear response to symmetric strains by at least two orders of magnitude. These observations establish orthogonal antisymmetric strain as a powerful tuning parameter for nematic order.