Precision Controlled Detwinning of Orthorhombic Twin Domains in Underdoped BaFe₂As₂

Underdoped BaFe2As2 exhibits an orthorhombic structural phase transition driven by electronic nematic ordering. In the past, the relation between lattice distortion and electronic anisotropy above the phase transition has been studied comprehensively by the technique of elastoresistivity. In contrast, the relation between spontaneous orthorhombicity ((a-b)/(a+b)) and spontaneous resistivity anisotropy ((Ra - Rb )/ (Ra + Rb)) below the phase transition has never been measured to the same level of precision. Several previous detwinning experiments have used samples under a constant uni-axial stress, which results in additional lattice distortion especially close to phase transition. Here we present x-ray diffraction data on a crystal sample in which in-situ tunable uniaxial stress is used to precisely control the detwinning of domains. Lattice constant data showing both the relative domain populations and the elastic response to stress are matched with simultaneous resistivity measurements to yield a precise determination of the domain resistivity anisotropy. Elastoresistivity tensor coefficients are also computed in the fully detwinned single domain state, which cannot be inferred from macroscopic strain measurements alone.