Uniaxial-strain mechanical detwinning of CaFe2As2 and BaFe2As2 crystals: Optical and transport study

The parent compounds of iron-arsenide superconductors, AFe2As2 (A=Ca, Sr, Ba), undergo a tetragonal to orthorhombic structural transition at a temperature TTO in the range 135-205 K depending on the alkaline-earth element. Below TTO the free standing crystals split into equally populated structural domains, which mask intrinsic, in-plane, anisotropic properties of the materials. Here we demonstrate a way of mechanically detwinning CaFe2As2 and BaFe2As2. The detwinning is nearly complete, as demonstrated by polarized light imaging and synchrotron x-ray measurements, and reversible, with twin pattern restored after strain release. Electrical resistivity measurements in the twinned and detwinned states show that resistivity, ρ, decreases along the orthorhombic ao axis but increases along the orthorhombic bo axis in both compounds. Immediately below TTO the ratio ρbo/ρao = 1.2 and 1.5 for Ca and Ba compounds, respectively. Contrary to CaFe2As2, BaFe2As2 reveals an anisotropy in the nominally tetragonal phase, suggesting that either fluctuations play a larger role above TTO in BaFe2As2 than in CaFe2As2 or that there is a higher temperature crossover or phase transition.