Hysteretic behavior in the optical response of the underdoped Fe-arsenide Ba( Fe 1 − x Co x ) 2 As 2 in the electronic nematic phase

We use a new technique that allows in situ variation of uniaxial stress to probe the polarization dependence of the optical reflectivity of the representative underdoped iron-arsenide $\text{Ba(}{\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x}{\text{)}}_{2}{\text{As}}_{2}$ through the tetragonal-to-orthorhombic structural transition and with respect to their electronic nematic phase. These measurements reveal a hysteretic behavior of the anisotropic optical response to uniaxial stress in the orthorhombic state associated with twin boundary motion, whereas the pressure-induced anisotropy of the optical response is reversible in the tetragonal state.