In Plane Resistivity Anisotropy in iron Chalcogenides*

FeTe suffers a bicolinear antiferromagnetic ordering, with a ($\frac{1}{2}$ 0) ordering wave-vector, in contrast to the ($\frac{1}{2}$ $\frac{1}{2}$) ordering wave-vector found in underdoped ``122'' and ``1111'' iron pnictides. At the optimal doping the static ($\frac{1}{2}$ 0) order disappears and a spin resonance at the ($\frac{1}{2}$ $\frac{1}{2}$) wave-vector emerges. Here we report measurements of the in-plane resistivity anisotropy of single crystals of Fe$_{1+\delta}$Te$_{1-x}$Se$_x$ for underdoped and optimally doped compositions. The underdoped compounds were partially detwinned by applying uni-axial strain along the ($\frac{1}{2}$ 0), revealing a larger resistivity along the antiferromagnetic ordering direction. However, for optimal doping uni-axial strain induces the largest resistivity anisotropy along the ($\frac{1}{2}$ $\frac{1}{2}$) direction, similar to the ``122'' family of compounds. This behaviour suggests that in addition to the presence of spin resonance, a divergent nematic susceptibility might be a key feature associated with optimal doping in iron based superconductors.