In-plane structural and electronic anisotropy in de-twinned (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$

The iron-pnictides undergo a tetragonal to orthorhombic structural transition below a doping - dependent temperature $T_s$. In the absence of external stress or strain, the orthorhombic phase is divided into four degenerate, equally populated, ``twin'' structural domains, obscuring direct measurement of in-plane anisotropy. This degeneracy may be broken through mild mechanical stress or strain leaving the sample de-twinned. The properties of detwinned (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ with x=0.1, 0.18 (hole under-doped) were discussed previously [1]. Here we report polarized-light microscopy and AC transport measurements of strain-detwinned (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ with a dopping range from x=0.15 to x=0.35. Our results provide new insight into a region of coexisting magnetic and superconducting order parameters. \\[4pt] [1] J. J. Ying, et al. Phys. Rev. Lett. \textbf{107} 067001 (2011).