Properties of de-twinned iron-arsenide SrFe$_2$As$_2$

The iron-pnictides, AFe$_2$As$_2$ (A = alkali earth metal), undergo a tetragonal to orthorhombic structural transition below $T_s$, which is in the range between 130~K to 210~K depending on the compound. To release elastic deformation the orthorhombic phase is spontaneously divided into four degenerate, equally populated, structural domains. This makes any measurements of in-plane anisotropy extremely difficult. Unlike high$-T_c$ Y-Ba-Cu-O which is orthorhombic already at room temperature, detwinning of pnictides is more difficult, because of lower $T_s$. We developed a technique of mechanical de-twinning of these materials that allows transport, x-ray and direct optical measurements [1]. Here we report polarized light microscopy, synchrotron X-ray analysis and AC transport measurements on SrFe$_2$As$_2$, which represents a clean case of first-order magnetic/structural transition. \\[4pt] [1] M. A. Tanatar, et al. Phys. Rev. B \textbf{81} 184508 (2010).