Imaging of doped iron pnictides across a structural phase transition

The emergent anisotropy through a structural-phase transition in an iron pnictide single crystal of Ba(Fe$_{0.987}$Au$_{0.012})_{2}$As$_{2}$ was studied using polarized laser light microscopy. The undoped parent of BaFe$_{2}$As$_{2}$'s crystal structure distorts from tetragonal to orthorhombic at the structural phase transition temperature $T_{S}$, which coincides with an antiferromagnetic transition, causing the formation of structural domains that can be observed as stripes across the sample. For Ba(Fe$_{0.987}$Au$_{0.012})_{2}$As$_{2}$, however, $T_{S}=$108 K and the Neel temperature $T_{N}=$100 K. We studied the disappearance of these domains as the sample was heated across these transitions. Images of the sample were taken using a defocused laser beam through fully crossed polarizers. The images were analyzed by aligning and averaging groups of images to reduce noise, by taking the difference of the images above and below $T_{S}$ to isolate the stripes from the background, and by using Fourier transformations and comparing them to those of simulated striped patterns.