using dark-field X-ray microscopy*

To gain a better understanding of the nematic phase in iron pnictides, we study underdoped Ba(Fe_1-xCu_x)₂As₂ (Cu-Ba122) using dark-field X-ray microscopy (DFXM). DFXM is a novel technique which enables one to image the real-space distribution of a selected diffraction peak emanating from a domain within the bulk of a sample. Cu-Ba122 undergoes a tetragonal-to-orthorhombic structural phase transition at low temperature due to electronic nematicity, at which point orthorhombic twin domains form along the ab-plane. By imaging a single domain, we observed (1) micron-scale periodic spatial modulations of diffraction intensity and strain within a domain, and (2) an increase in the amplitude and period of the spatial modulations as temperature is lowered.

These observations demonstrate the existence of a mesoscopic strain wave within the orthorhombic phase of iron pnictides that has so far been undetected in traditional X-ray diffraction due to its long wavelength. Our results also establish DFXM as a powerful new tool to probe the mesoscopic behavior of strongly-correlated quantum materials.