Data from: Discovery of spontaneous mesoscopic strain waves in nematic domains using dark-field X-ray microscopy

Electronic nematic order arises when correlated electrons spontaneously
break the rotational symmetry of a crystal lattice. When electronic
nematic order couples bilinearly to symmetry-breaking lattice strain, both
appear together at a single ferroelastic phase transition, producing
structural twin domains with distinct orientations of the nematic
director. While the effects of applied strain on these domains are well
established, the intrinsic behavior of spontaneous subdomain strain fields
has remained unexplored. Here, we report the discovery of spontaneous
mesoscopic strain waves within individual nematic domains of an iron-based
superconductor, observed using dark-field X-ray microscopy (DFXM). Using
this novel full-field imaging technique, we visualize subdomain strain
modulations emerging concurrently with nematic order. Elastic
compatibility relations governing inhomogeneous strains provide a natural
mechanism for the strain waves. Our findings reveal a broadly relevant
form of strain self-organization and position DFXM as a powerful probe of
the local interplay between lattice strain and electronic order.