Intertwined Dislocation Grids and Nematic Domains in an Iron-Based Superconductor Thin Film

A dislocation is a topological defect in a crystal lattice and can be associated with many functional properties. Dislocations can spontaneously form in epitaxial thin films as a strain relief mechanism. Here, we synthesize an iron-based superconductor thin film FeSe epitaxially on the SrTiO₃ substrate. Using scanning tunneling microscopy, we discover a structural modulation network, corresponding to a grid of edge dislocations at the film/substrate interface. We observe a striking change of the orientation of the dislocation lines as a function of the film thickness. Interestingly, since the edge dislocation grid gives rise to a spacially-varying uniaxial strain field, the formation of the nematic domains in FeSe is intimately intertwined with the dislocation grid. We further analyze their relationship by extracting the strain maps from the atomically-resolved topographs. Our finding provides an unexpected example of the formation and evolution of a dislocation grid in an epitaxial film, which is closely tied to the emergent nematic order of the iron-based superconductor FeSe.