Nanoscale decoupling of electronic nematicity and structural anisotropy in FeSe thin films*

In a material prone to a nematic instability, anisotropic strain provides a preferred symmetry-breaking direction for the electronic nematic state to follow. Here we discover that electronic nematicity can be locally decoupled from the underlying structural anisotropy in strain-engineered thin films of iron-selenide (FeSe). We use heteroepitaxial molecular beam epitaxy to grow FeSe with a nanoscale network of modulations that give rise to spatially varying strain. By analyzing small atomic displacements in scanning tunneling microscopy (STM) topographs, we map the local direction and magnitude of anisotropic strain. Concomitantly, by using spectroscopic imaging STM, we visualize how electronic nematic domains arrange themselves in response to a spatially inhomogeneous strain landscape. Overall, while the domains form so that the energy of nemato-elastic coupling is minimized, there are distinct regions where electronic nematic ordering fails to “flip” direction, even though the underlying structural anisotropy has been locally reversed. We conclude by discussing the implications of this finding.