Electronically driven nematicity in FeSe Films on SrTiO3

The intriguing role of nematicity in iron-based superconductors, defined as broken rotational symmetry below a characteristic temperature, is an intensely investigated contemporary subject. Nematicity is closely connected to the structural transition, however, it is highly doubtful that the lattice degree of freedom is responsible for its formation, given the accumulating evidence for the observed large anisotropy. Here we combine molecular beam epitaxy, angle-resolved photoemission spectroscopy and scanning tunneling microscopy together to study the nematicity in multilayer FeSe films on SrTiO3. Our results demonstrate direct connection between electronic anisotropy in momentum space and standing waves in real space at atomic scale. The lifting of orbital degeneracy of dxz/dyz bands gives rise to a pair of Dirac cone structures near the zone corner, which causes energy-independent unidirectional interference fringes, observed in real space as standing waves by scattering electrons off C2 domain walls and Se-defects. On the other hand, the formation of C2 nematic domain walls unexpectedly shows no correlation with lattice strain pattern, which is induced by the lattice mismatch between the film and substrate. Our results establish a clean case that the nematicity is driven by electronic rather than lattice degrees of freedom in FeSe films.