  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.
