  As a foundation of condensed matter physics, the normal states of most metals
are successfully described by Landau Fermi liquid theory with quasi-particles
and their Fermi surfaces (FSs). The FSs sometimes become deformed or gapped at
low temperatures owing to quasi-particle interactions, known as FS
instabilities. A notable example of a FS deformation that breaks only the
rotation symmetry, namely Pomeranchuk instability, is the d-wave FS distortion,
which is also proposed as one possible origin of electron nematicity in
iron-based superconductors. However, no clear evidence has been made for its
existence, mostly owing to the mixture of multiple orders. Here we report an
unequivocally observation of the Pomeranchuk nematic order in floating
monolayer (ML) FeSe on 1 ML-FeSe/SrTiO3 substrate. By using angle-resolve
photoemission spectroscopy, we find remarkably that the dxz and dyz bands are
degenerate at the Brillouin zone center (Gamma point), while their splitting is
even larger at zone corner (M point), in stark contrast to that in bulk FeSe.
Our detailed analysis show that the momentum-dependent nematic order in
floating monolayer FeSe is coming from the d-wave Pomeranchuk instability at M
point, shedding light on the origin of the ubiquitous nematicity in iron-based
superconductors. Our results establish the single-layer high-Tc superconductors
as an excellent material platform for investigating emergent quantum physics
under complex intertwinement.
