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.
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