d-Wave Fermi Surface Instability in the Nematic Phase of Two Monolayer FeSe/SrTiO₃

Nematicity, where electrons break rotational symmetry while preserving translational symmetry, is ubiquitous in strongly correlated quantum matters, including high-T_c cuprates and iron-based superconductors. A central question in nematicity is whether it is driven by Fermi surface instability in momentum space or orbital order (polarization) in real space, especially as nematicity intertwines with superconductivity. FeSe/SrTiO₃ (STO), where nematicity occurs without long-range magnetic order, is an ideal platform for studying the nature and origin of the electronic nematicity. Here, direct evidence of d-wave nematic order in two monolayer FeSe/STO using angle-resolved photoemission spectroscopy is presented, revealing a remarkable degeneracy of d_xz and d_yz bands at the Brillouin zone center, but a significant band separation at the zone corner. This momentum-dependent nematicity demonstrates that nematicity in FeSe/STO originates from the d-wave Fermi surface instability of the Pomeranchuk-type, offering insights into the relationship between nematicity and superconductivity. The results establish 2D FeSe thin film as a powerful platform for investigating quantum physics under complex intertwinement.