Pressure-dependent orbital-selective nematicity in FeSe

Multi-orbital/band electronic structure and orbital-dependent electron correlations critically shape emergent electronic states in correlated materials. Bulk FeSe exemplifies this through its enigmatic nematic phase, whose microscopic mechanism remains an outstanding question. Here, we perform comprehensive 57 Fe and 77 Se nuclear magnetic resonance (NMR) study on the evolution of the nematicity of bulk FeSe under hydrostatic pressure. At low pressures, alongside the nematicity involving on-site 3 d xz /3 d yz orbital polarization, a distinct non-local d_xy d xy nematicity emerges, indicating anisotropic intersite hopping of 3 d x y orbital electrons. With increasing pressure, while the on-site 3 d xz /3 d yz orbital polarization is gradually suppressed, the Lifshitz transition of the 3 d x y orbital hole pocket causes the d_xy d xy nematicity transforms into a spin-driven Ising-type nematicity above a characteristic pressure. This Fermi-surface crossover also influences the superconducting transition temperature T_c T c , signifying the pivotal role of the more-correlated 3 d x y orbital in Cooper pairing at high pressures. Our results establish the orbital-selective nature of nematicity in FeSe and its nontrivial tunability via hydrostatic pressure. We demonstrate that nematicity serves as a sensitive indicator of the underlying electronic structure and interactions in correlated electron systems.