Robust Orbital-Selective Flat Bands in Layered Transition-Metal Oxyhalides at Room Temperature

Flat electronic bands, which amplify electron correlations by quenching kinetic energy, provide an ideal foundation for exotic quantum phases. However, prevailing strategies—including geometrically frustrated lattices, moiré superlattices, and heavy-fermion systems—often suffer from inherent trade-offs among robustness, tunability, and orbital control, thereby limiting their broad applicability. Here, we report the experimental discovery of intrinsic orbital-selective flat bands in the layered transition-metal oxyhalide family M O X 2 ( M = Nb , Ta; X = Cl , Br, I), directly observed by angle-resolved photoemission spectroscopy at room temperature. Importantly, this materials platform exhibits pronounced tunability: The flat-band bandwidth and correlation strength can be systematically tuned via halogen substitution, while the Fermi level is controllable through electrostatic gating and surface charge transfer. Remarkably, the flat band remains stable upon dimensional reduction, persisting from the bulk crystal down to the few-layer limit. Our theoretical analysis reveals that this flat band transcends simple lattice geometry, originating instead from an orbital-driven synergetic Su-Schrieffer-Heeger-Lieb mechanism: a cooperative interplay between quasi-one-dimensional M − d z 2 Su-Schrieffer-Heeger chains and a two-dimensional M − d x 2 − y 2 Lieb-like sublattice, reinforced by Peierls dimerization. Together, these findings establish layered transition-metal oxyhalides as a versatile materials-by-design platform for systematically engineering intrinsic flat bands via chemical, electronic, and dimensional control, which provides a robust pathway to exploring room-temperature flat-band physics.