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, moire superlattices and heavy-fermion physics -- suffer from inherent trade-offs among robustness, tunability and orbital selectivity, limiting their broad applicability. Here, we unveil an intrinsic orbital-selective flat-band mechanism in the van der Waals materials NbOCl2 and TaOCl2, directly observed by angle-resolved photoemission spectroscopy (ARPES) and understood through density functional theory (DFT) and Wannier analysis. Crucially, we experimentally demonstrate that this momentum-independent flat band exhibits remarkable robustness, surviving from the bulk crystal down to the few-layer limit at room temperature. Our theoretical analysis traces its origin to the hybridization between Nb-dz2 orbital chains and the Lieb-like dx2-y2 sublattice, which is further reinforced by Peierls dimerization. Our findings not only establish transition-metal oxychlorides as a robust and tunable platform for flat-band-driven correlated phases under ambient conditions, but also uncover a new orbital-selective design principle for realizing flat bands in quantum materials.
@article{arxiv.2510.15080,
title = {Robust Orbital-Selective Flat Bands in Transition-Metal Oxychlorides},
author = {Xiangyu Luo and Ludovica Zullo and Sahaj Patel and Dongjin Oh and Qian Song and Asish K. Kundu and Anil Rajapitamahuni and Elio Vescovo and Natalia Olszowska and Rafal Kurleto and Dawid Wutke and Giorgio Sangiovanni and Riccardo Comin},
journal= {arXiv preprint arXiv:2510.15080},
year = {2025}
}