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

Xiangyu Luo1,*,†, Ludovica Zullo2,*,‡, Sahaj Patel1,*, Dongjin Oh1, Willa Mihalyi-Koch3, Emma Lian3, Jiaruo Li1, Qian Song1, Asish K. Kundu4 et al.

Anil Rajapitamahuni4, Elio Vescovo4, Natalia Olszowska5, Rafał Kurleto5, Dawid Wutke5, Xavier Roy3, Giorgio Sangiovanni2, and Riccardo Comin1,§

  • *These authors contributed equally to this work.
  • †Contact author: xyluo@mit.edu
  • ‡Contact author: ludovica.zullo@uni-wuerzburg.de
  • §Contact author: rcomin@mit.edu
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Phys. Rev. X 16, 021041 – Published 21 May, 2026

DOI: https://doi.org/10.1103/p3dw-tbqp

Abstract

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 MOX2 (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−dz2 Su-Schrieffer-Heeger chains and a two-dimensional M−dx2−y2 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.

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Flat electronic bands that suppress electron kinetic energy are essential for amplifying interactions and generating exotic quantum phases, yet most current materials hosting these bands require extremely low temperatures or precise tuning that limits their application. We used angle-resolved photoemission spectroscopy to identify robust, intrinsic flat bands at room temperature within a family of layered transition-metal oxyhalides. Our analysis reveals that these bands arise from an orbital-selective localization mechanism, where electron hopping is strongly suppressed for specific orbitals, confining those electrons within the crystal lattice. We demonstrated that these bands remain stable even when the material is thinned to several atomic layers and are highly tunable through chemical substitution or electrical gating. These findings establish a versatile platform for exploring strongly correlated quantum matter under realistic, ambient conditions. Our work provides a scalable pathway for designing materials that harness flat-band physics for future quantum technologies and fundamental condensed matter research.

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Phys. Rev. X 16, 021041– Published 21 May, 2026
  • Received 14 October 2025
  • Revised 18 March 2026
  • Accepted 21 April 2026
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