Discovery of a Single-Band Mott Insulator in a van der Waals Flat-Band Compound

Abstract

The Mott insulator provides an excellent foundation for exploring a wide range of strongly correlated physical phenomena, such as high-temperature superconductivity, quantum spin liquid, and colossal magnetoresistance. A Mott insulator with the simplest degree of freedom is an ideal and highly desirable system for studying the fundamental physics of Mottness. In this study, we have unambiguously identified such an anticipated Mott insulator in a van der Waals layered compound Nb3Cl8. In the high-temperature phase, where interlayer coupling is negligible, density functional theory calculations for the monolayer of Nb3Cl8 suggest a half-filled flat band at the Fermi level, whereas angle-resolved photoemission spectroscopy experiments observe a large gap. This observation is perfectly reproduced by dynamical mean-field theory calculations considering strong electron correlations, indicating a correlation-driven Mott insulator state. Since this half-filled band derived from a single 2a1 orbital is isolated from all other bands, the monolayer of Nb3Cl8 is an ideal realization of the celebrated single-band Hubbard model. Upon decreasing the temperature, the bulk system undergoes a phase transition, where structural changes significantly enhance the interlayer coupling. This results in a bonding-antibonding splitting in the Hubbard bands, while the Mott gap remains dominant. Our discovery provides a simple and seminal model system for investigating Mott physics and other emerging correlated states.

23 pages, 4 figures, and the Supplemental Material

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• Magnetoelectric behavior of breathing kagomé monolayers of ${\mathrm{Nb}}_3{(\mathrm{Cl},\mathrm{Br},\mathrm{I})}_8$ from first-principles calculations
• Energy-lowering symmetry breaking creates a flat-band insulator in paramagnetic Nb3Cl8
• Molecular Orbital Electronic Instability in the van der Waals Kagome Semiconductor Nb${}_3$Cl${}_8$: Exploring Future Directions
• Direct evidence of intrinsic Mott state and its layer-parity oscillation in a breathing kagome crystal down to monolayer
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• Isolated nearly flat higher Chern band in monolayer transition metal trihalides
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• Quasi-particle propagation across semiconductor-Mott insulator interfaces
• Evidence of Mott Insulator with Thermally Induced Melting Behavior in Kagome Compound Nb3Cl8
• Antiferromagnetic Spin Fluctuations and Structural Transition in Cluster Mott Insulator Candidate Nb3Cl8 Revealed by 93Nb- and 35Cl-NMR
• From strong to weak correlations in breathing-mode kagome van der Waals materials: Nb${}_3$(F,Cl,Br,I)${}_8$ as a robust and versatile platform for many-body engineering
• Topological Aspects of Dirac Fermions in a Kagomé Lattice
• Emergent dynamical Kondo coherence and competing magnetic order in a correlated kagome flat-band metal CsCr6Sb6
• Dimensionality-Dependent Exciton Dispersion in a Single-Band Mott Insulator
• Robust Orbital-Selective Flat Bands in Transition-Metal Oxychlorides
• Dissipation-Driven Transition of Particles from Dispersive to Flat Bands
• Transmission through multiple Mott insulator - semiconductor wells