Abstract
Two-dimensional materials offer opportunities for unravelling unprecedented ordered states at single layer limit. Among such ordered states, Mott phase is rarely explored. Here, we report the Mott phase in van der Waals chromium (II) iodide (CrI2) films. High quality CrI2 films with atomically flat surface and macro size are grown on graphitized 6H-SiC(0001) substrate by molecular beam epitaxy. By in situ low temperature scanning tunneling microscopy and spectroscopy (STM/STS), we reveal that the film has a band gap as large as ~3.2 eV, which is nearly thickness independent. Density functional plus dynamic mean field theory calculations suggest that CrI2 films may be a strong Mott insulator with a ferromagnetically ordered ground state. The Mott phase is corroborated by the spectral band splitting, that is consistent with the extended Hubbard model, and gap reduction at charge dopants. Our study provides a platform for studying correlated electron states at single layer limit.
Highlights
In low-dimensional electronic systems, correlations among electrons are enhanced due to the quantum confinement effect, which favors the formation of long-range collective ordered states [1,2] that are unprecedented according to the MerminWagner theorem [3]
By in situ low-temperature scanning tunneling microscopy and spectroscopy, we reveal that the film has a band gap as large as ∼3.2 eV, which is nearly thickness independent
The density functional theory (DFT) calculations are carried out using the projector-augmented-wave method implemented in VASP [33,34] within the local density approximation (LDA) and LDA + U functional [35,36]
Summary
In low-dimensional electronic systems, correlations among electrons are enhanced due to the quantum confinement effect, which favors the formation of long-range collective ordered states [1,2] that are unprecedented according to the MerminWagner theorem [3]. The split Hubbard bands are constrained with orbital selections for optical excitation, which aids the development of strong excitons [17] To these ends, the exploration of a monolayer Mott insulator with Jahn-Teller effect is highly desirable for in-depth controlled study of rich exotic correlated states and for building functional devices. The strong Coulomb energy in d orbitals of Cr makes it a promising system for the realization of a single-layer Mott insulator with degeneracy-lifted Hubbard bands. While this crystal has been synthesized in bulk form, its thin-film layers. We report an orbital-dependent Hubbard band splitting on a single charge defect
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