Mott phase in a van der Waals transition-metal halide at single-layer limit

Abstract

Two-dimensional materials offer opportunities for unravelling unprecedented ordered states at the single-layer limit. Among such ordered states, the Mott phase is rarely explored. Here we study the Mott phase in van der Waals chromium (II) iodide (CrI2) films. High-quality CrI2 films with an 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, we reveal that the film has a band gap as large as ∼3.2eV, 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 and spectral weight transfer at charge dopants that is consistent with the extended Hubbard model. Our study provides a platform for studying correlated electron states at the single-layer limit.