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

TL;DR

This study demonstrates the existence of a Mott insulating phase in single-layer van der Waals CrI2 films, combining experimental STM/STS measurements with theoretical calculations to reveal strong electron correlations and magnetic order.

Contribution

First observation of a Mott phase in single-layer van der Waals chromium iodide films using combined experimental and theoretical approaches.

Findings

CrI2 films exhibit a large, thickness-independent band gap of ~3.2 eV.

Spectral band splitting and gap reduction at charge dopants support Mott insulator behavior.

CrI2 is predicted to be a ferromagnetically ordered Mott insulator.

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.