Interaction-driven topological phase transition in monolayer CrCl2 (pyrazine) 2

Abstract

The quadratic band crossing points (QBCPs) at the Fermi level in two dimensions have been proposed to be unstable under electron-electron interactions. The possible interaction-driven states include the quantum anomalous Hall (QAH) state and various nematic ordered states. In this paper, motivated by the discovery of ferromagnetic van der Waals layered metal-organic framework ${\mathrm{CrCl}}_{2}{(\mathrm{pyrazine})}_{2}$, we theoretically propose that a single layer of ${\mathrm{CrCl}}_{2}{(\mathrm{pyrazine})}_{2}$ might realize one or some of these interaction-driven states based on the QBCP protected by ${C}_{4}$ symmetry. By introducing short-range density-density type repulsion interactions into this system, we have found the phase diagram depending on different interaction ranges and strengths. The exotic phases include the staggered chiral flux state manifesting the QAH effect, the site-nematic insulator, and the site-nematic Dirac semimetal state. The QAH state is robust against perturbations breaking the QBCP but it is weakened by increasing temperature. The metal-organic framework is tunable by changing the transition-metal elements, which might improve the gap size and stability of this interaction-induced QAH state.