Unprotected quadratic band crossing points and quantum anomalous Hall effect in FeB2 monolayer

TL;DR

This paper predicts that FeB2 monolayer hosts quadratic band crossing points that can spontaneously lead to a quantum anomalous Hall insulator state due to Coulomb interactions, based on first-principles calculations.

Contribution

It reveals that FeB2 monolayer exhibits nonmagnetic QBCPs that become a QAH insulator through interaction-induced symmetry breaking, a novel mechanism for topological phase transition.

Findings

FeB2 monolayer has QBCPs at K point.

Coulomb interactions induce a transition to a QAH insulator.

QBCP consists of two Dirac points with same chirality.

Abstract

Quadratic band crossing points (QBCPs) and quantum anomalous Hall effect (QAHE) have attracted the attention of both theoretical and experimental researchers in recent years. Based on first-principle calculations, we find that the FeB$_2$ monolayer is a nonmagnetic semimetal with QBCPs at $K$. Through symmetry analysis and $\mathbf{k}\cdot\mathbf{p}$ invariant theory, we find that the QBCP is not protected by rotation symmetry and consists of two Dirac points with same chirality (Berry phase of $2\pi$). Once introducing Coulomb interactions, we find that there is a spontaneous-time-reversal-breaking instability of the spinful QBCPs, which gives rise to a $C=2$ QAH insulator with orbital moment ordering.