Unprotected quadratic band crossing points and quantum anomalous Hall effect in FeB 2 monolayer

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 k · p invariant theory, we find that the QBCP is not protected by rotation symmetry and consists of two Dirac points with the same chirality (Berry phase of 2 π ). 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.