Revisiting quadratic band crossing: from interaction-driven instability to intrinsic topology

TL;DR

This paper presents a new mechanism for realizing quantum anomalous Hall phases through band inversion and spin-orbit coupling, proposing specific monolayer compounds as promising materials for robust topological states.

Contribution

It introduces a general band inversion mechanism protected by atomic spin-orbit coupling, enabling intrinsic topological gaps in correlated materials, and identifies candidate monolayer compounds.

Findings

Band inversion between orbital doublets and isolated orbitals generates quadratic band crossing points.

Intrinsic atomic spin-orbit coupling gaps the crossing at the single-particle level.

Proposes monolayer $MNX_2$ compounds as realistic materials for robust QAH phases.

Abstract

The realization of robust quantum anomalous Hall (QAH) phases at elevated temperatures remains a central challenge in condensed matter physics. While quadratic band crossing points (QBCP) provide a promising route towards QAH states, existing proposals are largely confined to idealized models or hindered by interaction-driven competing orders. Here, we demonstrate that these limitations are not intrinsic to QBCP but arise from their specific implementation. We propose a general mechanism where band inversion between a symmetry-protected orbital doublet (e.g. $d_{xz},d_{yz}$) and an isolated orbital (e.g. $d_{z^2}$)-generically generates a QBCP with opposite curvature. This crossing is directly gapped at the single-particle level by intrinsic atomic spin-orbit coupling, while the underlying band inversion naturally shields the resulting topological gap against other interaction-driven instabilities. We further suggest monolayer compounds $MNX_2$ ($M$= Ni, Pd, Pt; $N$= Nb, Ta; $X$= S, Se, Te) as a realistic material class that intrinsically realizes this mechanism. These findings provide a concrete pathway toward robust QAH phases in correlated materials.