Engineering Ideal 2D Type-II Nodal Line Semimetals via Stacking and Intercalation of van der Waals Layers

TL;DR

This paper demonstrates a bottom-up method to engineer ideal type-II nodal line semimetals in van der Waals bilayers through atomic intercalation, enabling tunable electronic properties and spontaneous ferromagnetism.

Contribution

It introduces a novel design strategy using atomic intercalation in van der Waals bilayers to realize ideal type-II NLSMs with symmetry protection and tunable features.

Findings

Fluorine intercalation creates a symmetry-protected type-II nodal loop at the Fermi level.

External electric and strain fields can tune the electronic properties of the system.

The system exhibits a van Hove singularity leading to spontaneous ferromagnetism.

Abstract

Two-dimensional type-II topological semimetals (TSMs), characterized by strongly tilted Dirac cones, have attracted intense interest for their unconventional electronic properties and exotic transport behaviors. However, rational design remains challenging due to the sensitivity of band tilting to lattice geometry, atomic coordination, and symmetry constraints. Here, we present a bottom-up approach to engineer ideal type-II nodal line semimetals (NLSMs) in van der Waals bilayers via atomic intercalation. Using monolayer $h$-AlN as a prototype, we show that fluorine-intercalated bilayer AlN (F@BL-AlN) hosts a symmetry-protected type-II nodal loop precisely at the Fermi level, enabled by preserved mirror symmetry ($\mathcal{M}_z$) and tailored interlayer hybridization. First-principles calculations reveal that fluorine not only tunes interlayer coupling but also aligns the Fermi energy with the nodal line, stabilizing the type-II NLSM phase. The system exhibits tunable electronic properties under external electric and strain fields and features a van Hove singularity that induces spontaneous ferromagnetism, realizing a ferromagnetic topological semimetal state. This work provides a versatile platform for designing type-II NLSMs and offers practical guidance for their experimental realization.