Type-II Nodal Line Fermions in New Z2 Topological Semimetals AV6Sb6 (A=K, Rb,and Cs) with Kagome Bilayer

TL;DR

This paper predicts the existence of type-II Dirac nodal line fermions in AV6Sb6 kagome compounds, revealing their topological properties and robustness against spin-orbit coupling, thus expanding the understanding of topological fermions in kagome materials.

Contribution

It introduces a new class of kagome compounds with type-II Dirac nodal lines and demonstrates their topological features and stability, which was not previously known.

Findings

Presence of type-II Dirac nodal lines in AV6Sb6 compounds

Nodal lines are protected by crystal symmetry and have quantized Berry phase

Nodal lines are robust against spin-orbit coupling effects

Abstract

The recently discovered layered kagome metals AV3Sb5 (A=K, Rb, and Cs) attract intensive interest due to their intertwining with superconductivity, charge-density-wave state, and nontrivial band topology. In this work, we show by first-principles calculations and symmetry arguments that unconventional type-II Dirac nodal line fermions close to the fermi level are present in another latest class of experimentally synthesized kagome compounds AV6Sb6 (A=K, Rb, and Cs). These compounds possess a unique kagome (V3Sb)2 bilayer that dominates their electronic and topological properties, instead of the kagome V3Sb monolayer in AV3Sb5. Crystal symmetry guarantees that the type-II Dirac nodal lines with quantized Berry phase lie in reflection-invariant planes of the Brillouin zone. We further reveal that the type-II Dirac nodal lines remain nearly intact in the presence of spin-orbital coupling and can be categorized as a Z2 classification. The findings establish AV6Sb6 as a class of new fascinating prototypes, which will extend the knowledge of interplay between unconventionally topological fermions and exotic quantum ordered states in kagome systems.