Deterministic scheme for two-dimensional type-II Dirac points and experimental realization in acoustics

TL;DR

This paper introduces a simple, deterministic band-folding scheme to realize two-dimensional type-II Dirac points, demonstrated experimentally in acoustics, revealing unique anisotropic properties and tilted kink states linked to valley-Hall effects.

Contribution

The paper proposes a novel, deterministic method for creating type-II Dirac points and demonstrates its experimental realization in acoustic systems.

Findings

Type-II Dirac points visualized in acoustics

Observation of tilted kink states from valley-Hall properties

The scheme's generality and deterministic nature confirmed

Abstract

Low-energy electrons near Dirac/Weyl nodal points mimic massless relativistic fermions. However, as they are not constrained by Lorentz invariance, they can exhibit tipped-over type-II Dirac/Weyl cones which provide highly anisotropic physical properties and responses, creating unique possibilities. Recently, they have been observed in several quantum and classical systems. Yet, there is still no simple and deterministic strategy to realize them since their nodal points are accidental degeneracies, unlike symmetry-guaranteed type-I counterparts. Here, we propose a band-folding scheme for constructing type-II Dirac points, and we use a tight-binding analysis to unveil its generality and deterministic nature. Through realizations in acoustics, type-II Dirac points are experimentally visualized and investigated using near-field mappings. As a direct effect of tipped-over Dirac cones, strongly tilted kink states originating from their valley-Hall properties are also observed. This deterministic scheme could serve as platform for further investigations of intriguing physics associated with various strongly Lorentz-violating nodal points.