Lifshitz transition and van Hove singularity in a Topological Dirac Semimetal

TL;DR

This study uncovers a Lifshitz transition and van Hove singularity in the electronic structure of the 3D Dirac semimetal Na3Bi, revealing topological changes and a band saddle point that distinguish it from 2D graphene analogs.

Contribution

It provides the first experimental evidence of a Lifshitz point and saddle point singularity in a 3D Dirac semimetal, highlighting unique topological features.

Findings

Identification of a Lifshitz point in Na3Bi

Observation of a band saddle point singularity in 3D Dirac materials

Multiple Dirac nodes connected via a Lifshitz point

Abstract

A topological Dirac semimetal is a novel state of quantum matter which has recently attracted much attention as an apparent 3D version of graphene. In this paper, we report critically important results on the electronic structure of the 3D Dirac semimetal Na3Bi at a surface that reveals its nontrivial groundstate. Our studies, for the first time, reveal that the two 3D Dirac cones go through a topological change in the constant energy contour as a function of the binding energy, featuring a Lifshitz point, which is missing in a strict 3D analog of graphene (in other words Na3Bi is not a true 3D analog of graphene). Our results identify the first example of a band saddle point singularity in 3D Dirac materials. This is in contrast to its 2D analogs such as graphene and the helical Dirac surface states of a topological insulator. The observation of multiple Dirac nodes in Na3Bi connecting via a Lifshitz point along its crystalline rotational axis away from the Kramers point serves as a decisive signature for the symmetry-protected nature of the Dirac semimetal's topological groundstate.