Observation of a bulk 3D Dirac multiplet, Lifshitz transition, and nestled spin states in Na3Bi

TL;DR

This study provides experimental evidence that Na3Bi hosts a bulk 3D Dirac multiplet with a Lifshitz transition and nestled spin states, revealing complex electronic and spin properties crucial for exploring interacting Dirac physics.

Contribution

First experimental observation of bulk Dirac multiplet and Lifshitz transition in Na3Bi, uncovering novel spin and electronic states predicted by theory.

Findings

Identification of two bulk 3D Dirac nodes on opposite sides of Γ

Observation of a Fermi surface Lifshitz transition and saddle point

Evidence for weak 2D nontrivial spin-orbit surface states with helical spin polarization

Abstract

Symmetry or topology protected Dirac fermion states in two and three dimensions constitute novel quantum systems that exhibit exotic physical phenomena. However, none of the studied spin-orbit materials are suitable for realizing bulk multiplet Dirac states for the exploration of interacting Dirac physics. Here we present experimental evidence, for the first time, that the compound Na3Bi hosts a bulk spin-orbit Dirac multiplet and their interaction or overlap leads to a Lifshitz transition in momentum space - a condition for realizing interactions involving Dirac states. By carefully preparing the samples at a non-natural-cleavage (100) crystalline surface, we uncover many novel electronic and spin properties in Na3Bi by utilizing high resolution angle- and spin-resolved photoemission spectroscopy measurements. We observe two bulk 3D Dirac nodes that locate on the opposite sides of the bulk zone center point $\Gamma$, which exhibit a Fermi surface Lifshitz transition and a saddle point singularity. Furthermore, our data shows evidence for the possible existence of theoretically predicted weak 2D nontrivial spin-orbit surface state with helical spin polarization that are nestled between the two bulk Dirac cones, consistent with the theoretically calculated (100) surface-arc-modes. Our main experimental observation of a rich multiplet of Dirac structure and the Lifshitz transition opens the door for inducing electronic instabilities and correlated physical phenomena in Na3Bi, and paves the way for the engineering of novel topological states using Na3Bi predicted in recent theory.