Discovery of a Three-dimensional Topological Dirac Semimetal, Na3Bi

Abstract: Three-dimensional (3D) topological Dirac semimetals (TDSs) represent a novel state of quantum matter that can be viewed as '3D graphene'. In contrast to two-dimensional (2D) Dirac fermions in graphene or on the surface of 3D topological insulators, TDSs possess 3D Dirac fermions in the bulk. The TDS is also an important boundary state mediating numerous novel quantum states, such as topological insulators, Weyl semi-metals, Axion insulators and topological superconductors. By investigating the electronic structure of Na3Bi with angle resolved photoemission spectroscopy, we discovered 3D Dirac fermions with linear dispersions along all momentum directions for the first time. Furthermore, we demonstrated that the 3D Dirac fermions in Na3Bi were protected by the bulk crystal symmetry. Our results establish that Na3Bi is the first model system of 3D TDSs, which can also serve as an ideal platform for the systematic study of quantum phase transitions between rich novel topological quantum states.

• The paper presents ARPES results that reveal linear band dispersions at two Dirac points with measured anisotropic velocities, confirming robust 3D Dirac fermions in Na3Bi.
• It employs selective potassium doping to illustrate that the bulk Dirac fermions remain stable despite surface state modifications, underscoring topological protection via crystal symmetry.
• The discovery positions Na3Bi as a model system for probing quantum phase transitions and realizing advanced spintronic applications in complex quantum materials.

Discovery of a Three-dimensional Topological Dirac Semimetal, Na3Bi

In the study of materials with unique electronic properties, the discovery of three-dimensional (3D) topological Dirac semimetals (TDSs) represents a significant advancement. The paper in question presents an in-depth investigation of Na3Bi, identified as a model system of such semimetals, highlighting its capacity to exhibit 3D Dirac fermions protected by bulk crystal symmetry. Utilizing angle-resolved photoemission spectroscopy (ARPES), the authors have empirically demonstrated the presence of these 3D Dirac fermions, offering both theoretical insights and empirical evidence essential to comprehending the real-life dynamics and applications of TDSs.

The significance of Na3Bi lies in its role as an archetype for understanding TDSs, which can be conceptually considered 3D analogues of graphene. Unlike the largely two-dimensional (2D) Dirac fermions found in graphene, TDSs contain 3D Dirac fermions characterized by linear dispersion along all three crystallographic directions. The ability of Na3Bi to host these fermions suggests potential for realizing multiple topological quantum states such as topological insulators, Weyl semimetals, and axion insulators, thereby positioning TDSs as transitional states in complex quantum systems.

The identification of robust bulk Dirac fermions in Na3Bi is underpinned by detailed ARPES measurements. The study reports linear band dispersions at two discrete Dirac points, with weak in-plane anisotropy but significant out-of-plane anisotropy. This intrinsic anisotropy, quantified with velocities $V_x \approx V_y \approx 3.74 \times 10^5$ m/s and $V_z \approx 2.89 \times 10^4$ m/s, reflects the distinctive electronic structure of Na3Bi and aligns with theoretical predictions. Additionally, the stability of these Dirac points against surface perturbations is illustrated by selective potassium doping experiments, which leave bulk characteristics unchanged despite changes to surface states, thus underscoring the topological protection afforded by bulk crystal symmetries.

The discovery of Na3Bi as a 3D TDS provides substantial implications for both theoretical and practical developments in condensed matter physics and materials science. The material’s long Fermi-wavelength presents possibilities for enhancing RKKY interactions, facilitating magnetic state realization through sparse magnetic doping, and potentially advancing spintronic technologies. As an ideal platform to explore quantum phase transitions, Na3Bi also permits systematic study of the transitions from TDSs to other emergent quantum states.

Future research directions could include exploring the interplay of symmetry and electronic topology in more complex heterostructures and superlattices based on Na3Bi. Furthermore, the paper opens pathways for the experimental realization of phenomena such as giant diamagnetism and quantum magnetoresistance, which are associated with topological phases in these complex systems. As the understanding of 3D TDSs progresses, leveraging materials like Na3Bi could lead to pivotal breakthroughs in quantum materials and their applications in next-generation technology.