Observation of quasi-two-dimensional Dirac fermions in ZrTe5

Since the discovery of graphene, layered materials have attracted extensive interest owing to their unique electronic and optical characteristics. Among them, Dirac semimetals, one of the most appealing categories, have been a long-sought objective in layered systems beyond graphene. Recently, layered pentatelluride ZrTe5 was found to host signatures of a Dirac semimetal. However, the low Fermi level in ZrTe5 strongly hinders a comprehensive understanding of the whole picture of electronic states through photoemission measurements, especially in the conduction band. Here, we report the observation of Dirac fermions in ZrTe5 through magneto-optics and magneto-transport. By applying a magnetic field, we observe a dependence of the inter-Landau-level resonance and Shubnikov–de Haas (SdH) oscillations with a nontrivial Berry phase, both of which are hallmarks of Dirac fermions. The angle-dependent SdH oscillations show a clear quasi-two-dimensional feature with a highly anisotropic Fermi surface and band topology, in stark contrast to the three-dimensional (3D) Dirac semimetal such as Cd3As2. This is further confirmed by the angle-dependent Berry phase measurements and the observation of bulk quantum Hall effect (QHE) plateaus. The unique band dispersion is theoretically understood: the system is at the critical point between a 3D Dirac semimetal and a topological insulator phase. With the confined interlayer dispersion and reducible dimensionality, our work establishes ZrTe5 as an ideal platform for exploring the exotic physical phenomena of Dirac fermions.