  Since the discovery of graphene, layered materials have attracted extensive
interests owing to their unique electronic and optical characteristics. Among
them, Dirac semimetal, one of the most appealing categories, has been a
long-sought objective in layered systems beyond graphene. Recently, layered
pentatelluride ZrTe5 was found to host signatures of 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
magnetic field, we observe a square-root-B dependence of inter-Landau-level
resonance and Shubnikov-de Haas (SdH) oscillations with non-trivial Berry
phase, both of which are hallmarks of Dirac fermions. The angular-dependent SdH
oscillations show a clear quasi-two-dimensional feature with highly anisotropic
Fermi surface and band topology, in stark contrast to the 3D Dirac semimetal
such as Cd3As2. This is further confirmed by the angle-dependent Berry phase
measurements and the observation of bulk quantum Hall 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 exotic physical phenomena
of Dirac fermions.
