Electrical transport in nano-thick ZrTe$_5$ sheets: from three to two dimensions

ZrTe$_5$ is a new topological material. It exhibits an infamous resistivity anomaly, of which the origin remains elusive. Recently, single layer ZrTe$_5$ is predicted to be a two-dimensional topological insulator, while experiments have suggested that bulk ZrTe$_5$ may be a three-dimensional topological Dirac semimetal. We report the first transport study on ultra thin ZrTe$_5$ flakes down to 10 nm. A significant modulation of the resistivity anomaly by thickness and gating has been observed. Remarkably, the metallic behavior, occurring only below about 150 K in bulk, persists to over 320 K for flakes less than 20 nm thick. We have identified contributions from a semiconducting hole band and a semimetallic band. As the thickness is reduced, the semiconducting band is shifted away from the Fermi level, while the semimetallic band remains. Quantum oscillations reveal that the semimetallic band is a Dirac band with a nontrival Berry's phase $\pi$. By incorporating the Dirac band, the resistivity anomaly of the material can be well explained. Moreover, the effect of the thickness offers a way to tune the energy bands in ZrTe$_5$.