Electrical transport in nanothick ZrTe5 sheets: From three to two dimensions

ZrTe5 is a newly discovered topological material. Shortly after a single layer ZrTe5 had been predicted to be a two-dimensional topological insulator, a handful of experiments have been carried out on bulk ZrTe5 crystals, which however suggest that its bulk form may be a three-dimensional topological Dirac semimetal. We report a transport study on ultrathin ZrTe5 flakes down to 10 nm. A significant modulation of the characteristic resistivity maximum in the temperature dependence by thickness 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. Furthermore, the resistivity maximum can be greatly tuned by ionic gating. Combined with the Hall resistance, we identify contributions from a semiconducting and a semimetallic band. The enhancement of the metallic state in thin flakes are a consequence of shifting of the energy bands. Our results suggest that the band structure sensitively depends on the film thickness, which may explain the divergent experimental observations on bulk materials.