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

摘要

ZrTe$_5$ is a newly discovered topological material. Shortly after a singlelayer ZrTe$_5$ had been predicted to be a two-dimensional topologicalinsulator, a handful of experiments have been carried out on bulk ZrTe$_5$crystals, which however suggest that its bulk form may be a three-dimensionaltopological Dirac semimetal. We report the first transport study on ultra thinZrTe$_5$ flakes down to 10 nm. A significant modulation of the characteristicresistivity maximum in the temperature dependence by thickness has beenobserved. Remarkably, the metallic behavior, occurring only below about 150 Kin 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 theHall resistance, we identify contributions from a semiconducting and asemimetallic bands. The enhancement of the metallic state in thin flakes areconsequence of shifting of the energy bands. Our results suggest that the bandstructure sensitively depends on the film thickness, which may explain thedivergent experimental observations on bulk materials.