  We report thickness-tuned electrical transport and Hall resistivity in highly
anisotropic three-dimensional Dirac semimetal ZrTe5 nanosheets. We find that
when the thickness of the nanosheet is blow about 40 nm, the system takes a
clear transition from topological semimetal with two bands carriers to a single
band with conventional hole carriers. The resistivity peak temperature T*
decreases systematically with decreasing thickness down to about 40 nm, then
shifts up with the further decrease of the thickness. Analysis of the data
below 40 nm indicates that the hole carriers completely dominate the transport
in the entire temperature range, regardless of the temperature being below or
above T*. By further tracking the carrier density, we find that the Fermi level
shifts consecutively downward from conduction band to the valence band as
decreasing the thickness. Our experiments unambiguously reveal a highly
thickness-tuned transition of band topology in ZrTe5 nanosheets.
