Transport in the ultra-quantum limit in topological pentatelluride HfTe₅

Topological pentatellurides, ZrTe₅/HfTe₅, show intriguing quantum transport properties. One of the unique characteristics is the extremely small Fermi surface consisting of massive Dirac fermions. This allows to reach the quantum limit of the system at a relatively low magnetic fields (B). Under the magnetic fields far above the quantum limit, all electrons are spin polarized and pushed into the zeroth Landau level. The increasing Zeeman energy and cyclotron energy under the magnetic field may lead to band inversion as the zeroth Landau bands cross with each other, as seen in a recent magneto-infrared spectroscopy study. We have investigated the quantum transport properties of HfTe₅ under pulsed magnetic fields and found signatures of enhanced conductivity and sign changes of Hall resistivity, pointing to a Liftshitz transition, under perpendicular magnetic fields. With the in-plane magnetic field, where B is parallel to the current direction, the longitudinal magnetoresistance shows a negative magnetoresistance that depends on temperature and magnetic field orientation. This might be understood by the Zeeman splitting of the bands. Our quantum transport results shed light on how the electronic property evolves in the ultra-quantum limit in HfTe₅.