Fermiology of the Dirac type-II semimetal candidates (Ni,Zr)Te-2 using de Haas-van Alphen oscillations

We have investigated the Fermi surface properties of the Dirac type-II semimetal candidates (Ni,Zr)Te-2 using torque magnetometry with applied fields up to 35 T. Magnetization shows clear de Haas-van Alphen (dHvA) oscillations above 20 T. The dHvA oscillations are smooth and well defined and consist of one distinct frequency (F-alpha similar to 530 T) in ZrTe2 and three ((F) over bar (alpha) similar to 72 T, (F) over bar (beta) similar to 425 T, and (F) over bar (gamma) similar to 630 T) in NiTe2. The Berry phase phi was determined by constructing the Landau level fan diagram. It is found that phi similar to 0 and pi for F-alpha and (F) over bar (beta), respectively, for ZrTe2 and NiTe2. This strongly suggests that the Dirac fermions make a dominant contribution to the transport properties of NiTe2, whereas topologically trivial fermions dominate those in ZrTe2. The presence of lighter effective mass m* = 0.13m(e) in NiTe2 compared to m* = 0.26m(e) in ZrTe2, where m(e) is an electron's rest mass, further confirms the presence of Dirac fermions in NiTe2. Our density functional theory calculations find that while both systems host type-II Dirac dispersions along the out-of-plane direction, their relative positions and the natures of the dispersions are different. The Dirac cone is closer to the Fermi energy E-F (similar to 100 meV above) in NiTe2, whereas it is far (similar to 500 meV) above E-F for ZrTe2. This is consistent with our experimental finding of a nontrivial Berry phase and dominant contribution from lighter electrons in the quantum oscillation signal for only NiTe2. These findings suggest that the proximity of the Dirac cone to EF in topological compounds is crucial for observing the effect from Dirac quasiparticles in their electrical transport or magnetic properties.