| Recently,a series of topological semimetal materials including Dirac semimetal(DSM),Weyl semimetal(WSM)and Node-Line semimetal(NLSM),have been predicted and studied.Topological semimetals possess the band crossing points,where the energy gap only closes at those isolated k-points or lines.When a loop encloses the crossing point,the line integral of the Berry connection along this closed loop results in a Berry phase ?.In 2D systems,graphene and surface state of topological insulator can be viewed as the topological semimetal,where the low energy band around the Dirac point can be described by the 2D massless Dirac equation.When we carried out this work,topological insulators had been discovered and many expected physical effects of the surface state,such as Aharonov-Bohm(AB);quantum Hall effect and quantum anomalous Hall effect have been observed.In the meantime,many new topological semimetals are quickly rising.However,of the contribution of the surface state is poor because the bulk carriers contribute to the transport dominantly.Hence,it is urgent to look for new topological materials with dominant Dirac fermions.We chose the 3D Dirac semimetal Cd3As2 nanowire and the Node-line semimetal ZrSiSe bulk crystal,which have the crossing points in the bulk and are very different from topological insulators.As expected,we observed interesting quantum oscillations in the magnetotransport.Main results are summarized in the following.(1)We have synthesized the high-quality Cd3As2 nanowires through the chemical vapor deposition(CVD)method and carried out the electrical transport on a nanowire with the 180 nm diameter.The Shubnikov-de Haas(SdH)effect was observed even up to 100 K.There is only a frequence F=28.27 T of SdH oscillation,fitting to which leads to the effective mass m*=0.052 me.Nontrivial ? Berry's phase is obtained by the Landau level fan diagram analysis,revealing the dominant transport of topological Dirac fermions.The angle-dependent cross-sectional area of the Fermi surface obtained from the angle-dependent SdH oscillations indicates the isotropy of the bulk Dirac transport.The large mean free path estimated from SdH oscillations suggests the Cd3As2 nanowire a promising platform for the 1D transport of Dirac semimetals.(2)We grow the ZrSiSe bulk crystal of Node-Line semimetal using chemical vapor transport(CVT)method.We carried out the magnetization and magnetoresistance measurement on the ZrSiSe crystals.The carrier density estimated from Hall signal is almost 1021 cm-3,which is far higher than the reported Dirac materials(graphene,topological surface states).The high carrier density is attribute to the Node-Line state.The de Haas-van Alphen oscillations are clearly observed at 1 T.When the magnetic field is applied along the c direction of ZrSiSe crystal,there is only an F=210 T oscillation.However,there are almost eight frequencies when the magnetic field is aligned along the ab plane of ZrSiSe crystals.This shows the anisotropy of Fermi surface and the fitting of the Berry phase reveals nontrival Dirac fermions.At a fixed value of magnetic field,the magnetoresistance displays fourfold character as changing the field direction.We measured the magnetoresistance at 3 T while changing the direction in 3D space systematically.The magnetoresistance(MR)is found to reach the minimum while magnetic field along[100]family directions and reach the maximum while magnetic field along[011]family directions.We measured the SdH oscillation of different angles at bc-plane and ac-plane of ZrSiSe crystal,and we found there is a 3D Fermi surface corresponding F=30 T oscillation.The oscillation frequency of 209 T comes from an oblate ellipsoid,whose cross-section area is a circle. |
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