Study On Transport Properties Of Topological Semimetals

As a new kind of topological material,topological semimetals get lots of attention.According to the magnetic monopole surrounded in Fermi surface,topological semimetals can be classified into Dirac semimetal,Weyl semimetal,node-line semimetal and triply degenerate semimetal and so on.Na3Bi and Cd3As2 were predicted to be Dirac semimetal by the groups of Zhong Fang,Xi Dai and Hongming Weng of Institude of Physics,Chinese Academy of Sciences.The theory was confirmed in 2014,which was the first discovery of Dirac semimetal in the world.Although topological semimetals have been developed in less than ten years,they have became one of the active research derections in condensed matter physics due to their rich physical phenomena and potential applications.The discovery and understanding of new Dirac materials are of great significant to the improvement of the theoretical seiries of condensed matter physics.In this shesis,we presented systematic transport properties study on the topological semimetals Nb3SiTe6,SrAs3,and ?-As.Specific researches are summarized as follows:(1)We used mechanical stripping method to strip nanothin sheeet samples from laminar ternary compound for transport measurement.We presented the magnetoresistance and Hall resistance of the thin flakes with magnetic field up to 33 T.The flakes showed strong angle-dependent MR properties and unsaturated quasilinear dependence accompanied with SdH oscillations.Furthermore,by analysis of SdH oscillation,we observed a nontrivial ?-Berry phase in the SdH oscillations which suggested the nontrivial topological nature of Nb3SiTe6.The angle-dependent quantum oscillations revealed a two-dimensional Fermi surface of Nb3SiTe6.All these transport results provided important information on the electronic structures and nontrivial topological nature of Nb3SiTe6.(2)We made the systematically transport measurement of the topological node-line semimetal SrAs3.Hall resistance mesurement revealed the coexistence of hole and electron,and the electon concentration was one order larger than the hole concentration,which indicated that there is no electon-hole complementarity in SrAs3.In the analysis of quantum oscillations,we obtained two carrier pockets with different effettive mass and Berry phase.Furthermore,we observed robust negative longituudinal magnetoresistance(NMR)in both direction a and c when the magnetic field is aligned parallel to the applied current(i.e.,B//E),which might due to chiral anomaly.Our study provided clearly tansport evidence of the existance of Dirac fermions in SrAs3.(3)?-As which is a black phosphorene analogue has recently attracted attentions in searching 2D materials with high mobility and excellent contact with electronic materials.ARPES experiment and the first-principle calculations revealed a topological insulator state,but recently the SdH oscillation studies on the ?-As single crystals indicated that it has a strong signature of Dirac fermions' characters.Here we carried on systematically both the dHvA and SdH effect measurements on?-As single crystals under high magnetic field.By the analysis of quantum oscillation and band structure calculated,we studied the electron structure and Fermi surface topology of ?-As.