Research On The Transport Properties Of New Topological Semimetals

Topological material state of topological insulators and topological semimetals have become a hot research in condensed state physics recent years.The quantum materials not only provide an ideal platform to research fundamental physics,but also have vast potential applications due to their novel properties and unique electronic structure.Search for new topological materials is helpful to understand the topological state and improve the topological theory.So far,there are mainly two ways in the research of topological material state.On the one hand,with the development of topological theory,new topological materials are predicted ever and again and then confirmed by ARPES and related transport measurements,which enhances our comprehension of new topological material state.On the other hand,new effects induced by topological state are always found in confirmed topological materials experimentally and theoretically,which can become a powerful tool to investigate topological materials.Our research are mainly about the transport properties of topological semimetals by magnetoresistance and quantum oscillations.We expect to find the transport evidences to confirm the topological state for new topological semimetals.On the other hand,we also find new effects induced by topological state in the confirmed topological materials.The thesis consists of five chapters.Chapter one:IntroductionThere are three parts in this chapter,which systematically introduces the content about topological insulators,topological semimetals and quantum oscillations.Firstly,we reviewed the development history of topological insulators,and then we introduced the concept,related theories,topological characters and research progress of topological insulators.In the second section of this chapter,we systematically introduced the concept,related theories,topological characters and research progress of Dirac semimetals and Weyl semimetals.In the end,we also introduced some new-style topological semimetals,such as node-line semimetals,topological semimetal with Dirac-node-arc,magnetic Dirac semimetals and triple point topological semimetals.In the third section of this chapter,we systematically introduced the principles and applications of quantum oscillations.At last,this chapter was summarized.Chapter two:the research on transport properties of the topological semimetals,PdTe2 and PtTe2In this chapter,we introduced the results of transport measurements for the topological semimetals,PdTe2 and PtTe2.The results of the quantum oscillation and magnetoresistance measurements show that despite of the complicated Fermi surfaces in PdTe2,only one branch dominates their transport properties.After quantitative analysis,we found that the dominated Fermi surface belongs the three identical arm-like Fermi surfaces which connect two big electronic Fermi surfaces.For PtTe2,which shares the same crystal structure and electronic structure,its magnetoresistance obeys the Kohler’s ruler well.So this single-band transport behavior may be common to this of materials.Chapter three:the research on transport properties of the magnetic topological semimetals,NdSb.In this chapter,we introduced the results of theoretical calculation,magnetoresistance and magnetism measurements of NdSb.High field magnetoresistance and magnetism data of NdSb shows that there is a transition of antiferromagnetic state to ferromagnetic state in about 10T.Then we did theoretical calculation for those two magnetic state,respectively.And the results shows that the antiferromagnetic state is a Dirac semimetal state.We detected the negative magnetoresistance induced by chiral anomaly in the antiferromagnetic state,which confirms the magnetic Dirac semimetal state in NdSb.Besides,the results of quantum oscillations shows that there is a Fermi surface reconstruction with the field induced magnetic transition,which may be related to the coupling of the f electrons to free electrons.Chapter four:planar Hall effect in type-Ⅱ Weyl semimetal WTe2In this chapter,we introduced the planar Hall effect(PHE)measured in type-ⅡWeyl semimetal WTe2.The PHE and anisotropic magnetoresistance are well fitted by the formulas deduced from chiral anomaly.After careful analysis,we confirmed that the PHE in WTe2 is induced by chiral anomaly.The PHE in WTe2 is very obvious,which can be easily detected experimentally.Our related work not only provides another transport evidence of Weyl semimetal state for WTe2,but also confirms that the PHE is powerful tool to detect the topological semimetal state.Chapter five:topological nature of the Dirac-node-arc semimetal PtSn4 probed by quantum oscillationsIn this chapter,we introduced the results of quantum oscillations of PtSn4.Through systematic measurement of quantum oscillation,we found that two dominated branches have some topological characters such as very small effective carrier mass and very high quantum mobility.By quantitative comparison with previous ARPES results,we confirmed that the two dominated branches belong to the Fermi surfaces surrounding the Dirac-node-arcs.