Single Crystals Growth And Physical Properties Of Layered Transition Metal Compounds SrCdBi₂ And YbMnSb₂

Topological materials have been studied extensively due to their unique physical properties,which are one of the research focuses in the field of condensed matter physics.Topological materials can be divided into topological insulators and topological semimetals that consist of Dirac semimetals,Weyl semimetals and node-line semimetals.In Dirac semimetals,the transport behaviors of electrons obey Dirac equation,and Dirac cones with linear dispersion appear in the bulk state while Fermi arc which connecting two Dirac cones appears on the surface state.The nontrivial properties provide opportunities for Dirac semimetals to be applied in spin electronics,quantum computer and so on.Recently,both experimental and theoretical researches indicate that 112-type manganese-based stibnides/bismuthides AMnX₂（A=Ca/Sr/Ba/Eu;X=Sb/Bi）are emerging candidates for Dirac semimetals.They are layered compounds with quasi-two-dimensional crystal structure and consist of alternative layers of a X square sheet and an edge-sharing MnX₄ tetrahedron,which are separated by A ions,and Dirac fermions are believed to exist in the X sheet.In addition,some compounds without magnetism also belong to 112-type systems,such as SrZnSb₂,BaCdSb₂,and BaZnBi₂.The lack of interaction between magnetism and quantum states make we study intrinsic topological physical properties more easily.In this thesis,we have successfully grown two 112-type transition metal compounds SrCdBi₂ and YbMnSb₂ by self-flux method and investigated their crystal structures,magnetism and transport properties systematically.Our results indicate that SrCdBi₂ possesses non-saturating linear magnetoresistance,high carrier mobility and shows magnetic field induced metal-to-insulator transition under low temperature.Besides,we have observed quantum oscillation in YbMnSb₂.These nontrivial physical properties suggest that SrCdBi₂ and YbMnSb₂ are hopeful candidates for Dirac semimetal with Dirac fermions.The thesis contains the following five chapters:In the first chapter,we briefly introduce several topological materials,including two-dimensional topological insulators,three-dimensional topological insulators,Weyl semimetals and Dirac semimetals.Moreover,we also introduce a few normal properties in topological materials,involving magnetoresistance effect,weak localization,weak anti-localization and quantum oscillation.In the second chapter,we give a overview of two methods of single crystal growth,which are Bridgman method and flux method,and several kinds of characterization techniques,including X-ray diffraction,scanning electron microscope,electrical transport measurement,magnetism measurement,and specific heat measurement.In the third chapter,we report the single crystal growth process and physical properties of transition metal bismuthide SrCdBi₂.Single crystals of SrCdBi₂ were grown by self-flux method with Cd and Bi.Electrical transport measurements indicate that SrCdBi₂ is a multiband system with large carrier mobility,in which electrons dominate the transport process.The temperature dependence of in-plane resistivity present metal-like behavior and SrCdBi₂ possesses quasi-two-dimensional electronic structure.Under low temperature and strong magnetic field,metal-to-insulator transition and non-saturating linear magnetoresistance was observed.The nontrivial physical properties suggest that SrCdBi₂ is a suitable candidate for Dirac semimetal without magnetism.The magnetic susceptibility of SrCdBi₂ shows weak diamagnetism and reveals an anomaly at about 210 K,which is also observed in resistivity and carrier density.We attribute the appearance of the anomaly to disorder or formation of domain.In the fourth chapter,we report the single crystal growth process and physical properties of transition metal stibnide YbMnSb₂.Single crystals of YbMnSb₂ were grown by self-flux method with Sb.We found that its in-plane resistivity is metal-like and is anisotropic under various magnetic field directions.Furthermore,YbMnSb₂ is a multiband system with quasi-two-dimensional electronic structure and electrons are dominant in the transport process.A antiferromagnetic transition was appeared at about 340 K and the easy magnetization axis is along c axis.Notably,we observed obvious de Haas-van Alphen oscillation and Shubnikov-de Haas oscillation in the magnetic field-dependent magnetic moment and magnetoresistance,respectively.Therefore,YbMnSb₂ is believed to be a candidate for antiferromagnetic Dirac semimetal.In the fifth chapter,we summarize the overall content and give a brief prospect of subsequent work on SrCdBi₂ and YbMnSb₂.