Exploration And Physical Properties Of Topological Materials And Low-dimensional Transition Metal Compounds

Topology and superconductivity are two important research topics in condensed matter physics and material science.Especially in recent years,topological materials have attracted extensive attention due to their novel band structures and physical properties.According to the characteristics of band structures,topological materials can be generally divided into topological insulators,topological superconductors and topological semimetals.Topological semimetals are also classified into several categories,such as Dirac semimetals,Weyl semimetals and nodal-line semimetals,and their categories become more and more extensive.These new topological electronic states usually lead to novel magneto-transport properties,such as large magnetoresistance,high carrier mobility and chiral-anomaly induced negative magnetoresistance,etc.In this dissertation,we have grown the high-quality single crystals of MoGe₂ for the first time.Subsequently,we studied the electromagnetic transport properties,and found that MoGe₂ is a topological semimetal.Secondly,the discovery of iron-based superconductivity has given rise to intense emotion to explore high temperature superconductivity after the copper oxide high temperature superconductors.Most of these materials have quasi-two-dimensional crystal structures,in which copper oxygen octahedron and iron arsenic tetrahedron are considered to be the genes for high temperature superconductivity,and hence those low dimensional transition metal materials with antiferromagnetic order are still at the forefront of the exploration of unconventional superconductivity.In this dissertation,we also synthesized a variety of new low-dimensional transition metal compounds,including quasi-one-dimensional fluorides Ba₃F₂MSe₃（M=Cd,Zn）with MSe₄tetrahedron,quasi-two-dimensional layered transition metal compounds Sr₄Fe₂O₆CdS₂ and Sr₄Mn₂O₆CdS₂ with CdS₂ layers,which provide important platforms for further exploring new high temperature superconductors.This dissertation is mainly divided into the following six parts:In chapter 1,first we introduce the development history of topological materials,including the theoretical and experimental research of topological insulators and topological semimetals,as well as their novel physical properties such as large magnetoresistance and negative magnetoresistance effect.Next,we introduce the low-dimensional transition metal compounds,mainly focusing on the iron-based superconductors and the low-dimensional compounds with MQ₂（M=transition metal;Q=S,Se）layer.Then,we briefly expounds the purpose and significance of the topics.Finally,the experimental methods including crystal growth and physical properties characterization are briefly described.In chapter 2,we present the single crystal growth of the topological material MoGe₂,and discuss experimental data of basic physical properties.High-quality MoGe₂ single crystals are grown by self-flux method.It is found that when the magnetic field is applied perpendicular to the electric current,MoGe₂ exhibits a resistivity-upturn behavior,similar to“metal-insulator transition”,and a large magnetoresistance at low temperature.Hall effect analysis indicates that the large magnetoresistance of MoGe₂ results from the carrier compensation mechanism and high carrier mobility.However,when the magnetic field is parallel to the current direction,MoGe₂ shows a weak antilocalization effect and negative magnetoresistance due to the chiral anomaly of Weyl nodes.Our results reveal that MoGe₂ is a topological semimetal.In chapter 3,we describe the synthesis and characterization of Ba₃F₂MSe₃（M=Zn,Cd）with quasi-one-dimensional chain structure.The new transition metal compounds Ba₃F₂MSe₃ are synthesized by solid-state reaction for the first time,and their structures are accurately analyzed and determined.The measurements of magnetic susceptibility,specific heat,spectrum and band calculation show that Ba₃F₂Zn Se₃ and Ba₃F₂CdSe₃are direct band gap semiconductors with band gaps of2.0 e V and 1.5 e V,respectively.In chapter 4,we describe the synthesis and characterization of a quasi-two-dimensional layered transition metal compound Sr₄Fe₂O₆CdS₂.The single crystal and polycrystalline samples of Sr₄Fe₂O₆CdS₂ are synthesized via solid-state reaction.Sr₄Fe₂O₆CdS₂ belongs to tetragonal structure with P4₂/nmc（No.137）space group,which is composed of alternating stacking Fe O₅ square pyramid,CdS₂ layer and Sr layer along the c-axis.The experimental and theoretical results show that Sr₄Fe₂O₆CdS₂ is a magnetic semiconductor with a band gap of 0.75 e V,which orders antiferromagnetically at 2.4 K.In chapter 5,we describe the synthesis and characterization of a quasi-two-dimensional layered compound Sr₄Mn₂O₆CdS₂.In Sr₄Fe₂O₆CdS₂,the element Fe is substituted by Mn,and then we obtain a new compound Sr₄Mn₂O₆CdS₂.The experimental analyses show that Sr₄Mn₂O₆CdS₂ is also a magnetic semiconductor with a band gap of 0.63 e V,while it orders antiferromagnetically at 91 K with a spin glass transition below 14 K.In chapter 6,we briefly summarize the research contents of this paper and outlook the further research of these materials.