Crystal Growth And Magnetrotransport Properties Of Topological Materials

Topological materials have attracted wide attention for its unique energy band structure and novel physical properties since the discovery of topological insulators.So far,the topological materials that have been discovered can be divided into topological insulators and topological semimetals,according to the difference of their band structures.Topological insulators have insulating states in the bulk but have conducting surface states.In topological insulators,protected by the time-inversion symmetry and affected by the spin-orbital-coupling interaction,there exist non-dissipative carriers with linear dispersion relation on the surface state,which bring about rich physical properties and a wide range of applications.Recently,the first principle calculation predicts the existence of a new class of topological material-topological semimetal.In topological semimetals,Dirac cones or Weyl cones with linear dispersion in the bulk state are connected by the Fermi arc in the surface state.Soon,this prediction was confirmed by lots of experiments.So far,the discovered Dirac semimetals mainly include Cd3As2,Na3 Bi,etc.,and the well studied Weyl semimetals mainly include the TaAs family.Due to the unique band structure in these materials,there always exist many unusual transport properties such as extremely large magnetoresistance,field-induced metal-insulator transition,high carrier mobility,and chiral anomaly induced negative magnetoresistance,topological non-trivial Berry phase,and so on.In this paper,we systematically studied the magnetic transport properties of topological materials including TaAs,HfTe₅ and gray arsenic at low temperature.Among them,TaAs is the first discovered Weyl semimetal.We first proved the nontrivial topological properties in Ta As by transport experiments,the results consist with the Weyl semi-metal state predicted by the first principle calculation.HfTe₅ is a good thermoelectric material.It was found that there exists a metal-to-insulator transition with the pronounced resistivity peak near 50-90 K.We studied the transport properties in HfTe₅ and proved that the metal-to-insulator transition in it relates to a topological phase transition,and moreover,transport results show that HfTe₅ is a Dirac semimetal near the pronounced resistivity peak.We discovered that the magnetoresistance in arsenic is significantly larger than that of the vast majority of other materials.The results of transport properties show that the extremely large magnetoresistance behavior in arsenic can be explained by the carrier compensation mechanism.In addition,the transport results also show that gray arsenic is likely to be in the Dirac semi-metallic state at low temperature.This dissertation is divided into the following four chapters:In the first chapter,firstly we introduce the topological materials according to their band structures and discovery time.Then,we present the properties of materials,especially topological materials,at low temperatures.Lastly,we give a brief overview about the growth method of single crystals.In the second chapter,we present the magnetic transport properties of TaAs,which is the first discovered Weyl semimetal.The first-principles calculations show that TaAs is a proposed Weyl semimetal,in which there exist massless Weyl fermions.We grew high-quality Ta As single crystals by chemical vapor transport method and studied the transport properties in it at low temperatures.Transport measurement results illustrate that the single crystal of Ta As exhibit large magnetoresistance at low temperature.The negative magnetoresistance effect caused by chiral anomaly appears when the direction of current and magnetic field is parallel.Further studies show that the concentrations of electrons and holes are different in Ta As.By analyzing the SdH oscillations in the magnetoresistance,we find very small Fermi surface perpendicular to the magnetic field,where topological non-trivial carriers may exist.All the transport experiment results support the theoretical expectation that TaAs is a Weyl semimetal.In the third chapter,we systematically studied the magnetic transport properties of HfTe₅ single crystal at different temperatures.HfTe₅ possess a temperature-induced metal-to-insulator transition with pronounced resistivity peak around 50K-90 K,which is confused for decades.Recently,the first-principles calculation results predict that the HfTe₅ bulk material sits in the critical state between the strong topological insulator and weak topological insulator.We measured the magnetic transport properties at different temperatures by using the high quality HfTe₅ single crystal grown by chemical vapor transport method.The results show that the topological phase transition indeed occurs with the change of temperature and the Dirac semi-metal state only exist near the characteristic temperature TP.In the fourth chapter,we systematically introduced the magnetic transport properties of the extremely large magnetoresistance material-gray arsenic.We discovered the extremely large magnetoresistance property in gray arsenic.So far,the magnetoresistance in the gray arsenic ranks second only after bismuth.The Hall effect measurements show that the extremely large magnetoresistance property in gray arsenic can be explained by the carrier compensation mechanism.The results of SdH oscillation analysis show that there exist topological non-trivial carriers in the bulk band of arsenic.The negative magnetoresistance effect caused by chiral anomaly at low temperature was observed when the direction of current and magnetic field parallel to each other.All the transport results indicate that gray arsenic is a potential Dirac semimetal.A summary and prospect of the research are attached in the end of the thesis.