Study On The Transport And Magnetic Properties Of Type ? Weyl Semimetal WTe₂ And Transition Metal Thiophosphite Material MPS₃

In recent years,research on topological materials has been extensive.Topological materials is a class of materials with a non-trivial topological band structure.It has become an attractive area of condensed matter physics to study of topological semimetals.Since the discovery of topological semimetals,we have discovered many strange properties in them.Such as negative magnetoresistance induced by chiral anomaly,extremely large magnetoresistance,high mobility and anomalous Hall effect.Many strange properties make topological semi-metals have a good prospect for applications.The main content of this dissertation is the layered transition metal chalcogenide WTe₂ and layered transition metal thiophosphites MPS₃?M = Mn,Ni,Fe?as the research objects,and their transport properties and magnetic properties are studied.Among them,the layered transition metal chalcogenide WTe₂ is predicted to be a type-? Weyl semimetal.By measuring the transport properties of WTe₂ single crystals,we sought to find the origin of its extremely large magnetoresistance.And for the first time,we measured the planar Hall effect in the WTe₂,giving its topological evidence from the transport.In addition,theoretical calculations show that the layered transition metal thiophosphites MPS₃ may have multiple Dirac cones in the electronic structure with half-full eg orbit,and therefore has attracted our research interest.Our magnetic measurements on a series of MPS₃ shows that they are antiferromagnetic materials at low temperatures.This paper is divided into the following chapters:The first chapter gives a brief overview of the topological material from topological insulators to topological semimetals according to the time they have been discovered.Then some magnetic transport properties of some materials at low temperature are introduced,which provides a background for the discussion of the extremely large unsaturated magnetoresistance of type-? Weyl semimetal WTe₂.The second chapter focuses on the research progress of the type-? Weyl semimetal WTe₂ in recent years.It mainly includes the latest research progresses on its non-saturated extremely large magnetoresistance,high-pressure phase transitions and topological non-trivial states.In the third chapter,non-stoichiometry effect on the extremely large magnetoresistance is systematically investigated for the Weyl semimetal WTe₂.Magnetoresistanceand Hall resistivity are measured for the as-grown samples with a slight difference in Te vacancies and the annealed samples with increased Tevacancies.The fittings to a two-carrier model show that the magnetoresistance is strongly dependent on the residual resistivity ratio?i.e.,the degree of non-stoichiometry?,which is eventually understood in terms of electron doping which not only breaks the balance between electron-type and hole-type carrier densities but also reduces the average carrier mobility.Thus,compensation effect and ultrahigh mobility are probably the main driving force of the extreme magnetoresistance in WTe₂In the fourth chapter,we report the first experimental observation of giant planar Hall effect in type-? Weyl semimetal WTe₂.Our comprehensive analyses of the experimental data demonstrate that the detected planar Hall effects is originated from the chiral anomaly of Weyl fermions.Unlike the somewhat elusive negative magnetoresistance,the planar Hall effects is robust and easy to be detected in type-II Weyl semimetal.This work reveals that the planar Hall effect is an effective transport probe to determine the topological semimetals,especially in type-II Weyl semimetals.In the fifth chapter,we introduce a series of single crystals of MPS₃?M=Mn,Fe,Ni?grown by chemical vapor transport method.X-ray diffraction and energy dispersive spectroscopy results show that the as-grown samples are of pure phase and good quality.All three samples are antiferromagnetic at low temperature.When magnetic field is parallel to sample plane,there is a broad peak in the susceptibility vs temperature curve existing above Neel temperature,which may be linked to short range orders caused by quasi-two-dimensional?2D?magnetic exchange.These phenomena indicate that MPS₃ system is a good platform for studying the magnetic properties of 2D materials.Previous theoretical and experimental results show that the system has a wide band gap and a large resistivity.Therefore,the MPS₃ system has a great application prospect in the dielectric application of 2D layered materials.At the end of the paper,we made a brief summary and made a further look at the future work.