Properties Of Topological Semimetal TiSi And The Electric Field Effect Of Layered Nitride MNX

The topological insulator have received extensive attention,since the discovery of quantum hall effect in 1980.Topological insulator has an insulated state in the body and its surface state is a topologically protected metal state with linear dispersion relation.Unlike Topological insulator,topological semimetal has a Dirac cone or a Weyl cone with a linear dispersion relation in its bulk state.Due to its special energy band structure,topological semimetal has some unique physical properties such as field-induced metal-to-insulator-like transition,negative magneto-resistance,small cross-sectional area of Fermi-surface,nontrivial Berry phase and so on.According to the degeneracy and momentum space distribution of the nodal points,the topological semimetals can be classified into three categories:Dirac semimetals,Weyl semimetals and nodal line semimetals.However,only a few materials were investigated as nodal line semimetals,such as PbTaSe₂,ZrSiS,Be,PtSn₄ and TlTaSe₂.Since 1960,people began to modulate the properties of materials by electric field effect.However,due to the low breakdown voltage of dielectric materials,accumulated carrier density on sample surface is very low,so conventional field effect transistor can only modulate the physical properties of materials in a limited range.Recently,field effect transistors using ionic liquids as dielectrics have attracted wide attention.This device can gather extremely high concentration carriers on the sample surface,so it can modulation the physical properties of materials in a wide range.MNX series materials have the same layered structure as cuprate superconductors and Iron-based superconductors.Its parent phase is an insulator,which can be metalized and become superconductor through metal ion intercalation or forming X vacancies.In this thesis,we have systematically studied the magnetic transport properties of a topological semimetal TiSi and the electric field effect of MNX materials.The main contents are as follows:The first chapter briefly introduces topological materials and superconducting materials.Firstly,according to the classification of topological materials,topological insulator and topological semimetal are introduced,Then the development of superconductivity,cuprate superconductor,Iron-based superconductor and MNX superconducting materials are introduced.The second chapter introduces the magnetotransport Properties of a topological semimetal TiSi.TiSi was theoretically predicted to be a nodal line semimetal and the nodal ring is very close to the Fermi surface,so it is a perfect platform to explore the exotic physical properties of nodal line semimetals.We successfully grow the single crystals of TiSi by flux method and studied its basic physical properties.The resistance test under magnetic field showed that TiSi has a magnetic field induced metal-to-insulator-like transition at low temperature.The analysis of the nonlinear Hall resistivity indicates that TiSi is a multiband system with low carrier densities and high mobilities.The dHvA oscillation analysis of TiSi reveals that TiSi has a small cross-sectional area of Fermi surface,a light cyclotron effective mass and a nontrivial Berry phase.All these results manifest that Dirac fermions exist in TiSi and TiSi has nontrivial topological characteristics.The third chapter introduces the electric field effect's study of single crystals of Zr/HfNCl.We studied the electric field effect of single crystals of Zr/HfNCl by using DEME-TFSI as dielectric.It is found that electric field effect can induce superconducting state in samples,and the temperature of applied voltage has a great influence on the superconducting characteristics.At 220 K,the superconducting state induced by electric field effect will disappear after prolonged relaxation or application of reverse voltage.However,at 250 K,the induced superconducting state will not disappear.We carefully studied the superconducting state induced by electric field at two temperatures.For these phenomena,we propose a reasonable scenario:At low temperatures,the shift of Cl on sample surface caused by electric field leads to a local electron doping,which results in induced superconductivity.At high temperatures,Electric field effect causes partial Cl on sample surface to escape from the sample.This results in the formation of Cl vacancies on the sample surface,which leads to irreversible superconductivity.The fourth chapter introduces the electric field effect's study of polycrystalline samples of MNX.We used three kinds of ionic liquids as dielectrics and found that electric field effect can induced superconductivity in polycrystalline samples of MNX.Then,we used aqueous solution as dielectric to study the electric field effect of polycrystalline samples of MNX.It is found that most metal salt solutions can induce superconductivity into MNX samples,while pure water and Yb⁺³ salt solutions cannot.Therefore,when aqueous solution is used as dielectric,superconductivity may not be caused by the formation of X vacancies,but caused by the intercalation compound formed by metal ions entering MNX samples.At the end of the paper,we summarized the research results of this paper and made a simple prospect for the future work.