Exploration And Study Of Layered Transition Metal Nitrogen Or Oxide Compounds And Semimetal Cd

In recent years,layered ternary transition metal nitrogen or oxides M?N/O?Xs have attracted much attention due to their rich physical properties,such as superconducting and spin-Peierls transition.It is reported that the superconductors in MNX-type exhibit many unusual superconducting properties,such as the small Fermi level density of states,the weak electron–phonon coupling,quasi-2D superconducting characteristics and abnormal superconducting energy gap.Unfortunately,these superconductors are very sensitive to air,which slows their further development.In this dissertation,we studied the field-induced irreversible superconductivity in ZrNBr single crystals,which is stable in air and can provide better platform for exploring the superconducting mechanism in single d electron system.Meanwhile,we also explored the evolution of spin-Peierls transition in TiOCl by carrier intercalation.In addition,topological semimetals always show novel properties,such as large magnetoresistance?MR?,chiral-anomaly induced negative MR and have many potential applications in magnetic storage,magnetic sensors,and electronic spin devices.We prepared high quaility cadmium?Cd?single crystals to study the transport properties.The six main contents are listed as follows:In chapter 1,we introduce the recent research progress in MNX system.Firstly,we introduce the two crystal structures??-FeOCl type and?-SmSI type?and fundamental physical properties of MNXs.We also list several methods to achieve the superconducting transition in parent compounds,such as doping,carrier intercalation?including alkali metal or alkaline earth?,electrochemical intercalation,halogen atom deintercalation,gate-induced superconductivity.Then,the progresses of?-phase and?-phase superconductivity are presented clearly.We simply introduce the spin-Peierls transition and the current research progresses of TiOCl.Subsequently,we introduce three main different topological semimetals including Dirac semimetal,Weyl semimetal,node-line semimetal and the unconventional MR.In chapter 2,we report the field induced superconducting transition near 14 K and 11 K in layered ZrNBr single crystals.The appearance of superconductivity strongly depends on the direction and strength of the gate voltage.The transitions near14 K and 11 K show similar dependence of gate voltage and magnetic field,suggesting the corresponding superconducting phase of 11 K is not a disorder,but an unknown phase.The phenomenon of irreversible superconductivity occurs as increasing the process temperature?T_G?above 235 K.The measurements of upper critical field H_c2 for H||ab and H||c reveal characteristic behaviors of quasi-2D superconductors.In chapter 3,we firstly introduce the preparation method of TiOCl intercalation compounds,and then study the evolution of the electrical properties and spin-Peierls transition under alkaline earth metal or magnetic atom intercalation.The crystal structure of the intercalation compounds change from Pmmn to Immm.The resistance of intercalation compounds exhibits near-semiconductor behavior and the spin-Peierls transition near 65 K is strongly suppressed.The experiments provide a possible inspiration for achieving metallization or even superconducting transition of TiOCl.In chapter 4,we report a systematic study on the magnetotransport properties of the single crystal of Cadmium?Cd?.The theoretical calculation suggests that Cd is a topological insulator with the spin-orbit coupling?SOC?,and/or a high symmetry line semimetal without SOC.The resistivities for both directions?I//a and I//c?show field induced metal-to-insulator-like transitions and the transition temperature increases as increasing the strength of magnetic field.The MR exhibits near quadratic magnetic field dependence at low temperatures and respectively reaches up to1,140,000%and 350,000%at 2 K and 9 T for both directions.Interestingly,the negative longitudinal MR?only observed in the c-axis direction?strongly depends on the direction of the crystal axis and the electrode position on the single crystal.We speculate that this negative longitudinal MR might be associated with the dislocations formed in the process of crystal growing along c-axis,but the chiral anomaly effect cannot be excluded.Interestingly,both the MR and the Hall resistivity show significant oscillations with period of B rather than 1/B.As increasing the temperature,the oscillation period does not change,but the amplitude gradually decreases.In chapter 5,we report the electrical transport properties and magnetic properties of NbSiAs single crystals crystallizing in the layered PbFCl-type structure.The resistivity exhibits metal behavior in the temperature range of 2 K to 300 K,and no transition was observed at low temperatures.The magnetic susceptibility exhibits anomalous diamagnetic behavior,indicating that the effective electron mass in the system is much smaller than the mass of electrons?m*<<m?.These experimental results show that the NbSiAs is a normal metal.In chapter 6,we briefly summarize the content of this thesis and outlook the further research and application of these materials.