The Transport Study Of Strong Spin Orbit Coupling Material Of Sr₂IrO₄ And ZrBi₂

Spin orbit coupling is an interaction between the spin angular moment and orbit angular moment.In quantum mechanics,the spin orbit coupling can be described as an additional interaction in the Hamitonian.For normal systems,the interaction energy is much weaker than other interactions and be treated as a perturbation,so the spin orbit coupling influences little to the properties.But for a system containing heavy atoms,the spin orbit interaction becomes large and can influence the ground state,so many new phenomena occur in these unique systems.This paper focuses on the strong spin orbit coupling materials,the former two chapters give an introduction of spin orbit coupling and some unique states in strong spin orbit coupling materials,the later three chapters are my experiments.In chapter one,we firstly talk about the origin and classification of spin orbit coupling interaction.After that,we list and introduce some interesting phenomena occurred in the strong spin orbit copling materials,such as topological insulator,topological semimetal,topological superconductor,Axion insulator,Slator insulator and quantum spin liquid state.In chapter two,we focus on the descriptions of iridates and Sr₂IrO₄.The 5d transiton metal oxides are an unique system to study the comparable energies of spin orbit coupling,field crystal splitting energy and electron electron Coulomb repulsions.Thanks to the interplay and competition of these three interactions,many nontrival phenomena have been realized or predicted in certain correlation regime.Sr₂IrO₄ is a spin orbit coupling induced Mott insulator and shares a similar structure with the high temperature superconductor parent La₂CuO₄ both structurally and electrically.Considering those,Sr₂IrAO₄ was predicted to be a new kind of high temperature superconductor parent in many theoretical studies.But so far,no one convince that.The two promising studies are the observed gapless d-wave like STS spectroscopy and Fermi arc in ARPES measurements,while no other evidences show that the signs come from unconventional superconductivity.From the doping and high pressure experiments of Sr2Ir04,it is concluded that the insulating nature of polycrystalline Sr₂IrO₄ samples are robust to chemical doping and pressure.The descriptions from chapter three to chapter five are my experiments.The first work is the study of Ga doped Sr₂IrO₄.We successfully synthesized a series of Ga doped Sr₂IrO₄ polycrystals.XRD measurement indicates no structure change was caused through doping,while XPS and Hall resistivity measurements show the electron carriers were introduced.The electrical measurement shows Ga doping enhances conductivity.When doping content reaches to 5%,a metal insulator transition appears.The enhanced conductivity could be attributed to the reduced energy gap and the increased carrier concentrations.The magnetic transport measurements show Ga doping decreases the weak ferromagnetism of Sr₂IrO₄.In the doping content of 2.5%-5%,a sudden increased magnetic moment as temperature decreasing in M-T curve were observed below the Curie temperature.The suppressed ferromagnetism and sudden increased magnetic moment were considered to be caused by the dilute Ir³⁺.In chapter four,we study the effects of Ir-O-Ir bond angle and carrier concentration on the transport of Sr₂IrO₄.We successfully synthesized two series of doped Sr₂IrO₄ polycrystals,which are Ba doped samples and La doped samples.The XRD and Hall resistivity measurements show that the Ba doping leads to significant straightening of the Ir-O-Ir bond angle while it does not introduce too much extra charge carriers.On the other hand,the La doping has less influence on the Ir-O-Ir bond angle but it efficiently introduces electron charge carriers into the Sr₂IrO₄ compound.Accordingly,the transport measurements show that the La doping in the Sr₂IrO₄ system results in an enhanced conductance and a reduced magnetic moment compared with those in Ba doped samples.From the above results,we conclude that the bended Ir-O-Ir bond angle plays a less important role in the robustness of the insulating state than the carrier type and concentration.In chapter five,we show the study of a kind of bismuth compound,ZrBi₂.ZrBi₂ single crystal were synthesized through self-flux method.The electrical measurement shows no sign of superconductivity down to 2K,the magnetoresistance reaches to 400%at 2K when the applied field is 8.5T.The magnetoresistances measured at different temperature obey the Kohler’s rule,suggesting only a single scattering rate on the Fermi surface.The chapter six are the conclusions and outlooks.