Study On Topological Superconductivity Of Sr_xBi₂Se₃ And Superconductivity Of BiS₂-based Superconductors

Layered compounds,represented by copper oxide and iron-based superconductors,are important directions for exploring new superconducting materials.Recently,a new layered superconducting system with the same superconducting BiS₂ layers named BiS₂-base superconductors has been reported.Bi2Se3 and Bi2Te3,which also have a layered structure,are typical three-dimensional topological insulators due to strong spin-orbit coupling.Superconductivity can be induced in Bi2Se3 and Bi2Te3 by high pressure or doping,which becomes a promising approach for realizing topological superconductivity.In this paper,we successfully induced superconductivity in Bi2Se3 by intercalation of strontium atoms,and investigated the effect of quenching and magnetic element doping on the superconductivity of Bi2OS₂.This thesis is divided into four chapters.In chapter 1,after giving a brief introduction to the basic concepts,novel electronic properties,and material realizations of topological insulators and bulk topological superconductors,we focus the discussion on recent progress in the exploration of topological superconductors based on topological insulators.In detail,the crystal and electronic structures,model Hamiltonians,and possible nematic odd-parity superconductivity in CuxBi2Se3 and SrxBi2Se3 are discussed.We also briefly introduce other candidates of bulk topological superconductors,including NbxBi2Se3.In chapter 2,we comprehensively review the diversity of the crystal structure and physical properties of the BiS₂-based superconductors.We pay our attention to the impact of carriers doping,high pressure and chemical pressure on superconductivity,and the coexistence of superconductivity and magnetism in the BiS₂ family.In chapter 3,by intercalation of alkaline earth metal Sr in the topological insulator Bi2Se3,we obtained high quality SrxBi2Se3 single crystal samples for the first time in the world,which showed bulk superconductivity with a transition temperature of Tc?2.5 K.Hall measurment showed SrxBi2Se3 is n-type,with carrier density??2×1019 cm-3?one order of magnitude lower than that of CuxBi2Se3??2×1020 cm 3?.From analysis of the Shubnikov-de Hass oscillations,we found transport evidence of the existence of surface states and elucidated the fact that the electronic structure of SrxBi2Se3 is closer to Bi2Se3 than CuxBi2Se3.Moreover,SrxBi2Se3 superconcuctor is stable in the air,which is important in both fundamental investigation and real application.The existence of topological surface states,lower Fermi level,more robust bulk superconductivity compared to CuxBi2Se3 make SrxBi2Se3 an ideal platform for investigating topological superconductivity.In chapter 4,we successfully induced superconductivity with TC?4.7 K by quenching the previously reported insulating compound Bi2OS₂ in high tempreture.Measurements of Hall effect and the obey of Kohler' rule indicate its single-band charactre,with low n-type carrier density?1019 cm-3?.Analysis with XRD measurement indicates a structural phase transition occurs,which is likely to form a new superconducting phase.By studying the vortex dynamics behavior of the quenched Bi2OS₂,we found that its critical current density?JC?is 3-4 times larger than that of Bi4O4S3,and point pinning dominates the pinning mechanism.We think that this is because the quenching process makes the sample retain many impurities as the point pinning center.Then we studied the effect of Mn doping in quenched Bi2OS₂ superconductor,and found not only the suppression of superconductivity,but also the coexistence of superconductivity and anti ferromagnetism.We attruibuted this phenomenon to Mn doping both in the superconducting BiS₂ layers and in the spacer layers.