Research Of Li,Ir,Pb-doped Topological Insulator Bi₂Se₃ And Compositeds With FeSe

Recently,topological insulators（TIs）and superconducting materials have become hot topics in the reseach of physics.TIs are materials with a bulk band gap but have a conducting surface state.Its surface states are protected by time-reversal symmetry and not easy to be affected by general impurities.Bi₂Se₃ has relatively large bulk energy gap,simple band structure and a single Dirac cone in the Brillouin zone,which makes it stands out in numerous TIs.The ferromagnetism in TIs will break surface states,induce spin-orbit coupling gap and realize Quantum Anomalous Hall Effect（QAHE）,by making use of which,quantum devices are expected to be made.The QAHE has been confirmed in 2013,but it is difficult to observe it due to small size band gap and low Curie temperature of the material.Therefore,scientific research workers need to find the material that can achieve high temperature QAHE.Since having features of zero resistance,diamagnetism and Josephson,superconducting materials have broad application prospects.Iron-based superconductor materials have been found in recent years,much attention have been paid to them due to their high superconducting transition temperature,which break through the limit by BCS theory.FeSe has a simple chemical formula,easy preparation in the laboratory and dose not contain toxic heavy metal elements,these characteristics make it research widely.Majorana fermion may be found in the heterostructure between a TI and a superconductor,which is expected to realize the application of quantum computers.According to those above,the full content is divided into two parts:The first part studies the performance of Bi₂Se₃ samples doped with Li,Ir,Pb,respectively.In the Li-doped samples,obvious metal-insulator（M-I）transformation were observed under low temperature.Surface state conductivity is the dominant under low temperature;A small amount of Li can inhibit phonon scattering effect and high concentration of Li enhanced phonon scattering;In the curves of magnetic field dependent susceptibility,magnetic properties obviously changed from diamagnetic of pure Bi₂Se₃ to ferromagnetic of doping samples,suggesting that Li-doped samples can induce ferromagnetic.In the Ir/Pb-doped samples,all samples show the same rhombohedral layered structure;Most of doping atoms substitute Bi in low doping content and then intercalate into the clearances between layers of Bi₂Se₃ in high concentration;The resistivity increases first and then decreases with the doping content.The second part maks preparation of FeSe and Bi₂Se₃ mixed samples.In the FeSe-doped Bi₂Se₃ samples,all samples exhibit the same weakly metallic behavior;The sample of high doping content（7wt%）showed negative magnetoresistance in the test temperature zone;All doped samples showed ferromagnetism and the ferromagnetic enhanced with increasing FeSe content;It was considered that the origin of ferromagnetism is magnetic Fe-Se compounds.In the Bi₂Se₃-doped samples,the superconducting transition temperature（Tc,0）increases first and then decreases with the doping content;Low and high magnetic tests showed that superconducting phase coexisting with ferromagnetic phase were found in all samples,diamagnetic of samples first enhanced then weakend with increasing the doping content.