The Study Of Superconductivity And Structure Of Bismuth-Chalcogenide Superconductors

Only two members of high-transition-temperature superconductors have been reported, the cuprate and iron-based superconductors. Though the high-transition-temperature superconduting mechanism is still unclear, the common features of their structure and research experience may throw some light on the way to ex-ploration of novel superconductors. In the first chapter, we briefly introduce some fundamental concept and theory of the superconductivity, and then pay attention to the research history and latest progress of cuprate and iron-based hight-transition-temperature superconductors.Both the cuprate and iron-based high-transition-temperature superconduc-tors are 2D layered materials, composed of an alternate stacking of a common superconduting layer and a spacer layer along c-axis. Therefore, materials with a layered crystal structure have been intensively studied as a promising approach to the exploration of new high-transition-temperature superconductors. The recent-ly reported BiCh2-based (Ch:S. Se) superconductors also have the 2D layered structure, and share the experience of iron-based high-transition-temperature su-perconductors in the research methods. Bismuth-Chalcogenide compounds attract a lot of attentions, because they contain most of the topological insulators, such as the Bi2Se3, Bi2Te3 and Sb2Te3. Superconductivity could be induced by inter-calation of metal atom or large external pressure. For example, CuτBi2Se3 and SxTBi2Se3 have been focused with interest, as the promising topological supercon-ductor candidates. In the second chapter, we classify the Bismuth-Chalcogenide superconductors as different system, and then introduce their structure, super-conducting properties and the newest research progress in detail.We have accomplished some interesting work on the research of Bismuth-Chalcogenide superconductor system. For the Bi-O-S structure superconductors, we confirmed the previous reported Bi4O4S3 superconductor actually consists of two different phases. Bi3O2S3 superconductive phase and Bi2OS2 insulated phase. We denoted Bi3O2S3 as a conventional type-Ⅱ superconductor with a multi-band structure by the detailed research of its structure, superconducting properties, and the F-doped effect. We also transformed the insulated Bi2OS2 into a super-conductor by F-doping, and found the best doping ratio was 0.24, about half of the ReO1-*xFxBiS2.This work is showed in the third chapter.LaO1-xIFxBiSe2 single crystals have been synthesised by the assist of the flux CsCl and KCl. We found that F doping can increase the superconductivity, and the largest doping ratio saturates at about 0.5. The upper critical magnetic fields of LaO1-xFxBiSe2 single crystal along different directions existed a large deviation, which means there is an obvious anisotropy. The value of anisotropic constant γs is estimated about 30. The result of magnetic susceptibility and heat capacity measurement provide evidences for bulk superconductivity, and the superconducting volume fraction is almost 100%. The fourth chapter introduces this work in detail.Our group have reported a 2.9K superconductivity in SrxBi2Se3 for the first time in the international. The structure remains the same with Bi2Se3. and a Dirac-fermion-like surface state is found by the Sd-H oscillation under high mag-netic field. These facts make it possible to be a topological superconductor candi-date. We studied its superconductivity and found a fact that it has almost 100% superconducting volume fraction, and very stable in the air atmosphere. The ex-ternal pressure effect can suppress the superconductivity efficiently, and decrease the carrier densitv.