Single Crystal Growth And Transport Properties In BiS₂ Based Superconductors

The exploring and study of physical property for novel layered superconductors have always been the hot frontier in the field of superconductivity, even in Condensed matter physics. And the discovery of hight-Tc cuprates and Fe-based superconduc-tors has caused great climaxes in researches of superconductivity again and again. In June 2012, the novel layered superconductor Bi4O4S3 was discovered which has at-tracted much attention of scientists. By using the technologies and experiences in hight-Tc cuprates and Fe-based superconductors, scientists extended this system quick-ly into a new and large system-BiS2-based superconductors. The researches on BiS2-based superconductors are conducive to understanding of superconducting mechanis-m and exploring of novel layered superconductors. This thesis centres around single crystal growth, transport properties and physical properties under pressure in BiS2-based superconductors. After that we will introduce the results of pressure effect in (Li1-xFex)OHFeSe.The history of physics of superconductivity and the superconducting mechanism will be introduced in the first part of the thesis, and then we present the discovery of BiS2-based superconductors and related research progress. After that the physical properties under pressure in BiS2-based superconductors will be introduced.In the second part, the relevant knowledge about high-pressure technology and single crystal orientation will be presented. First, we will introduce the developmen-t course of high-pressure technology and mention that high-pressure technology is very important for superconductivity. And structures, principles and characteristic-s of piston-cylinder type high-pressure installation and diamond anvil cell(DAC) will be illuminated. There are two methods of pressure calibration:using the standard sample and ruby fluorescence technology. Finally, we will present the relevant knowl-edge about single crystal orientation and introduce the principle of Laue photographic method single crystal orientation.The third part of the thesis is the work on single crystal growth and physical prop-erty of NdO1-xFxBiS2. The single crystals were grown by mixture flux method, and the magnetic and resistive measurements reveal the intrinsic superconductivity. Anal-ysis based on the anisotropic Ginzburg-Landau theory gives a very large anisotropy. Measurements of excess conductivity, in-plane angle-dependent resistance and Nernst reveal a giant superconducting fluctuation. And two gap feature were observed by s-canning tunneling spectroscopy. Finally, the semiconducting behavior of normal state under magnetic is analysed.The fourth part is about correlational studies of on LaOo.5F0.5BiSe2 single crystals. By using a hydrostatic pressure, the ground state and superconductivity in LaO0.5Fo.5BiSe2 could be tuned. We found that the normal state resistivity and superconducting transi-tion temperature show nonmonotonic changes with the external pressure. The the nor-mal state resistivity decreased with the increase of pressure, and there is a semiconductor-metal transition. The data of superconducting transition temperature at various pressure reveal two distinct SC phases. The lower Tc phase is suppressed with the increase of pressure, and then a new higher Tc phase emerges above critical pressure.In the fifth part, we will introduce the pressure effect of (Li1-xFex)OHFeSe single crystals. Recently, a new intercalated iron-selenide superconductor (Li1-xFex)OHFeSe has been synthesized. We have successfully grown the single crystals by using the hy-drothermal ion-exchange method. By applying a hydrostatic pressure, the supercon-ducting transition temperature is suppressed continuously. Though comparing the re-sults of K0.8Fe1.7Se2 under pressure, we think that spacing between the adjacent FeSe-tetrahedra layers is crucial to the suppression of superconductivity.A detailed summary is presented in the end.