Growth And Superconductivity Of Single Crystal La_1-xTa_xOBbiS₂ Superconductor

Since the superconductor has special properties,a lot of scientists have effort on the depth study of it for more than one hundred years.And the layered superconductors is the focus.The new layered bismuth-based superconductors have been discovered in June 2012 and attracted close attention quickly.Through the experience of copper oxide and Fe-based high-temperature superconductors,a series of BiS₂-based superconductors have been come out in a short time and formed a large family of BiS₂-based superconductors.The superconducting mechanism of the layered high-temperature superconductors have not been clearly understood today,but the study of the new layered superconductors could provide us a lot of useful information.In this essay,we focus on the novel layered BiS₂-based superconductor of the La_1-xTa_xOBiS₂,which grown by a high-temperature flux method,explore the best growth process of the single crystals and characterize the properties of them,including the cystal structure,microstructure,anisotropy,magnetic and electrical transport properties.Additionally,we try to explore the possible reason of the superconductivity in the LaOBiS₂.The first chapter of this essay is about brief introduction,including the discovery of superconductivity,reviewing the study history of superconductor for more than one hundred years and the microcosmic mechanism of superconductor.There have a highlight about the relevant literature review,the research of polycrystal and single crystal of BiS₂-based superconductors.Finally,the research content and structure of this essay have been elaborated.The second chapter mainly describes the sample preparation methods,experimental reagents,experimental equipment,sample analysis methods involved in the experimental process and the growth process of the single crystal La_1-xTa_xOBiS₂ superconductor.There have a description of the solid state reaction method and a high-temperature flux method.And then,the method of characterization is introduced,including crystal structure,microstructure,magnetic properties and electrical transport properties of the samples.Finally,there have a detailed description of the growth process of single crystal La_1-xTa_xOBiS₂ superconductor.The third chapter mainly describes the growth of the single crystal La_1-xTa_xOBiS₂ superconductor.We obtain the millimeter single crystal successfully by a high-temperature flux method and optimize the growth process,including the doping amount,the type of flux,the type of raw material,the type of reaction vessel and the heat treatment process.The crystal structure of the La_1-xTa_xOBiS₂ is tetragonal CeOBiS₂ type structure,belong to the space group of P4/nmm and ·exist some impurities.The single crystal has a nucleation automatically,growing bigger through two-dimensional along the ab plane and the final shape is the sheet.Finally,the M-T result shows that,when the x = 0.1,the Tc of the bigger single cystal and small one is 3.02 K and 2.92 K.the Tirr is 2.36 K?2.48 K,but when thex =0.15,the Tc is 3.06 K,the Tirr is 2.39 K.The fourth chapter mainly describes the crystal structure,phase composition,mechanism of phase formation,microstructure,anisotropy and superconductivity of the LaOBiS₂ single crystal and polycrystal,which grown by the solid state reaction method and a high-temperature flux method,and try to explore the possible reason of the superconductivity in it.The XPS result shows that there have any other elements in the pure single crystal and the purity is very high.The crystal structure of the LaOBiS₂ is tetragonal CeOBiS₂ type structure,belong to the space group of P4/nmm and the Tc is about 3 K.The single crystal XRD shows that there have three possible crystal structure of the LaOBiS₂ including tetragonal,monoclinic and triclinic,and the part of samples have some defects.We find that the fermi surface of these three type structure is in band gap and have no metal feature,but the density of state is increasing at the fermi surface by introducing the point defect and the quality increase quickly.Finally,we predict that the possible reason of the superconductivity in the LaOBiS₂ is the point defect or the unstable crystal structure or both.By use of the transition temperature Tc and the irreversible transition temperature Tirr,we acquire the magnetic anisotropy parameter y of the pure LaOBiS₂ single crystal is 3.1 and 7.24,which shows the weak magnetic anisotropy.The last chapter studies the effect of different heat treatment temperature on the properties of the polycrystalline La_0.9Ta_0.1OBiS₂ superconductor which prepared by the solid state reaction method,optimizes the heat treatment temperature and obtains the phase diagram.The research shows that,the superconductor has excellent performance when the heat treatment temperature is 950 ?.The crystal structure is tetragonal CeOBiS₂ type,belong to the space group of P4/nmm and exist some impurities.The grain is smaller,compact and the connection is also better.The sample has the biggest RRR,which shows the metal property of the sample are obvious at low temperature,and the gap of metal-semiconductor is the lowest,thus the temperature at which the structural phase transition occurs is the lowest.At the same time,the upper critical magnetic field Hc2?0?is 1.94 T and the coherence length ? is 13.03 nm.