The Exploration Of Cuprate Superconductors Under High Pressure

Since Onnes firstly discovered the superconductivity in 1911,the research on superconductivity has never stopped.Many superconductors,especially unconventional superconductors,including heavy fermion superconductors,organic superconductors,cuprate superconductors and iron superconductors were discoverd,which has greatly promoted the development of superconductivity.The application of superconductivity is limited by the superconducting transition temperature.Only below the transition temperature,the sample can become superconductive.Thus,liquid nitrogen and liquid helium are usually needed as cooling cryogent.However,helium resource is rather scarce on earth.Thus a superconductor with higher T_c,especially above the temperature of liquid nitrogen is highly desired for applications.The two basic properties of superconductivity are zero resistance and perfect diamagnetism.A superconductor with zero resisitance means that it can carry nondissipative supercurrent.For a superconductor,the irreversibility line H_irr(T)separates the phase diagram into zero and finite resistive dissipation.Usually a superconductor with a higher irreversibility magnetic field can carry higher nondissipative supercurrent.In the cuprate family,for Hg-and Tl-based systems,the toxic elements Hg and Tl strongly limit the high-power applications of these materials.The Bi-based system is nontoxic,but the layered structure and huge anisotropy do not allow a high irreversibility field at the liquid nitrogen temperature.The irreversibility field and superconducting current density decrease rapidly with increasing temperature in the moderate temperature region for the Bi-based system.The Y-based YBa₂Cu₃O₇-?is nontoxic and has a high irreversibility field,and is thought to be a promising material for applications.Howevre,it is extremely difficult to produce a long superconducting wire for the short coherent length,this is why we can not realize large-scale applications up to now.Thus,it is very crucial to find a nontoxic superconductor with T_c beyond liquid nitrogen boiling temperature and high irreversibility magnetic field for applications.As an extreme physical condition,high pressure synthesis has the advantages of improving the reacting efficiency and synthesizing materials which are difficult to be synthesized under ambient pressure.Thus it plays an important role in exploration of new superconductors.In this thesis,we report the successful synthesis of a nontoxic cuprate superconductor(Cu,C)Ba₂Ca₃Cu₄O11+?under high pressure and high temperature.And we find that it has the highest irreversibility magnetic field in liquid nitrogen temperature region,which is thought to be promising for large scale applications.Meanwhile,we also fabricate a new nontoxic cuprate superconducting system Ga-Ba-Ca-Cu-O with the highest transition temperature of 116K.The achievements and significance of this thesis are summarized as follows:We successfully synthesized the nontoxic cuprate superconductor(Cu,C)Ba₂Ca₃Cu₄O11+?with volume fraction higher than 90%under high pressure and high temperature,the transition temperature is about 116K.By choosing the criterion of resistivity of 1%?_n(T_c),we can determine the irreversibility field,which is 15 T at about 82K,5T at 94.2K and 3T at 98.6K.We also give a comparison of the irreversibility lines between our samples and other cuprate systems(including polycrystals,films/single crystals with H//c).We can see that the irreversibility field of(Cu,C)Ba₂Ca₃Cu₄O11+?is the highest in the temperature region of 77K to 116K.Meanwhile,we find that we could not reach superconductivity without carbon involved in the last step of high-pressure and high-temperature sintering.If we could synthesize the sample under ambient pressure or grow(Cu,C)Ba₂Ca₃Cu₄O11+?thin films,we can probably make superconducting wires and high field magnet based on(Cu,C)Ba₂Ca₃Cu₄O11+?.Thus,this material has great potential for applications.In addition,we have synthesized a new nontoxic cuprate system Ga-Ba-Ca-Cu-O by solid state reaction method under 3.7GPa.We find that there are two superconducting phases in the sample.The phase with T_c of 82K corresponds to GaBa₂Ca₅Cu₆O₁₄₊?and the phase with T_c of 113K/116K(T_c varies depending on the oxygen content)may correspond to GaBa₂Ca₃Cu₄O₁₀₊?or GaBa₂Ca₂Cu₃O₈₊?.Then,we synthesized the sample under a pressure up to 10 GPa and measured the the temperature dependence of magnetic susceptibility of the sample.The morphology characterization and composition analysis on the samples were completed by using SEM.The result of SEM confirms that the phase with T_c of 116K actually corresponds to GaBa₂Ca₃Cu₄O₁₀₊?.And the photographs of SEM show that there are some small single crystals with size of 10-20?m in the sample.If larger single crystals can be systhesized,one can measure the intrinsic properties.The discovery of this superconducting system may provide a new platform for the study of superconducting mechanism of cuprates.