Vortex Dynamics Of FeSe_xTe_1-x Single Crystal And Bi-based Oxides

In 2008,the superconductivity with Tc=26 K in iron-based superconductor(SC)LaO1-xFxFeAs(1111)was discoveried by Hosono,subsequently the superconducting transition temperature was quickly raised to 56 K by means of substitution and pressure.After the discovery of SC in 1111 system,the other type SCs such as(Ba,K)Fe2As2(122),LiFeAs(111)and FeSe(11)were discoveried.The iron-based SCs display smaller anisotropy and larger coherence length compared with cuprate compounds.These characteristics make the iron-based SCs to be potential candidates for practical application.Ba1-xKxBiO3-?(BKBO)and BaPb1-xBixO3-?(BPBO)are the SCs which were doped in the BaBiO3.They display simple structure,larger coherence length and isotropy.In this thesis,the structure,magnetic properties and superconductivity of FeSexTe1-x single crystal,Ba0.67K0.33BiO3-? single crystal and BaPb1-xBixO3-?polycrystal were investigated in detail.Based on the results of upper critical field Hc2,critical current density Jc,pinning energy and the relaxtion rate,we discussed the vortex dynamics.In chapter one,the history of the research on superconductivity and the vortex dynamics theory were introduced,then the background knowledge of FeSexTe1-x,BKBO and BPBO SCs were introduced.In chapter two,we introduced the method for preparing the FeSexTe1-x single crystals,Ba0.67K0.33BiO3-? single crystal and BaPb1-xBixO3-? polycrystals and the experiment method.In chapter three,we introduced the characterization of FeSexTe1-x single crystals.The upper critical field Hc2,anisotropy,flux pinning energy U of FeSexTe1-x(x =0.5,0.4),based on the electrical transport measurements in different magnetic fields rotated from the c-axis to ab-plane of the crystal were discussed.The result indicated that the crystals have a higher upper critical field and a smaller anisotropy.By measuring the dynamic magnetization relaxtions we discussed the vortex dynamics of FeSe0.5Te0.5 single crystal.The critical current density based on the magnetic hystersis loop(MHL)is very large.Our experiments suggested that the second peak on MHL could be observed and we discussed this phenomenon.Analysis on the pinning force density indicated that there was only on pinning mechanism in the FeSeo.sTeo.s single crystal,and the pinning mechanism maybe normal cores pinning.In chapter four,we introduced the vortex dynamics of Ba0.67K0.33BiO3-? single crystal.The angular dependence of resistivity p-0(where 0 is the angle between the c-axis and the magnetic field H)at H = 0.5 T,T = 25.5 K was measured.We fitted the curve of p-0 by different models and the result indicated that the Ba0.67K0.33BiO3-?single crystal maybe have a four-fold symmetry.In addition,the upper critical field Hc2,anisotropy,flux pinning energy of Ba0.67K0.33BiO3-?,based on the electrical transport measurements in two different magnetic fields(0= 90°,135°)were discussed.In chapter five,we investigated the vortex dynamic of the BaPb1-xBixO3-?(x =0.1-0.3)samples.We obtained the phase diagram by measuring the electrical transport which could be divided into three regions:metal,semiconducting and superconducting.In the superconducting region,the doping x dependence of critical temperature could be fitted well by a parabolic equation,1-Tc/Tcmax= A(x-xc)2 with A=120.68 and xc = 0.223.The upper critical field Hc2,flux pinning energy of BaPb1-xBixO3-?(x = 0.175,0.20,0.23,0.25),based on the electrical transport measurements in different magnetic fields were discussed.The pinning energy was enhanced by Bi-doping.In low field,a possible strong collective flux pinning of 2D defects might be produced by stripe-like nanoscale structure and/or Bi3+1 and Bi5+ ions induced dislocations.Above crossover field,a weak pinning of linear-like defects and/or point defects dominated in higher field.