Vortex Dynamics For Iron-based Superconductors

The iron-based superconductors display high superconducting transition temperature, metallic characteristic, smaller anisotropy and larger coherence length compared with high Tc cuprate compounds, which in principle benefits for current transport across grain boundaries. These characteristics make the iron-based materials to be potential candidates for practical application in the field of superconductivity. Therefore, further investigations on the vortex dynamics and fluctuation conductivity behavior, especially making clear the relationship between the microstructure structure (grain boundary and dislocation, etc.) and the pinning mechanism are of great significance, which has been become one of the most new hot topics in the condensed matter physics.In this thesis, the angular dependences of upper critical field HC2, vortex glass transition field Hg and H*, and flux pinning energy U of BaFe1.9Ni0.1As2single crystals, based on the electrical transport measurements in different magnetic fields rotated from the caxis to ab-plane of the crystal were discussed. The vortex phase diagrams H-T and H-6were obtained (where θ is the angle between the c-axis and the magnetic field H). The effect of grain boundaries on the pinning mechanism is also discussed. Furthermore, the oxygen annealing effects on the superconductivity of Fe1.06Te0.6Se0.4, as well as the fluctuation conductivity of the phase-separated K0.78Fe1.75Se2compound are also investigated.In chapter one, the crystal structure, upper critical field, irreversible field, pinning potential, critical current density, magnetic relaxation and conductivity fluctuation for different families of iron-based superconductors were introduced. The vortex dynamics behaviors, including the δTc and δl pinning mechanism and superconducting phase diagram were also displayed. In addition, we also introduced the effect of different annealing conditions on the superconductivity of Fe1+y/Te1-xSex single crystals.In chapter two, the angular dependences of upper critical field Hc2, vortex glass transition field Hg and H*, and flux pinning energy U of BaFe1.9Nio.1As2single crystals were studied, based on the electrical transport measurements in different magnetic fields rotated from the c-axis to ab-plane of the crystal. The vortex phase diagrams H-T and H-θ were obtained. It is found that the dominant pinning mechanism is the δ8Tc pinning in the high crystalline quality BaFe1.9Ni0.1As2sample without appreciable defects, which can fit the data of the critical current density well. However, with introducing defects such as grain boundaries and decreasing the superconducting transition temperature Tc, the δl pinning mechanism will emerge into the system.In chapter three, the oxygen annealing effects on the superconductivity of Fe1.06Te0.6Se0.4were systematically investigated by measuring the resistivity, magnetic susceptibility, Hall coefficient and X-ray photoelectron spectroscopy. An increase in carrier density nH due to oxygen annealing was observed by Hall coefficient. Furthermore, through the X-ray photoelectron spectroscopy experiments, it was found that the oxygen annealing changes Fe0and Te0states to Fe2+/3+and Te4+respectively, while the valence variation of Se is negligible. It is found that the specimen annealed in oxygen shows a clear superconductivity enhancement, which is closely related to the increase of carrier density nH as well as the changes of valence states in Fe and Te ions.In chapter four, the fluctuation conductivity of the BaFe1.9Ni0.1As2and phase-separated K0.78Fe1.75Se2single crystals were systematically investigated by measuring the magnetoresistance at various fields with different field directions. It is found that the fluctuation of conductivity in K0.78Fe1.75Se2single crystal reveals the3D scaling behavior for H//c, but matches the2D scaling form for H//ab, displaying a3D-2D crossover behavior with increasing the angle9from c-axis to ab-plane, where θ is the angle between the c-axis and the magnetic field H. As for the BaFe1.9Ni0.1As2single crystal, the fluctuation conductivity always displays the3D scaling behavior for all the angles. These results indicate that the dimensionality of the superconductivity in K0.78Fe1.75Se2material is strongly affected by the field direction due to the phase separation.In chapter five, the magnetic relaxation behaviors of K0.78Fe1.75Se2, FeTeo.6Seo.4and BaFe1.9Ni0.1As2single crystals were studied by measuring the relaxation behaviors of magnetization. The temperature and field dependencies of pinning potential U0and reduced relaxation rate of S were obtained by the Anderson-Kim formula. With increasing temperature and magnetic field, the pinning potential U0decreases but the reduced relaxation rate of S increases for the K0.78Fe1.75Se2and FeTe0.6Se0.4single crystals. As for the BaFe1.9Ni0.1As2single crystal with apparent fish tail effect, pinning potential U0gradually increases first, and then decreases with increasing temperature/field, reaches a maximum at certain temperature/field. These results indicates that the fish tail effect in BaFe1.9Ni0.1As2single crystal is closely related to the temperature/field dependencies of the pinning potential and reduced relaxation rate.