The Synthesis And Flux Dynamics Of Iron-based Superconductor (Li_1-xFe_x)OHFeSe

As the second high temperature superconductors after the copper oxide superconductors,iron-based superconductors have got great attention of the condensed matter physics in recent years because of its high transition temperature,high upper critical field and strong current transport capacity.As an important branch of superconducting physics research,flux dynamics has provided a good theoretical foundation for exploring the origin of flux pinning,the characteristics of vortex motion in superconductors,and the application of high temperature superconductor in strong electric field.The superconductor(Li_1–xFe_x)OHFeSe is a newly synthesized FeSe-based superconducting material.Due to its novel synthetic method,absence of obvious phase separation,and coexistence of antiferromagnetism and superconductivity,it provides a new platform to explore the mechanism of iron-based superconductors.In this paper,we have synthesized high quality(Li_1–xFe_x)OHFeSe single crystals by optimizing the experimental conditions of hydrothermal ion exchange method,and studied its transport properties,anisotropy,flux pinning mechanism,vortex phase transition,magnetic relaxation and flux dynamics characteristics.The specific research results are as follows:?1?Based on the optimized experimental parameters,we have carried out systematic research on the influence of the concentration of selenourea on the superconducting properties of(Li_1–xFe_x)OHFeSe single crystals.The results show that the optimized concentration of selenourea is 0.8 mol/L,such as the initial superconductivity transition temperature is 41.7 K,the temperature of the magnetic susceptibility measured by DC magnetic susceptibility is 40.5 K,and the residual resistivity ratio is estimated to be 11,indicating the excellent quality of the obtained(Li_1–xFe_x)OHFeSe single crystals.The critical current density is as large as 2.5×105 A/cm2 at 2 K and self field,but it decrease fast with the increasing temperature.?2?According to the Werthamer-Helfand-Hohenberg theory,the calculated upper critical field is H_c2^ab=614T and H_c2^c=63T,so the anisotropy value is 9.7.The big out-plane anisotropy behavior versus temperature has been explored according to the anisotropic Ginzburg–Landau theory,and the anisotropy factor ? displays linear behavior towards temperature.The flux pinning potential is as large as 495 K and 2261 K at 1 T in H//c and H//ab orientation,respectively.However,the flux pinning potentials are very sensitive to the applied magnetic field and fast decrease with the increasing magnetic field in both directions,indicating the weak flux pinning ability.According to the characteristics of the magnetic flux pinning density and the temperature dependence of the normalized critical current density under different magnetic fields,the flux pinning mechanism is determined as dl pinning via spatial fluctuation of charge carrier mean free path.?3?According to the theory of vortex glass and thermally activated vortex motion,we have obtained the vortex phase diagram of the mixed state and found a large vortex liquid region.The results of dynamic magnetic relaxation measurement show that the magnetic relaxation rate is as large as 15.3% at 1 T and 10 K,and has a strong dependence on magnetic and temperature.This is mainly due to the vortex thermally activation due to the poor vortex pinning ability.