Comparative Study Of Vortex Dynamics In Ba_0.6K_0.4Fe₂As₂ And CaKFe₄As₄ Single Crystals

The vortex dynamics is an important part in the research of superconductivity.In type II superconductors,the magnetic field will penetrate into the superconductor in the form of quantized flux when the magnetic field is larger than the lower critical field Hci.And then the so-called mixed state can be formed.Many important researches on the vortex dynamics have been carried out in cuprate high-Tc superconductors.Iron-based high-Tc superconductors have similar layer structure as cuprate superconductors,but they are different in some cases.This may affect the vortex properties.Hence,the study on vortex dynamics in iron-based superconductors is very necessary and important.The vortex dynamics are studied in two hole-doped materials in this thesis,i.e.,Ba0.6K0.4Fe2As2 and CaKFe4As4.The results of vortex dynamics in these two materials may provide fruitful information to understand the vortex dynamics in iron-based superconductor.In chapter I,the history and basic properties of superconductor especially the iron-based superconductor are introduced.The basic concepts and models of vortex dynamics are introduced after that.Finally,we also introduce the second peak effect in magnetic hysteresis loops,which is also related to vortex properties.In chapter II,we investigate the vortex dynamics in Ba0.6K0.4Fe2As2 single crystal.The M(H)curves show the second-peak effect which is more pronounced at high temperatures close to critical temperature Tc.The calculated transient current density exceeds 105 A/cm2 at 2 K.And we also study the vortex dynamics by dynamic and conventional relaxation methods.The weak temperature and magnetic field dependence of relaxation rate suggest the strong pinning for vortices in this material in intermediate temperature or field region.The time dependent magnetization curves can be scaled by Maley's method,which indicates that the collective vortex pinning model is applicable in the material.And we also calculate the glass exponent ?.Finally,we present the phase diagram of this sample.In chapter III,the vortex dynamics of CaKFe4As4 are studied.The structure of this material are a bit different from Ba0.6K0.4Fe2As2 although they are both hole doped samples.The M(H)curves don't show clear fishtail effect,but they behave differently at different temperatures.And the peak effect of JS(T)curve at constant magnetic field is observed,which is similar with the second peak effect of magnetization versus magnetic field.The methods of dynamic and conventional magnetization relaxation are also adopted to analyze the vortex dynamics in this sample.The relaxation rates show strong temperature dependence.And we also find that the Maley's method doesn't work very well in this material.The phase diagram is displayed in the end.Chapter IV is a summary of this thesis.We summarize the similarity and difference of vortex dynamics between the two samples.