Research On The Doping Effect And Transport Properties For Iron-based Superconductors

Iron-based superconductors（IBSs）have attracted much research attention because of their fascinating properties.Among IBSs,11-system has the simplest structure,making it to be an ideal material to probe the mechanism of high temperature superconductivity.Meanwhile,topological superconductivity,Majorana zero energy model,and the superconducting coupling strength in the BCS-BEC crossover region were discovered in Fe Te Se,which have triggered a research boom in it.In this paper,the behaviors of upper critical field and flux motion in Fe_1+yTe_0.6Se_0.4 affected by the excess iron have been systematically studied.In addition,1111-system has the highest bulk T_c among IBS and very large upper critical fields,which has great application potential.Among the 1111-system,large-size fluorine-based superconductor Ca Fe As F single crystals can be prepared,which is advantage for studying its intrinsic properties.The main contents and results of this dissertation are as follows:1.Upper critical field H_c2（T）of Fe_1+yTe_0.6Se_0.4 single crystals with controlled amounts of excess Fe were investigated by resistivity measurements over a wide range of temperatures and magnetic fields.H_c2（T）with magnetic fields along c axis(H_c2^c（T）)was fitted using the two-band model,and the change in the SC coupling strength was discussed.Meanwhile,H_c2（T）with magnetic fields in the ab plane(H_c2^ab（T）)was fitted using the WHH model and a strong paramagnetic effect was discovered.A crossover behavior at low temperature for H_c2^c（T）and H_c2^ab（T）indicates the presence of a strong anisotropic spin-paramagnetic effect.To understand the strong anisotropic spin-paramagnetic effect,a spin-locking model was proposed,and our results support the local-pairing scenario.Finally,based on the estimation of the coherence length at 0 Kξ_c（0 K）～9.5?,it is speculated that the Fe_1+yTe_1-xSe_x could be a quasi-two-dimensional superconductor.2.We found that the in-plane anisotropy of critical current density J_c2/J_c1 is～1under self-field,while increases when temperature approaches T_c.The out-of-plane anisotropy of J_c2/J_c3 is estimated as large as 7-11.The discrepancy between the in-plane and out-of-plane anisotropy is discussed to be from the lack of pinning centers in layers.A non-scaling behavior of（reduced field h dependence of）reduced pinning force f_p-h curves caused by the magnetization relaxation is found.The critical state J_c and pinning force F_p regained by vortex dynamic analysis manifest that the pinning type in the annealed Fe_1+yTe_0.6Se_0.4 single crystals is normal point pinning.Our results suggest that the transport current should be applied in ab-plane at temperatures lower than 10 K for high power applications.3.The millimeter-size Ca Fe_0.89Co_0.11As F single crystals were synthesized by self-flux method.The phase transition from vortex glass state to the normal state was systematically studied through the magnetotransport measurements,and some characteristic temperatures and fields(H_c2,T_k,T^*,T_g,and T_fc)were obtained.In the thermally activated flux flow region,three dimensional vortex nature was confirmed.In the zero-resistivity region,the second magnetization peak（SMP）involving elastic-plastic（E-P）phase transition was observed through the magnetization measurements,and the magnetic field of SMP(H_sp)was obtained.Meanwhile,the dominant normal point pinning in Ca Fe_0.89Co_0.11As F was confirmed.Finally,the vortex phase diagram of Ca Fe_0.89Co_0.11As F was established.Details on the various vortex region and relevant physical scenarios were discussed.These preliminary results not only enrich the vortex phase diagram of iron-based superconductors,but also provide some information for the understanding of vortex dynamics of the 1111 system.