Preparations And Physical Properties Of Iron-Based Intercalated Superconducting Single Crystals

Iron-based superconductors（IBSs）have opened up the new chapter in the study of high temperature superconductivity after cuprates superconductors,and the synthesis of intercalated superconductors have hewed out the new direction for exploring the mechanism of high temperature superconductivity.The ACa₂Fe₄As₄F₂（A=K,Rb,Cs）single crystals are recently reported new type of Fe As-based self-doped intercalated superconductors.It consists of 1111-and 122-type structures through orderly stacking,forming a unique double Fe₂As₂ layers structure separated by insulating Ca₂F₂ layers.The high anisotropy of upper critical magnetic field and resistivity between in-plane and out-of-plane in normal state indicate extremely quasi-two-dimensional electronic behavior.The obvious resistivity tail in the superconducting transition region under magnetic field and the relatively low irreversible field indicate the weak interlayer coupling and significant flux motion.The angle-resolved photoelectron spectroscopy reveals the bilayer splitting effect contributed by the interlayer coupling.These characteristics are closer to the physical properties of cuprates superconductors.Thus,it provides a new platform for exploring the pairing symmetry of iron-based superconductors and elucidating the mechanism of high temperature superconductivity.We prepared high quality RbCa₂Fe₄As₄F₂single crystals with superconducting transition temperature T_c～31 K by self-flux method,studied the Ni doping effect on the superconductivity of the 12442 system,and explored the flux dynamics of RbCa₂Fe₄As₄F₂ single crystals in detail.In addition,because of the electrical neutral layer and weak attraction force between layers of 11system,the FeSe-based intercalated superconductors can be synthesized by soft chemical method and show great controllability in superconductivity,which is the current research hotspot.Based on the hydrothermal ion de-intercalation method,we prepared full-range S-doped FeSe_1-xS_x（0≤x≤1）single crystals and plotted the complete phase diagram of FeSe_1-xS_x system.The main contents and conclusions of this paper are as follows:（1）We studied the Ni doping effect of RbCa₂Fe₄As₄F₂ single crystals.Firstly,a series of Ni-doped RbCa₂(Fe_1-xNi_x)₄As₄F₂（0≤x≤0.1）single crystals have been successfully prepared by self-flux method.It is found that Ni dopants not only introduce extra electrons that modify the topology of Fermi surface,but also act as impurity scattering centers that contribute to the pair breaking effect.The superconducting transition temperature T_c is suppressed with a rate ofΔT_c/Ni-1%=-2.7 K.Such suppression of T_c and those in other similar hole doped IBSs,such as Ba_0.6K_0.4Fe₂As₂,Ba_0.5K_0.5Fe₂As₂,and Eu RbFe₄As₄ with multiple nodelss gaps,can be well scaled together.Combining with relevant experimental data reported so far,we speculate that the pairing symmetry in 12442 system is very likely to be nodeless s±-wave.Upon Ni doping,the coherence lengthξ_c（0）is gradually increased and becomes larger than the Fe₂As₂ interbilayer distance when x>0.07,indicating that the nature of superconductivity changes from quasi two-dimensional to three-dimensional.The anisotropy of the upper critical field which first increases from 6.7 at the pristine sample to 8.1 at x=0.03 close to T_c and then systematically decreases to 3.7 at x=0.09 shows a nonmonotonic dependence on doping.（2）We studied the flux pinning behavior of RbCa₂Fe₄As₄F₂ single crystals.Based on the magneto-transport and magnetization measurements,it is found that the resistive transition under magnetic fields shows a foot-like kink at a characteristic temperature T_s,followed by a resistive tail in nearly zero resistivity region.Such behavior is ascribed to a vortex slush transition at T_s,below which the vortex state has short-range vortex lattice correlation,and then a second-order transition into the vortex glass phase occurs with further decreasing temperature.Above T_s,the Arrhenius plot of resistivity shows two linear regions that are separated by a crossover line T_cr（H）,which is associated with a crossover from collective to plastic pinning or different flux pinning behaviors resulted from different types of defect.In addition,the magnetic hysteresis loops reveal a second magnetization peak（SMP）,and the peak H_sp shows a non-monotonic temperature-dependent relationship,which anomalous behavior has never been observed in IBSs.Finally,the vortex phase diagram of RbCa₂Fe₄As₄F₂ is constructed according to the measured results.（3）We investigated the effect of disorder on the second magnetization peak（SMP）effect of RbCa₂Fe₄As₄F₂ single crystals by three selected RbCa₂Fe₄As₄F₂single crystals with the same T_c but different disorder levels.Although their self-field critical current density J_c at 3 K are basically similar,slightly more than 10⁶ A cm^-2,however,only the sample S2 owning moderate disorder level shows significant SMP,which manifests the benefit of a moderate amount of disorder in superconducting RbCa₂Fe₄As₄F₂ single crystals.The evolution of the normalized pinning force density f_p against the reduced field h=H/H_irr matches well with the behavior of non-monotonic SMP,and demonstrates the type of pinning changing from the dominant density weak pinning at low temperatures to strong pinning at high temperatures mainly caused by natural stacking fault of intergrowth layers.The normalized magnetic relaxation indicates a fast vortex creep,and SMP is proved to be strongly associated with the elastic to plastic（E-P）vortex crossover.The anomalous SMP driven by the E-P phase transition gradually evolves into a step-like transition near the irreversible field,which connects the SMP of high temperature superconductors at low field and that of low temperature superconductors at high field,revealing the possible common origin from E-P phase transition.（4）We synthesized the full-range S-doped FeSe_1-xS_x（0≤x≤1）single crystals via a hydrothermal method and the complete phase diagram has been achieved based on the resistivity and magnetization measurements,which provides a sample platform and phase diagram reference for subsequent study of the system.The nematicity is found to be suppressed with the increase of x,and a small superconducting dome appears inside the nematic phase.Outside the nematic phase,the superconductivity is continuously suppressed and reaches a minimum T_c at x=0.45,beyond this point,T_cslowly increases until x=1.Furthermore,the normal state resistivity evolves from non-Fermi-to Fermi-liquid behavior with S doping at low temperatures,accompanied by a reduction in electronic correlations.An anomalously resistive upturn at a characteristic temperature T^* in the intermediate region 0.31≤x≤0.71 is observed.T^* shows a domelike behavior with a maximum value at x=0.45,which is opposite to the evolution of T_c,indicating the competition between T^*and superconductivity.