The Study On The Superconducting Properties And Magnetism For The Doping And Intercalation Of The FeSe-based Superconductors

The FeSe superconductors can further improve T_c by the chemical doping or applied pressure.Currently,the study of FeSe-based superconducting materials is mainly to obtain higher transition temperatures materials by intercalation method.This paper aims to study the doping and intercalation effects to the properties and mechanism in FeSe based superconductors.Crystal structure,magnetic properties,electrical transport properties,superconducting fluctuations,and quasiparticle relaxation dynamics has been investigated in the FeSe system.In addition,the ultrafast quasiparticle dynamics of the FeSe_0.9S_0.1 single-crystal samples were investigated.This paper mainly involves S and Te elements doped in the FeSe,and(Li_0.8Fe_0.2)OH intercalation in FeSe and FeSe_1-xTe_x,studies the influence of different doping concentrations or intercalation on its crystal structure and superconducting properties,and carries out the study of quasiparticle dynamics using ultrafast spectroscopy for some samples.The FeSe_0.94 single crystals have been synthesized by using the high-temperature KCl flux technology.There are two phases in the sample found by the XRD.The Tc of FeSe_0.94 is about 9.5 K based on the magnetic and transport measurements.The coexistence of the superconductivity and the weak ferromagnetism is observed under the low temperature region.The structural phase transition is observed under the high temperature region.The FeSe_0.9S_0.1 single crystal has been synthesized by chemical vapor transport technique.The T_cof the sample is 9.5 K determined by the resistivity measurements.The structural phase transition at T_s=72 K is observed in the resistivity curves.The result has revealed that the T_sis reduced by the doping.The Fe1.05Se1_-xTex single crystals have been synthesized using the self-flux method.The effects of different doping concentration on the crystal structure and superconducting properties of the sample have been investigated.Te doping on the Se sites in FeSe has enlarged the lattice parameters.The Tc has increased by doping.Resistance broadening effect in the resistivity curve has been observed with the increasing magnetic field,which has been studied by the thermally activated flux flow model.Peak effects were observed in the hysteresis loops and the critical current density of the Fe_1.05Se_0.5Te_0.5 samples.The（Li0.8Fe0.2）OHFeSe single crystals has been successfully synthesized by the ion exchange method.The peak effect has been observed in the hysteresis loops and the critical current density.The resistivity broadening region is studied,and the pinning potential is obtained.The phase diagram is studied by the vortex glass theory.There is a transition from vortex glass phase to vortex liquid phase to normal state in the phase diagram.Superconductivity fluctuations above T_c of(Li_0.8Fe_0.2)OHFeSe are found by the resistivity and magnetic measurements.The quasiparticle dynamics of(Li_0.8Fe_0.2)OHFeSe and FeSe_0.9S_0.1has been investigated by femtosecond transient optical spectroscopy to analyze the corresponding amplitude and relaxation time with temperature.The appearance of the superconductivity and the opening of the superconducting energy gap are reflected in the slow process component by analyzing the amplitude and relaxation time of the(Li_0.8Fe_0.2)OHFeSe.The A_1g phonon mode was obtained by extracting the phonon oscillatory signal and performing a fast Fourier transformation.The electron-phonon coupling strength of(Li_0.8Fe_0.2)OHFeSe was estimated using the Allen model.The quasiparticle dynamics and longitudinal acoustic coherent phonon softening of FeSe_0.9S_0.1 has been investigated by femtosecond transient optical spectroscopy.The process of relaxation components with the changing temperature has been obtained by fitting analysis.The structural phase transition has been proved by analyzing the anomalous changes in the amplitude and decay time,which is consistent with the resistivity measurements.The electron-phonon coupling constant is obtained by the A_1g mode phonon.