Iron-based Superconductors Ca(Fe_0.88Co_0.12)AsF And Ba(Fe_0.943Co_0.057)₂As₂

Iron-based superconductors show excellent performance in superconducting critical transition temperature,upper critical magnetic field,anisotropy and grain boundary current-carrying capacity.It is found that physical pressure can improve the superconducting transition temperature of iron-based superconductors,which provides a pathway for further exploration of the superconducting mechanism.In this paper,diamond anvil technology is used to systematically study the electromagnetic transport operation of iron-based superconducting Ca(Fe_0.88Co_0.12)As F and Ba(Fe_0.94Co_0.06)₂As₂with cobalt doping under high pressure.The thesis mainly uses the integrated physical property measurement system(PPMS)and magnetic measurement system(MPMS)to study the properties of resistivity-temperature,superconducting upper critical magnetic field,hall effect and magnetization under high pressure,and preliminarily discusses the physical mechanism of electromagnetic transport.In the Ca Fe_0.88Co_0.12As F single crystal,it was found that under atmospheric pressure,its superconducting transition temperature T_c^onsetwas 21.9 K(?_n90%).At pressure of 3.24?9.1 GPa and temperature range of 300?0 K,the resistivities decreased firstly,then rose and finally went into the superconducting state.At the pressure of 10.6?17.4 GPa,the resistivities first decreased and then rose,the superconducting state disappeared in the end.As pressure increased,the resistivity decreased and the superconducting transition temperature moved to the lower temperature,indicating the pressure suppressed the superconductivity.The reason might be that the energy gap between the Fermi surface and the adjacent conduction band bottom was too large,and the superposition between the Fermi surface and the conduction band bottom could not be achieved by increasing pressure.With the increase of magnetic fields,the superconducting transition temperature decreased.The reason is that the spin of the electron is affected by the strong magnetic field,the cooper pair is destroyed,and the superconductivity is suppressed or even completely disappeared.The empirical formula H_c2(T)=H_c2(0)(1-T/T_c)ⁿwas used to fit the upper critical magnetic field curves under different pressures,and it was found that the curves had obvious rising curvature at low temperature,which was a multi-segment feature of superconductivity.By calculation:H_c2(0)=10.6?17.1 T,n=1.22?1.48.The hall coefficients at different pressures and temperatures were all in the order of 10^-9m³/C.The hall coefficients decreased with the increase of temperature,indicating that the decrease of temperature accelerated the transfer of electrons from the charge reservoir layer to the iron-arsenic layer,providing carriers for the realization of superconductivity.Through the measurement of magnetization and electric transport,the arch T_c-P phase diagram was finally obtained.It was found that the variation trend of electric transport T_cwas basically the same as that of magnetic transport T_c,m,and the electric transport curve was slightly higher than that of magnetic transport.The study of Ba(Fe_0.94Co_0.06)₂As₂found that:before going into the superconducting state with pressure among 1.56?7.89 GPa,the resistivities dropped with the decrease of temperature,which was manifested as the metal behavior of electrical transport.When the pressure was1.56GPa,the spin density wave phase transition temperature T_sdwwas 31 K.As the pressure continued to increase,the resistivities upwarping disappeared,indicating that the pressure inhibited the spin density wave phase transition.According to the T_c-P fitting curve,the superconducting transition temperature first increased and then decreased with the increasing pressure.When pressure was 3.72 GPa,the superconducting transition temperature of the sample was the highest,T_c=27.8 K.The empirical formula H_c2(T)=H_c2(0)[1-(T/T_c)²]/[1+(T/T_c)²]was used to obtain the fitting curves of Ba(Fe_0.94Co_0.06)₂As₂in the upper critical magnetic field.By calculating the fitting curves under the pressure of 1.56,2.28 and 3.72 GPa,the range of H_c2(0)of the upper critical magnetic field was determined to be 50.23?56.62 T.The hall coefficients of the sample were always negative.The hall coefficients decreased as the pressure increased.Thus,the superconductivity is easier to achieve.The variation trend of the T_c,m-P curves obtained by magnetization were basically consistent with that of the T_c-P curves in the process of electrical transport.