Effects Of Element Doping And Pressure On The Crystal Structure And Electronic Structure Of AFe₂As₂（A=Ba,Sr） Iron-based Superconductor

The crystal structure,magnetic structure and electronic state of iron-based superconductors are similar to those of copper based superconductors.However,from the perspective of electronic structure,it belongs to multi band superconductor（conventional superconductor）similar to magnesium diboride.Iron based superconductor matrix is more metallic,which is a bridge between copper based superconductor and traditional metal superconductor.It provides a new carrier for studying the micro mechanism of superconductivity.The changes of crystal and electronic structures of 122 type iron-based superconductors Ba Fe₂As₂and Sr Fe₂As₂under element doping and pressure are systematically studied by using first principle calculation.The main work and conclusions are as follows:1.The main characteristics of structural changes for transition metal element doped AFe₂As₂（A=Ba,Sr）system and the relationship between As-Fe-As angle,average Fe-Fe distance and AFM order suppression were studied.The transition metal elements Co,Ni,Rh,Pd,Ir and Pt doped AFe₂As₂（A=Ba,Sr）system has superconductivity.The reason for superconductivity caused by transition metal doping is explained here.The results show that after doping,except for Co doped Ba Fe₂As₂system,all average Fe-Fe distance becomes longer,which suppresses the AFM order.The doping of transition metal elements makes the nesting of hole type Fermi surface in the center of Brillouin region evolve into the nesting between hole type and electron type Fermi surface.The Fermi level moves from the hole band to the electron band,and the superconducting phase comes from the broadening of the hybrid band near the Fermi level.2.The structural evolution and electronic structure changes caused by K,Ni and P substituting at different positions in AFe₂As₂（A=Ba,Sr）system were systematically studied.K substituted Ba for hole doping,Ni substituted Fe for electron doping and P substituted As for Isovalent doping.The effects of K,Ni and P doping on the reconstruction of Fermi surface and the change of band were studied.The results show that after K,Ni and P doping,the Fe-As bond length becomes shorter and the As-Fe-As angle becomes smaller,which is an important factor to suppress the Fe magnetic moment.By analyzing the total electron density and maximum value of electron density field before and after doping,it can be seen that the electron density near As atom is enhanced,and doping makes the isosurface with higher electron density between Fe and As interconnected.3.The structural evolution and electronic structure of AFe₂As₂（A=Ba,Sr）system under hydrostatic pressure and uniaxial pressure were systematically studied.The results show that under the four pressurized environments of hydrostatic pressure and uniaxial pressure along the coordinate axes a,b and c,the ability of uniaxial pressure environment along the c axis to suppress AFM order and cause Lifshitz transition is the strongest,followed by hydrostatic pressure environment,and the ability of uniaxial pressure environment along the a or b axis is the weakest.The results explain the fluctuation of structural parameters and superconducting critical pressure in pressurization experiment.The increase of Fe-Fe average distance in pressure environment is conducive to the suppression of AFM order in AFe₂As₂（A=Ba,Sr）.The hybrid band width of Fe 3d and As 4p near the Fermi level increases with the suppression of Fe magnetic moment.The average Fe-Fe distance is closely related to AFM suppression and Fe-As bond length.4.The effect of Co and Pt doped AFe₂As₂（A=Ba,Sr）system on magnetic fluctuation suppression was studied by Heisenberg model.The results show that the ground states of Ba(Fe_1-xCo_x)₂As₂（x=0,0.125）and Sr(Fe_1-xPt_x)₂As₂（x=0,0.125）systems are collinear AFM order.By comparing the atomic coordinates before and after Fe site doping AFe₂As₂（A=Ba,Sr）,it is found that the Fe atom basically remains stationary but the position of As atom changes.There is a strong AFM superexchange between the nearest neighbor（J₁）and next nearest neighbor（J₂）atoms of Fe in Ba Fe₂As₂.In Co doped Ba Fe₂As₂system,the Fe magnetic moment decreases,and the values of J₁and J₂decrease at the same time,indicating that the AFM order is suppressed.The decrease of J₁/2J₂means that the collinear AFM order has an advantage in competition with other magnetic order,which is conducive to suppressing the magnetic fluctuation of Ba(Fe_1-xCo_x)₂As₂system.Heisenberg model calculation shows that the collinear AFM order in Pt doped Sr Fe₂As₂is the ground state.The system energy under four different magnetic states shows that the collinear AFM order energy is the lowest.After doping Pt,the value of J₁/2J₂of Sr Fe₂As₂system increases less,and Pt doping will not bring strong magnetic fluctuation,which is consistent with the previous experimental data.