Theoretical Study Of Fermi Surface And Local Density Of States Induced By A Nonmagnetic Impurity In Fe-based Superconductors

Since the discovery of a series of Fe-based superconductors in 2008,high temperature superconductivity in these materials has been a hot topic in condensed matter physics. Similar to the cuprate superconductors, the Fe-based superconductors also have a layer structure. However, the band structure and the Fermi surfaces are more complex than those in cuprate superconductors. The electronic states of the iron-based superconductors can be changed by doping impurities or applying pressure.In this thesis, based on the two-orbital four-band tight-binding model, we investigate the effect of kinetic parameters associated with the Fe-As(Se, Te) bonds on the Fermi surfaces in the Fe-based superconductors. By adjusting the kinetic parameters,we obtain the trends of two hole Fermi surfaces(α-and β-bands) around Γ point and two electron Fermi surfaces(γ-and δ-bands) around the Μ point, i.e., the kinetic parameters t1, t2 and t3 control the shape and size of these hole and electron Fermi surfaces while the kinetic parameters t4 fixes the distance between the two hole(electron) Fermi surfaces.Based on the knowledge of mean field BCS theory and nonmagnetic impurities,the total Hamiltonian of the system is constructed. The local density of states induced by a nonmagnetic impurity is calculated by taking the Bogoliubov transformation and employing Green’s function method.This thesis is divided into four chapters:In the first chapter, we mainly introduce the basic properties of superconductors,four kinds of the Fe-based superconductors and their Fermi surfaces and crystal structures.In the second chapter, based on a two-orbital four-band tight-binding model,we study the influence of kinetic parameters on the Fermi surfaces.In the third chapter, we briefly introduced the scanning tunneling microscope(STM) experiments and the influence of impurities on Fe-based superconductors.We also calculated local density of states induced by a nonmagnetic impurity by taking the Bogoliubov transformation and applying Green’s function method.In the last chapter, we give a summary of this thesis.