Theoretical Study On Local Density Of States Induced By A Defect In Iron-based Superconductors

The discovery of the iron-based superconductivity has attracted great attention in the condensed matter community.Similar to cuprate superconductors,iron-based superconductors also have a layer structure.The superconductivity comes from Cooper pairs in the Fe-Fe plane.Now,iron-based superconductors have many families,which possess common Fe2X2(X=As,Se,Te,etc.)layers within their crystal structures.The parent compounds of iron-based superconductors are bad metals with an antiferromagnetic order.When holes or electrons are doped,the compounds become superconducting with increasing concentration of dopant.Different from the hole single band cuprates,the iron-based superconductors are the hole and electron mixed multiband superconductors.In addition to the cuprates,the Iron-based superconductors are another family of high temperature superconductors,which provides us a new platform to study high temperature superconductivity.In general,the as-grown crystals contain defects,e.g.impurities and vacancies,in the experiments.The zero-energy bound states induced by the interstitial Fe atom have been reported by the STM.Nonmagnetic impurities and As vacancies also have a certain influence on the local density of states in the iron-based superconductors.In this thesis,based on a two-orbital four-band tight binding model,we investigate the impacts of a single nonmagnetic impurity and an As vacancy on the local density of states in the iron-based superconductors with the S?wave symmetry,respectively.In-gap bound states induced by a nonmagnetic impurity can be used to distinguish the S and S?wave pairing symmetry in iron-based superconductors.Based on a two-orbital four-band tight binding model,the effects of a single nonmagnetic impurity on the local density of states in the iron-based superconductors with the S?wave symmetry are studied.The dependence of the intensities and positions of impurityresonance peaks and the strength of impurity potential is obtained.The resonance peaks at positive and negative energies are symmetric in a certain range of the impurity potential.The resonance peak at positive energy splits into two peaks when Vs > 0.40 e V.An impurity with Vs=0.70 e V induces four impurity resonance peaks on the next-nearest-neighbor site.Such in-gap bound states could be observed by scanning tunneling microscopy experiments.Based on the two-orbital four-band tight binding model,we propose the Hamiltonian for an As vacancy in the iron-based superconductors.The total Hamiltonian is diagonalized by the canonical transformation and Bogoliubov transformation.After a series of tedious but straightforward calculations,the expressions of the local density of states on the sublattice A and sublattice B are finally obtained by solving the equations of motion of the fermion annihilation operator and the Green function,respectively.