Impurity Effect In Electron-doped High-Tc Superconductors

High-Tc superconductivity has attracted broad interest in the past thirty years.Usually the superconductivity can be realized by doping either holes or electrons to the parent compound.At present,the hole-doped compounds have been studied intensively both theoretically and experimentally while the electron-doped ones have been relatively less explored.Actually,the phase diagrams of these two kinds of compounds are rather different,namely,for electron-doped ones,the antiferromagnetic region is rather large and the antiferromagnetic order and superconducting order may coexist.It is interesting and important to study theoretically the physical properties for the antiferromagnetic region and the coexisting region of the electron-doped high-Tc superconductors.In the present work,we study theoretically the quasiparticle states around a nonmagnetic impurities in electron-doped high temperature superconductors based on the Bogoliubov-de Gennes equations.In the antiferromagnetic state,one in-gap impurity resonance state is found.For stronger impurity potentials the impurity resonance peak moves toward the gap edge and finally disappears for a strong impurity.As the doping density increases,the antiferromagnetic order and superconducting order coexist.In this coexisting state,the in-gap resonance peaks are rather robust and appear in pairs that are lying symmetric with the Fermi energy.In addition,the position and intensity of the scattering peaks are related to the impurity scattering potential and the next-nearest neighbor hopping term.For a rather strong impurity,the resonance peaks shift to near the gap edge.When further increasing the doping density,the system become the pure superconducting state.In pure superconducting state,the resonance peaks still appear in pairs,however,the peak intensities of the pure superconductors are weaker than those in the coexisting state.For both the coexisting state and the pure superconducting state,the two resonance peaks may occasionally merge into one zero-energy peak.The spin-resolved local density of state are also investigated and may be used to detect possible antiferromagnetic order.