Direct determination of localized impurity levels located in the blocking layers ofBi2Sr2CaCu2Oyusing scanning tunneling microscopy/spectroscopy

Abstract

Local electronic states of the high-$T_c$ superconductor ${\mathrm{Bi}}_{2-x}{\mathrm{Pb}}_x{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_y$ single crystals with $x=0.6$ have been studied using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS). We found that they are modified by substitutional impurities occupying the Sr (or Ca) and Bi site. Individual impurities could be imaged selectively by STM; the Sr(Ca)-site impurities could be seen only for a positive sample bias, whereas the Pb atoms substituting for the Bi sites could be imaged with larger corrugations for a negative bias. STS measurements revealed that the former behaves as a donorlike dopant with an energy of $+1.7\mathrm{eV}$, consistent with a chemical view indicating that excess ${\mathrm{Bi}}^{3+}$ ions are substituted in the ${\mathrm{Sr}}^{2+}$ sites. On the other hand, the latter gives rise to an acceptorlike state at $-1.4\mathrm{eV}$ near the band edge, which is thought to promote hole doping into the $\mathrm{Cu}{\mathrm{O}}_2$ layer.