Direct determination of localized impurity levels located in the blocking layers of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_y$ using scanning tunneling microscopy/spectroscopy

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.

Figure 1

STM images of Pb-Bi2212 taken at (a) positive and (b) negative bias voltages as indicated in the each image at $4.3\mathrm{K}$. The tunneling current was fixed at $0.4\mathrm{nA}$.

Figure 2

Narrow-scan STM images of Pb-Bi2212 at $4.3\mathrm{K}$. The tunneling conditions were (a) $V_s=0.1\mathrm{V}$ and $I_t=0.5\mathrm{nA}$, (b) $V_s=1.8\mathrm{V}$ and $I_t=1.0\mathrm{nA}$, and (c) $V_s=-1.8\mathrm{V}$ and $I_t=0.6\mathrm{nA}$.

Figure 3

Differential conductance images of Pb-Bi2212 at (a) positive and (b) negative bias voltages at $4.3\mathrm{K}$. The differential conductance values were normalized at (a) $+0.2\mathrm{V}$ and (b) $-0.2\mathrm{V}$. Tunneling conditions were (a) $V_s=1.8\mathrm{V}$ and $I_t=0.4\mathrm{nA}$, and (b) $V_s=-1.8\mathrm{V}$ and $I_t=0.6\mathrm{nA}$.

Figure 4

Tunneling spectra obtained for various impurity sites imaged as shown in Fig. 3. (a) Tunneling spectra taken for an Sr(Ca)-site impurity (dotted line). (b) Tunneling spectra taken for a Pb atom (dotted line). The solid lines are “normal” spectra taken at locations far away from impurity sites. These spectra are normalized to (a) $0.2\mathrm{V}$ and(b) $-0.2\mathrm{V}$. The insets in (a) and (b) show high-resolution spectra around the Fermi level, obtained for the Sr(Ca)-site impurity and the Pb atom, respectively.