Scanning Josephson Tunneling Microscopy of Single-Crystal ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ with a Conventional Superconducting Tip

We have performed both Josephson and quasiparticle tunneling in vacuum tunnel junctions formed between a conventional superconducting scanning tunneling microscope tip and overdoped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ single crystals. A Josephson current is observed with a peak centered at a small finite voltage due to the thermal-fluctuation-dominated superconducting phase dynamics. Josephson measurements at different surface locations yield local values for the Josephson $I_C{R}_N$ product. Corresponding energy gap measurements were also performed and a surprising inverse correlation was observed between the local $I_C{R}_N$ product and the local energy gap.

Figure 1

$I\mathrm{\text{−}}V$ characteristics of $\mathrm{Pb}/\mathrm{I}/\mathrm{\text{overdoped}}$ BSCCO ($T_C=79\mathrm{K}$) STM Josephson junctions at $T=2.1\mathrm{K}$. The Pb gap is clearly seen around $V=1.4\mathrm{mV}$. Inset: $dI/dV$ spectrum (black line) measured before low $R_N$ measurements, showing sharp coherence peaks with $\Delta =37\mathrm{meV}$. $dI/dV$ measured after low $R_N$ measurements (red line) indicates an LDOS change due to the high current density.

Figure 2

(a) Low bias $I\mathrm{\text{−}}V$ characteristics of Fig. 1 for various $R_N$ at $T=2.1\mathrm{K}$. (b) Averaged $I\mathrm{\text{−}}V$ characteristic near zero bias for quasiparticle background (red line). One of the observed $I\mathrm{\text{−}}V$ curves is shown by the black line. (c) Thermally fluctuated Josephson currents peaked at $V_p$ as derived by subtracting quasiparticle background [Fig. 2b] from the $I\mathrm{\text{−}}V$ curves [Fig. 2a]. The data are represented by the lines and the symbols represent two-parameter fits to the phase diffusion model.

Figure 3

Plot of $I_C/\sqrt{k_B{T}_n/e}$ vs $G_N$. The slope is equal to $I_C{R}_N/\sqrt{k_B{T}_n/e}$. Using the fitted slope and substituting the previously determined $T_n$, the Josephson product at this surface point is found to be $I_C{R}_N=335\mu \mathrm{V}$.

Figure 4

$I_C{R}_N$ as a function of $\Delta$. Each data point represents a separate measurement from a different location over 4 different samples. The $I_C{R}_N$ value derived from Fig. 3 is one of the black square symbols denoted with an arrow. All other points are derived similarly. $I_C{R}_N$ appears to be a maximum for $\Delta$ between 40 and 45 meV, while it decreases for larger and smaller $\Delta$. Sketches of $T_C$ and $T^{*}$ from the Emery-Kivelson model are shown by dotted and dashed lines, respectively. The vertical scale for the model curves is arbitrary.