Scanning Josephson spectroscopy on the atomic scale

The Josephson effect provides a direct method to probe the strength of the pairing interaction in superconductors. By measuring the phase fluctuating Josephson current between a superconducting tip of a scanning tunneling microscope and a BCS superconductor with isolated magnetic adatoms on its surface, we demonstrate that the spatial variation of the pairing order parameter can be characterized on the atomic scale. This system provides an example where the local pairing potential suppression is not directly reflected in the spectra measured via quasiparticle tunneling. Spectroscopy with such superconducting tips also shows signatures of previously unexplored Andreev processes through individual impurity-bound Shiba states. The atomic resolution achieved here establishes scanning Josephson spectroscopy as a promising technique for the study of novel superconducting phases.

Figure 1

(a) $dI/dV$ of Pb(110) with a normal tip resolving two superconducting gaps. Inset: Atomic scale topography of the Pb(110) surface. (b) $dI/dV$ of Pb(110) with a superconducting Pb tip. (c) Superconducting tip spectroscopy on Pb(110), normalized to $R_N$. Curves are offset for clarity. As junction resistance is decreased, features due to Andreev reflections $\left(eV=\pm {\mathrm{\Delta }}_{\mathrm{Pb}}\right)$ and the Josepshon effect $\left(V=0\right)$ become more pronounced. Due to the quality of the tip, the two superconducting gaps of Pb are not clearly resolved. Inset: Schematic of an Andreev process between two superconductors with the same gap, occurring at a threshold $e\left|V\right|={\mathrm{\Delta }}_{\text{Pb}}$. (d) $IV$ characteristics of the STM Josephson junction around zero bias, for different normal state resistance $R_N$. (e) Oscillations in $dI/dV$ (offset for clarity, normalized to $R_N$) due to photon-assisted Cooper pair tunneling with characteristic frequency of ${\nu }_0\approx 23$ GHz, determined by fitting the bias of the peaks as a function of oscillation number (inset). (f) Fit to $IV$ and $dI/dV$ (bottom right inset) data for $R_N=250 \mathrm{k}\mathrm{\Omega }$ using the $P\left(E\right)$ theory. Top left inset: Values of critical current extracted from $P\left(E\right)$ fits (blue dots), in agreement with the Ambegaokar-Baratoff formula (black). All point spectra in this figure were acquired at a parking bias of $V=-5$ mV at which the normal state junction resistance was determined (well outside the superconducting gap).

Figure 2

(a) Topography of Pb after evaporating a submonolayer of Fe. Individual Fe adatoms tend to be centered on a trench (type $A$) or row (type $B$) of the Pb(110) surface as marked. Suppression of pair current [(b),(f)] and $dI/dV$ [(c),(g)] on the Fe adatom (blue) compared to on Pb (black) at $1 \mathrm{M}\mathrm{\Omega }$ junction impedance for type $A$ [(b)–(e)] and type $B$ [(f)–(i)] adatoms. Simultaneous topographies [(d),(h)] and differential conductance maps [(e),(i)] at $V=0$ for $R_N=1 \mathrm{M}\mathrm{\Omega }$ demonstrate the spatial suppression of the Josephson effect over the Fe adatoms. Parking conditions $V=-5$ mV and $I=5$ nA for data in (b)–(i), ensuring that $R_N$ is the same over both the adatom and bare Pb. Different superconducting tips were used for the point spectra and the conductance maps, but the $I_c$ suppression is consistent across measurements.

Figure 3

(a) $dI/dV$ with a superconducting tip ($V=-5$ mV, $I=10$ nA) on a type A impurity, plotted on a log scale, showing the appearance of Shiba states at ${\mathrm{\Delta }}_{\mathrm{Pb}}+E_{S1}=\pm 2.06$ meV and ${\mathrm{\Delta }}_{\mathrm{Pb}}+E_{S2}=+2.28$ meV (marked by arrows). (b) A zoom-in of $dI/dV$ at lower junction resistances highlights multiple Andreev reflections through the Shiba state observed at energies $\pm ({\mathrm{\Delta }}_{\mathrm{Pb}}+E_{S1})/2$. Parking bias $V=-5$ mV for all spectra. [(c)-(f)] Conductance maps at $eV=\pm ({\mathrm{\Delta }}_{\mathrm{Pb}}+E_{S1})$ and $eV=\pm ({\mathrm{\Delta }}_{\mathrm{Pb}}+E_{S1})/2$ taken simultaneously with the topograph in the inset of (b). Corresponding spatial patterns between the Shiba state ($V=2$ mV) and the Andreev reflection state ($V=1$ mV), and similarly for states at negative bias are observed. Scale bar is $5 \AA$. (g) Schematic for the Andreev reflection process with threshold voltage $eV=\pm ({\mathrm{\Delta }}_{\mathrm{Pb}}+E_{S1})/2$, where the relevant energy gap is that of the sample.