Kondo-Enhanced Andreev Tunneling in InAs Nanowire Quantum Dots

We report measurements of the nonlinear conductance of InAs nanowire quantum dots coupled to superconducting leads. We observe a clear alternation between odd and even occupation of the dot, with subgap peaks at $|V_{\mathrm{sd}}|=\Delta /e$ markedly stronger (weaker) than the quasiparticle tunneling peaks at $|V_{\mathrm{sd}}|=2\Delta /e$ for odd (even) occupation. We attribute the enhanced $\Delta$ peak to an interplay between Kondo correlations and Andreev tunneling in dots with an odd number of spins, and we substantiate this interpretation by a poor man’s scaling analysis.

Figure 1

(a) SEM image of a typical nanowire device. (b) Linear conductance in the Kondo regime for temperatures $T=750\mathrm{mK}(\mathrm{\text{black line}})-T=950\mathrm{mK}(\mathrm{\text{red line}})$ and at 300 mK (dashed line). The qualitative temperature dependence of the valley conductances for temperatures above $T_c$ are indicated by arrows. Leftmost inset shows $dI/d{V}_{\mathrm{sd}}$ vs $V_{\mathrm{sd}}$ and $V_g$ for the region of ${\kappa }_1$ at $800\mathrm{mK}(>T_c)$. At 300 mK, i.e., below $T_c$ (rightmost inset) the Kondo ridge is suppressed. (c), (d) Schematic illustration of the processes which lead to peaks at $|V_{\mathrm{sd}}|=2\Delta /e$ and $|V_{\mathrm{sd}}|=\Delta /e$, respectively.

Figure 2

(a) Stability diagram for the $V_g$ region of ${\kappa }_1$ and ${\kappa }_2$ and $|V_{\mathrm{sd}}|\le 0.35\mathrm{mV}$ exhibiting a pronounced even-odd periodicity in the subgap structure. The approximate positions of the dominant peaks at $V_{\mathrm{sd}}=\pm \Delta /e$, $\pm 2\Delta /e$ are indicated. (b) $dI/d{V}_{\mathrm{sd}}$ vs $V_{\mathrm{sd}}$ through the middle of two neighboring diamonds [lines in (a)] showing the enhanced peaks at $|V_{\mathrm{sd}}|\approx \Delta /e$ for the Kondo diamond (curves offset for clarity). (c) Bias spectroscopy for a larger $V_g$ region showing the enhanced $\Delta$ peak for four Kondo diamonds. Main panel shows the linear conductance (rightmost axis) and the distance between $\pm \Delta$ and $\pm 2\Delta$ subgap peaks in the middle of the diamonds [leftmost axis, symbols as in (b)]. Horizontal dashed lines indicate the position of $2\Delta$ and $4\Delta$.

Figure 3

(a) $dI/d{V}_{\mathrm{sd}}$ vs $V_{\mathrm{sd}}$ through ${\kappa }_1$ for different temperatures 310 mK–1700 mK (offset for clarity). Bold trace shows the Kondo peak at $T=800\mathrm{mK}$ and the formation of the $\Delta /e$ peak can be followed. Symbols correspond to traces in (b) showing the suppression of the valley conductance (crosses) and the continuing increase of the average heights of the $\Delta$ peaks below $T_c$ (squares). Solid line shows a fit to the gap function ${\Delta }_{\mathrm{BCS}}(T)$. (c), (d) As in (a), (b) from another device.