Transport Measurement of Andreev Bound States in a Kondo-Correlated Quantum Dot

Abstract: We report transport measurements of gate-tunable Andreev bound states in a carbon nanotube quantum dot coupled to two superconducting leads. In particular, we observe clear features of two types of Kondo ridges, which can be understood in terms of the interplay between the Kondo effect and superconductivity. In the first type (type I), the coupling is strong and the Kondo effect is dominant. Levels of the Andreev bound states display anti-crossing in the middle of the ridge. On the other hand, crossing of the two Andreev bound states is shown in the second type (type II) together with the 0-$\pi$ transition of the Josephson junction. Our scenario is well understood in terms of only a single dimensionless parameter, $k_BT_K^{min}/\Delta$, where $T_K^{min}$ and $\Delta$ are the minimum Kondo temperature of a ridge and the superconducting order parameter, respectively. Our observation is consistent with measurements of the critical current, and is supported by numerical renormalization group calculations.