Nonequilibrium Transport Measurement of Andreev Bound States Near the 0-Π Transition in a Superconductor-Carbon Nanotube-Superconductor Junction

We report non-equilibrium transport measurement of gate tunable
Andreev bound states in
a nanotube quantum dot coupled to two superconducting leads.
In particular, we observe clear features of the two different types of
Kondo ridges,
which can be understood in terms of the interplay between the Kondo
effect and the superconductivity.
In the first type of the Kondo ridge (Type I), the coupling is strong
and the Kondo effect is dominant.
The 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 of the Kondo ridge (type II), which is directly
related to the 0-Π transition of the Josephson junction.
Our scenario is well understood in terms of only a single
dimensionless parameter, k_B T_K^{min}/Δ, where T_K^{min}
and Δ are the minimum Kondo temperature of a ridge and the
superconducting order parameter, respectively. Our observation is also
confirmed by the numerical renormalization group calculation.