Universality of the two-stage Kondo effect in carbon nanotube quantum dots

We investigate the Kondo effect in nanometer-sized carbon nanotube quantum dots fabricated by electromigration. Here, we report on a pronounced dip within the Kondo peak observed at zero magnetic field. The zero-bias conductance exhibits a nonmonotonic temperature dependence with two characteristic energy scales. This behavior is ascribed to a two-stage Kondo effect occurring in an even charge state of the quantum dot. We demonstrate the universal behavior of the out-of-equilibrium conductance in this regime, and we extract the parameters of the universal scaling function.