Two-Stage Kondo Effect and Kondo Box Level Spectroscopy in a Carbon Nanotube

The concept of the Kondo box describes a single spin, antiferromagnetically coupled to a quantum dot with a finite level spacing. Here, a Kondo box is formed in a carbon nanotube interacting with a localized electron. We investigate the spins of its first few eigenstates and compare them to a recent theory. In an open Kondo-box, strongly coupled to the leads, we observe a non-monotonic temperature dependence of the nanotube conductance, which results from a competition between the Kondo-box singlet and the conventional Kondo state that couples the nanotube to the leads.