  We investigate a tunable two-impurity Kondo system in a strongly correlated
carbon nanotube double quantum dot, accessing the full range of charge regimes.
In the regime where both dots contain an unpaired electron, the system
approaches the two-impurity Kondo model. At zero magnetic field the interdot
coupling disrupts the Kondo physics and a local singlet state arises, but we
are able to tune the crossover to a Kondo screened phase by application of a
magnetic field. All results show good agreement with a numerical
renormalization group study of the device.
