Inelastic cotunneling mediated singlet-triplet transition in carbon nanotubes

We investigate electronic transport through single-wall carbon-nanotube quantum dots weakly coupled to metallic leads in the Coulomb-blockade regime. While sequential tunneling is suppressed, the transport, due to correlated tunneling of two electrons cotunneling, is possible. We report on a pronounced current peak for the singlet-triplet transition mediated by inelastic cotunneling processes that allows for their separation from the elastic components. Using the second-order perturbation theory in the tunnel-coupling strength we are able to fit it into the experimental data, explain the details of the line shape, and extract values of parameters that describe pseudospin asymmetry.