Non-equilibrium singlet-triplet Kondo effect in carbon nanotubes

The Kondo effect is a many-body phenomenon arising due to conduction electrons scattering off a localized spin(1). Coherent spin-flip scattering off such a quantum impurity correlates the conduction electrons, and at low temperature this leads to a zero-bias conductance anomaly(2,3). This has become a common signature in bias spectroscopy of single-electron transistors, observed in GaAs quantum dots(4-9) as well as in various single-molecule transistors(10-15). Although the zero-bias Kondo effect is well established, the extent to which Kondo correlations persist in non-equilibrium situations where inelastic processes induce decoherence remains uncertain. Here we report on a pronounced conductance peak observed at finite bias voltage in a carbon-nanotube quantum dot in the spin-singlet ground state. We explain this finite-bias conductance anomaly by a non-equilibrium Kondo effect involving excitations into a spin-triplet state. Excellent agreement between calculated and measured nonlinear conductance is obtained, thus strongly supporting the correlated nature of this non-equilibrium resonance.