Abstract
The Kondo effect is a many-body phenomenon arising due to conduction electrons scattering off a localized spin1. Coherent spin-flip scattering off such a quantum impurity correlates the conduction electrons, and at low temperature this leads to a zero-bias conductance anomaly2,3. This has become a common signature in bias spectroscopy of single-electron transistors, observed in GaAs quantum dots4,5,6,7,8,9 as well as in various single-molecule transistors10,11,12,13,14,15. 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.
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Acknowledgements
We thank L. DiCarlo and W. F. Koehl for experimental contributions and D. H. Cobden and V. Körting for useful discussions. This research was supported by the Center for Functional Nanostructures of the DFG (J.P., P.W.), the European Commission through project FP6-003673 CANEL of the IST Priority (J.P.), ARO/ARDA (DAAD19-02-1-0039), NSF-NIRT (EIA-0210736) (N.M., C.M.M.) and the Danish Technical Research Council (J.N.).
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Paaske, J., Rosch, A., Wölfle, P. et al. Non-equilibrium singlet–triplet Kondo effect in carbon nanotubes. Nature Phys 2, 460–464 (2006). https://doi.org/10.1038/nphys340
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DOI: https://doi.org/10.1038/nphys340
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