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Kondo effect in an integer-spin quantum dot

Abstract

The Kondo effect—a many-body phenomenon in condensed-matter physics involving the interaction between a localized spin and free electrons—was discovered in metals containing small amounts of magnetic impurities, although it is now recognized to be of fundamental importance in a wide class of correlated electron systems1,2. In fabricated structures, the control of single, localized spins is of technological relevance for nanoscale electronics3,4. Experiments have already demonstrated artificial realizations of isolated magnetic impurities at metallic surfaces5,6, nanoscale magnets7, controlled transitions between two-electron singlet and triplet states8, and a tunable Kondo effect in semiconductor quantum dots9,10,11,12. Here we report an unexpected Kondo effect in a few-electron quantum dot containing singlet and triplet spin states, whose energy difference can be tuned with a magnetic field. We observe the effect for an even number of electrons, when the singlet and triplet states are degenerate. The characteristic energy scale is much larger than in the ordinary spin-1/2 case.

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Figure 1: Spin-flip processes leading to ordinary and singlet–triplet Kondo effect in a quantum dot.
Figure 2: Sample description, energy spectrum and magnetic-field evolution of the ground state.
Figure 3: Zero-bias resonance and T-dependence of the conductance at the singlet–triplet degeneracy.
Figure 4: Singlet–triplet energy separation tuned by a magnetic field.

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Acknowledgements

We thank Yu. V. Nazarov, K. Maijala, S. M. Cronenwett, J. E. Mooij and Y. Tokura for discussions. We acknowledge financial support from the Specially Promoted Research, Grant-in-Aid for Scientific Research, from the Ministry of Education, Science and Culture in Japan, from the Dutch Organisation for Fundamental Research on Matter (FOM), from the NEDO joint research program, and from the EU via a TMR network.

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Correspondence to S. De Franceschi.

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Sasaki, S., De Franceschi, S., Elzerman, J. et al. Kondo effect in an integer-spin quantum dot. Nature 405, 764–767 (2000). https://doi.org/10.1038/35015509

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