Kondo effect in an integer-spin quantum dot

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.