The Kondo effect is a key many-body phenomenon in condensed matter physics.It concerns the interaction between a localised spin and free electrons.Discovered in metals containing small amounts of magnetic impurities, it is nowa fundamental mechanism in a wide class of correlated electron systems. Controlover single, localised spins has become relevant also in fabricated structuresdue to the rapid developments in nano-electronics. Experiments have alreadydemonstrated artificial realisations of isolated magnetic impurities atmetallic surfaces, nanometer-scale magnets, controlled transitions betweentwo-electron singlet and triplet states, and a tunable Kondo effect insemiconductor quantum dots. Here, we report an unexpected Kondo effect realisedin a few-electron quantum dot containing singlet and triplet spin states whoseenergy difference can be tuned with a magnetic field. This effect occurs for aneven number of electrons at the degeneracy between singlet and triplet states.The characteristic energy scale is found to be much larger than for theordinary spin-1/2 case.
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