A strong Kondo effect in vertical quantum dot holding an odd number of electrons and spin 1/2 was investigated. The estimated Kondo temperature for 'doublet-doublet' degeneracy was found to be similar to that for singlet-triplet degeneracy with an even electron number, indicating that a total of fourfold spin and orbital degeneracy accounts for enhancement of the Kondo temperature. An orbital degeneracy for odd N and S = 1/2 dramatically enhanced the Kondo effect. The results show that an SU(4) model at orbital degeneracy is used to qualitatively reproduce experimental observation.
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