| Budget Amount *help |
¥1,850,000 (Direct Cost: ¥1,700,000、Indirect Cost: ¥150,000)
Fiscal Year 2007: ¥650,000 (Direct Cost: ¥500,000、Indirect Cost: ¥150,000)
Fiscal Year 2006: ¥1,200,000 (Direct Cost: ¥1,200,000)
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| Research Abstract |
(1) We studied the nonequilibrium Kondo effect in quantum dots under finite bias voltage. Specifically, we have reformulated the density matrix which describes the nonequilibrium steady state, and have obtained the reduced density matrix, by integrating out one of the two conduction channels. This formulation enables us to treat the current flowing state with a single-channel model, and may be applied to construct new nonperturbative approaches to the nonequilibrium state.
(2) We have studied the Kondo effect in quantum dots connected to superconductors. In the Kondo regime where the dot has a well-defined local moment, a simple scaling argu-ment predicts that whether the ground state becomes a singlet or a magnetic doublet is determined by the value of the Kondo temperature and superconducting gap. We have pointed out that the deviation from the scaling behavior is important in quantum dots, because the Coulomb interaction and the superconducting gap can be the same order. We have shown that some important features of the deviation are seen in the superconducting atomic limit, which is realized at large gaps.
(3) We have calculated the single-particle Green's function for the quantum dots connected to the superconductors, using the numerical renormalization group. Our results clarify the precise features of the Andreev bound state, which emerges inside the superconducting gap, and the density of states near the gap.
(4) We have also studied the transport properties of. the super-dot-normal junctions. It is shown that the low-lying energy states can be described by a generalized version of the local Fermi-liquid theory, owing to the coupling with the normal lead. Using the numerical renormalization group, we have also calculated the renormalized parameters for the quasi-particles, and have clarified the features of the competition between the Kondo effect and the Andreev reflection.
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