Mikio Eto

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 systems. In fabricated structures, the control of single, localized spins is of technological relevance for nanoscale electronics. Experiments have already demonstrated artificial realizations of isolated magnetic impurities at metallic surfaces, nanoscale magnets, controlled transitions between two-electron singlet and triplet states, and a tunable Kondo effect in semiconductor quantum dots. 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.

Enhancement of kondo effect in quantum dots with an even number of electrons

We investigate the Kondo effect in a quantum dot with almost degenerate spin-singlet and triplet states for an even number of electrons. We show that the Kondo temperature as a function of the energy difference between the states Delta reaches its maximum around Delta = 0 and decreases with increasing Delta. The Kondo effect is thus enhanced by competition between singlet and triplet states. Our results explain recent experimental findings. We evaluate the linear conductance in the perturbative regime.

Electronic structures of few electrons in a quantum dot under magnetic fields

The many-body effect on the electronic structures of a few electrons confined in a quantum dot is calculated, under magnetic fields, by the exact diagonalization method. When the Coulomb interaction is comparable to the one-electron level spacings and the magnetic field is not too large, the peak positions of the Coulomb oscillation reflect a shell structure of the one-electron levels, which is in semi-quantitative agreement with experimental results [Tarucha et al.: Phys. Rev. Lett. 77 (1996) 3613]. Under strong magnetic fields, the correlation effect results in transitions of the ground state. These transitions can be observed as cusps of the magnetic-field dependence of the peak positions, in the presence of the Zeeman effect.

1A1g to 3B1g conversion at the onset of superconductivity in La2-xSrxCuO4 due to the apical oxygen effect

By the first-principles variational calculations we show that, with the increase of Sr concentration, the ground state of CuO 6 octahedron in La 2- x Sr x CuO 4 changes from 1 A 1 g to 3 B 1 g near the observed onset concentration of superconductivity while the ground state of the CuO 4 cluster in Nd 2- x Ce x CuO 4 is always 3 B 1 g in the presence of dopant electrons. Based on this result, two kinds of phase diagrams are predicted for hole- and electron-doped superconductors. These results support the spin polaron mechanism proposed by Kamimura et al .

Conductance through Quantum Dot Dimer Below the Kondo Temperature

The conductance through two quantum dots connected in series is studied below the Kondo temperature, based on the slave boson formalism of the Anderson model. The transport properties are characterized by the ratio of the magnitude of the tunneling coupling between two dots to the width of the level broadening. When the ratio is less than unity, the Kondo resonant states are formed between each dot and an external lead, and the conductance is determined by the hopping between the two resonant states. When the ratio is larger than unity, these Kondo resonances are split into the bonding and antibonding peaks, which are located below and above the Fermi level in the leads, respectively, for low gate voltages. As a result, the conductance is suppressed. The conductance has a maximum of 2e 2 /h when the bonding peak is matched with the Fermi level by controlling the gate voltage.

Spin Polarization at Semiconductor Point Contacts in Absence of Magnetic Field

Semiconductor point contacts can be a useful tool for producing spin-polarized currents in the presence of spin–orbit (SO) interaction. Neither magnetic fields nor magnetic materials are required. ...

Resonant tunneling and Fano resonance in quantum dots with electron-phonon interaction

We theoretically study the resonant tunneling and Fano resonance in quantum dots with electron-phonon (e-ph) interaction. We examine the bias-voltage

First principles investigation of the electronic structures of copper oxide superconductors by the MCSCF cluster method

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Electrical detection and magnetic-field control of spin states in phosphorus-doped silicon

dependence of the decoherence, using the Keldysh Green function method and perturbation with respect to the e-ph interaction. With optical phonons of energy

Mean-field theory of the Kondo effect in quantum dots with an even number of electrons

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