Kicheon Kang

Kondo resonance, Coulomb blockade, and Andreev transport through a quantum dot

We study resonant tunneling through an interacting quantum dot coupled to normal metallic and superconducting leads. We show that large Coulomb interaction gives rise to novel effects in Andreev transport. Adopting an exact relation for the Greens function, we find that at zero temperature, the linear response conductance is enhanced due to Kondo-Andreev resonance in the Kondo limit, while it is suppressed in the empty site limit. In the Coulomb blockaded region, on the other hand, the conductance is reduced more than the corresponding conductance with normal leads because large charging energy suppresses Andreev reflection.

PERTURBATION TREATMENT FOR TRANSPORT THROUGH A QUANTUM DOT

Resonant tunnelling through an Anderson impurity is investigated by employing a new perturbation scheme at nonequilibrium. This new approach gives the correct weak and strong coupling limit in

Spin fluctuation and persistent current in a mesoscopic ring coupled to a quantum dot

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Photoemission spectra, hybridization, and Coulomb correlation in RT2 (RY,Ce,Pr,Nd,Sm,Gd; TFe,Co,Ni)

by introducing adjustable parameters in the self-energy and imposing self-consistency of the occupation number of the impurity. We have found that the zero-temperature linear response conductance agrees well with that obtained from the exact sum rule. At finite temperature the conductance shows a nonzero minimum at the Kondo valley, as shown in recent experiments. The effects of an applied bias voltage on the single-particle density of states and on the differential conductances are discussed for Kondo and non-Kondo systems.

Correlation effects on the electron-phonon interaction

We investigate the persistent current influenced by the spin fluctuations in a mesoscopic ring weakly coupled to a quantum dot. It is shown that the Kondo effect gives rise to some unusual features of the persistent current in the limit where the charge transfer between two subsystems is suppressed. Various aspects of the crossover from a delocalized to a localized dot limit are discussed in relation with the effect of the coherent response of the Kondo cloud to the Aharonov-Bohm flux.

Effect of charge fluctuations on the persistent current through a quantum dot

Abstract The effects of the R 4f/T 3d hybridization and the Coulomb correlation in the R 4f and T 3d photoemission spectra of RT 2 compounds (RY,Ce,Pr,Nd,Sm,Gd; TFe,Co,Ni) have investigated. The calculated R 4f/T 3d hybridization parameters decrease with an increasing atomic number from RCe to Gd for the same T, and from RCo 2 to RNi 2 for the same R. These trends are consistent with those in the photoemission spectra. The calculated quasi-particle spectral densities indicate the importance of the on-site Coulomb correlation of the T 3d electrons in the electronic structures of RT 2 .

Electric Aharonov-Bohm effect without a loop in a Cooper pair box

Abstract The electron-phonon coupling constant (λph) is evaluated for the Hubbard Hamiltonian system. It is found that the Coulomb correlation in the Hubbard model gives rise to a phonon hardening and an enhancement of λph. It is alos found that λph becomes larger than the electron-spin-fluctuation coupling constant λsf for a correlated system with a nearly filled band.

Vacuum-Fluctuation-Induced Dephasing of a Qubit in Circuit Quantum Electrodynamics

We study coherent charge transfer between an Aharonov-Bohm ring and a side-attached quantum dot. The charge fluctuation between the two substructures is shown to give rise to an algebraic suppression of the persistent current circulating the ring as the size of the ring becomes relatively large. The charge fluctuation at resonance provides a transition between diamagnetic and paramagnetic states. Universal scaling, the crossover behavior of a persistent current from a continuous to a discrete energy limit in the ring, is also discussed.

Ultimate charge sensitivity and efficiency of a quantum point contact with a superposed input state

We predict the force-free scalar Aharonov-Bohm effect of a Cooper pair box in an electric field at a distance without forming a closed path of the interfering charges. The superposition of different charge states plays a major role in eliminating the closed loop, which is distinct from the original topological Aharonov-Bohm effect. The phase shift is determined by the charge-state-dependent local field interaction energy. In addition, our proposed setup does not require a pulse experiment for fast switching of a potential, which eliminates the major experimental obstacle for observing the ideal electric Aharonov-Bohm effect.

Quantum-limited charge detection with two quantum point contacts

We investigate the measurement-induced dephasing of a qubit coupled with a single-mode cavity in the vacuum limit. Dephasing of the qubit state takes place through the ntanglement of the qubit and the single probe photon sent to the cavity, while the cavity mode never occupies a photon. We find that the qubit state is dephased even if the cavity is always in the vacuum state. This dephasing is caused purely by the interaction between the qubit and the vacuum field. We also show that this vacuum-fluctuation-induced dephasing takes place much faster than the spontaneous decay of the qubit excited state, and therefore our prediction is observable in a real experiment.