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We analyze the effects of a distortion of the nuclear potential of a molecular quantum dot (QD) as well as a shift in its equilibrium position on nonequilibrium-vibration-assisted tunneling through the QD with a single level ([variant_greek_epsilon]d) coupled to the vibrational mode. For this purpose, we derive an explicit analytical expression for the Franck–Condon (FC) factor for a displaced-distorted oscillator surface of the molecule and establish rate equations in the joint electron-phonon representation to examine the current-voltage characteristics and zero-frequency shot noise and skewness as well. Our numerical analysis shows that the distortion has two important effects. The first one is that it breaks the symmetry between the excitation spectra of the charge states, leading to asymmetric tunneling properties with respect to [variant_greek_epsilon]d>0 and [variant_greek_epsilon]d<0. Second, distortion (frequency change in the oscillator) significantly changes the voltage-activated cascaded transition mechanism and consequently gives rise to a different nonequilibrium vibrational distribution from that of the case without distortion. Taken in conjunction with strongly modified FC factors due to distortion, this results in some new transport features: the appearance of strong negative differential conductance even for a single-level QD with symmetric tunnel couplings, a giant Fano factor even for a molecule with an extremely weak electron-phonon interaction, and enhanced skewness that can have a large negative value under certain conditions. [ABSTRACT FROM AUTHOR]

We examine the shot noise spectrum properties of coherent resonant tunneling in coupled quantum dots (CQDs) in both series and parallel arrangements by means of quantum rate equations and MacDonald’s formula. Our results show that for a series-CQD with a relatively high dot-dot hopping Ω and Ω/Γ>=1 (Γ denotes the dot-lead tunnel-coupling strength), the noise spectrum exhibits a dip at the Rabi frequency 2Ω in the case of noninteracting electrons, but the dip is supplanted by a peak in the case of strong Coulomb repulsion; furthermore, it becomes a dip again for a completely symmetric parallel-CQD by tuning the enclosed magnetic flux. [ABSTRACT FROM AUTHOR]

Presents the results of a study regarding the quantum transport through double-dot Aharonov-Bohm interferometry in Coulomb blockade regime. Results of employing the modified quantum rate equations and the slave-boson approach in the study; Description of the quantum dot; Discussion of the Coulomb correlation effect at arbitrary temperature.

The authors analyze resonant tunneling subject to a nonadiabatic time-dependent bias voltage through an asymmetric single molecular quantum dot with coupling between the electronic and vibrational degrees of freedom using a Tien-Gordon-type rate equation. The results clearly exhibit the appearance of photon-assisted satellites in the current-voltage characteristics and the elimination of hot-phonon-induced negative differential conductance with increasing ac driving amplitude for an asymmetric system. This can be ascribed to an ac-induced suppression of unequilibrated (hot) phonons in an asymmetric system. [ABSTRACT FROM AUTHOR]

We study the full counting statistics (FCS) of electron tunneling through a multiterminal quantum dot in the Kondo regime within the slave-boson mean-field theory. By employing the method of Gogolin and Komnik for calculating the FCS generating function based on the nonequilibrium Green's function [Phys. Rev. B 73, 195301 (2006)]. we obtain the counting field λ-dependent self-consistent equations for the mean values of the slave-boson fields and the explicit expression for the derivative of the adiabatic potential of the system with respect to the counting fields. Performing a perturbative expansion to the first order of λ, we find an extra contribution to the shot noise due to the bias-induced Bose field fluctuation, and then confirm that the nonequilibrium particle number fluctuation plays an important role in the current noise of the Kondo dot: enhancement of the current autocorrelation and a positive current cross-correlation. [ABSTRACT FROM AUTHOR]

We present a theoretical investigation for the full counting statistics of the Andreev tunneling through a quantum dot (QD) embedded between superconducting (SC) and normal leads in the presence of a strong on-site electron-phonon interaction using nonequilibrium Green function method. For this purpose, we generalize the dressed tunneling approximation (DTA) recently developed in dealing with inelastic tunneling in a normal QD system to the Andreev transport issue. This method takes account of vibrational effect in evaluation of electronic tunneling self energy in comparison with other simple approaches and meanwhile allows us to derive an explicit analytical formula for the cumulant generating function at the subgap region. We then analyze the interplay of polaronic and SC proximity effects on the Andreev reflection spectrum, current-voltage characteristics, and current fluctuations of the hybrid system. Our main findings include: (1) no phonon side peaks in the linear Andreev conductance; (2) a negative differential conductance stemming from the suppressed Andreev reflection spectrum; (3) a novel inelastic resonant peak in the differential conductance due to phonon assisted Andreev reflection; (4) enhancement or suppression of shot noise for the symmetric or asymmetric tunnel-coupling system, respectively. [ABSTRACT FROM AUTHOR]