Rong Lü

Tunable Fano effect in parallel-coupled double quantum dot system

With the help of the Green function technique and the equation of motion approach, the electronic transport through a parallel-coupled double quantum dot (DQD) is theoretically studied. Owing to the interdot coupling, the bonding and antibonding states of the artificial quantum-dot molecule may constitute an appropriate basis set. Based on this picture, the Fano interference in the conductance spectra of the DQD system is readily explained. The possibility of manipulating the Fano line shape in the tunneling spectra of the DQD system is explored by tuning the dot-lead coupling, the interdot coupling, the magnetic flux threading the ring connecting dots and leads, and the flux difference between two subrings. It has been found that by making use of various tunings, the direction of the asymmetric tail of Fano line shape may be flipped by external fields and the continuous conductance spectra may be magnetically manipulated with the line shape retained. More importantly, by adjusting the magnetic flux, the function of two molecular states can be exchanged, giving rise to a swap effect, which might play a role as a qubit in the quantum computation.

Effects of electron-phonon interaction on nonequilibrium transport through a single-molecule transistor

On the basis of the nonequilibrium Greens function and nonperturbative canonical transformation for the local electron-phonon interaction (EPI), the quantum transport through a single-molecule transistor (SMT) has been investigated with particular attention paid to the joint effect of the EPI and SMT-lead coupling on the spectral function and conductance. In addition to the usual EPI-induced renormalized effects (such as the redshift, sharpening, and phonon sidebands of the SMT level), owing to improved disentangling the electron-phonon system it has been found that the profile of the spectral function of the SMT electron is sensitive to lead chemical potentials, thus can readily be manipulated by tuning the bias as well as the SMT-gate voltage. As a consequence, the broken particle-hole symmetry in this system can be clearly recognized through the phonon sidebands in the spectral function. These EPI effects also manifest themselves in the nonequilibrium transport properties of the SMT, particularly at low temperature.

Fano effect through parallel-coupled double Coulomb islands

By means of the nonequilibrium Green function and equation of motion method, the electronic transport is theoretically studied through a parallel-coupled double quantum dot (DQD) in the presence of on-dot Coulomb interaction U. With focus on the quantum interference in the U-dominant parallel-coupled DQD, we find two types of Fano interferences in the conductance spectra. If the one-particle DQD bonding and antibonding bands are well separated from their Coulomb blockade counterparts, the main features of Fano interference in usual DQD systems are recovered with minor revisions. The most interesting is the hybridization between the antibonding state and the Coulomb counterpart of the bonding state, which gives rises to two new channels for Fano resonance. The Fano interference in the Coulomb hybridized systems can be controlled by the electrostatic and magnetic approaches, and exhibits properties quite different from what are reported in the noninteracting Fano–Anderson model.

Phonon-assisted Kondo effect in a single-molecule transistor out of equilibrium

The joint effect of the electron–phonon interaction and Kondo effect on nonequilibrium transport through a single molecule transistor is investigated by using the improved canonical transformation scheme and extended equation of motion approach. Two types of Kondo phonon-satellite with different asymmetric shapes are fully confirmed in the spectral function, and are related to the electron spin singlet or hole spin singlet, respectively. Moreover, when a moderate Zeeman splitting is caused by a local magnetic field, the Kondo satellites in the spin-resolved spectral function are found to disappear on one side of the main peak, and they disappear on the opposite side for the opposite spin component. All these peculiar signatures that manifest themselves in the nonlinear differential conductance are explained with a clear physics picture.

The Study on the Filling of Atoms in a Carbon Nanotube

The filling of Pb, C, Al and S atoms into Carbon nanotubes is studied by use of the discrete variational method within the local density functional theory. It is found that Lead atom inclines to enter into tubes, while Carbon atom rather stays at the mouth of nanotubes, which is in good agreement with experiments. The effect of nanotube length and diameter on the filling is discussed.

Interplay between Fano resonance and PT symmetry in non-Hermitian discrete systems

We study the effect of PT-symmetric complex potentials on the transport properties of non-Hermitian systems, which consist of an infinite linear chain and two side-coupled defect points with PT-symmetric complex on-site potentials. By analytically solving the scattering problem of two typical models, which display standard Fano resonances in the absence of non-Hermitian terms, we find that the PT-symmetric imaginary potentials can lead to some pronounced effects on transport properties of our systems, including changes from the perfect reflection to perfect transmission, and rich behaviors for the absence or existence of the prefect reflection at one and two resonant frequencies. Our study can help us to understand the interplay between the Fano resonance and PT symmetry in non-Hermitian discrete systems, which may be realizable in optical waveguide experiments.

Double-layer Bose-Einstein condensates with a large number of vortices

In this paper we systematically study the double-layer vortex lattice model, which is proposed to illustrate the interplay between the physics of a fast rotating Bose-Einstein condensate and the macroscopic quantum tunneling. The phase diagram of the system is obtained. We find that under certain conditions the system will exhibit a phase transition which is a consequence of the competition between interlayer coherent hopping and the interlayer density-density interaction. In one phase the vortices in one layer coincide with those in the other layer. In another phase two sets of vortex lattices are staggered, and as a result the quantum tunneling between two layers is suppressed. To obtain the phase diagram we use the quantum Hall mean field and Thomas-Fermi mean field theories. Two different criteria for the transition taking place are obtained, which reveals some fundamental differences between these two mean-field states. The sliding mode excitation is also discussed.

Time-dependent electron transport through molecular quantum dots in the presence of external irradiation

We present a fully nonequilibrium calculation of the low-temperature transport properties of a single molecular quantum dot coupled to the local phonon mode when an ac field is applied to the gate. The resonant behaviour is shown in the time-averaged differential conductance as the ac frequency matches the frequency of the local phonon mode, which is a direct consequence of the satellite-phonon-peak structure in the dot electron spectral function. Different step structure with and without the external irradiation is found in the I-V curves, and oscillation behaviour is found in the step height as a function of the irradiation intensity.

Magnetization quantum tunneling at excited levels for a biaxial spin system in an arbitrarily directed magnetic field

The quantum tunneling of the magnetization vector between excited levels are studied theoretically in single-domain ferromagnetic nanoparticles with biaxial crystal symmetry placed in an external magnetic field at an arbitarily directed angle in the ZX plane. The temperature dependences of the tunneling frequency and the decay rate are clearly shown for each case.

Generation and detection of spin current in the three-terminal quantum dot.

We propose a novel device composed of a quantum dot tunneling coupled to ferromagnetic, superconducting, and normal-metal leads. This device can generate, manipulate, and detect pure spin current through the interplay between the spin-polarized quantum transport and the Andreev reflection. The spin current in this device is a well-defined conserved current since there is no need for any spin-orbit coupling. The proposed device is realizable using present nanotechnology.