Splitting of the Fano resonance by Coulomb interactions in a two-dot quantum ring

We study the splitting of the Fano resonance in a Aharonov–Bohm interferometer with a quantum dot in each of its arms. Both intra- and inter-dot Coulomb repulsions are taken into account by employing the Keldysh nonequilibrium Green’s function technique. The single narrow Fano resonance in the noninteracting case is split into two in the presence of either intra- or inter-dot Coulomb interaction. We find that four Fano peaks emerge in the conductance or local density of states spectra when the two kinds of interactions exist simultaneously. Such behavior holds true for the accompanying broad Breit–Wigner type resonance. We also show that the positions of the Fano peaks can be tuned with the aid of the magnetic flux penetrating through the ring, which might have practical applications in device design or quantum computation.     

Transport properties of a double quantum dot molecule in the presence of impurity effects

In this Letter, we studied the electronic transport through a parallel-coupled double quantum dot (DQD) molecule including impurity effects at zero temperature. The linear conductance can be calculated by using the Green's function method. An obvious Fano resonance arising from the impurity state in the quantum dot is observed for the symmetric dot-lead coupling structure in the absence of the magnetic flux through the quantum device. When the magnetic flux is presented, two groups of conductance peaks appear in the linear conductance spectra. Each group is decomposed into one Breit-Wigner and one Fano resonances. Tuning the system parameters, we can control effectively the shapes of these conductance peaks. The Aharonov-Bohm (AB) oscillation for the magnetic flux is also studied. The oscillation period of the linear conductance with π, 2π or 4π may be observed by tuning the interdot tunneling coupling or the dot-impurity coupling strengths.     

Fano effect in a T-shaped double quantum dot structure in the presence of Rashba spin-orbit coupling

The influence of Rashba spin-orbit coupling on the Fano lineshape of the conductance spectrum in a T-shaped double quantum dot structure is theoretically studied. By second-quantizing the electron Hamiltonian in this structure, it is found that the Rashba interaction brings about a spin-flip interdot hopping term. With the enhancement of the Rashba interaction, this term separates the two resonant peaks in the conductance spectrum from each other. More importantly, it causes the broadening of the narrow Fano peak, and the narrowing of the broader peak. Finally, the asymmetric Fano lineshape changes into a symmetric profile in the global conductance spectrum.     

Fano effect of a parallel-coupled triple Rashba quantum dot system

Using the nonequilibrium Keldysh Green's function technique, the Fano effect of a parallel-coupled triple Rashba quantum dot system is investigated. The conductance as a function of electron energy is numerically calculated. Compared with the case of a parallel-coupled double quantum dot system, two additional Fano resonance peaks occur in the conductance spectrum. By adjusting the structural parameters, the two Fano resonance peaks may change into the resonance peaks. In addition, the influence of Rashba spin-orbit interaction on the conductance is studied.     

Andreev reflection in a quantum dot coupled to ferromagnetic and superconductor leads under magnetic fields

By employing the nonequilibrium Green's function, we investigate the spin-dependent linear Andreev reflection (AR) resonant tunneling through a quantum dot connected to a ferromagnetic lead and a superconducting lead, where the magnetization direction in the ferromagnetic lead can be tuned by one. We focus our attention on the effects of the magnetic fields on the AR conductance. One high conductance peak and one low conductance peak are developed in the linear AR conductance when a stronger magnetic field is considered. The interplay between the spin-flip scattering and the magnetic fields on the AR conductance are also studied.     

Spin-dependent shot noise of inelastic transport through molecular quantum dots

Here we present a theoretical analysis of the effect of inelastic electron scattering on spin-dependent transport characteristics (conductance, current–voltage dependence, magnetoresistance, shot noise spectrum, Fano factor) for magnetic nanojunction. Such device is composed of molecular quantum dot (with discrete energy levels) connected to ferromagnetic electrodes (treated within the wide-band approximation), where molecular vibrations are modeled as dispersionless phonons. Non-perturbative computational scheme, used in this work, is based on the Green's function theory within the framework of mapping technique (GFT–MT), which transforms the many-body electron–phonon interaction problem into a single-electron multi-channel scattering problem. The consequence of the localized electron–phonon coupling is polaron formation. It is shown that polaron shift and additional peaks in the transmission function completely change the shape of considered transport characteristics.     

Photon-assisted Shot noise of the double quantum dot interferometer in weak Kondo regime

The shot noise of a parallel double quantum dot (DQD) system under the perturbation of microwave fields is investigated in the weak Kondo regime. Peak-valley structures exhibit in the differential conductance and shot noise, and side resonant peaks emerge around the Kondo peak due to the absorption and emission of photons. The shot noise is sensitively dependent on the adjusting approach through changing the gate voltages. Large resonant Fano factor accompanying photon-induced side peaks appear by simultaneously varying the two gate voltages. The photon suppression and enhancement of shot noise have been evaluated corresponding to the coherent and incoherent current correlation. The destructive interference causes the suppression of shot noise by changing the Aharonov–Bohm phase.     

Charging effects in biased molecular devices

The influence of charging effects on the transport characteristics of a molecular wire bridging two metallic electrodes in the limit of weak contacts is studied by the generalized Breit–Wigner formula. Molecule is modeled as a quantum dot with discrete energy levels, while the coupling to the electrodes is treated within a broad-band theory. Owing to this model we find self-consistent occupation of particular energy levels and orbital energies of the wire in the presence of transport. The nonlinear conductance and current–voltage characteristics are investigated as a function of bias voltage in the case of symmetric and asymmetric coupling to the electrodes. It is shown that the shape of those curves are determined by the combined effect of the electronic structure of the molecule and by electron–electron repulsion.     

Conductance in an Aharonov-Bohm Interferometer with Parallel-Coupled Double Dots

We present a theoretical study of the conductance in an Aharonov-Bohm interferometer containing two coupled quantum dots. The interdot tunneling divides the interferometer into two coupled subrings, where opposite magnetic fluxes are threaded separately while the net flux is kept zero. Using the Green function technique we derive the expression of the linear conductance. It is found that the Aharonov-Bohm effect still exists, and when the level of each dot is aligned, the exchange of the Fano and Breit-Wigner resonances in the conductance can be achieved by tuning the magnetic flux. When the two levels are mismatched the exchange may not happen. Further, for some specific asymmetric systems where the coupling strengths between the two dots and the leads are not equal, the flux can change the Fano resonance into an antiresonance, which is absent in symmetric systems.     

Correlation-induced resonances and population switching in a quantum-dot coulomb valley

Strong correlation effects on electron transport through a spinless quantum dot are considered. When two single-particle levels in the quantum dot are degenerate, a conserved pseudospin degree of freedom appears for generic tunneling matrix elements between the quantum dot and leads. Local fluctuations of the pseudospin in the quantum dot give rise to a pair of asymmetric conductance peaks near the center of a Coulomb valley. An exact relation to the population switching is provided.     

Spin-flip mesoscopic transport through a quantum dot coupled to carbon nanotube terminals

We investigate mesoscopic transport through a system that consists of a central quantum dot (QD) and two single-wall carbon nanotube (SWCN) leads in the presence of a rotating magnetic field. The spin-flip effect is induced by the rotating magnetic field, and the tunnelling current is sensitively related to the spin-flip effect. We present the calculations of charge and spin current components to show the intimate relations to the SWCN leads. Zeeman effect is important when the applied magnetic field is strong enough. The current characteristics are quite different when the source-drain bias is zero (eV=0) and nonzero (eV≠0). The asymmetric peak and valley of spin current versus gate voltage exhibit Fano resonance. Multi-resonant peaks of spin current versus photon energy ħω reflect the structure of CN quantum wires, as well as the resonant photon absorption and emission effect. The matching-mismatching of channels in the CN leads and QD results in novel spin current structure by tuning the frequency.     

Spin-dependent conductance in FM/quantum-dot/FM tunneling system

We present a theoretical study of the spin-dependent conductance spectra in a FM/semiconductor quantum-dot (QD)/FM system. Both the Rashba spin-orbit (SO) coupling in the QD and spin-flip scattering caused by magnetic barrier impurities are taken into account. It is found that in the single-level QD system with parallel magnetic moments in the two FM leads, due to the interference between different tunneling paths through the spin-degenerate level, a dip or a narrow resonant peak can appear in the conductance spectra, which depends on the property of the spin-flip scattering. When the magnetizations of the two FM leads are noncollinear, the resonant peak can be transformed into a dip. The Rashba SO coupling manifests itself by a Rashba phase factor, which changes the phase information of every tunneling path and can greatly modulate the conductance. When the QD has multiple levels, the Rashba interlevel spin-flip effect appears, which changes the topological property of the structure. Its interplay with the Rashba phase can directly tune the coupling strengths between dot and leads, and can result in switching from resonance into antiresonance in the conductance spectra.     

Time-dependent electron transport through an Aharonov–Bohm ring embedded with two quantum dots

The time-dependent electron transport through an Aharonov–Bohm ring embedded with two quantum dots in the presence of external microwave (MW) fields are investigated theoretically by using the nonequilibrium Green's function method. Whether the MW field can induce or suppress the Fano resonance depends on the part to which the field is applied. When the MW field is applied only to the two quantum dots, the photon-assisted Fano peaks appear at the sidebands of the original Fano peak. The existence of the original Fano peak or the photon-assisted ones can be controlled by the field strength. When the MW field is applied only to one lead, the original Fano peak is suppressed by the MW field, and the negative current caused by the electron–photon pump effects is found.     

Spacing and width of Coulomb blockade peaks in a silicon quantum dot

We present an experimental study of the fluctuations of Coulomb blockade peak positions of a quantum dot. The dot is defined by patterning the two-dimensional electron gas of a silicon MOSFET structure using stacked gates. The ratio of charging energy to single-particle energy is considerably larger than in comparable GaAs/AlGaAs quantum dots. The statistical distribution of the conductance peak spacings in the Coulomb blockade regime was found to be unimodal and does not follow the Wigner surmise. The fluctuations of the spacings are much larger than the typical single-particle level spacing and thus clearly contradict the expectation of random matrix theory. Measurements of the natural line width of a set of several adjacent conductance peaks suggest that all of the peaks in the set are dominated by electrons being transported through a single-broad energy level.     

两端线型双量子点分子Aharonov-Bohm干涉仪电输运

设计一个两端线型双量子点分子Aharonov-Bohm (A-B)干涉仪. 采用非平衡格林函数技术, 理论研究无含时外场作用下的体系电导和引入含时外场作用下的体系平均电流. 在不考虑含时外场时, 调节点间耦合强度或磁通可以诱导电导共振峰劈裂. 控制穿过A-B干涉仪磁通的有无, 实现了共振峰电导数值在0与1之间的数字转换, 为制造量子开关提供了一个新的物理方案. 同时借助磁通和Rashba自旋轨道相互作用, 获得了自旋过滤. 当体系引入含时外场时, 平均电流曲线展示了旁带效应. 改变含时外场的振幅, 实现了体系平均电流的大小与位置的有效控制, 而调节含时外场的频率, 则可以实现平均电流峰与谷之间的可逆转换. 通过调节磁通与Rashba自旋轨道相互作用, 与自旋相关的平均电流亦得到有效控制. 研究结果为开发利用耦合多量子点链嵌入A-B 干涉仪体系电输运性质提供了新的认知. 上述结果可望对未来的量子器件设计与量子计算发挥重要的指导作用.     

Fano and Kondo resonance in electronic current through nanodevices

Electronic transport through a quantum dot strongly coupled to electrodes is studied within a model with two conduction channels. It is shown that multiple scattering and interference of transmitted waves through both channels lead to Fano resonance associated with Kondo resonance. Interference effects are also pronouncedly seen in transport through the Aharonov-Bohm ring with the Kondo dot, where the current characteristics continuously evolve with the magnetic flux.     

Thermopower of a multilevel quantum dot coupled with leads in Coulomb blockade

We study the thermopower of a multilevel quantum dot which is coupled with the two leads. From our theoretic results, the thermopower of a multilevel quantum dot shows an oscillatory dependence on the gate voltage, which has been found in a lot of experiment data. The Fano effect of the electronic transport through the multilevel quantum dot is also shown as an obvious asymmetric line shape of the thermopower which come from the interference between the resonant and nonresonant multilevel paths of the conductive electrons. In addition, at the higher temperature, to thermopower, not conductance, it is the multilevel that is much easier to do contribution to the Fano effect.     

Intradot spin-flip Andreev reflection tunneling through a ferromagnet-quantum dot-superconductor system with ac field

We investigate Andreev reflection (AR) tunneling through a ferromagnet-quantum dot-superconductor (F-QD-S) system in the presence of an external ac field. The intradot spin-flip scattering in the QD is involved. Using the nonequilibrium Green function and BCS quasiparticle spectrum for superconductor, time-averaged AR conductance is formulated. The competition between the intradot spin-flip scattering and photon-assisted tunneling dominates the resonant behaviors of the time-averaged AR conductance. For weak intradot spin-flip scattering strengths, the AR conductance shows a series of equal interval resonant levels. However, the single-peak at main resonant level develops into a well-resolved double-peak resonance at a strong intradot spin-flip scattering strength. Remarkable, multiple-photon-assisted tunneling that generates photonic sideband peaks with a variable interval has been found. In addition, the AR conductance-bias voltage characteristic shows a transition between the single-peak to double-peak resonance as the ratio of the two tunneling strengths varies.     

Transport through artificial single-molecule magnets: Spin-pair state sequential tunneling and Kondo effects

The transport properties of an artificial single-molecule magnet based on a CdTe quantum dot doped with a single Mn+2 ion(S=5/2) are investigated by the non-equilibrium Green function method.We consider a minimal model where the Mn-hole exchange coupling is strongly anisotropic so that spin-flip is suppressed and the impurity spin S and a hole spin s entering the quantum dot are coupled into spin pair states with(2S+1) sublevels.In the sequential tunneling regime,the differential conductance exhibits(2S+1) possible peaks,corresponding to resonance tunneling via(2S+1) sublevels.At low temperature,Kondo physics dominates transport and(2S+1) Kondo peaks occur in the local density of states and conductance.These peaks originate from the spin-singlet state formed by the holes in the leads and on the dot via higher-order processes and are related to the parallel and antiparallel spin pair states.     

Fano effect and Andreev bound states in T-shape double quantum dots

In this Letter, we investigate the transport through a T-shaped double quantum dot coupled to two normal metal leads left and right and a superconducting lead. Analytical expressions of Andreev transmission and local density of states of the system at zero temperature have been obtained. We study the role of the superconducting lead in the quantum interferometric features of the double quantum dot. We report for first time the Fano effect produced by Andreev bound states in a side quantum dot. Our results show that as a consequence of quantum interference and proximity effect, the transmission from normal to normal lead exhibits Fano resonances due to Andreev bound states. We find that this interference effect allows us to study the Andreev bound states in the changes in the conductance between two normal leads.