Charge dynamics effects in conductance through a large semi-open quantum dot

Fano lineshapes in resonant transmission in a quantum dot imply interference between localized and extended states. The influence of the charge accumulated at the localized levels, which screens the external gate voltage acting on the conduction channel is investigated. The modified Fano q parameter and the resonant conduction is derived starting from a microscopic Hamiltonian. The latest experiments on ‘charge sensing’ and ‘Coulomb modified Fano sensing’ compare well with the results of the present model.     

Spin-flip effects in a parallel-coupled double quantum dot molecule

We investigate theoretically the electronic transport through a parallel-coupled double quantum dot (DQD) molecule attached to metallic electrodes, in which the spin-flip scattering on each quantum dot is considered. Special attention is paid to the effects of the intradot spin-flip processes on the linear conductance by using the equation of motion approach for Green’s functions. When a weak spin-flip scattering on each quantum dot is present, the single Fano peak splits into two Fano peaks, and the Breit–Wigner resonance may be suppressed slightly. When the spin-flip scattering strength on each quantum dot becomes strong, the linear conductance spectrum consists of two Breit–Wigner peaks and two Fano peaks due to the quantum interference effects. The positions and shapes of these resonant peaks can be controlled by using the magnetic flux through the quantum device.     

Quantum interference effect of resonant transport in nano-scale systems

We propose a new method of analyzing the quantum interference effect of the resonant transport in ballistic open systems. The new method is to obtain the resonant eigenvalues by computing the norm of the retarded and advanced Green's functions. Using the method, we illustrate for a fullerene and an AB-ring the relation between each resonant state and each asymmetric conductance peak, namely the Fano peak. We show that the combination of resonant states determines the symmetry of a conductance peak and that the Fano peak is caused by the asymmetry of the numerator of the conductance around a resonance. The Fano peak appears not only due to the quantum interference effect as often claimed, but more generally due to the resonant transport.     

等离子体纳米结构中的Fano共振效应及其应用

Fano共振效应是一种具有非对称线型的共振散射现象,起源于共振过程和非共振过程的量子干涉效应。近年来,在等离子体纳米结构中Fano共振现象也被发现,并成为纳米光子学的一个研究热点。等离子体Fano共振通常具有较窄的光谱线宽,且不能直接与入射光耦合,只能局域在近场,强的近场局域特性可以获得巨大的表面电磁场增强。由于等离子体Fano共振独特的光学特性,已经被应用到单分子探测、高灵敏度传感、增强光谱、完美吸收、电磁诱导透明和慢光光子学器件等众多领域当中。     

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.     

Coulomb-modified Fano resonance in a one-lead quantum dot

We investigate a tunable Fano interferometer consisting of a quantum dot coupled via tunneling to a one-dimensional channel. In addition to Fano resonance, the channel shows strong Coulomb response to the dot, with a single electron modulating channel conductance by factors up to 100. Where these effects coexist, line shapes with up to four extrema are found. A model of Coulomb-modified Fano resonance is developed and gives excellent agreement with experiment.     

Quantum transport across multilevel quantum dot

The conductance across a quantum dot can be influenced by levels localized in the dot and having little hybridization with the conduction channel. Fano lineshapes arising in resonant transmission measurements, imply interference between the localized and extended states. By applying a magnetic orthogonal field, the total spin of a quantum dot can be tuned. Electron correlations drive the dot through level crossings to higher spin states. Such crossings can give rise to Kondo conductance when the dot is at Coulomb blockade close to a magnetic field induced level degeneracy. In a previous work [P. Stefański, A. Tagliacozzo, B.R. Bulka, Phys. Rev. Lett. 93 (2004) 186805] we have shown that a Fano-like pattern also appears when the continuum of the conduction states originates from a broad Kondo resonance. A bunch of localized core levels, weakly coupled to the Kondo resonance, imprints the broad Kondo peak with Fano lineshapes. A signature of the presence of correlations in the quantum dot is discussed.     

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.     

外力驱动下耦合弹簧振子的法诺共振

法诺共振是物理体系中普遍存在的一种非对称共振现象,它最早起源于量子物理,其微观图像是原子谱线中窄的分离态与宽的连续态之间的相干干涉.本文利用经典力学体系中两个弹簧振子的耦合,使其中一个弹簧振子受到周期性外力的驱动,成功类比了量子力学中的法诺共振现象.通过分析每个弹簧振子的动力学方程,严格求解它们的振动公式,从而得到每个...     

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.     

含stubs量子波导系统的电子自旋极化输运性质

理论上研究了含stubs的Rashba自旋轨道耦合（spin-orbit coupling, SOC）量子波导系统的自旋极化输运性质. 利用晶格格林函数方法,发现由于stubs和SOC产生的势阱使系统中出现束缚态,这些束缚态与传播态之间相互干涉导致电导中出现Fano共振结构,同时在对应的自旋极化率中也出现Fano共振或反共振结构. 此外,由于系统结构的突变使电子被反向散射和量子干涉效应,电导中出现一系列的共振峰. 但是,当系统加上外磁场后,所有这些效应都被抑制, 系统重新出现量子化电导, 同时自旋电导也出 关键词： 量子波导 自旋极化输运 自旋轨道耦合     

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.     

ac field-induced Fano resonances in a parallel-coupled double quantum dot system

The effects of an ac electric field on the Fano resonance in a parallel-coupled double quantum dot system are investigated theoretically. The field can induce the photon-assisted Fano resonances for both symmetrical and asymmetrical parallel configurations. The magnitude and position of the photon-assisted Fano peak can be tuned by the ac field strength and frequency, respectively. Furthermore, the Fano resonance can appear with increasing the field frequency for both the symmetrical and asymmetrical configurations. This provides an efficient mechanism to control the Fano resonance. The photon-electron pumping effects for the symmetrical and asymmetrical cases are also studied in the weak- and strong-coupling regime.     

Entanglement via tunable Fano-type interference in asymmetric semiconductor quantum wells

Entanglement is realized in asymmetric coupled double quantum wells (DQWs) trapped in a doubly resonant cavity by means of Fano-type interference through a tunneling barrier, which is different from the previous studies on entanglement induced by strong external driven fields in atomic media. We investigate the generation and evolution of entanglement and show that the strength of Fano interference can influence effectively the degree of the entanglement between two cavity modes and the enhanced entanglement can be generated in this DQW system. The present investigation may provide research opportunities in quantum entangled experiments in the DQW solid-state nanostructures and may result in a substantial impact on the technology for entanglement engineering in quantum information processing.     

Switching an electrical current with atoms: the reproducible operation of a multi-atom relay

The demonstration of a multi-atom quantum point contact relay is reported, which can be reversibly switched between a quantized conducting ‘on-state’ and an insulating ‘off-state’ by applying an electrochemical control potential to a separate, third electrode, the control or gate electrode. The transition occurs directly from the conducting ‘on-state’ at 5 G₀ (G₀=2e²/h being the conductance quantum) to the insulating ‘off-state’. No stable intermediate levels are observed during the switching process, indicating a reproducible bistable reconfiguration of one single multi-atom contact rather than a deposition and dissolution of different parallel contacts. The results at the same time demonstrate the feasibility and reproducible operation of a configurable electronic device based on a multi-atom contact, which exhibits the functionality of an atomic relay or a transistor, opening intriguing perspectives for electronics and logics on the atomic scale.     

Kondo Resonance versus Fano Interference in Double Quantum Dots Coupled to a Two-Lead One-Ring System

We analyse the transport properties of a coupled double quantum dot （DQD） device with one of the dots （QD1） coupled to metallic leads and the other （QD2） embedded in an Aharonov-Bhom （A-B） ring by means of the slaveboson mean-field theory. It is found that in this system, the Kondo resonance and the Fano interference exist simultaneously, the enhancing Kondo effect and the increasing hopping of the QD2-Ring destroy the localized electron state in the QD2 for the QD1-leads, and accordingly, the Fano interference between the DQD-leads and the QD1-leads are suppressed. Under some conditions, the Fano interference can be quenched fully and the single Kondo resonance of the QD1-leads comes into being. Moreover, when the magnetic flux of the A-B ring is zero, the influence of the parity of the A-B ring on the transport properties is very weak, but this influence becomes more obvious with non-zero magnetic flux. Thus this model may be a candidate for future device applications.     

Quantum waveguide theory for hole transport in mesoscopic structures

A quantum waveguide theory is proposed for hole transport in the mesoscopic structures, including the band mixing effect. We found that due to the interference between the ‘light’ hole and ‘heavy’ wave, the transmission and reflection coefficients oscillate more irregularly as a function of incident wave vector geometry parameters. Furthermore conversion between the heavy hole and light hole states occurs at the intersection.     

Magnetic phase diagram of double-layered La2−2xCa1+2xMn2O7 manganite

Double-layered manganite La_2−2xCa_1+2xMn₂O₇ have been synthesized for compositions ‘x’=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 by solid state reaction method. From X-ray diffraction study, their crystal structures were found to be tetragonal perovskite with lattice parameters decreasing with increasing ‘x’. The decreasing lattice parameters affect the balance between in-plane, intra-bilayer and inter-bilayer exchange interactions, which is reflected on magnetotransport properties. The metal-to-insulator transition temperature is found to vary with composition and peaked around ‘x’=0.3. From ac-susceptibility study, 2D-ferromagnetic ordering was observed at higher temperatures for all compositions whereas 3D-ferromagnetic ordering was observed at quite low temperatures. In low-temperature region, decreasing susceptibility shows antiferromagnetic state for all compositions. On the basis of electrical and magnetic properties, a magnetic phase diagram is given.     

Stable and resonant Cooper pairs

A Bethe-Salpeter treatment of Cooper pairs (CPs) based on an ideal Fermi gas (IFG) ‘sea’ yields the familiar stable negative-energy, infinite-lifetime two-particle bound-state if two-hole CPs are ignored. Otherwise, it gives purely imaginary energies and thus unphysical solutions. However, a Bogoliubov transformation to the BCS ground state yields the trivial sound solution plus a nontrivial one. The latter are two-fermion ‘moving CPs’ as positive-energy stable objects for zero center-of-mass momentum (CMM), and finite-lifetime resonant ones for nonzero CMM with a linear dispersion leading term. Hence, Bose-Einstein condensation of such pairs may occur in exactly two dimensions as it cannot with quadratic dispersion.