Anisotropic property in interacting quantum wires embedded in (110) plane

We investigate the theoretically combined effect of spin-orbit interactions and Coulomb interaction on the ground state and transport property of a quantum wire oriented along different crystallographic directions in the (110) plane. We find that the electron’s ground state exhibits phase transition among spin density wave, charge density wave, singlet superconductivity and metamagnetism, which can be controlled by changing the crystallographic orientation, the strengths of the spin-orbit interactions and the Coulomb interaction. The ac conductance exhibits a significant anisotropic behavior and a out-of-plane spin polarization which can be tuned by an in-plane electric field.     

Activation behavior and dielectric relaxation in polyvinyl alcohol and multiwall carbon nanotube composite films

The dc and ac electrical transport property of Polyvinyl Alcohol-Multiwall Carbon Nanotubes (PVA-MWNT) composites has been investigated within a temperature range and in the frequency range 20Hz-1MHz. The temperature variation of dc conductivity gives the presence of two different activation energies. The dielectric properties of the samples have been explained in terms of electric modulus vector.The dielectric relaxation has been explained in terms of interfacial polarization occurring in between the insulating PVA matrix and MWNT conductive filler. The variation of the relaxation time with temperature also indicates the presence of two different activation energies.     

Conductance of a Quantum Dot in the Presence of a Phonon Field

We theoretically investigate the electrical transport property of a quantum dot with longitudinal optical phonons. The conductance through the dot connected to two leads is calculated by the nonequilibrium Green function within the Landauer-Büttiker framework. The numerical examples of the conductance with different electron-phonon coupling strengths show that the presence of a phonon field typically results in the suppression of the main peak accompanied by some phonon side peaks. Both the main peak and the side peaks axe sensitive to the electron-phonon coupling strength, which is related to temperature. Our results for this system are consistent with some related previous works but the calculation is comparatively simple.     

Enhanced spin polarization in an asymmetric magnetic graphene superlattice

The authors investigate the spin-resolved transport through an asymmetrical magnetic graphene superlattice (MGS) consisting of the periodic barriers with abnormal one in height. To quantitatively depict the asymmetrical MGS, an asymmetry factor has been introduced to measure the height change of the abnormal barrier. It is shown that the spin filter effect is strongly enhanced by the barrier asymmetry both in the Klein and the classical tunneling regimes. In the presence of abnormal barrier, the conductance with certain spin direction is suppressed with respect to different tunneling regimes, and thus high spin polarization with opposite sign can be achieved.     

Shot noise suppression in a series graphene tunnel barrier structure

The shot noise properties in a graphene-based multi-quantum well structure are investigated theoretically. It is found that when the number of the potential barriers (quantum wells) is big enough and the width of the barriers and the wells is shorter than the mean free path, another Dirac-like point at which the Fermi energy equals half of the barrier height appears. The transport is almost forbidden at this new Dirac-like point, the conductivity gets the minimum, and the Fano factor approaches 1/3. With the random potential barrier being taken into consideration, the conductance enhances clearly, meanwhile the Fano factor is suppressed much more.     

点间耦合强度对三耦合量子点系统微分电导的影响

本文利用非平衡格林函数运动方程方法,研究了与两个电极耦合在一起的三耦合量子点系统的微分电导及量子干涉的AB振荡问题.通过理论计算发现,由于量子点上的局域态密度的不同从而导致系统电导或隧穿性质的不同,而且量子点间耦合强度、量子点能级等都会对输运性质产生影响. 关键词： 量子点 非平衡格林函数 运动方程 局域态密度     

Temperature Dependence of Abnormal Fano Resonance in Photon-Assisted Transport Through a Side-Coupled Quantum Dot

We investigate transport through a perfect quantum wire with a side-coupled quantum dot under an ac field. Time-averaged complex conductance is formulated by using the nonequilibrium Green function (NGF) method. We find that the electron-photon interaction together with the quantum interference of electron wave function can lead to anti-resonance in the conductance, which is then useful for tuning coherence and phases of electrons. Meanwhile, we study the temperature dependence of the conductance. Interestingly, a peak-structure can be developed at the Fano resonance levels with increasing temperatures.     

An approach based on the free-electron theory to calculate electron conductance of perfect metallic nanowires

For the first time, the quantum free-electron theory is utilized for determining the quantized electrical conductance in a perfect metallic multi-atom nanowire. In this formulation, both spin-up and spin-down conduction electrons are considered so that our final result comes from a series combination of the contributions to the conductance from the two possible types of spin-oriented electron, assuming a symmetric distribution of the involved counter-ions. In addition, several aspects related to the atom-lead coupling are discussed.     

Electrical properties of thermally evaporated nickel-dimethylglyoxime thin films

Thin Bis-(dimethylglyoximato)nickel(II) [Ni(DMG)₂] films of amorphous and crystalline structures were prepared by vacuum deposition on Si (P) substrates. The films were characterised by X-ray fluorescence and X-ray diffraction. The constructed Al/Ni(DMG)₂/Si(P) metal-insulator-semiconductor devices were characterised by the measurement of the gate-voltage dependence of their capacitance and ac conductance, from which the surface states density D_it of insulator/semiconductor interface and the density of the fixed charges in the oxide were determined. The ac electrical conduction and dielectric properties of the Ni(DMG)₂-Silicon structure were studied at room temperature. The data of the ac measurements of the annealed films follow the correlated barrier-hopping CBH mode, from which the fundamental absorption bandgap, the minimum hopping distance, and other parameters of the model were determined.     

Theory of ac quantum conduction through an ideal constriction

A scattering theory is formulated for time-dependent (ac) transport through quantum constrictions or quantum point contacts. This is an extension of the standard scattering treatment for the time-independent (dc) case where quantized conductance steps and resonances occur. For an ideal constriction, the first-order transmission sidebands are derived when a time-dependent sinusoidal potential is applied. The frequency dependence of the conductance is discussed, and possible experiments are suggested.     

Resonant transport through a carbon nanotube junction exposed to an ac field

The electron transport through a carbon nanotube (CNT) double barrier junction exposed to an external electromagnetic field is studied. The electron spectrum in the quantum well (QW) formed by the junction bears relativistic features. We examine how the ac field affects the level quantization versus the ac field parameters and chirality. We find that the transport through the junction changes dramatically versus the ac field frequency and amplitude. These changes are pronounced in the junction's differential conductance, which allows judgment about the role of relativistic effects in the CNT QW structures.     

Spin-dependent transport through quantum dot with inelastic interactions

Using the Keldysh nonequilibrium Green function method, we theoretically investigate the electron transport properties of a quantum dot coupled to two ferromagnetic electrodes, with inelastic electron-phonon interaction and spin flip scattering present in the quantum dot. It is found that the electron-phonon interaction reduces the current, induces new satellite polaronic peaks in the differential conductance spectrum, and at the same time leads to oscillatory tunneling magnetoresistance effect. Spin flip scattering suppresses the zero-bias conductance peak and splits it into two, with different behaviors for parallel and anti-parallel magnetic configuration of the two electrodes. Consequently, a negative tunneling magnetoresistance effect may occur in the resonant tunneling region, with increasing spin flip scattering rate.     

Ac response of a coupled double quantum dot

The effect of phase-breaking process on the ac response of a coupled double quantum dot is studied in this paper based on the nonequilibrium Green function formalism. A general expression is derived for the ac current in the presence of electron--phonon interaction. The ac conductance is numerically computed and the results are compared with those in [Anatram M P and Datta S 1995 Phys. Rev. B 51 7632]. Our results reveal that the inter-dot electron tunnelling interplays with that between dots and electron reservoirs, and contributes prominently to the ac current when inter-dot tunnelling coupling is much larger than the tunnelling coupling between dots and electron reservoirs. In addition, the phase-breaking process is found to have a significant effect on the ac transport through the coupled double dot.     

Coherent motion of vortices driven by both dc and ac current in a Josephson junction ratchet array

We have studied the vortex dynamics in a ratchet array of Josephson junctions in the presence of magnetic field of 1/5 flux quantum per plaquette. The ratchet potential consists of both alternate critical currents for all the vertical junctions and alternate shunt capacitances for all the horizontal junctions. The vortices driven by an ac current in some parameters are found to show the directional motion as well as the asymmetric current-voltage characteristics. We use the time-dependent vorticity and the time-dependent vorticity-vorticity correlation function to analyze the motion of vortices on a few fractional Shapiro steps. We have found that vortices on a fractional Shapiro n/5-step move coherently through n plaquettes during a single ac cycle. The asymmetric features of the ratchet array gradually disappear as finite temperature increases.     

Quantum dots as scatterers in electronic transport: interference and correlations

Conductance through a system consisting of a wire with side-attached quantum dots is calculated. Such geometry of the device allows to study the coexistence of quantum interference, electron correlations and their influence on conductance. We underline the differences between ‘classical’ Fano resonance in which the resonant channel is of single-particle nature and ‘many-body’ Fano resonance with the resonant channel formed by Kondo effect. The influence of electron-electron interactions on the Fano resonance shape is also analyzed.     

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.     

Optoelectronic Properties for Armchair-Edge Graphene Nanoribbons

We study theoretically the electronic and transport property for an armchair-edge graphene nanoribbon （GNR.） with 12 and 11 transversal atomic lines, respectively. The ONR. is irradiated under an external longitudinal polarized high-frequency electromagnetic field at low temperatures. Within the framework of linear response theory in the perturbative regime, we examine the joint density of states and the real conductance of the system. It is demonstrated that, by numerical examples, some new photon-assisted intersubband transitions over a certain range of field frequency exist with different selection rules from those of both zigzag-edge GNR. and single-walled carbon nanotube. This opto-electron property dependence of armchair-edge GNR. on field frequency may be used to detect the high-frequency electromagnetic irradiation.     

Phonon-assisted Kondo effect in single-molecule quantum dots coupled to ferromagnetic leads

Based on the infinite-U Anderson model spin-polarized transport through the tunnel magnetoresistance (TMR) system of single-molecule quantum dot is investigated under the interplay of strong electron correlation and electron-phonon (e-ph) coupling. The spectral density and the nonlinear differential conductance are studied using the extended non-equilibrium Green's function method through calculating the dot-level splitting self-consistently. The results exhibit that a serial of peaks emerge on the two sides of the main Kondo peak for the antiparallel magnetic configuration of electrodes, while for the parallel case both the main and phonon-assisted satellite Kondo peaks all split up into two asymmetric peaks even at zero-bias. Correspondingly, the nonlinear differential conductance displays a set of satellite-peaks around the Kondo-peak in the presence of the e-ph interaction. Furthermore, extra maxima and minima appear in the TMR curve. The TMR alternates between the positive and the negative values along with the variation of bias voltage.     

Electron transport in carbon nanotube quantum dots in an axial magnetic field

The quantum conductance of the quantum dots (QDs) made of two kinds of primary carbon nanotubes (CNTs), i.e., armchair and zigzag CNTs, threaded by an axial magnetic field, has been studied by using the tight binding approximation and constant interaction model. It is found that under increasing axial magnetic field, each conductance shell of the zigzag CNT-QDs could split into two groups with each group of two peaks moving up or down, respectively. And the up- and down-moving two peaks would re-group with other two peaks, down- and up-moving, in the neighboring shell, forming a new four-peak shell, and then re-splitting, re-grouping again due to the Aharonov-Bohm effect, which is in agreement with those of experiments. But, in contrast, the conductance shells of the armchair CNT-QDs do not split by the magnetic field. Our subsequent theoretical studies show further that the above phenomena, i.e., the conductance shell-splitting, re-grouping, and re-splitting again with increasing the magnetic field exist in all the CNT-QDs except for the armchair one.     

Functional renormalization group approach to the singlet-triplet transition in quantum dots

We present a functional renormalization group approach to the zero bias transport properties of a quantum dot with two different orbitals and in the presence of Hund's coupling. Tuning the energy separation of the orbital states, the quantum dot can be driven through a singlet-triplet transition. Our approach, based on the approach by Karrasch et?al (2006 Phys. Rev. B 73 235337), which we apply to spin-dependent interactions, recovers the key characteristics of the quantum dot transport properties with very little numerical effort. We present results on the conductance in the vicinity of the transition and compare our results both with previous numerical renormalization group results and with predictions of the perturbative renormalization group.