Electronic transmission through a quantum wire by side-attached nanowires

A system of arrays of nanowires side-coupled to a quantum wire is studied. Transport through the quantum wire is investigated by using a noninteracting Anderson tunneling Hamiltonian. An analytical expression of the conductance at zero temperature is given, showing a band with alternating forbidden and allowed minibands due to the discrete structure of the nanowires. A generalization of the odd–even parity symmetry is found in this system, whose conductance exhibits a forbidden miniband in the center of the band for an odd number of sites in the nanowires, while shows an allowed band for an even number.     

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.     

通过带有侧向耦合量子点的量子线中的光辅助隧穿

应用非平衡格林函数方法,研究了带有微波调制的侧向耦合量子点的量子线中的光辅助隧穿.在考虑了量子干涉和微波场的情况下,得出并讨论了电子传榆幅度和相位方面的信息.电子传输幅度显示出一系列的反共振峰(对应图中的谷结构).峰值的高度与振荡的微波场的幅度和频率有关,而峰的位置只与微波场的频率有关.在有限温的情况下,反共振峰值的高度随着温度的增加而减小,当温度足够高时,反共振峰会消失,特别地,在一定的温度下,低温下谷的地方会演变成峰.     

Spin-dependent thermoelectric transport through double quantum dots

We study the thermoelectric transport through a double-quantum-dot system with spin-dependent interdot coupling and ferromagnetic electrodes by means of the non-equilibrium Green’s function in the linear response regime.It is found that the thermoelectric coefficients are strongly dependent on the splitting of the interdot coupling,the relative magnetic configurations,and the spin polarization of leads.In particular,the thermoelectric efficiency can reach a considerable value in the parallel configuration when the effective interdot coupling and the tunnel coupling between the quantum dots and the leads for the spin-down electrons are small.Moreover,the thermoelectric efficiency increases with the intradot Coulomb interaction increasing and can reach very high values at appropriate temperatures.In the presence of the magnetic field,the spin accumulation in the leads strongly suppresses the thermoelectric efficiency,and a pure spin thermopower can be obtained.     

Spin-dependent transport of holes in microstructures periodically modulated by diluted magnetic semiconductor sections

We study spin transport of holes through microstructures modulated periodically by diluted magnetic semiconductor (DMS) sections, stubless or stubbed. The stubs are symmetric or asymmetric and the magnetizations of consecutive DMS sections are parallel or antiparallel. The transmission coefficients of holes with spin up (T⁺) or down (T^-) are drastically different since the spins feel different potential profiles in the DMS sections. As a result, nearly square-wave patterns, or wide plateaus and oscillations, can be obtained for the transmission and the spin polarization as functions of the incident energy or of various parameters of the periodically repeated unit. Results for simple and composite units with and without deviations from perfect periodicity are reported. Some of the structures considered exhibit a strong spin-filtering behavior.     

Spin-dependent Fano effect through parallel-coupled double quantum dots

Based on the nonequilibrium Green' function method, the spin-dependent Fano effect through parallel-coupled double quantum dots has been investigated by taking account of both Rashba spin-orbit interaction and intradot Coulomb interaction. It is shown that the quantum interference through the bonding, antibonding states and through their Coulomb blockade counterparts may result in two Breit-Wigner resonances and two Fano resonances in the conductance spectra. Moreover, the Fano lineshape of the two spin components can be modulated by Rashba spin-orbit interaction when the magnetic flux is switched on.     

Spin-dependent transport through an interacting quantum dot system

Using an equation of motion technique, we report on a theoretical analysis of transport characteristics of a spin-valve system formed by a quantum dot coupled to ferromagnetic leads, whose magnetic moments are oriented at an angle θ with respect to each other, and a mesoscopic ring by the Anderson Hamiltonian. We analyse the density of states of this system, and our results reveal that the density of states show some noticeable characteristics depending on the relative angle θ of magnetic moment M, and the spin-polarised strength P in ferromagnetic leads, and also the magnetic flux Φ and the number of lattice sites N_R in the mesoscopic ring. These effects might have some potential applications in spintronics.     

STM tunneling through a quantum wire with a side-attached impurity

The STM tunneling through a quantum wire (QW) with a side-attached impurity (atom, island) is investigated using a tight-binding model and the non-equilibrium Keldysh Green function method. The impurity can be coupled to one or more QW atoms. The presence of the impurity strongly modifies the local density of states of the wire atoms, thus influences the STM tunneling through all the wire atoms. The transport properties of the impurity itself are also investigated mainly as a function of the wire length and the way it is coupled to the wire. It is shown that the properties of the impurity itself and the way it is coupled to the wire strongly influence the STM tunneling, the density of states and differential conductance.     

THz-field-induced electronic transmission step-structure for a quantum wire

We study theoretically the low-temperature electronic transport property of a straight quantum wire under the irradiation of a finite-range transversely polarized external terahertz (THz) electromagnetic (EM) field. Using the freeelectron model and the scattering matrix approach, we show an unusual behaviour of the electronic transmission of this system. A sharp step-structure appears in the electronic transmission probability as the EM field strength increases to a threshold value when a coherent EM field is applied. We demonstrate that this effect physically comes from the inelastic scattering of electrons with lateral photons through intersubband transitions.     

Optical coherent transient effects in a magnetized semiconductor quantum wire

Effects of an external magnetic field on free induction decay (FID) and the Stark effect in GaAs/GaAlAs quantum wires have been investigated analytically. Our results show that both FID and the Stark effect become enhanced due to the presence of a magnetic field. We have also seen that the magnetic field plays an important role in wider wires while in thinner wires, the geometric confinement dominates over the magnetic effects. The results are found to be in good qualitative agreement with that available in the literature.     

Trapping of an electron in the transmission through two quantum dots coupled by a wire

We consider single-channel transmission through a double quantum dot that consists of two identical single dots coupled by a wire. The numerical solution for the scattering wave function shows that the resonance width of a few of the states may vanish when the width (or length) of the wire and the energy of the incident particle each take a certain value. In such a case, a particle is trapped inside the wire as the numerical visualization of the scattering wave function shows. To understand these numerical results, we explore a simple model with a small number of states, which allows us to consider the problem analytically. If the eigenenergies of the closed system cross the energies of the transmission zeroes, the wire effectively decouples from the rest of the system and traps the particle.     

Spin-dependent quantum transport through an Aharonov--Bohm structure spin splitter

Using the tight-binding model approximation, this paper investigates theoretically spin-dependent quantum transport through an Aharonov-Bohm （AB） interferometer. An external magnetic field is applied to produce the spinpolarization and spin current. The AB interferometer, acting as a spin splitter, separates the opposite spin polarization current. By adjusting the energy and the direction of the magnetic field, large spin-polarized current can be obtained.     

Spin-dependent Breit-Wigner and Fano resonances in photon-assisted electron transport through a semiconductor heterostructure

We theoretically investigate the electron transmission through a seven-layer semiconductor heterostructure with the Dresselhaus spin-orbit coupling under two applied oscillating fields. Numerical results show that both of the spin-dependent symmetric Breit-Wigner and the asymmetric Fano resonances appear and that the properties of these two types of resonance peaks are dependent on the amplitude and the relative phases of the two applied oscillating fields. The modulation of the spin-polarization efficiency of transmitted electrons by the relative phase is also discussed.     

Persistent current through a semiconductor quantum dot with Gaussian confinement

The persistent diamagnetic current in a GaAs quantum dot with Gaussian confinement is calculated. It is shown that except at very low temperature or at high temperature, the persistent current increases with decreasing temperature. It is also shown that as a function of the dot size, the diamagnetic current exhibits a maximum at a certain confinement length. It is furthermore shown that for a shallow potential, the persistent current shows an interesting maximum structure as a function of the depth of the potential. At low temperature, the peak structure is pretty sharp but becomes broader and broader with increasing temperature.     

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.     

Ballistic conductance in kane type semiconductor quantum wire

The energy spectrum, ballistic conductance of an electron on the surface of a Kane type semiconductor hollow cylinder has been calculated by using the Kane equation with an additional term that takes into account the spin-orbit (SO) interaction. This term, known as Rashba term, occurs for asymmetric quantum wells, where two directions on the normal n are physically nonequivalent. If Rashba spin-orbital interaction is incorporated into energy spectrum, it leads to the emergence of new extrema. We obtained electron energy spectrum, which depends on the sign of the effective spin orbital constant. The energy spectrum of electrons has two branches when the magnetic field does not exist. One of these branches has only one minimum while the other branch has one maximum around k = 0 and two minima. The external magnetic field can control these extrema which occur in the event transport. The results were used to obtain the ballistic conductance at finite temperature of the Kane type hollow cylinder. It has been found that the presence of additional local extremum points in the subband of the electronic spectrum leads to a nonmonotonic dependence of the ballistic conductance of the system on the chemical potential. The g-factor of electrons was observed to depend on Rashba parameter in a linear manner. The effect of finite temperature smears out the sharp steps in the zero-temperature conductance.     

Spin-dependent shot noise in diluted magnetic semiconductor/semiconductor heterostructures with a nonmagnetic barrier

We investigate quantum size effect on the spin-dependent shot noise in the diluted magnetic semiconductor (DMS)/semiconductor heterostructure with a nonmagnetic semiconductor (NMS) barrier in the presence of external magnetic and electric fields. The results demonstrate that the NMS barrier plays a quite different role from the DMS layer in the electron transport process. It is found that spin-down shot noise shows relatively regular oscillations as the width of DMS layer increases, while the spin-up shot noise deceases monotonically. However, as the width of NMS layer increases, the spin-down shot noise displays irregular oscillations at first and then decreases while the spin-up shot noise decreases at a quite different rate. The results indicate that the shot noise can be used as a sensitive probe in detecting material type and its size.