Spin-polarized transport through a time-periodic non-magnetic semiconductor heterostructure

Spin-dependent Floquet scattering theory is developed to investigate the photon-assisted spin-polarized electron transport through a semiconductor heterostructure in the presence of an external electric field. Spin-dependent Fano resonances and spin-polarized electron transport through a laser irradiated time-periodic non-magnetic heterostructure in the presence of Dresselhaus spin-orbit interaction and a gate-controlled Rashba spin-orbit interaction are investigated. The electric field due to laser along with the spin-orbit interactions help to get spin-dependent Fano resonances in the conductance, whereas the external bias can be appropriately adjusted to get a near 80% spin-polarized electron transmission through heterostructures. The resultant nature of the Floquet scattering depends on the relative strength of these two electric fields.     

Electron transport for a laser-irradiated quantum channel with Rashba spin--orbit coupling

We investigate theoretically the electron transport for a two-level quantum channel (wire) with Rashba spin--orbit coupling under the irradiation of a longitudinally-polarized external laser field at low temperatures. Using the method of equation of motion for Keldysh nonequilibrium Green function, we examine the time-averaged spin polarized conductance for the system with photon polarization parallel to the wire direction. By analytical analysis and a few numerical examples, the interplay effects of the external laser field and the Rashba spin--orbit coupling on the spin-polarized conductance for the system are demonstrated and discussed. It is found that the longitudinally-polarized laser field can adjust the spin polarization rate and produce some photon sideband resonances of the conductance for the system.     

Quantum Thermal Transport through Extremely Cold Dielectric Chains

In the framework of Green＇s function theory out of equilibrium, a Landauer-Buttiker （LB） formula for thermal conductance is derived. A simplified model for describing extremely cold dielectric chains is proposed for the first time. Further we apply the present LB formula for studying thermal conductance at low-lying modes, emerging in dielectric atom chains. We find that quantum thermal conductance undergoes an anomalous transition due to new quasiparticle excitations, resulting from nonlinear atom-atom interactions. This theoretical prediction is in excellent agreement with a high-accuracy measurement to thermal conductance quantum.     

Melt-mediated coalescence of solution-deposited ZnO nanoparticles by excimer laser annealing for thin-film transistor fabrication

Nanoparticle solutions are considered promising for realizing low cost printable high performance flexible electronics. In this letter, excimer laser annealing (ELA) was employed to induce melting of solution-deposited ZnO nanoparticles and form electrically conductive porous films. The properties of the films were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, DC conductance, and photoluminescence measurements. Thin-film field-effect transistors have been fabricated by ELA without the use of conventional vacuum or any high temperature thermal annealing processes. The transistors show n-type accumulation mode behavior with mobility greater than 0.1 cm²/V s and current on/off ratios of more than 10⁴. Optimization and control of the laser processing parameters minimized thermal impact on the substrate. This technique can be beneficial in the fabrication of metal oxide based electronics on heat sensitive flexible plastic substrates using low-cost, large-area solution processing combined with direct printing techniques.     

Modeling of a Silicon Nanowire pH Sensor with Nanoscale Side Gate Voltage

A silicon nanowire （Si-NW） sensor for pH detection is presented. The conductance of the device is analytically obtained, demonstrating that the conductance increases with decreasing oxide thickness. To calculate the electrical conductance of the sensor, the diffusion-drift model and nonlinear Poisson-Boltzmann equation are applied. To improve the conductance and sensitivity, a Si-NW sensor with nanoscaie side gate voltage is offered and its characteristics are theoretically achieved. It is revealed that the conductance and sensor sensitivity can be enhanced by adding appropriate side gate voltages. This effect is compared to a similar fabricated structure in the literature, which has a wire with a rectangular cross section. Finally, the effect of NW length on sensor performance is investigated and an inverse relation between sensor sensitivity and NW length is achieved.     

Nanobubbles in solid-state nanopores

From conductance and noise studies, we infer that nanometer-sized gaseous bubbles (nanobubbles) are the dominant noise source in solid-state nanopores. We study the ionic conductance through solid-state nanopores as they are moved through the focus of an infrared laser beam. The resulting conductance profiles show strong variations in both the magnitude of the conductance and in the low-frequency noise when a single nanopore is measured multiple times. Differences up to 5 orders of magnitude are found in the current power spectral density. In addition, we measure an unexpected double-peak ionic conductance profile. A simple model of a cylindrical nanopore that contains a nanobubble explains the measured profile and accounts for the observed variations in the magnitude of the conductance.     

Influence of laser light on electronic transport through atomic-size contacts

This Letter reports on the influence of laser irradiation onto the electrical conductance of gold nanocontacts established with the mechanically controllable break-junction technique. We concentrate on the study of reversible conductance changes which can be as high as 200%. We investigate the dependence on the initial conductance of the contacts, and on the wavelength, the intensity, and the position of the laser spot with respect to the sample. Under most conditions an enhancement of the conductance is observed. Several physical mechanisms which might contribute to the observed effect including thermal expansion, rectification, plasmon excitation, and photon-assisted transport are discussed, among which the two latter ones are most likely the dominating ones.     

Resonant Photon-Assisted Structure of Conductance through a Quantum Wire

We investigate theoretically the electron transport properties of a two-sublevel quantum wire irradiated by a strong laser field resonant with the quasiparticle transition at low temperatures. Using the method of the Keldysh equation of motion for nonequilibrium Green function, we examine the time-averaged conductance for the system with photon polarization parallel and perpendicular to the tunnelling current direction, respectively. We demonstrate that, by analysing some numerical examples, a feature of absolute negative conductance appears in the parallel case, while the conductance shows a symmetry distributed peaks in the Derpendicular case.     

亚纳秒光脉冲下GaAs光电开关行为的理论分析

本文对GaAs光电开关在亚纳秒光脉冲(650ps左右)触发下的性能作了理论分析。用计算机数值分析方法计算了光电开关的电导随光脉冲的变化过程,分析了光脉冲的能量、半宽度的涨落引起的光电开关的触发跳动、响应时间及幅度的变化规律。在合理选用参数时,用SPICE设计得到了上升沿为5kV/ns,触发跳动小于5ps的斜坡电压,从而证实了光电开关在亚纳秒光脉冲作用下的可行性。     

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.     

Resonance effect of a magnetic quantum ring on the conductance of a two-dimensional sample

The resonant tunneling of electrons through a magnetic quantum ring in a two-dimensional sample and the effect of this resonant tunneling on the conductivity of the sample are studied in this work. The ballistic conductance of the sample is calculated for several values of the ratio (inner radius)/(outer radius) of the ring. The limit of a conductance plateau is recovered for approximately equal radii and previous results for the conductance of a magnetic quantum dot are recovered in the case of an approximately zero inner radius.     

Time-dependent electron transport in HgTe/CdTe quantum wells

Based on the Floquet theory and Keldysh's nonequilibrium Green's function methods, we study the electron transport through the HgTe/CdTe quantum wells (QWs) irradiated by a monochromatic laser field. We find that when the laser field is applied, the edge states are split into a series of sidebands. When the Fermi level lies among these sidebands, the quantized plateau of the conductance is destroyed. Instead, the conductance versus the radiation frequency exhibits the successive oscillation peaks corresponding to the resonant tunneling through the sidebands of the edge states. The resonant interaction between the quasiparticles and the radiation field opens the gaps in the crossing region of the sidebands, which can be tuned by the radiation strength and frequency. This leads to the shift of the oscillation peaks in the conductance. We also show that the amplitudes of the oscillation peaks in the conductance are governed by the radiation strength and frequency.     

Contact conductance between graphene and quantum wires

The contact conductance between graphene and two quantum wires which serve as the leads to connect graphene and electron reservoirs is theoretically studied. Our investigation indicates that the contact conductance depends sensitively on the graphene-lead coupling configuration. When each quantum wire couples solely to one carbon atom, the contact conductance vanishes at the Dirac point if the two carbon atoms coupling to the two leads belong to the same sublattice of graphene. We find that such a feature arises from the chirality of the Dirac electron in graphene. Such a chirality associated with conductance zero disappears when a quantum wire couples to multiple carbon atoms. The general result irrelevant to the coupling configuration is that the contact conductance decays rapidly with the increase of the distance between the two leads. In addition, in the weak graphene-lead coupling limit, when the distance between the two leads is much larger than the size of the graphene-lead contact areas and the incident electron energy is close to the Dirac point, the contact conductance is proportional to the square of the product of the two graphene-lead contact areas, and inversely proportional to the square of the distance between the two leads.     

Single electron tunneling at large conductance: The semiclassical approach

We study the linear conductance of single electron devices showing Coulomb blockade phenomena. Our approach is based on a formally exact path integral representation describing electron tunneling nonperturbatively. The electromagnetic environment of the device is treated in terms of the Caldeira-Leggett model. We obtain the linear conductance from the Kubo formula leading to a formally exact expression which is evaluated in the semiclassical limit. Specifically we consider three models. First, the influence of an electromagnetic environment of arbitrary impedance on a single tunnel junction is studied focusing on the limits of large tunneling conductance and high to moderately low temperatures. The predictions are compared with recent experimental data. Second, the conductance of an array of N tunnel junctions is determined in dependence on the length N of the array and the environmental impedance. Finally, we consider a single electron transistor and compare our results for large tunneling conductance with experimental findings. Received 2 February 2000     

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.     

Resonance Transport of Graphene Nanoribbon T-Shaped Junctions

We investigate the transport properties of T-shaped junctions composed of armchair graphene nanoribbons of different widths. Three types of junction geometries are considered. The junction conductance strongly depends on the atomic features of the junction geometry. When the shoulders of the junction have zigzag type edges, sharp conductance resonances usually appear in the low energy region around the Dirac point, and a conductance gap emerges. When the shoulders of the junction have armchair type edges, the conductance resonance behavior is weakened significantly, and the metal-metal-metal junction structures show semimetallic behaviors. The contact resistance also changes notably due to the various interface geometries of the junction.     

A new approach to the quantized electrical conductance

The quanta of electrical conductance is derived for a one-dimensional electron gas both by making use of the quasi-classical motion of a quantum fluid and by using arguments related to the uncertainty principle. The result is extended to a nanowire of finite cross section area and to electrons in magnetic field, and the quantization of the electrical conductance is shown. An additional application is made to the two-dimensional electron gas.     

Effects of Contact Geometry on Electron Transport of 1,4-Diaminobenzene

The equilibrium electron transport of 1,4-diaminobenzene sandwiched between two Au electrodes is simulated by using a first principles analysis. The results show that equilibrium conductance increases with the molecule- electrode distance decreasing, and a platform occurs at the distance varying from 1.4 A to 1.9 A, implying the insensitiveness of 1,4-diaminobenzene equilibrium conductance to molecule-electrode distance. This is helpful to understand the improved reliability and reproducibility of conductance measurements using amines.     

Anderson transition in 1D systems with spatial disorder

A simple Kronig-Penney model for 1D mesoscopic systems with δ peak potentials is used to study numerically the influence of spatial disorder on conductance fluctuations and distribution at different regimes. The Lévy laws are used to investigate the statistical properties of the eigenstates. It is found that an Anderson transition occurs even in 1D meaning that the disorder can also provide constructive quantum interferences. The critical disorder W_c for this transition is estimated. In these 1D systems, the metallic phase is well characterized by a Gaussian conductance distribution. Indeed, the results relative to conductance distribution are in good agreement with the previous works in 2D and 3D systems for other models. At this transition, the conductance probability distribution has a system size independent shape with large fluctuations in good agreement with previous works.     

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.