Quantum Wire Model Research Articles

Theoretical Study of Nonequilibrium Electron Transport and Charge Distribution in a Three-Site Quantum Wire

We derive the nonequilibrium transport property formulas for a three-site quantum wire model using Keldysh formalism. Some rigorous formulas in the case of noninteraction are provided for direct calculations. On the basis of the numerical calculations, we investigate the differential and total conductances, transport current, and on-site electronic charges of a wire in some special cases. For a uniform-ingredient wire, if the temperature T=0 K, it shows that, when site–site couplings in the wire are stronger than wire–electrode couplings, resonant tunneling transport takes place and the phenomenon of conductance quantization can be easily observed. In the opposite case, these quantum effects on transport disappear gradually with the increase in the strength of wire–electrode couplings. We also discuss the charge distributions in the three sites of the wire and the characteristics of the charge barrier (Schottky barrier) regardless of Coulomb interaction. If T>0 K, all the line shapes of the transport properties become smoother than those at T=0 K owing to thermal fluctuations. For a wire containing impurities, the line shapes of the transport properties change because of the change of system electronic states.

Simulation of the charge transport in DNA

AbstractIn this research, charge transport characteristics in a double‐stranded DNA are discussed. Based on the polaron drift model, DNA was characterized by a 1D quantum wire model and the phonon scattering rates were calculated by deformation potential theory. Furthermore, using the calculated pho‐ non scattering rates, the carrier dynamics in DNA were characterized by the Monte Carlo simulation technique. The simulation result shows a rapid charge transport from the initial point. The validity of our modeling is also discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Charge Transport Analysis in DNA from the Aspect of Phonon Scattering

Charge transport characteristics in a double-stranded DNA is discussed. Phonon scattering rates in DNA were calculated by a 1D quantum wire model. In addition, using the calculated phonon scattering rates, the carrier dynamics in DNA was simulated by Monte Carlo method. The simulation result shows rapid spread of charges from the initial point

Absolute Continuity of the Spectrum for Periodically Modulated Leaky Wires in $${\mathbb{R}^{3}}$$

We consider a model of leaky quantum wire in three dimensions. The Hamiltonian is a singular perturbation of the Laplacian supported by a line with the coupling which is bounded and periodically modulated along the line. We demonstrate that such a system has a purely absolutely continuous spectrum and its negative part has band structure with an at most finite number of gaps. This result is extended also to the situation when there is an infinite number of the lines supporting the perturbations arranged periodically in one direction.

Electronic transmission in a model quantum wire with side-coupled quasiperiodic chains: Fano resonance and related issues

We present exact results for the transmission coefficient of a linear lattice at one or more sites of which we attach a Fibonacci quasiperiodic chain. Two cases have been discussed, viz, when a single quasiperiodic chain is coupled to a site of the host lattice and, when more than one dangling chains are grafted periodically along the backbone. Our interest is to observe the effect of increasing the size of the attached quasiperiodic chain on the transmission profile of the model wire. We find clear signature that, with a side coupled semi-infinite Fibonacci chain, the Cantor set structure of its energy spectrum should generate interesting multifractal character in the transmission spectrum of the host lattice. This gives us an opportunity to control the conductance of such systems and to devise novel switching mechanism that can act over arbitrarily small scales of energy. The Fano profiles in resonance are observed at various intervals of energy as well. Moreover, an increase in the number of such dangling chains may lead to the design of a kind of spin filters. This aspect is discussed.

Conductivity of quantum wires in uniform magnetic fields

The features of the de conductivity of quantum wires in longitudinal and transverse magnetic fields are studied for degenerate and nondegenerate electron gas. The conductivity is calculated on the basis of the Kubo formalism with regard to the elastic scattering of charge carriers at long-wavelength lattice vibrations. The final theoretical results for the conductivity are compared to the experimental data. The suggested model of quantum wires allows, among other things, an interpretation of the nonmonotonic dependence of the transverse magnetoresistance on the magnetic field.

Modeling Raman scattering in porous silicon

In this work, we model the Raman scattering by phonons using the Born potential and the Green's function formalism, which takes into account the long-range correlation of atomic vibrations. The porous silicon is viewed as a sponge, in which periodical column pores are dug in direction [001] from crystalline silicon, i.e., a supercell model is used to calculate the Raman response. The results show that the main Raman peak shifts to lower energies when the porosity increases, and for square pores it asymptotically approaches to a limit value of 475 cm–1. Finally, the supercell results are compared with the quantum wire model, in which the main Raman peaks move to higher energies as the width of the wires grows. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical Properties of InGaN Epitaxial Layers Studied Using a Disordered Quantum-Wire Model

We present a theoretical study on the optical properties of InGaN epitaxial layers based on a disordered quantum-wire model. By taking the disorder effect and the carrier relaxation into consideration, we calculate the absorption and time-dependent photoluminescence spectra of InN nanowires in GaN matrix. The theoretical results reveal that the main features of thick InGaN epilayers that are grown under constant growth conditions may be explained qualitatively by the theory of disordered quantum wires.

Spin polarization in a model quantum wire with a cross-section

Numerical investigation of the electronic structure of a small closed quantum system fabricated on the two-dimensional electron gas forming on the interference between GaAs and AlxGa1-xAs is reported. The Kohn-Sham spin-density-functional theory is applied. Self-consistent results show that the quantum system assumes unequal numbers of electrons for spin up and spin down because of the existence of bound states. The profiles of effective potential energies for spin up and spin down electrons are then found to be very different, which will result in different transmission coefficients. It is notable that this spin polarization is caused by the geometry of the nanostructure.

Effects of Interface Roughness and Embedded Nanostructures on Device Properties

AbstractWe give an overview of the open-boundary planar supercell stack method (OPSSM), as a means for treating 3D quantum transport in mesoscopic tunnel structures. The flexibility of the method allows us to examine a variety of physical phenomena relevant to quantum transport. In this work we focus on the effects of interface roughness and embedded nanostructures in tunnel devices. Four representative applications of OPSSM are discussed: (1) interface roughness in double barrier resonant tunneling structures, (2) self-organized InAs quantum dot insertions in GaAs/AlAs double barrier structures, (3) tunneling characteristics of ultra-thin oxides with interface roughness, and, (4) embedded quantum wire model of dielectric breakdown. These examples demonstrate scattering and localization effects under different biasing conditions.

Planar quantum wire-like nanostructures in a perpendicular magnetic field

A bend-imitating model of multiply bent 2D quantum wires in a perpendicular magnetic field is proposed. According to this model each circular-like bend of a continuous quantum wire can be treated as some multichannel point scatterer with the scattering ability depending on the bending angle, bending radius, magnetic field and time. The problem of electronic localization caused by bending is studied and the magneto-delocalizing effect is predicted.

Photoluminescence and structure properties from μc-Si:H and μc-Si:H-PS samples

Microcrystalline and porous on microcrystalline silicon thin films were produced. The photoluminescence characteristics of porous on microcrystalline silicon were studied and compared with those from microcrystalline and porous silicon grown on silicon wafers. Under steady state it is possible to excite these samples with visible light at room temperature. This excitation gives rise to a red photoluminescence band, which is similar to that of porous silicon excited under the same conditions. MicroRaman and transmission spectroscopy shows that the porous silicon layer was produced below the microcrystalline silicon thin film that acts as a shield for excitation and emission of radiation with a wavelength smaller than 500 nm. Sample thickness, crystallite size and crystalline fraction are determined. The results are discussed in terms of the theoretical framework based in the fluctuating quantum wire model for porous silicon.

Temperature dependence of conductivity in multi-subband quantum wires

Tomonaga-Luttinger liquid theory is extended to multi-subband quasi-one-dimensional systems in order to apply it to a realistic model of quantum wires. We use standard thermal Green's method and discuss the temperature dependence of the conductivity. With this method we can treat the long-range coulomb interaction between electrons, and in this case the conductivity behaves like T α , where α is roughly equal to the inverse of the number of the occupied subbands.

Conductance increase by electron-phonon interaction in quantum wires.

We investigate the influence of electron-phonon interactions on the DC-conductance $\Gamma $ of a quantum wire in the limit of one occupied subband. At zero temperature, a Tomonaga-Luttinger-like renormalization of $\Gamma $ to a value slightly larger than $2e^{2}/h$ is calculated for a realistic quantum wire model.

Polarization of porous silicon luminescence

The polarization characteristics of light emitting porous Si films is investigated. The degree of polarization increases to well over 20% when the excitation wavelength is at the blue edge of the emission band and decreases with decreasing excitation wavelength. The steady-state value of the degree of polarization is established in less than 1 μs. The experimental results bear qualitative agreement with a quantum-wire model.

Stability of two-electron bound states in a model quantum wire

The stability of two-electron bound states is examined in a model quantum wire. It is emphasized that the stability crucially depends on the Coulomb screening effect. If the screening length for the electron-electron interaction is of the order of 10 nm or less, two-electron bound states can be easily formed.

Surface and optical analyses of porous silicon membranes

Surface and optical analytical techniques of x-ray photoelectron spectroscopy, Auger electron spectroscopy, secondary ion mass spectrometry, Fourier transform infrared transmission, photoluminescence and Raman scattering have been used to study porous Si membranes with visible light emission. The main components of surface layers were found to be SiO2 plus Si clusters and some contamination elements. The depth distributions of these impurities over the entire 200 mu m thick membrane were also profiled. Porous Si membranes show a large number of infrared active modes due to the oxidized and organized surfaces of the pores. Their Raman features are markedly distinct from that of crystalline, microcrystalline and amorphous Si. Their anomalous Raman temperature behaviour is explained by the quantum wire model and strain effects. The combination of surface and optical analyses offers better understanding for the properties of porous Si.

Elastic electron transmission by barriers in a three-dimensional model quantum wire.

Numerical solutions of the time-dependent and time-independent Schr\odinger equations for electron tunneling in a three-dimensional model quantum wire have been carried out and used to calculate transmission probabilities over a range of energies. The model simulates a quantum heterostructure corresponding to a cylindrically symetric wire constructed from two different materials such that square barriers to the axial (z) motion occur in the disjoint region B=(${\mathit{z}}_{1}$\ensuremath{\le}z\ensuremath{\le}${\mathit{z}}_{2}$)\ensuremath{\cup}(${\mathit{z}}_{3}$\ensuremath{\le}z\ensuremath{\le}${\mathit{z}}_{4}$). In addition, the electron is bound harmonically in the radial distance r transverse to the longitudinal axis z; the electron vibrational frequency changes discontinuously from ${\mathrm{\ensuremath{\omega}}}_{\mathit{A}}$ in the disjoint region A=(-\ensuremath{\infty}z${\mathit{z}}_{1}$)\ensuremath{\cup}(${\mathit{z}}_{2}$z${\mathit{z}}_{3}$)\ensuremath{\cup}(${\mathit{z}}_{4}$z\ensuremath{\infty}) to ${\mathrm{\ensuremath{\omega}}}_{\mathit{B}}$ in the disjoint region B.

Interpretation of photoluminescence excitation spectroscopy of porous Si layers

Photoluminescence excitation (PLE) measurements allow one to obtain direct information on the absorption processes in porous Si leading to the strong visible luminescence which has recently been reported. This technique does not need a free-standing porous Si sample, but the effect of layer thickness should be included in the interpretation of PLE data. In our PLE spectra, two separate absorption edges are observed and the initial one shifts to higher energies with storage time. The trends of these changes are consistent with the quantum wire model.

Photoluminescence, Structure, and Composition of Laterally Anodized Porous Si

We have studied the photoluminescence (PL), structure, and composition of laterally anodized porous Si. Broad PL peaks were observed centered between ∼620–720 nm with strong intensities measured from 500 to 860 nm. Macroscopic variations in PL intensities and peak positions are explained in terms of the structure and anodization process. Structural studies suggest that the PL appears to originate from a multilayered porous Si structure in which the top two layers are amorphous. X‐ray diffraction spectra also suggest the presence of a significant amorphous phase. In addition to high concentrations of B and N, we have measured extremely high concentrations (≫1020 cm−3) of H, C, O, and F. Our results indicate that laterally anodized porous Si does not fit the crystalline Si quantum wire model prevalent in the literature, suggesting that some other structure is responsible for the observed luminescence.