ac-Conductivity of aperiodic chains re-examined

ac-Conductivity of infinite quasiperiodic chanins have been calculated using the Miller-Abrahams equations and a real space decimation technique. Our approach deals with the infinite lattice directly, instead of calculating the conductivity for periodic repetitions of long quasiperiodic segments, as has been done recently. We point out an interesting observation that, some aperiodic chains show the same analytical behaviour in the low frequency ac-conductivity as an ordered chain, whereas, some others behave in a totally different way. We explain this observation from a renormalization group point of view.     

Hot electron relaxation in one-dimensional models: exact polaron dynamics versus relaxation in the presence of a Fermi sea

We present the exact solution for the time evolution of the electron and phonon momentum distribution for a one-dimensional polaron model with alinear electronic energy dispersion. The electron momentum distribution is shown to obey aMarkovian quantum kinetic equation. Numerical results for the polaron model are compared to the corresponding exact results, when the negative momentum states are filled in the initial state. The presence of this Fermi sea modifies the dynamics except in the short time regime. The different, long time dynamics might show up in comparison of hot electron relaxation of undoped and doped semiconductors.     

Properties of Graphene Based Parametric Pump

The adiabatic parametric electron pump of the infinite zigzag graphene ribbons and the infinite armchair graphene ribbons is investigated by the tight binding method. The pumping signals are added by two gates around the ribbons. It is shown that the de current can be pumped out by cyclically varying the two gate voltages and the pumped current strongly depends on the driving frequency, the pumping amplitude and the phase difference of the gate voltages. The pumped current is mediated by the graphene energy levels and its peaks occur around the energies where transmission coefficients and density of states are large. The pump current may give one peak or two opposite peaks corresponding to each transmission peak or transmission pair peaks. The height and width of the current peaks increase with the amplitude of the pumping driving voltages. The pumped current is antisymmetric about the phase difference Ф=π and for small pumping amplitude the pumped current is a sinusoidal function of the phase difference. Some graphene ribbons, although with different widths, have very similar contours of the transmission coefficients and give the same pumped current figures.     

Improved expression for calculating electron transport cross sections

We report a simplified correction for the electron transport cross sections (TCSs) for a number of selected atomic targets ranging from H to U and electron energies between 50 and 4000 eV. The correction has been made to the approximate analytical expression of transport cross sections derived by Jablonski [A. Jablonski, Phys. Rev. B 58 (1998) 16470] where an argued parameter is introduced. The latter is obtained from a polynomial fit. The energy dependence of the percentage deviation between TCSs from the corrected expression and those obtained from other sources is presented. The TCSs calculated in the present work showed better agreement with accurate values of TCSs than those reported in earlier publications. This may facilitate the evaluation of parameters needed for quantitative Auger-electron spectroscopy and X-ray photoelectron spectroscopy.     

Statistical model for the effects of dephasing on transport properties of large samples

We present a statistical model for the effects of dephasing on the transport properties of large devices. The physical picture is different from earlier models which assume that dephasing happens continuously throughout the sample, whereas we model the dephasing in a statistical sense, assuming a distribution of completely phase randomizing regions between which the transport is coherent and described by the nonequilibrium Green’s function method. Thus the sample is effectively divided into smaller parts making the numerical treatment more efficient. As a first application the conductances of ordered and disordered linear tight-binding chains are calculated and compared to the results of other phenomenological models in the literature.     

Non-universality of dephasing in quantum transport

We investigate the influence of the environment on coherent quantum transport. While random processes with classical stochastic potentials can be treated in the concept of phase memory, this is not possible for fully quantized system-plus-reservoir models. In the latter case, the decay of coherences is not only depending on the coupled dynamics, but also crucially on the special geometry of the physical system in regard. We introduce a microscopic model with localized environmental modes where the geometry dependence is removed and calculate the coherence length.     

Theory of hot-electron quantum diffusion in semiconductors

In the linear response regime close to equilibrium, the fluctuation-dissipation theorem relates linear transport coefficients via the well-known Green–Kubo or Einstein relation. The latter embodies a deep connection between fluctuations causing diffusion and dissipation, which are responsible for a finite mobility. Far from equilibrium, however, the Einstein relation is no longer valid so that both the mobility and diffusivity gain their own physical integrity. Consequently, beyond a linear response, both quantities have to be described by different approaches. Unfortunately, there is a strong imbalance of research activities devoted to the study of both transport mechanisms in semiconductors. On one hand, the rich physics of high-field quantum drift in semiconducting structures has a long history and has reached a high level of sophistication. On the other hand, there are only comparatively few and unsystematic studies that cover quantum diffusion of carriers under high-field conditions. This review aims at reducing this gap by presenting a unified approach to quantum drift and quantum diffusion. Starting from a semi-phenomenological basis, a quantum theory of transport coefficients is developed for one- as well as multi-band models. Physical implications are illustrated by selected applications whereby the quantum character of the approach is emphasized. Furthermore, the basic unified treatment of transport coefficients is extended by accounting for the two-time dependence of one-particle correlation functions in quantum statistics. As an application, a phononless transport mechanism is identified, which solely originates from the double-time nature of the evolution. Finally, additional examples are presented that illustrate the important role played by quantum diffusion in semiconductor physics.     

A comparison of confining potentials in the quantum wire problem

A simple numerical method for solving a two-terminal quantum electronic waveguide problem is presented. The method can be adapted to a quantum wire cavity of irregular geometry and/or non-constant potential field. We compare the circular bend wire with parabolic confining potential profile to the commonly used hard wall confinement. We find an energy scaling which makes the results correspond closely.     

Electron-mediated entanglement in semiconductor phonon systems

Phonons in condensed matter systems are usually treated as a decohering thermal bath. We study the entanglement between the phononic modes which is created by the interaction with a fermionic system (electrons) whose degrees of freedom are traced out as a thermal bath. The resulting picture thus reverses the usual scheme and aims at highlighting the possibility of exploiting bosonic degrees of freedom in condensed matter systems for new quantum computing protocols.     

Influence of negative pressure on thermoelectric properties of Sb2Te3

We investigated the influence of negative pressure on the electrical conductivity, the Seebeck coefficient, and the power factor of Sb₂Te₃. We performed first-principles calculations with the linearized-augmented plane-wave method considering negative hydrostatic pressure in the range from zero to −2 GPa and doping for electrons and holes up to 10²⁰ cm⁻³. Our results predict a significant increase of the Seebeck coefficient and the power factor under negative pressure for certain doping concentrations.     

Interband quantum kinetics with static disorder scattering I: Direct CPA solution for a rectangular-pulse-excited semiconductor

For a two band model semiconductor alloy with the disorder potential concentrated to the conduction band, the photoexcitation by a long rectangular pulse represents a case soluble in the coherent potential approximation. Explicit analytic expressions for the transient electron distribution are derived using the nonequilibrium Green functions. The evanescent coherent component is gradually superseded by the incoherent distribution whose saturation value is obtained using the Ward identity.     

Spin polarization in parallel double dots with spin-orbit interaction

Spin polarization in parallel double quantum dots embedded in arms of Aharonov-Bohm interferometer is investigated. The spin-orbit interaction exists in quantum dots. We find that the spin polarization is quite large even with a weak spin-orbit interaction. The direction and the strength of the spin polarization are well controllable and manipulatable by simply varying the strength of spin-orbit interaction or the energy levels in quantum dots. Moreover, electron-electron interaction strengthens the spin accumulation when the energy levels of the two quantum dots are identical. As the energy levels are unequal, electron-electron interaction cannot increase the spin accumulation. It is worth mentioning that the device is free of a magnetic field or a ferromagnetic material and it can be easily realized with present technology.     

Current partition in multiprobe conductors in the presence of slowly oscillating external potentials

In the presence of a static potential drop a carrier stream incident at a contact of the sample is partitioned into the other contacts according to the transmission probabilities of the sample. The bare response to oscillating potentials, on the other hand, violates current conservation due to the piling up of unscreened charges in the sample, and has to be modified by taking the induced screening potential into account. We present a novel derivation of the conductance response to oscillating external chemical potentials, find the response to an arbitrary internal potential in terms of functional derivatives with respect to the local potential of the scattering matrix of the conductor, and determine the screening potential for slowly oscillating potentials from the condition of local charge neutrality. We find that the current partitioning depends on ratios of local densities of states which reflect the injection and emission properties of the contacts of the sample.     

Localization and dephasing driven by magnetic fluctuations in low carrier density colossal magnetoresistance materials

Localization and dephasing of conduction electrons in a low carrier density ferromagnet due to scattering on magnetic fluctuations is considered. We claim the existence of the “mobility edge”, which separates the states with fast diffusion and the states with slow diffusion; the latter is determined by the dephasing time. When the “mobility edge” crosses the Fermi energy a large and sharp change of conductivity is observed. The theory provides an explanation for the observed temperature dependence of conductivity in ferromagnetic semiconductors and manganite pyrochlores. Received 17 January 1999 and Received in final form 12 March 1999     

Thermal Rectification Effect of an Interacting Quantum Dot

We investigate the nonlinear thermal transport properties of a single interacting quantum dot with two energy levels tunnel-coupled to two electrodes using nonequilibrium Green function method and Hartree-Fock decoupling approximation. In the case of asymmetric tunnel-couplings to two electrodes, for example, when the upper level of the quantum dot is open for transport, whereas the lower level is blocked, our calculations predict a strong asymmetry for the heat （energy） current, which shows that the quantum dot system may act as a thermal rectifier in this specific situation.     

Optimal block-tridiagonalization of matrices for coherent charge transport

Numerical quantum transport calculations are commonly based on a tight-binding formulation. A wide class of quantum transport algorithms require the tight-binding Hamiltonian to be in the form of a block-tridiagonal matrix. Here, we develop a matrix reordering algorithm based on graph partitioning techniques that yields the optimal block-tridiagonal form for quantum transport. The reordered Hamiltonian can lead to significant performance gains in transport calculations, and allows to apply conventional two-terminal algorithms to arbitrarily complex geometries, including multi-terminal structures. The block-tridiagonalization algorithm can thus be the foundation for a generic quantum transport code, applicable to arbitrary tight-binding systems. We demonstrate the power of this approach by applying the block-tridiagonalization algorithm together with the recursive Green’s function algorithm to various examples of mesoscopic transport in two-dimensional electron gases in semiconductors and graphene.     

Gas adsorption on a single walled carbon nanotube-model simulation

We simulate the conduction variation of a gas-adsorbed carbon nanotube by a hybridization model, which has been previously used to simulate the gas adsorption on a nanographite ribbon. Two energy parameters, hybridization interaction and orbital energy level, are employed to simulate and distinguish the adsorbed gases. Two mechanisms, carrier localization and charge distribution, coexist in the gas adsorption process and provide a qualitative explanation for the current increase or decrease in gas adsorption experiments for the carbon nanotube.     

Interface-induced dephasing of Mie plasmon polaritons

Surface and, in particular, interface effects influence all physical and chemical properties of nanostructured matter. Mie surface plasmon polaritons (MPPs) in metallic nanoparticles are excellent and sensitive sensors for optical investigation of these effects of realistic particles since their lifetimes due to dephasing (decoherence) effects and their resonance energies drastically depend upon the chemistry and topology of their surfaces/interfaces. A survey is given over some results of our own long term research on MPPs which started, in fact, as early as 1969. Theoretical models and experiments concerning the A parameter, the δ n parameter, MPP phase decoherence and static and dynamic interface charge transfer effects (“chemical interface damping”) are briefly summarized. The effect of radiation damping is disregarded throughout: we assume the particle sizes to be small enough to justify this simplification, which makes it easier to draw conclusions from the MPPs on nanomaterial properties. Obviously, there is a wide field for future research concerning particle interfaces on the basis of Mie’s theory. On the other hand, all of these effects have to be incorporated into Mie’s theory to obtain a “modern” version which is reliable on a quantitative level to describe experimental data.     

Microwave-induced dephasing in one-dimensional metal wires

We report on the effect of monochromatic microwave (MW) radiation on the weak-localization corrections to the conductivity of quasi-one-dimensional silver wires. Because of the improved electron cooling in the wires, the MW-induced dephasing is observed without a concomitant overheating of electrons over wide ranges of the MW power P(MW) and frequency f. The observed dependences of the conductivity and MW-induced dephasing rate on P(MW) and f are in agreement with the theory by Altshuler, Aronov, and Khmelnitsky [Solid State Commun. 39, 619 (1981)]. Our results suggest that in the low-temperature experiments with 1D wires, saturation of the temperature dependence of the dephasing time can be caused by an MW electromagnetic noise with a sub-pW power.     

Analytical approach to the quantum-phase transition in the one-dimensional spinless Holstein model

We study the one-dimensional Holstein model of spinless fermions interacting with dispersion-less phonons by using a recently developed projector-based renormalization method (PRM). At half-filling the system shows a metal-insulator transition to a Peierls distorted state at a critical electron-phonon coupling where both phases are described within the same theoretical framework. The transition is accompanied by a phonon softening at the Brillouin zone boundary and a gap in the electronic spectrum. For different filling, the phonon softening appears away from the Brillouin zone boundary and thus reflects a different type of broken symmetry state.