Percolation and the electron–electron interaction in an array of antidots

A square lattice of microcontacts with a period of 1 μm in a dense low-mobility two-dimensional electron gas is studied experimentally and numerically. At the variation of the gate voltage V _g , the conductivity of the array varies by five orders of magnitude in the temperature range T from 1.4 to 77 K in good agreement with the formula σ(V _g ) = (V _g ?V _g ^* (T))^β with β = 4. The saturation of σ(T) at low temperatures is absent because of the electron–electron interaction. A random-lattice model with a phenomenological potential in microcontacts reproduces the dependence σ(T, V _g ) and makes it possible to determine the fraction of microcontacts x(V _g , T) with conductances higher than σ. It is found that the dependence x(V _g ) is nonlinear and the critical exponent in the formula σ ∝ ? (x - 1/2) ^t in the range 1.3 < t(T, V _g ) < β.     

Tailoring electron–phonon interaction in nanostructures

Efficient design of optoelectronic devices based on electron intersubband transitions depends critically on the knowledge of the intersubband relaxation times which in turn, depends on electron scattering with LO and acoustic phonons. In this article the intersubband scattering time associated with electron–acoustic-phonon interaction has been discussed in terms of phonon mode quantization and phonon confinement with describing the acoustic phonon dispersion relation in detail by introducing the cut-off frequency for each mode. It has been shown that the quantization of acoustic phonon modes lead to an enhancement in electron–phonon scattering time in AlGaAs quantum well structures. Based on the presented model, a new tailoring method has presented to adjust the electron–phonon scattering time in intersubband-transition-based structures while keeping the electronic properties unaltered. Also, we illustrated that for a quantum well with subband energy separation of ∼30 meV, the intersubband scattering time with acoustic-phonon-assisted transitions could be tailored from ∼120 ps to increased value of ∼400 ps or reduced value of ∼45 ps by inserting a 1 nm-thickacoustically soft or hard layers, respectively, while keeping the same the initial energy separation.     

Dispersion and the electron–phonon interaction in a single heterostructure

We investigate the electron–phonon interaction in a polar–polar single heterostructure through the use of the linear combination of hybrid phonon modes, considering the role of longitudinal optical, transverse optical and interface modes, using a continuum model that accounts for both mechanical and electrical continuity over a heterostructure interface. We discuss the use of other models for such systems, such as the bulk phonon (3DP) and dielectric continuum (DC) models, using previously developed sum-rules to explain the limitations on their validity. We find that our linear combination (LC) model gives an excellent agreement with scattering rates previously derived using the 3DP and DC models when the lattice dispersion is weak enough to be ignored, however, when there is a noticeable lattice dispersion, the LC model returns a different answer, suggesting that interface modes play a much greater part in the scattering characteristics of the system under certain conditions. We also discuss the remote phonon effect in polar/polar heterostructures.     

Pulse–pulse interaction in dispersion-managed fiber systems with nonlinear amplifiers

The pulse–pulse interaction in a dispersion-managed fiber system is studied for the case when a nonlinear gain and spectral filtering are included into the dispersion-gain map. In this system, the pulse of any shape converges quickly to a dispersion-managed soliton. Using the technique of interaction plane, we have found stable bound states of two pulses. The effects we have studied may significantly reduce the chances of pulse coalescence in specially designed optical transmission lines.     

Effect of electron–phonon interaction on surface states in wurtzite nitride semiconductors under pressure

A variational theory is proposed to study the electronic surface states in semi-infinite wurtzite nitride semiconductors under the hydrostatic pressure. The electronic surface state energy level is calculated, by taking the effects of the electron–Surface–Optical–phonon interaction, structural anisotropy and the hydrostatic pressure into account. The numerical computation has been performed for the electronic surface state energy levels, coupling constants and the average penetrating depths of the electronic surface state wave functions under the hydrostatic pressure for wurtzite GaN, AlN and InN, respectively. The results show that electron–Surface–Optical–phonon interaction lowers the electronic surface state energy levels. It is also found that the electronic surface state energy levels decrease with the hydrostatic pressure in wurtzite GaN and AlN. But for wurtzite InN, the case is contrary. It is shown that the hydrostatic pressure raised the influence of electron–phonon interaction on the electronic surface states obviously. The effect of electron–Surface–Optical–phonon interaction under the hydrostatic pressure on the electronic surface states cannot be neglected, in specially, for materials with strong electron–phonon coupling and wide band gap.     

Electronic Raman scattering and the electron–phonon interaction in YB6

Electronic Raman scattering in YB₆ and in its structural and electronic analog LaB₆ has been studied in the temperature range of 10–730 K. The experimental spectra have been compared to those calculated on the basis of ab initio band structures with renormalization owing to the electron–phonon interaction. Good agreement between the calculation and experiment for LaB₆ has been obtained throughout the entire temperature range. This allows the determination of the coupling constant λ _ep = 0.25. To satisfactorily describe the spectra of electronic light scattering in YB₆, it is necessary to introduce an additional electron relaxation channel. In this case, the estimate of the electron–phonon coupling constant λ _ep is no more than 0.4; for this reason, a high superconducting transition temperature cannot be explained only by the phonon mechanism.     

Exotic electron states and tunable magneto-transport in a fractal Aharonov–Bohm interferometer

A Sierpinski gasket fractal network model is studied in respect of its electronic spectrum and magneto-transport when each ‘arm’ of the gasket is replaced by a diamond shaped Aharonov–Bohm interferometer, threaded by a uniform magnetic flux. Within the framework of a tight binding model for non-interacting, spinless electrons and a real space renormalization group method we unravel a class of extended and localized electronic states. In particular, we demonstrate the existence of extreme localization of electronic states at a special finite set of energy eigenvalues, and an infinite set of energy eigenvalues where the localization gets ‘delayed’ in space (staggered localization). These eigenstates exhibit a multitude of localization areas. The two terminal transmission coefficient and its dependence on the magnetic flux threading each basic Aharonov–Bohm interferometer is studied in details. Sharp switch on–switch off effects that can be tuned by controlling the flux from outside, are discussed. Our results are analytically exact.     

Dark states and Aharonov-Bohm oscillations in multi-quantum-dot systems

We study the formation of dark states and the Aharonov-Bohm effect in symmetrically/asymmetrically coupled three-and four-quantum-dot systems.It is found that without a transverse magnetic field,destructive interference can trap an electron in a dark state.However,the introduction of a transverse magnetic field can disrupt the dark state,giving rise to oscillation in current.For symmetrically structured quantum-dot systems,the oscillation has a period of one flux quanta.But for asymmetrically structured dot systems,the period of oscillation is halved.In addition,the dephasing due to charge noise also blocks the formation of dark states,while it does not change the period of oscillation.     

Hydrodynamic analysis of electrostatic counter-streaming instability in a spin-polarized electron–positron–ion plasma

In this study, we present linear analysis of electrostatic counter-streaming instability in spin-polarized electron–positron–ion (e-p-i) plasma. With the aid of the separate spin evolution-quantum hydrodynamic (SSE-QHD) model, we derive the dispersion relation of counter-streaming instability. We numerically solve the dispersion and find four wave solutions: Langmuir wave, positron acoustic mode, and two electron and positron spin-dependent waves. It is noted that coupling of streaming and spin effects excites Langmuir instability and positron acoustic mode instability. However, in the absence of spin effect, only Langmuir instability will survive in e-p-i plasma. We have also discussed the effects of positron concentration, streaming speed, and spin polarization on the real frequency of waves and the growth rate. The present study may be helpful for understanding longitudinal wave propagation and instabilities in dense magnetized environments.     

Magnetoelectric effect induced by electron–electron interaction in three dimensional topological insulators

We compute the magnetoelectric response of an interacting topological insulator in three space dimensions with a short range interaction between electrons in different orbitals. We show that in the presence of interactions and inverted bands the chiral phase is gauged away and replaced by a topological angle (θ-term) which is determined by saddle point of the interacting action and the Fujikawa integration measure. The magnetoelectric response breaks time reversal symmetry which is restored at strong interactions. The effect is equivalent to the one in four dimensions without interaction; it can be observed by measuring the Faraday rotation under external stress.     

Dynamics of electron–hole plasmas and localized excitons in highly excited GaN-based ternary alloys

We have studied photoluminescence (PL) spectra of GaN crystals and InGaN ternary alloys at low temperatures as a function of the femtosecond laser excitation intensity. With an increase of the intensity, the broad PL due to electron–hole plasmas (EHP) appears below the biexciton PL in the GaN sample. On the other hand, the broad EHP PL appears above the localized exciton PL in the InGaN sample. The intensity dependence of PL properties of InGaN crystals is completely different from that of GaN crystals. The effect of alloy disorder on PL processes in ternary alloys is discussed.     

High intensity laser–matter interaction and electron plasma acceleration

The basic principles of the electron acceleration in laser produced plasmas and the related secondary sources of energetic radiation with a particular attention to betatron radiation are presented.     

Fast electron beam–plasma interaction in single-walled carbon nanotubes

A theoretical study on the plasmon-polariton modes coupled with a fast electron beam inside a metallic single-walled carbon nanotube is presented. The Maxwell’s equations coupled with a linearized hydrodynamic model for the nanotube’s charge oscillations are used. By considering the electron beam effects, general expression of dispersion relation of electromagnetic modes on nanotube’s surface is obtained. It is shown numerically that by considering the electron beam effects, the polariton frequency shifts to lower values.     

Theoretical examination of electron–phonon interaction and superconductivity in the hexagonal pnictide SrPtAs

We have calculated the structural and electronic properties of SrPtAs in a hexagonal KZnAs-type of crystal structure using a generalized gradient approximation of the density functional theory and the ab initio planewave pseudopotential method. These results are used to further calculate the phonon dispersions curves and the phonon density of states using a linear response approach based on the density functional theory. Using the electronic and phonon results, the electron–phonon coupling is computed to be of the intermediate strength of 0.78. In large part, this is contributed by the phonon modes dominated by the vibrations of Pt and As atoms. The superconducting critical temperature is estimated to be 1.9 K, in good accord with its experimental value of 2.4 K.     

Features of quasiparticle decay in 2D electronic systems with spin–orbit interaction

The dependence of the width of the spectral function of electrons and holes on the wavevector and excitation energy in a 2D electron system with spin-orbit interaction caused by structural inversion asymmetry is analyzed in the G ⁰ W ⁰ approximation. It is shown that an additional (relative to the generation of electron-hole pairs) channel of hole decay due to emission of a plasmon appears in the case of low electron density. Noticeable spin asymmetry of the spectral function width appears in the region of electron excitations.     

Anderson localization in strongly coupled disordered electron–phonon systems

Based on the statistical dynamic mean-field theory, we investigate, in a generic model for a strongly coupled disordered electron–phonon system, the competition between polaron formation and Anderson localization. The statistical dynamic mean-field approximation maps the lattice problem to an ensemble of self-consistently embedded impurity problems. It is a probabilistic approach, focusing on the distribution instead of the average values for observables of interest. We solve the self-consistent equations of the theory with a Monte Carlo sampling technique, representing distributions for random variables by random samples, and discuss various ways to determine mobility edges from the random sample for the local Green function. Specifically, we give, as a function of the ‘polaron parameters’, such as adiabaticity and electron–phonon coupling constants, a detailed discussion of the localization properties of a single polaron, using a bare electron as a reference system.     

Effects of squeezing-antisqueezing in strongly coupled electron–phonon systems

The effects of squeezing-antisqueezing resulting from the motion and density fluctuation of the electrons on the properties of both electrons and phonons have been studied by using a new variational ansatz with correlated displacement and squeezing in strongly coupled electron–phonon systems. The effects results in (1) reduction of the ground state energy, and enhancement of stability of the systems, (2) increase of the binding energy of the polaron occurred and weakening of growing speed of polaron narrowing of electron band, (3) increase of the charge density wave order and (4) suppression of increased tendency of anomalous quantum fluctuation of the phonons in the systems. The antisqueezed effect plays an important role in determining the properties of the electrons and phonons in the strongly coupled electron–phonon systems.     

The effects of electron–phonon interaction on anisotropic RKKY interaction in graphene nanoribbon

We study the Ruderman–Kittle–Kasuya–Yosida (RKKY) interaction in doped armchair graphene nanoribbon. The effects of both external magnetic field and electron-Holstein phonon on RKKY interaction have been addressed. RKKY interaction as a function of distance between localized moments has been analyzed. It has been shown that a magnetic field along the z-axis mediates an anisotropic interaction which corresponds to a XXZ model interaction between two magnetic moments. In order to calculate the exchange interaction along arbitrary direction between two magnetic moments, we should obtain both transverse and longitudinal static spin susceptibilities of armchair graphene nanoribbon in the presence of electron-phonon coupling and magnetic field. The spin susceptibility components are calculated using the spin dependent Green’s function approach for Holstein model Hamiltonian. The effects of spin polarization on the dependence of exchange interaction on distance between moments are investigated via calculating correlation function of spin density operators. Our results show the influences of magnetic field on the spatial behavior of in-plane and longitudinal RKKY interactions are different in the presence of magnetic field.