Dc conductance of ordered and disordered silicene superlattices

This paper studies the conductance of charge carriers through silicene-based superlattices consisting of monolayer silicene by means of transfer matrix method. At first, we consider the ordered superlattices and drive analytically the transmission probability of Dirac fermions. We show that the number of resonance picks increases with increasing the number of superlattice barriers. In order to the best understand of the appearance of the picks, we exactly studied transmission properties of the silicene superlattice. Also, the effect of disorder on the probability of transmission through the system of various sizes is studied. The short-range correlated disorder is applied on the thickness of electron doped silicene strips as quantum barriers which fluctuates around their mean values. We show that the oscillating conductance as a function of barriers hight suppresses with imposing the disorder in the silicene superlattice. Also, the effect of structural parameters on the conductance of the system is studied.     

Photoluminescence of strained-layer superlattices

The strained-layer superlattices (SLS's) of In_xGa_1?xAsGaAs and the single hetero-structure of In_xGa_1?xAs on GaAs were grown by MBE method. The samples obtained have a perfect surface morphology. The alloy composition of In_xGa_1?xAs layer and the growth rate were determined with high accuracy by in situ observation of the intensity oscillation of RHEED pattern. Photoluminescence peak energies of SLS's are in agreement with the calculated value by the Kronig-Penny analysis.     

Photoluminescence spectra of doping superlattices

Pulsed and cw photoluminescence measurements have been performed on a variety of GaAs doping superlattices with unequal n- and p- doping levels. At 6 K, samples with a p- to n-type dopant ratio greater than 2 and potential well depths between 150 and 500 meV exhibit a two-peak spectrum over a range of excitation intensities sufficient to generate enough carriers to partially compensate the internal electric potential. The peak separation is dependent on the superlattice parameters and scales with the depth of the potential wells. Measured separations as large as 150 meV are inconsistent with an explanation based on quantized valence or conduction minibands. As the sample temperature is increased to 50 K, the higher energy emission decreases with respect to the low-energy peak, indicating that this emission results from recombination involving a metastable state that can be readily depopulated by phonon interaction. A model is proposed which ascribes the high-energy peak to recombination to acceptor states near the boundary of the p-doped layers.     

Electronic bandstructure of disordered superlattices

The disordering of semiconductor superlattices is analysed in the spatial Fourier domain. It is demonstrated that disordering is equivalent to suppressing the higher-order Fourier coefficients of the alloy profile. Two superlattice bandstructure algorithms are developed in the Fourier domain which allow the electronic envelope function and energy to be determined. The first employs the effective mass approximation. The second is a 14-bandk·pwhich includes nonparabolicity, anisotropy and noncentrosymmetry which are particularly relevant in the determination of nonlinear optical coefficients.     

Photoluminescence internal quantum yield in superlattices

We show that the internal quantum yield of a GaAs/AlAs superlattice (SL) can be directly obtained from the height of the step observed in the photoluminescence (PL) excitation spectrum of the underlying GaAs layer when the excitation energy is scanned across the SL transition. From a study of the dependence of the SL and GaAs PL on the excitation power density and energy and on temperature, we are able to specify the conditions of applicability of this method. The importance of power non-linearities, reabsorption and photon recycling effects are discussed.     

Photoluminescence of strained-layer superlattices under high excitations

Photoluminescence and laser oscillation from In_0.2Ga_0.8As/GaAs strained-layer superlattices were measured under pulse-light excitations. Double-peak spectra were obtained, in contrast to the lattice-matched super-lattices.     

Lattice vibrations in ordered and disordered thiourea

Raman scattering and infrared reflection spectra of single crystals of thiourea were recorded at several temperatures between 2 K and 240 K in the spectral region (03500) cm^–1. The intention was to search for changes in frequency, linewidth, and intensity of optical phonon modes near the various phase transitions which are known to exist in this material. All observed phonon modes have been tabulated according to a group-theoretical enumeration, and they are attributed to internal motions of the molecular groups and external lattice vibrations. A survey of the temperature dependence of all observed lines is given.Some modes are in fact influenced by the phase transitions in a characteristic manner. The observed temperature dependences can be interpreted by a simple pseudospinphonon coupling model (following contribution II).     

Lattice vibrations in ordered and disordered thiourea

The critical dynamics of the Syozi model for dilute ferromagnetism is considered by the use of master equations. The dynamics is soluble as it is assumed that the time scale of motion on the sublattice on which the impurities move is so much faster than on the other sublattice that fast relaxing variables may be adiabatically eliminated, leaving a new soluble master equation. It is found that the linear and non-linear relaxation of magnetization exponents ^(l) and ^(nl) increase on dilution to ^(l)/(1–) and ^(nl)/(1–) respectively ( is the specific heat exponent for the pure system, which itself changes on dilution to –/(1–)). Thus if the exponents for the pure system obey the scaling law of Rácz and Fisher ^(nl)= ^(l)– ( is the magnetization exponent which changes on dilution to /(1–)) then so do the exponents for the diluted system. Similarly the exponent for spin diffusion changes on dilution to /(1–).     

Photoluminescence studies of GaAs/AlAs short period superlattices

Low-temperature pressure-dependent photoluminescence measurements on short-period GaAs/AlAs superlattice structures are presented. Measurements show that the lowest energy conduction-band states are in the AlAs layers and the highest energy valence-band states are located in the GaAs layers. This result is supported by the following three experimental observations: (1) the observed pressure coefficient for the conduction-band to valence-band transition energy is negative, (2) the magnetic mass of this transition is “heavy”, and (3) the band-to-band absorption coefficient appears to be small. These experimental observations are in agreement with predictions of tight-binding calculations.     

Effects of Coulomb interaction in intentionally disordered semiconductor superlattices

The interplay between disorder and electron–electron interaction is studied using measurements of the vertical dc conductivity of intentionally disordered GaAs/Al_0.3Ga_0.7As superlattices. At low temperatures a quasimetallic behavior is observed even for large disorder which is attributed to an inhomogeneous charge distribution reducing the disorder potential. At low doping levels and low temperatures, exchange–correlation interaction leads to an inhomogeneous charge distribution over the wells of ordered superlattices similar to a one-dimensional Wigner lattice.     

On some kinetic phenomena in ordered and disordered substances

A new temperature behaviour of nuclear magnetization is predicted. The relaxation theory in disordered ferromagnets is presented. The scattering of quasiparticles on two-level systems is described. A nonlinear susceptibility of ferromagnets is calculated.