Influence of degeneracy on momentum relaxation times in semiconductors

We have studied the effect of degeneracy on momentum relaxation times under ohmic as well as non-ohmic conditions. We find that a proper momentum relaxation time, within the framework of the Boltzmann transport equation, can no longer be defined for isotropic but inelastic scattering when the carriers are hot whereas under ohmic conditions it can be defined and is appreciably altered by degeneracy. For elastic scattering the momentum relaxation time is found to be unaffected by degeneracy for both ohmic and non-ohmic regimes.     

Lidar observation of the bubble trace in the sea surface layer

The evolutions of the signals of the elastic (Mie) and Raman backscattering of the Nd:YAG laser second-harmonic radiation in the upper layer of the sea under cavitational perturbation induced by the passage of a high-speed boat are compared under the conditions of the outdoor experiment. It is shown that the signal of the elastic scattering on bubbles returns to the background level within the time of ∼10 min. The relaxation time of the spontaneous Raman scattering signal can be an order of magnitude larger than this value (the evaluated value under the particular conditions of this experiment is ∼120 min). Interpretation is given to this difference in attenuation time of the elastic and inelastic scattering signals at the cavitational perturbation of the sea.     

Energy losses of 2D electron gas due to near-surface acoustic phonon scattering

We have shown that for quantum wells placed close to the stress-free surface of the semiconductor heterostructure, the energy relaxation rate of two-dimensional electrons interacting with acoustic phonons at low temperatures (Bloch–Grüneisen regime) is changed considerably in comparison with that of a two-dimensional electron gas placed in a bulk of semiconductor. The relaxation rate is enhanced in the case of a semiconductor–vacuum system and is suppressed in the case of the surface covered by a thin metal film. The enhanced energy loss is caused by additional scattering at localized and reflected acoustic waves, and the decrease appears due to suppression of piezoelectric scattering in the vicinity of the metal.     

Anharmonic processes of scattering and relaxation of slow quasi-transverse phonons in cubic crystals

Relaxation of slow quasi-transverse phonons in anharmonic processes of scattering in cubic crystals with positive (Ge, Si, diamond) and negative (KCl, NaCl) anisotropies of the second-order elastic moduli has been considered. The dependences of the relaxation rates on the direction of the wave vector of phonons in scattering processes with the participation of three quasi-transverse phonons (the TTT relaxation mechanisms) are analyzed within the anisotropic continuum model. It is shown that the TTT relaxation mechanisms in crystals are associated with their cubic anisotropy, which is responsible for the interaction between noncollinear phonons. The dominant contribution to the phonon relaxation comes from large-angle scattering. For crystals with significant anisotropy of the elastic energy (Ge, Si, KCl, NaCl), the total contribution of the TTT relaxation mechanisms to the total relaxation rate exceeds the contribution of the Landau-Rumer mechanism either by several factors or by one to two orders of magnitude depending on the direction. The dominant role of the TTT relaxation mechanisms as compared to the Landau-Rumer mechanism is governed, to a considerable extent, by the second-order elastic moduli. The total relaxation rates of slow quasi-transverse phonons are determined. It is demonstrated that, when the anharmonic processes of scattering play the dominant role, the inclusion of one of the relaxation mechanisms (the Landau-Rumer mechanism or the mechanisms of relaxation of the slow quasi-transverse mode by two slow or two fast modes) is insufficient for describing the anisotropy of the total relaxation rates in cubic crystals.     

Exciton energy relaxation on acoustic phonons in double-quantum-well structures

The paper analyzes the rate of energy relaxation involving acoustic phonon emission between exciton states in a double quantum well. A theoretical study is made of the part played by two mechanisms, one of which is a one-step transition with emission of an acoustic phonon and the other is a two-step transition, which includes elastic exciton scattering from interface nonuniformities followed by energy relaxation within an exciton subband. The rate of the two-step transition in real double quantum wells is shown to be higher than that of the one-step transition. As follows from calculations, the fast energy relaxation between exciton states is determined by the elastic scattering of phonons from the interface.     

Substrate and chain size dependence of near surface dynamics of glassy polymers

We use nanohole relaxation to study the surface relaxation of films of glassy isotactic poly (methyl methacrylate) (i-PMMA) films. These measurements allow us to obtain the time dependent relaxation function at a number of different sample temperatures for the first 2-3 nm of the free surface in a system often used as a model system for the effect of the substrate on thin film dynamics. The surface is observed to relax at temperatures up to 42 K below the bulk Tg value, even on systems where the thin film Tg is known to be greater than the bulk value. We are able to determine the range over which the substrate directly affects the free surface relaxation, and determine a surprisingly large (Mw independent) limiting thickness of approximately 180 nm where the free surface relaxation is not affected by the substrate. For thick films (h>200 nm) we find an unexpected linear Mw dependence of the near surface relaxation time.     

Elastic scattering and surface effects on Auger electron angular distribution and attenuation law

We have studied, through Monte Carlo (MC) simulations, the effect of elastic scattering and the presence of a real surface on the angular distribution and attenuation of the no-energy-loss electron emission. We performed two kinds of MC approaches, with and without surface, altering the ratio among elastic and inelastic scattering cross sections to enhance the elastic scattering effect. We found that angular distribution is always affected by the elastic scattering, meanwhile the attenuation law shape is only altered in the presence of a real surface and elastic scattering together. We also compared simulation results with analytical model predictions.     

Anomalous Heat and Momentum Transport Arising from Surface Roughness in a Normal <Superscript>3</Superscript>He Slab

I discuss heat and momentum transport in a mesoscopic film of ³He, confined by rough walls in the normal Fermi liquid state. Inelastic binary quasiparticle scattering mediated by elastic scattering from the surface roughness gives rise to a coherent “mixed” scattering channel that drives anomalous transport over a range of temperature. I calculate the thermal conductivity and viscosity of the film in this regime and derive these in terms of the film thickness and autocorrelation function of the surface roughness, which enters the formulation as an independent input. This calculation can be useful in understanding and isolating the effects of confinement and surface roughness, especially in the context of exploring the superfluid state in the film.     

Dynamic anisotropy and heterogeneity of polystyrene thin films as studied by inelastic neutron scattering

We studied the dynamic anisotropy and heterogeniety of polystyrene thin films in glassy state with inelastic neutorn scattering. Adjusting the scattering vector to the directions parallel and perpendicular to the film surface, we observed the elastic scattering intensities as a function of temperature. It was found for the 200 A film that the elastic intesity decreased with increasing temperature more rapidly in the perpendicular direction than in the pararell direction, showing the higher mobility in the perpendicular direction. However, such dynamical anisotropy was not observed in the 1000 A film. The decrease in the mobility was observed with the film thickness in both the directions. These results were explained in terms of an interface hard layer. We also evelauated the dynamical heterogeniety from the non-Gaussian parameter A0, which increased with decreasing the film thickness, showing the increase in the dynamical heterogeneity. Assuming a simple bi-layer model consisting of the interface hard layer and the bulk-like layer, we analyzed the thickness dependence of the non-Gaussian parameter A0 and the mean square displacement (u2) to find that the hard layer has a thickness of approximately 130 A and a mean square displacement of approximately 0.018 A2 at 230 K.     

Surface visco-elastic moduli of a Langmuir film of a polystyrene-polydimethyl siloxane block copolymer at the air-ethyl benzoate interface

Surface quasi-elastic light scattering has been applied to a spread film of a block copolymer of polystyrene and polydimethyl siloxane. The influence of surface concentration (surface pressure) at a fixed surface wave number has been explored. The capillary wave frequency and damping showed a similar dependence on the surface concentration as values obtained earlier, but due to a more appropriate analysis of the correlation functions, surface visco-elastic moduli obtained were distinctly different. By correlating the values obtained with the variations in solvated polystyrene layer thickness from neutron reflectometry, the maximum in dilational modulus was shown to occur at the same nominal surface concentration where the layer begins to stretch and take on brush-like behaviour. This same surface concentration is where the relaxation time of the spread film also has a maximum value, the relaxation time being calculated using the standard linear model of visco-elasticity, which was found to fit the frequency dependence of the surface tension and dilational moduli at the resonant nominal surface concentration of 3.1 mg m^-2. Received 21 August 2001 and Received in final form 11 January 2002     

Relaxation between electrons and surface phonons of a homogeneously photoexcited metal film

The energy relaxation between the hot degenerate electrons of a homogeneously photoexcited metal film and the surface phonons (phonon wave vectors in two dimensions) is considered under Debye approximation. The state of electrons and phonons is described by equilibrium Fermi and Bose functions with different temperatures. Two cases for electron scattering by the metal surface, namely specular and diffuse scattering, are considered.     

Inelastic neutron scattering study of low energy excitations in polymer thin films

We report inelastic neutron scattering measurements on polystyrene thin films in a glassy state in the meV region. We found in elastic scattering that the mean square displacement decreased with film thickness, and hence the corresponding force constant f increased. In inelastic and quasielastic scattering, we observed the so-called boson peak at around 1.5 meV and the picosecond fast process for the first time in thin films, both of which decreased in intensity with film thickness. These results were discussed in terms of the potential hardening due to the confinement of polymer chains and/or the interfacial dead layer.     

Weakly bound dimers of fermionic atoms

We discuss the behavior of weakly bound bosonic dimers formed in a two-component cold Fermi gas at a large positive scattering length a for the interspecies interaction. We find the exact solution for the dimer-dimer elastic scattering and obtain a strong decrease of their collisional relaxation and decay with increasing a. The large ratio of the elastic to inelastic rate is promising for achieving Bose-Einstein condensation of the dimers and cooling the condensed gas to very low temperatures.     

The elastic scattering effect on transmitted and reflected Al thin film Auger yields

We have studied, using a Monte Carlo simulation code, the effect of elastic scattering on reflected and transmitted Al thin film Auger yields. We found that the behaviour of both types of yields, when the energy of impinging electrons or the film thickness are changed, can be understood in terms of the slowing down and the elastic scattering of impinging electrons. We have also found that, when working with thin film, and under certain conditions, the backscattering effect may be neglected.     

Dynamical pattern formation for diblock copolymer films under stretch by moving walls

We propose a computational method that takes into account the dynamical influence of moving rigid walls over the pattern formation for thin films of diblock copolymers. The competition between the surface field energy and elastic stretching energy, and the effects of the molecular relaxation on pattern formation are studied. Finally, it is also observed that stretching the film enhances the ordering of patterns in it.     

Deformation electron-phonon coupling in disordered semiconductors and nanostructures

We study the electron-phonon relaxation (dephasing) rate in disordered semiconductors and low-dimensional structures. The relaxation is determined by the interference of electron scattering via the deformation potential and elastic electron scattering from impurities and defects. We have found that in contrast with the destructive interference in metals, which results in the Pippard ineffectiveness condition for the electron-phonon interaction, the interference in semiconducting structures substantially enhances the effective electron-phonon coupling. The obtained results provide an explanation to energy relaxation in silicon structures.     

Instability-driven quantum dots

When a film is strained in two dimensions, it can relax by developing a corrugation in the third dimension. We review here the resulting morphological instability that occurs by surface diffusion, called the Asaro–Tiller–Grinfel'd instability (ATG), especially on the paradigmatic silicon/germanium system. The instability is dictated by the balance between the elastic relaxation induced by the morphological evolution, and its surface energy cost. We focus here on its development at the nanoscales in epitaxial systems when a crystal film is coherently deposited on a substrate with a different lattice parameter, thence inducing epitaxial stresses. It eventually leads to the self-organization of quantum dots whose localization is dictated by the instability long-time dynamics. In these systems, new effects, such as film/substrate wetting or crystalline anisotropy, come into play and lead to a variety of behaviors.     

Features of nuclear reactions induced by ultracold neutrons passing through thin films

The probabilities of elastic scattering and capture of ultracold neutrons in thin films are calculated in terms of the time quantum theory. It is shown that at a neutron wavelength exceeding considerably the film thickness, the macroscopic probability of the (n, γ)-reaction can decrease as compared to the macroscopic probability of elastic scattering.     

Effects of anisotropic scattering on electronic transport properties

The role of anisotropic scattering in transport properties is studied within the framework of the anisotropic relaxation time. The concept of the relaxation time is introduced in a rigorous way, without approximation. Its physical meaning and its connection with linear response theory is discussed. The manner in which electron-impurity scattering and electronphonon scattering influence the relaxation time is considered as well as the role of such anisotropies in the de Haas-van Alphen effect, Hall effect, deviations from Mathiessen's rule, and magnetoresistance. Examples are taken from recent experimental and theoretical work. Some common pitfalls surrounding the use of the anisotropic relaxation time are pointed out.     

Effects of anisotropic scattering on electronic transport properties

The role of anisotropic scattering in transport properties is studied within the framework of the anisotropic relaxation time. The concept of the relaxation time is introduced in a rigorous way, without approximation. Its physical meaning and its connection with linear response theory is discussed. The manner in which electron-impurity scattering and electronphonon scattering influence the relaxation time is considered as well as the role of such anisotropies in the de Haas-van Alphen effect, Hall effect, deviations from Mathiessen's rule, and magnetoresistance. Examples are taken from recent experimental and theoretical work. Some common pitfalls surrounding the use of the anisotropic relaxation time are pointed out.