Resonant tunneling of electrons through virtual interference states formed as a result of reflection from the boundary of strongly doped region of GaAs

The paper is devoted to analysis of the electron transport through one-barrier GaAs/AlAs/GaAs heterostructures. The oscillating component of transport characteristics of symmetric one-barrier GaAs/AlAs/GaAs heterostructures with spacers, which is associated with resonance tunneling of electrons via virtual states formed in the spacer region of the structures due to reflection of electrons from the n^?-GaAs/n⁺-GaAs interface and their subsequent interference. It is shown that electrons are predominantly reflected coherently from the boundary of the strongly doped region as in the case of one-dimensional averaged potential of randomly arranged (beginning from this boundary) impurities. It is shown that low-energy virtual resonances are suppressed due to electron scattering as a result of their interaction with longitudinal optical (LO) phonons in the spacer region.     

Photoelectron spectra of thin films of the high-T c superconductor YBa2Cu3O7−δ

The results of calculations of the energy band structure of thin films are used to obtain theoretical photoelectron spectra of the valence band of the compound YBa₂Cu₃O_7−δ for δ=0 and δ=1 at various photon energies; the spectra are averaged over the photoelectron emission angles. The principal structural features of the spectra are determined by the d states of copper atoms, while the variation of the shape of the spectra with increasing photon energy is attributed to a relative decrease in the contribution from the p states of oxygen atoms. The density of d states around the Fermi level for YBa₂Cu₃O₆ films is observed to increase relative to YBa₂Cu₃O₇ films. Fiz. Tverd. Tela (St. Petersburg) 39, 437–440 (March 1997)     

Direct formation of InGaAs/GaAs quantum dots during submonolayer epitaxies from molecular beams

Direct formation of InGaAs/GaAs quantum dots is studied comparatively by Reflection High Energy Electron Diffraction, Scanning Tunneling Microscopy and Photoluminescence methods. It is found that submonolayer migration enhanced epitaxy growth mode allows a reduction in the dispersion of quantum dots size distribution as compared to submonolayer molecular beam epitaxy. In situ and ex situ results obtained are in agreement with each other and give a conceptual kinetic picture of the growth processes during epitaxy from molecular beams.     

Magnetic properties and the mechanism of formation of the uncompensated magnetic moment of antiferromagnetic ferrihydrite nanoparticles of a bacterial origin

The magnetic properties of the superparamagnetic ferrihydrite nanoparticles that form as a result of the vital activity of Klebsiella oxytoca bacteria are studied. Both an initial powder with an average number of iron atoms N _Fe ～ 2000–2500 in a particle and this powder after annealing at 140°C for 3 h in air are investigated. The following substantial modifications of the magnetic properties of the ferrihydrite nanoparticles are detected after annealing: the superparamagnetic blocking temperature increases from 23 to 49.5 K, and the average magnetic moment of a particle increases (as follows from the results of processing of magnetization curves). The particles have antiferromagnetic ordering, and the magnetic moment resulting in the superparamagnetism of the system appears due to random spin decompensation inside the particle. For this mechanism, the number of uncompensated spins is proportional to the number of magnetically active atoms raised to the one-half power, and this relation holds true for the samples under study at a good accuracy. The possible causes of the detected shift of magnetic hysteresis loops at low temperatures upon field cooling are discussed.     

Ab initio calculation and synchrotron X-ray spectroscopy investigations of tin oxides near the Sn <Emphasis Type="Italic">L</Emphasis><Subscript>3</Subscript> absorption edges

The results of theoretical and experimental investigations of the electron-energy structure of the conduction band of tin oxides have been presented. The Sn L ₃ X-ray absorption near-edge fine structure (XANES) has been calculated for the first time for single crystals of metallic tin and tin monoxide, as well as for the orthorhombic and tetragonal phases of tin dioxide, using the linearized augmented plane wave (LAPW) method. The fine structure of the XANES spectra has been compared with the specific features of the energy distribution of the local partial densities of states of the tin compounds under investigation. A joint analysis of the results of the simulation and the experimental X-ray synchrotron Sn L ₃ XANES spectra of commercial bulk samples of metallic tin and tin oxides SnO and SnO₂ has been carried out.     

Three-dimensional numerical simulation of the operation of the rotating-detonation chamber

The aim of this work is to apply three-dimensional numerical simulation to determining the conditions of the stable operation of the rotating-detonation chamber (RDC), the thermal state of the chamber walls, as well as the most important parameters of the flow at the inlet and outlet, keeping in mind the possibility of placing the RDC between a compressor and a turbine in a prospective gas turbine installation. The model is based on a system of three-dimensional unsteady Reynolds-averaged Navier-Stokes, energy, and species conservation equations for a multicomponent reacting gas mixture supplemented by a turbulence model. The system is solved using a combined algorithm based on the finite-volume and particle methods. The capabilities of the computer program are demonstrated by the example of a circular RDC with inner and outer walls 260 and 306 mm in diameter and with axial introduction of a hydrogen-air mixture through an annular gap at the bottom of the chamber (with a relative area of 0.6). The detonation wave spun over the bottom at a frequency of ??126000 rpm. Calculations have shown that such an RDC can operate in a steady mode with one detonation wave.     

Swelling,elasticity and structure of hydrogels prepared via bis-macromonomers of poly(ethylene oxide)

The effect of swelling on the shear modulus was studied for hydrogels prepared by radical polymerization of methacrylate-terminated poly(ethylene oxide) (PEO) bis-macromonomers of different molecular weight. Gels made of long chains (M = 12000 or 6000) display classical softening upon swelling, whereas gels made of shorter chains (M = 4000 or 2000) remain rigid or even stiffen. The abnormal behaviour is explained by a specific character of network junctions presented by polymethacrylate chains in which each unit is linked with a PEO network chain. It is assumed that the interactions among densely grafted PEO chains result in their stretching on polymerization and non-affine deformation on swelling, which stiffen the gel. This is verified by the data on copolymer (macromonomers - 2-hydroxyethyl methacrylate) gels that have lesser densities of PEO chains attached to the junctions and show weaker stiffening on swelling. The osmotic pressure of gels was estimated from the swelling pressure and shear modulus. Similar to the mixing pressure of equivalent PEO solutions, it varies as the 9/4 power of polymer concentration. At the same time, it is lower than the mixing pressure. This indicates that the junctions make only quantitative changes in the osmotic properties of macromonomer chains.