Low-temperature magnetoresistance due to weak localization in lightly doped semiconductors

The first observation of low-temperature magnetoresistance (MR) of interference nature in the case of a light doping is reported. The MR occurs in n- and p-type Ge samples at a frequency of 10 GHz at temperatures below 30 K in weak magnetic fields on the background of the classical MR effect associated with electrons in different valleys (n-Ge) and with heavy and light holes (p-Ge).     

Low-temperature variation of magnetic order in a nonmagnetic n-Ge:As semiconductor in the vicinity of the metal-insulator phase transition

The phenomenon of the low-temperature transition from antiferro- to ferromagnetic ordering of impurity spins in a nonmagnetic compensated n-Ge:As semiconductor near the metal-insulator phase transformation has been experimentally observed. The effect is manifested by rather sharp changes in the spin density and g-factor in the electron spin resonance spectra. As the relative content of a compensating impurity (gallium) is reduced below 0.7, the transition temperature begins to decrease and, at a degree of compensation below 0.3, drops below the studied temperature range (i.e., below 2 K).     

Electrostatic models of insulator-metal and metal-insulator concentration phase transitions in Ge and Si crystals doped by hydrogen-like impurities

Two electrostatic models have been developed that allow calculation of the critical concentration of hydrogen-like impurities in three-dimensional crystalline semiconductors corresponding to the insulator-metal and metal-insulator transition in the zero temperature limit. The insulator-metal transition manifests itself as a divergence of the static permittivity observed in lightly compensated semiconductors as the concentration of polarizable impurities increases to the critical level. The metal-insulator transition is signaled by the divergence of the dc electrical resistivity in heavily doped semiconductors as the compensation of the majority impurity increases (or its concentration decreases). The critical impurity concentration corresponds to the coincidence of the percolation level for the majority carriers with the Fermi level. The results of the calculations made with these models fit the experimental data obtained for n-and p-type silicon and germanium within a broad range of their doping levels and impurity compensation.     

A kinematical study of Kirchhoff-Rayleigh flow

Summary A method of calculating the separated flow of a viscous fluid is proposed, which allows to split up properly the boundary condition problem from the viscous phenomena. The theory is developed for the flow past a plate and yields wakes of finite extension having an underpressure which depends directly on the amount of vorticity diffusion and dissipation occurring in the fluid. Application of the method to real flows shows good agreement between the calculated and the measured velocity distributions in front of the plate and in the wake.

---

Résumé Une méthode de calcul de l'écoulement décollé d'un fluide visqueux est proposée qui permet de séparer clairement le problème aux limites des phénomènes visqueux. La théorie est développée pour l'écoulement autour d'une plaque et donne des sillages de longueur finie ayant une dépression de culot directement dépendante de l'intensité de la diffusion et dissipation de la vorticité se produisant dans le fluide. L'application de la méthode à des écoulements réels montre une bonne concordance entre les répartitions de vitesse calculées et mesurées sur le devant de la plaque et dans le sillage.

     

Gas transfer and water vapor condensation in cathode layers of air–hydrogen fuel cells

Model of heat and mass transfer in catalytic cathode layers of air–hydrogen fuel cells is developed on the basis of experimental data on the layers’ structure. The effect of carbon nanotubes is analyzed: their introducing into the catalytic layer increased the layer’s porosity. The derived analytical expressions allow estimating the carbon-nanotubes-content-dependence of the catalytic layer structure parameters, in particular, the gas channel characteristic dimensions and oxygen and water molecule diffusion coefficients. The simulation results showed that the adding of carbon nanotubes into the catalytic layer allows increasing the fuel cell power significantly, due to removal of limitations caused by water condensation process. The calculated results agree well with the previously obtained experimental data.     

Gas phase activation energy for unimolecular dissociation of biomolecular ions determined by focused RAdiation for gaseous multiphoton ENergy transfer (FRAGMENT)

We present a novel approach for the determination of activation energy for the unimolecular dissociation of a large (>50 atoms) ion, based on measurement of the unimolecular dissociation rate constant as a function of continuous-wave CO(2) laser intensity. Following a short ( approximately 1 s) induction period, CO(2) laser irradiation produces an essentially blackbody internal energy distribution, whose 'temperature' varies inversely with laser intensity. The only currently available method for measuring such activation energies is blackbody infrared radiative dissociation (BIRD). Compared with BIRD, FRAGMENT: (a) eliminates the need to heat the surrounding ion trap and vacuum chamber to each of several temperatures (each requiring hours for temperature equilibration); (b) offers a three-fold wider range of effective blackbody temperature; and (c) extends the range of applications to include initially cold ions (e.g., gas-phase H/D exchange). Our FRAGMENT-determined activation energy for dissociation of protonated bradykinin, 1.2 +/- 0.1 eV, agrees within experimental error to the value, 1.3 +/- 0.1 eV, previously reported by Williams et al. from BIRD experiments. Copyright 1999 John Wiley & Sons, Ltd.     

Antiferromagnetic spin glass in doped Ge near insulator-metal transition

A low-temperature (3–100 K) electron spin resonance (ESR) study of the spin system of neutral As donors in Ge showed that on the insulator side of the insulator-metal transition the single-spin density exponentially disappears as T → 0. Such spins are bound into pairs to give an antiferromagnetic (AF) phase. Upon increasing the temperature the AF phase is destroyed, the single-spin density and, as a result, the ESR absorption signal becomes stronger. The temperature dependences of the densities of the pairs and single spins are typical for a chaotic distribution of neutral donors. In this case, there is no Néel temperature. For a low degree of compensation, the crystal lattice of Ge with the AF phase is actually a nanostructured system characterized by anisotropic internal stresses that are the strongest along one of the [110] directions. These stresses give rise to the anisotropy of the g-factor which is responsible for experimentally observed splitting of the ESR line. The compensating impurities destroy the AF phase and reduce this splitting. Local stresses are present in this case, too, but now they appear because of the Coulomb interaction of oppositely charged impurities and have no preferred orientation.     

The Peierls phase transition in <Emphasis Type="Italic">n</Emphasis>-Ge caused by impurity interaction

The study of the electron paramagnetic resonance in Ge:As has revealed that the insulating state in uncompensated semiconductors is preserved near the insulator-metal phase transition because of the appearance of lattice distortions. The latter are caused by the interaction of the spins localized on impurity atoms due to the spin-Peierls transition. In Ge:As, this effect manifests itself in the concentration range n = n = 3 × 10¹⁷–3.7 × 10¹⁷ cm⁻³.