Two Coulomb waves theory for direct excitation

A theoretical model of Dewangan, in which the total scattering wave function is approximated by a distorted wave containing two Coulomb wave functions, is discussed and its relation with the Brauner-Briggs-Klar model for ionization is examined. An important feature of the theory is that it includes a second Born amplitude naturally and in addition, contains, albeit approximately, both real and imaginary parts of all higher order Born terms. The theory is applied to study the 1s→2s excitation of hydrogen by electrons in the energy range 54.4 to 400eV. The differential and integral cross sections predicted by the theory are compared with the results of other theories and experimental data at 54.4eV and a good agreement is found.     

Phonon growth in a many valley model semiconductor at low lattice temperatures

It is shown how the traditional method of neglecting the energy of acoustic phonons and approximating their distribution by the equipartition law leads to significant errors in the phonon growth rate in a many-valley model semiconductor when the lattice temperature is low.     

Periodic and partially periodic representations ofSU(N) q

The Gelfand-Zetlin basis is adapted toSU(N) _q forq a root of unit. Extra parameters are incorporated in the matrix elements of the generators to obtain all the invariants corresponding to the augmented center. A crucial identity is derived and proved, which guarantees the periodicity of the action of the generators. Full periodicity is relaxed by stages, some raising and lowering operators remaining injective while others become nilpotent with corresponding changes in the dimension of the representation. In the extreme case of highest weight representations. all the raising and lowering operators are nilpotent. As an alternative approach an auxiliary algebra giving all the periodic representations is presented. An explicit solution of this system forN=3, while fully equivalent to the G.-Z. basis, turns out to be much simpler.     

Triple differential cross sections for ionization of hydrogen atoms by positrons in a Schwinger variational calculation

Schwinger variational principle has been used to calculate triple differential cross-sections for ionization of hydrogen atoms by positrons at intermediate and high energies for Ehrhardt type asymmetric geometry. The results agree in general with the calculations of Brauneret al [8] and with the second Born calculation.     

Shocks in accretion flows with SPH

Summary We present the preliminary results of a time-dependent simulation, with the Smoothed-Particle Hydrodynamics tecnique, of the accretion of gas onto Schwartzschild black holes. The flow should exibit a stable shock for the parameters chosen. In the case of 1D axisymmetric simulations, we find that the predicted inner shock position is unstable. For 2D axisymmetric simulations, in the light of the first runs, we argue that in this case shocks may be a more common result.     

MITATT mode in DDR heterostructure Impatt

Large signal characterisation of double heterostructure DDR Impatt diode has been carried out in the millimeter-wave range considering the MITATT mode of operation. The structure of the device is p⁺-p₂-p₁-n₁-n₂-n⁺ where impact ionisation and tunneling takes place in the p₁-n₁ region. In this study we have considered two well-known heterostructures, e.g., InP/GaInAs/InP and InP/InGaAsP/InP and one nonconventional structure GaAs/InP/GaAs. The theoretical results of the performances of these devices as regards of output power, efficiency, and negative conductance revealed that the structures are quite promising as the source of power in the millimeter-wave range. The analysis may be used for other mm wave DDR heterostructure Impatts.     

Universal occurrence of soot superaggregates with a fractal dimension of 2.6 in heavily sooting laminar diffusion flames

Using small-angle light scattering we show that a new phase of soot with size ca. 10 microm and a fractal dimension of D approximately equal to 2.6 exists in laminar diffusion flames for a wide range of heavily sooting fuels. This new phase appears to be a supramicrometer extension of the well-known submicrometer, D approximately equal to 1.8 phase of soot formed via diffusion-limited cluster aggregation (DLCA). The occurrence of this new soot phase correlates with an empirical sooting index for fuels. This supports a creation scenario in which these supramicrometer aggregates are created via a percolation of the submicrometer, D approximately equal to 1.8 aggregates.