Lucio Demeio

Numerical simulations of perturbed Vlasov equilibria

In this work, the long‐time behavior of the solutions of the Vlasov–Poisson system when starting near a spatially homogeneous equilibrium is analyzed numerically. It is found that the asymptotic state toward which the system evolves is not a Bernstein–Green–Kruskal (BGK) equilibrium, but can rather be described by a superposition of BGK modes. Also, it is shown that the small oscillations at the plasma frequency after saturation in the one‐sided bump‐on‐tail instability are due to beating between the unstable mode and the least stable mode.

Wigner-function approach to multiband transport in semiconductors

In this work we present a one-dimensional, multi-band model for electron transport in semiconductors that makes use of the Wigner-function formalism and that allows for energy bands of any shape. A simplified two-band model is then derived from the general equations, by using the parabolic band approximation.

Effects of velocity changing collisions on line shapes of HF in Ar

The generalized Hess method (GHM) gives a line shape expression which is formally equivalent to the Rautian–Sobel’man hard collision model of Dicke narrowing, but differs radically in the definition of one of the relaxation terms. The relaxation term leading to pressure broadening is the same, but the term leading to Dicke narrowing and ultimately to Doppler line shapes at zero density differs in certain important respects: (1) in GHM it is a weighted sum of the pressure broadening coefficient and an optical diffusion coefficient and (2) there is no sharp distinction between ‘‘velocity changing’’ and ‘‘phase changing’’ collisions. The Dicke narrowing term should thus be understood as including both collision types irretrievably intermixed, with GHM providing a prescription for both relaxation terms. Applied to HF v=0→1, j→j±1 absorption spectra in a bath of Ar and using an accurate interaction potential obtained from spectra of the van der Waals complex and essentially exact close coupling scattering S ma...

Time dependent nucleation. II. A semiclassical approach

The continuum approximation of the Becker–Doring birth and death equations [B. Shizgal and J. C. Barrett, J. Chem. Phys. 91, 6505 (1989)] leads to a Fokker–Planck equation for the continuous cluster distribution. This linear Fokker–Planck equation is solved with the expansion of the cluster distribution function in the eigenfunctions of the Fokker–Planck operator. The Fokker–Planck eigenvalue problem can be transformed into an equivalent Schrodinger equation. In this paper, the semiclassical Wentzel–Kramers–Brillouin (WKB) method and the corresponding supersymmetric WKB method are employed in the determination of the eigenvalues and eigenfunctions of the equivalent Schrodinger equation. We compare the approximate results with those obtained with a standard discretization scheme. We obtain eigenvalues and eigenfunctions of the Fokker–Planck operator which are in good agreement with the exact ones. The nucleation fluxes and associated time lags are also considered.

Numerical simulations of BGK modes

Solutions of the full nonlinear Vlasov–Poisson system for a one-dimensional unmagnetized plasma that correspond to undamped travelling waves near Maxwellian equilibria are analysed numerically using the splitting scheme algorithm. The numerical results are clearly in favour of the existence of such waves and confirm that there is a critical phase velocity below which they cannot be constructed.

Quantum Corrections to Classical BGK Modes in Phase Space

In this paper we study the quantum corrections to the Bernstein‐Greene‐Kruskal equilibria of plasma physics by using perturbation methods near the classical solution. We obtain the solution for the Wigner function and for the self‐consistent potential to first order in the smallness parameter, which is the square of Plancks constant (in dimensionless form), and investigate the structure of the quantum phase space.

Electron degradation and thermalization in CH4 gas

The relaxation to equilibrium of an ensemble of electrons dilutely dispersed in a large excess of CH4 is studied with solutions of the Boltzmann equation. Elastic and vibrationally inelastic collision processes are included in the analysis. The relaxation time for the approach to equilibrium defined for the relaxation of the average electron energy is determined for two different cross section sets. The kinetic theory formalism, based on the Boltzmann equation, is compared with the formalism used in radiation chemistry and physics and based on the Spencer–Fano equation.

Multiband quantum transport models for semiconductor devices

The modeling of semiconductor devices, which is a very active and intense field of research, has to keep up with the speed at which the fabrication technology proceeds; the devices of the last generations have become increasingly smaller, reaching a size so small that quantum effects dominate their behaviour. Quantum effects such as resonant tunneling and other size-quantized effects cannot be described by classical or semiclassical theories; they need a full quantum description [Fre90, JAC92, KKFR89, MRS90, RBJ91, RBJ92]. A very important feature, which has appeared in the devices of the last generation and which requires a full quantum treatment, is the presence of the interband current: a contribution to the total current which arises from transitions between the conduction and the valence band states. Resonant interband tunneling diodes (RITDs) are examples of semiconductor devices which exploit this phenomenon; they are of big importance in nanotechnology for their applications to high-speed and miniaturized systems [YSDX91, SX89]. In the band diagram structure of these diodes there is a small region where the valence band edge lies above the conduction band edge (valence quantum well), making interband resonance possible.

Multiband, Non-Parabolic Wigner-Function Approach to Electron Transport in Semiconductors

Abstract In this work, we introduce a multiband transport model for quantum electron transport in semiconductors following the Wigner-function approach. By using the Bloch-Floquet decomposition of the density matrix, we obtain the Bloch-Floquet projections of the Wigner function and derive their evolution equations for energy bands of arbitrary shape. The equations of the model are very general and allow (in principle) the investigation of quantum processes in which interband transitions and/or non-parabolicity effects may occur. Finally, we present numerical applications for some particular cases in which the numerical solution can be obtained easily.

Response Scenario and Nonsmooth Features in the Nonlinear Dynamics of an Impacting Inverted Pendulum

In this work, we perform a systematic numerical investigation of the nonlinear dynamics of an inverted pendulum between lateral rebounding barriers. Three different families of considerably variable attractors-periodic, chaotic, and rest positions with subsequent chattering-are found. All of them are investigated, in detail, and the response scenario is determined by both bifurcation diagrams and behavior charts of single attractors, and overall maps. Attention is focused on local and global bifurcations that lead to the attractor-basin metamorphoses. Numerical results show the extreme richness of the dynamical response of the system, which is deemed to be of interest also in view of prospective mechanical applications.