A comprehensive analysis of the stopping power of antiprotons and negative muons in He and gas targets for projectile velocities (equivalent antiproton energies) ranging from about 0.1 to 10 au (0.25 keV to 2.5 MeV) is performed. Recent experimental data are contrasted with theoretical results obtained from different approaches. The electronic stopping power is evaluated within the coupled-state atomic-orbital method and the distorted-wave Born approximation as well as, for low projectile velocities, within a generalized adiabatic-ionization model that takes into account collisional-broadening effects. The departure of the antiproton stopping power from the proton stopping power (`Barkas effect'), observed for intermediate projectile velocities, is discussed. The contribution to the stopping power arising from energy transfer to the translational degrees of freedom of the target system (`nuclear stopping') is evaluated. Our analysis results in a good understanding of the stopping mechanisms of negative heavy particles in gases, in particular in He. Discrepancies between theory and experiment in the case are attributed to effects of the molecular structure of the target.

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A general method for solving the stationary one-electron, two-center Coulomb problem with a superimposed (uniform) strong magnetic field is described and applied. For arbitrary orientation of the field with respect to the line connecting the centers, the pertinent Schrodinger equation is solved by evaluating analytically the Hamiltonian matrix in a basis of (nonorthogonal) Hylleraas functions and solving numerically the generalized eigenvalue problem for this matrix. A detailed study of the properties of "magnetically dressed" (diatomic) one-electron molecular orbitals is performed by calculating energies and wave functions for the H2+ and (H-He)'+ systems for field strengths up to about 10 T. Molecular-orbital correlation diagrams are presented and discussed, in which dressed-orbital energies are displayed as a function of internuclear distance R at fixed angle 9 between field direction and internuclear axis, and as a function of 0 at fixed R. Equilibrium internuclear distances and total binding energies are calculated as functions of field strength for the magnetically dressed H2+ system in its lowest gerade and ungerade states at 0=0 and t9=90'. The influence of the magnetic field on molecular binding properties as well as on the separation behavior of molecular orbitals at large internuclear distances is illustrated by means of wave-function plots. Whenever possible, our results are compared to those of previous investigations. The convergence properties of our method are discussed.

Grain-boundary-limited transport in semiconducting SnO 2 thin films: Model and experimentsA recently developed model that unifies the ballistic and diffusive transport mechanisms is applied to the carrier transport across potential barriers at grain boundaries in microcrystalline semiconducting materials. In the unified model, the conductance depends on the detailed structure of the band edge profile and in a nonlinear way on the carrier mean free path. Equilibrium band edge profiles are calculated within the trapping model for samples made up of a linear chain of identical grains. Quantum corrections allowing for tunneling are included in the calculation of electron mobilities. The dependence of the mobilities on carrier mean free path, grain length, number of grains, and temperature is examined, and appreciable departures from the results of the thermionic-field-emission model are found. Specifically, the unified model is applied in an analysis of Hall mobility data for n-type c-Si thin films in the range of thermally activated transport. Owing mainly to the effect of tunneling, potential barrier heights derived from the data are substantially larger than the activation energies of the Hall mobilities. The specific features of the unified model, however, cannot be resolved within the rather large uncertainties of the analysis.

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For electron transport in parallel-plane semiconducting structures, a model is developed that unifies ballistic and diffusive transport and thus generalizes the Drude model. The unified model is valid for arbitrary magnitude of the mean free path and arbitrary shape of the conduction band edge profile. Universal formulas are obtained for the current-voltage characteristic in the nondegenerate case and for the zero-bias conductance in the degenerate case, which describe in a transparent manner the interplay of ballistic and diffusive transport. The semiclassical approach is adopted, but quantum corrections allowing for tunneling are included. Examples are considered, in particular the case of chains of grains in polycrystalline or microcrystalline semiconductors with grain size comparable to, or smaller than, the mean free path. Substantial deviations of the results of the unified model from those of the ballistic thermionic-emission model and of the drift-diffusion model are found. The formulation of the model is one-dimensional, but it is argued that its results should not differ substantially from those of a fully three-dimensional treatment. PACS number(s): 05.60.Cd,

A unified theoretical description of ballistic and diffusive carrier transport in parallel-plane semiconductor structures is developed within the semiclassical model. The approach is based on the introduction of a thermo-ballistic current consisting of carriers which move ballistically in the electric field provided by the band edge potential, and are thermalized at certain randomly distributed equilibration points by coupling to the background of impurity atoms and carriers in equilibrium. The sum of the thermo-ballistic and background currents is conserved, and is identified with the physical current. The current-voltage characteristic for nondegenerate systems and the zero-bias conductance for degenerate systems are expressed in terms of a reduced resistance. For arbitrary mean free path and arbitrary shape of the band edge potential profile, this quantity is determined from the solution of an integral equation, which also provides the quasi-Fermi level and the thermoballistic current. To illustrate the formalism, a number of simple examples are considered explicitly. The present work is compared with previous attempts towards a unified description of ballistic and diffusive transport.

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