F. X. Bronold

Radio-frequency discharges in oxygen: I. Particle-based modelling

In this series of three papers we present results from a combined experimental and theoretical, particle-based study to quantitatively describe capacitively coupled radio-frequency discharges in oxygen. The particle-in-cell Monte Carlo model on which the theoretical description is based is described in this paper. It treats space charge fields and transport processes on an equal footing with the most important plasma–chemical reactions. For given external voltage and pressure, the model determines the electric potential within the discharge and the distribution functions for electrons, negatively charged atomic oxygen and positively charged molecular oxygen. Previously used scattering and reaction cross section data are critically assessed and in some cases modified. To validate our model, we compare the densities in the bulk of the discharge with experimental data and find good agreement, indicating that essential aspects of an oxygen discharge are captured.

Electron-hole pair condensation at the semimetal-semiconductor transition: a BCS-BEC crossover scenario

Institut fu¨r Theoretische Physik, Universita¨t Leipzig, D-04109 Leipzig, Germany(Dated: January 30, 2012)We act on the suggestion that an excitonic insulator state might separate—at very lowtemperatures—a semimetal from a semiconductor and ask for the nature of these transitions. Basedon the analysis of electron-hole pairing in the extended Falicov-Kimball model, we show that tuningthe Coulomb attraction between both species, a continuous crossover between a BCS-like transitionof Cooper-type pairs and a Bose-Einstein condensation of preformed tightly-bound excitons mightbe achieved in a solid-state system. The precursor of this crossover in the normal state might causethe transport anomalies observed in several strongly correlated mixed-valence compounds.

Possibility of an excitonic insulator at the semiconductor-semimetal transition

We calculate the critical temperature below which an excitonic insulator exists at the pressure-induced semiconductor-semimetal transition. Our approach is based on an effective-mass model for valence and conduction band electrons interacting via a statically screened Coulomb potential. Assuming pressure to control the energy gap, we derive, in the spirit of a crossover from a Bose-Einstein (BE) to a BCS condensate, a set of equations that determines, as a function of the energy gap (pressure), the chemical potentials for the two bands, the screening wave number, and the critical temperature. We (i) show that in leading order the chemical potentials are not affected by the exciton states, (ii) verify that on the strong-coupling (semiconductor) side the critical temperatures obtained from the linearized gap equation coincide with the transition temperatures for a BEC of noninteracting bosons, (iii) demonstrate that mass asymmetry strongly suppresses BCS-type pairing, (iv) investigate the composition of the environment of the excitonic insulator, and (v) discuss in the context of our theory recent experimental claims for exciton condensation in

Mie scattering analog in graphene: Lensing, particle confinement, and depletion of Klein tunneling

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Magnetic-field dependence of electron spin relaxation in n-type semiconductors

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Mie scattering by a charged dielectric particle

Guided by the analogy to Mie scattering of light on small particles, we show that the propagation of a Dirac-electron wave in graphene can be manipulated by a circular gated region acting as a quantum dot. Large dots enable electron lensing, while for smaller dots resonant scattering entails electron confinement in quasibound states. Forward scattering and Klein tunneling can be almost switched off for small dots by a Fano resonance arising from the interference between resonant scattering and the background partition.

Surface States and the Charge of a Dust Particle in a Plasma

We present a theoretical investigation of the magnetic-field dependence of the longitudinal (T 1 ) and transverse (T 2 ) spin-relaxation times of conduction-band electrons in n-type III-V semiconductors. We find that the interplay between the Dyakonov-Perel process and an additional spin-relaxation channel, which originates from the electron wave-vector dependence of the electron g factor, yields a maximal T 2 at a finite magnetic field. We compare our results with existing experimental data on n-type GaAs and make specific additional predictions for the magnetic-field dependence of electron-spin lifetimes.

Wall Charge and Potential from a Microscopic Point of View

We study for a dielectric particle the effect of surplus electrons on the anomalous scattering of light arising from the transverse optical phonon resonance in the particles dielectric function. Excess electrons affect the polarizability of the particle by their phonon-limited conductivity, either in a surface layer (negative electron affinity) or the conduction band (positive electron affinity). We show that surplus electrons shift an extinction resonance in the infrared. This offers an optical way to measure the charge of the particle and to use it in a plasma as a minimally invasive electric probe.

Optical signatures of the charge of a dielectric particle in a plasma.

We investigate electron and ion surface states of a negatively charged dust particle in a gas discharge and identify the charge of the particle with the electron surface density bound in the polarization-induced short-range part of the particle potential. On that scale, ions do not affect the charge. They are trapped in the shallow states of the Coulomb tail of the potential and act only as screening charges. Using orbital-motion limited electron charging fluxes and the particle temperature as an adjustable parameter, we obtain excellent agreement with experimental data.

Electron surface layer at the interface of a plasma and a dielectric wall

Macroscopic objects floating in an ionized gas (plasma walls) accumulate electrons more efficiently than ions because the influx of electrons outruns the influx of ions. The floating potential acquired by plasma walls is thus negative with respect to the plasma potential. Until now plasma walls are typically treated as perfect absorbers for electrons and ions, irrespective of the microphysics at the surface responsible for charge deposition and extraction. This crude description, sufficient for present day technological plasmas, will run into problems in solid-state based gas discharges where, with continuing miniaturization, the wall becomes an integral part of the plasma device and the charge transfer across it has to be modelled more precisely. The purpose of this paper is to review our work, where we questioned the perfect absorber model and initiated a microscopic description of the charge transfer across plasma walls, put it into perspective, and indicate directions for future research (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)