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ABSTRACT: 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 quatum 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.
01/2013;
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ABSTRACT: 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 particle's 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.
Physical Review Letters 12/2012; 109(24):243903. · 7.37 Impact Factor
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ABSTRACT: 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.
04/2012;
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ABSTRACT: Plasma-boundaries floating in an ionized gas are usually negatively charged. They accumulate electrons more efficiently than
ions leading to the formation of a quasi-stationary electron film at the boundaries. We propose to interpret the build-up
of surface charges at inert plasma boundaries, where other surface modifications, for instance, implantation of particles
and reconstruction or destruction of the surface due to impact of high energy particles can be neglected, as a physisorption
process in front of the wall. The electron sticking coefficientse and the electron desorption timeτe, which play an important role in determining the quasi-stationary surface charge, and about which little is empirically and
theoretically known, can then be calculated from microscopic models for the electron-wall interaction. Irrespective of the
sophistication of the models, the static part of the electron-wall interaction determines the binding energy of the electron,
whereas inelastic processes at the wall determinese andτe. As an illustration, we calculate se and τe for a metal, using the simplest model in which the static part of the electron-metal interaction is approximated by the classical
image potential. Assuming electrons from the plasma to loose (gain) energy at the surface by creating (annihilating) electron-hole
pairs in the metal, which is treated as a jellium half-space with an infinitely high workfunction, we obtain se≈10-4 and τe≈10-2s. The product seτe≈10-6s has the order of magnitude expected from our earlier results for the charge of dust particles in a plasma but individuallyse is unexpectedly small and τe is somewhat large. The former is a consequence of the small matrix elements occurring in the simple model while the latter
is due to the large binding energy of the electron. More sophisticated theoretical investigations, but also experimental support,
are clearly needed because if se is indeed as small as our exploratory calculation suggests, it would have severe consequences for the understanding of the
formation of surface charges at plasma boundaries. To identify what we believe are key issues of the electronic microphysics
at inert plasma boundaries and to inspire other groups to join us on our journey is the purpose of this colloquial presentation.
The European Physical Journal D 04/2012; 54(3):519-544. · 1.48 Impact Factor
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ABSTRACT: We act on the suggestion that an excitonic insulator state might
separate---at very low temperatures---a semimetal from a semiconductor and ask
for the nature of these transitions. Based on the analysis of electron-hole
pairing in the extended Falicov-Kimball model, we show that tuning the Coulomb
attraction between both species, a continuous crossover between a BCS-like
transition of Cooper-type pairs and a Bose-Einstein condensation of preformed
tightly-bound excitons might be achieved in a solid-state system. The precursor
of this crossover in the normal state might cause the transport anomalies
observed in several strongly correlated mixed-valence compounds.
01/2012;
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ABSTRACT: We study the plasma-induced modifications of the potential and charge distribution across the interface of a plasma and a dielectric wall. For this purpose, the wall-bound surplus charge arising from the plasma is modeled as a quasistationary electron surface layer in thermal equilibrium with the wall. It satisfies Poisson's equation and minimizes the grand canonical potential of wall-thermalized excess electrons. Based on an effective model for a graded interface taking into account the image potential and the offset of the conduction band to the potential just outside the dielectric, we specifically calculate the modification of the potential and the distribution of the surplus electrons for MgO, SiO2, and Al2O3 surfaces in contact with a helium discharge. Depending on the electron affinity of the surface, we find two vastly different behaviors. For negative electron affinity, electrons do not penetrate into the wall and a quasi-two-dimensional electron gas is formed in the image potential, while, for positive electron affinity, electrons penetrate into the wall and a negative space-charge layer develops in the interior of the dielectric. We also investigate how the non-neutral electron surface layer—which can be understood as the ultimate boundary of a bounded gas discharge—merges with the neutral bulk of the dielectric.
Phys. Rev. B. 09/2011; 85(7).
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ABSTRACT: With an eye on plasma walls we investigate, within an effective model for the
two active electrons involved in the process, secondary electron emission due
to Auger de-excitation of metastable nitrogen $N_2(^3\sigma^+_u)$ molecules at
metallic surfaces. Modelling bound and unbound molecular states by a LCAO
approach and a two-center Coulomb wave, respectively, and the metallic states
by the eigenfunctions of a step potential we employ Keldysh Green's functions
to calculate the secondary electron emission coefficient $\gamma_e$ retaining
for the Auger self-energy the non-locality in time and the dependence on the
single particle quantum numbers. For the particular case of a tungsten surface
we find good agreement with experimental data indicating that the relevant
Auger physics is well captured by our easy-to-use model.
09/2011;
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ABSTRACT: Plasma walls accumulate electrons more efficiently than ions, leading to wall potentials which are negative with respect to the plasma potential. Theoretically, walls are usually treated as perfect absorber for electrons and ions, implying perfect sticking of the particles to the wall and infinitely long desorption times for particles stuck to the wall. For electrons, we question the perfect absorber model and calculate, specifically for a planar dielectric wall, the electron sticking coefficient s<sub>e</sub> and the electron desorption time τ<sub>e</sub>. For the uncharged wall, we find s<sub>e</sub> ≪ 1 and τ<sub>e</sub>≈ 10<sup>-4</sup> s. Thus, in the early stage of the build-up of the wall potential, when the wall is essentially uncharged, the wall is not a perfect absorber for electrons. For the charged wall, we find τ<sub>e</sub><sup>-1</sup>≈ 0. Thus, τ<sub>e</sub> approaches the perfect absorber value. But s<sub>e</sub> is still only of the order of 10<sup>-1</sup>. Calculating s<sub>e</sub> as a function of the wall potential and combining this expression with the quasi-stationary balance equations for the electron and ion surface densities, we find the self-consistent wall potential, including surface effects, to be 30% of the perfect absorber value.
IEEE Transactions on Plasma Science 03/2011; · 1.17 Impact Factor
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ABSTRACT: Based on the SO(2)-invariant slave-boson scheme, the static charge, orbital,
and excitonic susceptibilities in the extended Falicov-Kimball model are
calculated. Analyzing the phase without long-range order we find instabilities
towards charge order, orbital order, and the excitonic insulator (EI) phase.
The instability towards the EI is in agreement with the saddle-point phase
diagram. We also evaluate the dynamic excitonic susceptibility, which allows
the investigation of uncondensed excitons. We find qualitatively different
features of the exciton dispersion at the semimetal-EI and at the
semiconductor-EI transition supporting a crossover scenario between a BCS-type
electron-hole condensation and a Bose-Einstein condensation of preformed bound
electron-hole pairs.
02/2011;
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ABSTRACT: We study phonon-mediated adsorption and desorption of an electron at
dielectric surfaces with deep polarization-induced surface potentials where
multi-phonon transitions are responsible for electron energy relaxation.
Focusing on multi-phonon processes due to the nonlinearity of the coupling
between the external electron and the acoustic bulk phonon triggering the
transitions between surface states, we calculate electron desorption times for
graphite, MgO, CaO, (\text{Al}_2\text{O}_3), and (\text{SiO}_2) and electron
sticking coefficients for (\text{Al}_2\text{O}_3), CaO, and (\text{SiO}_2). To
reveal the kinetic stages of electron physisorption, we moreover study the time
evolution of the image state occupancy and the energy-resolved desorption flux.
Depending on the potential depth and the surface temperature we identify two
generic scenarios: (i)adsorption via trapping in shallow image states followed
by relaxation to the lowest image state and desorption from that state via a
cascade through the second strongly bound image state in not too deep
potentials and (ii)adsorption via trapping in shallow image states but followed
by a relaxation bottleneck retarding the transition to the lowest image state
and desorption from that state via a one step process to the continuum in deep
potentials.
02/2011;
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ABSTRACT: We study phonon-mediated temporary trapping of an electron in polarization-induced external surface states (image states) of a dielectric surface. Our approach is based on a quantum-kinetic equation for the occupancy of the image states. It allows us to distinguish between prompt and kinetic sticking. Because the depth of the image potential is much larger than the Debye energy multi-phonon processes are important. Taking two-phonon processes into account in cases where one-phonon processes yield a vanishing transition probability, as it is applicable, for instance, to graphite, we analyze the adsorption scenario as a function of potential depth and surface temperature and calculate prompt and kinetic sticking coefficients. We find rather small sticking coefficients, at most of the order of $10^{-3}$, and a significant suppression of the kinetic sticking coefficient due to a relaxation bottleneck inhibiting thermalization of the electron with the surface at short timescales. Comment: 10 pages, 7 figures
06/2010;
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ABSTRACT: A complete kinetic modeling of an ionized gas in contact with a surface requires the knowledge of the electron desorption time and the electron sticking coefficient. We calculate the desorption time for phonon-mediated desorption of an image-bound electron, as it occurs, for instance, on dielectric surfaces where desorption channels involving internal electronic degrees of freedom are closed. Because of the large depth of the polarization-induced surface potential with respect to the Debye energy multi-phonon processes are important. To obtain the desorption time, we use a quantum-kinetic rate equation for the occupancies of the bound surface states, taking two-phonon processes into account in cases where one-phonon processes yield a vanishing transition probability, as it is sufficient, for instance, for graphite. Besides producing an estimate for the desorption time of an electron image-bound to a graphite surface, we investigate the desorption scenario and show that desorption via cascades over bound states dominates unless direct one-phonon transitions from the lowest bound state to the continuum are possible. Comment: 20 pages, 8 figures
01/2010;
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ABSTRACT: We re-examine the three-dimensional spinless Falicov-Kimball model with dispersive $f$ electrons at half-filling, addressing the dispute about the formation of an excitonic condensate, which is closely related to the problem of electronic ferroelectricity. To this end, we work out a slave-boson functional integral representation of the suchlike extended Falicov-Kimball model that preserves the $SO(2)\otimes U(1)^{\otimes 2}$ invariance of the action. We find a spontaneous pairing of $c$ electrons with $f$ holes, building an excitonic insulator state at low temperatures, also for the case of initially non-degenerate orbitals. This is in contrast to recent predictions of scalar slave-boson mean-field theory but corroborates previous Hartree-Fock and RPA results. Our more precise treatment of correlation effects, however, leads to a substantial reduction of the critical temperature. The different behavior of the partial densities of states in the weak and strong inter-orbital Coulomb interaction regimes supports a BCS-BEC transition scenario. Comment: slightly revised version, 10 pages, 6 figures
12/2009;
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ABSTRACT: 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.
Physical Review Letters 11/2008; 101(17):175002. · 7.37 Impact Factor
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ABSTRACT: Motivated by the possibility of pressure-induced exciton condensation in intermediate-valence Tm[Se,Te] compounds we study the Falicov-Kimball model extended by a finite f-hole valence bandwidth. Calculating the Frenkel-type exciton propagator we obtain excitonic bound states above a characteristic value of the local interband Coulomb attraction. Depending on the system parameters coherence between c- and f-states may be established at low temperatures, leading to an excitonic insulator phase. We find strong evidence that the excitonic insulator typifies either a BCS condensate of electron-hole pairs (weak-coupling regime) or a Bose-Einstein condensate (BEC) of preformed excitons (strong-coupling regime), which points towards a BCS-BEC transition scenario as Coulomb correlations increase. Comment: 4 pages, 2 figures, completely revised version
07/2008;
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ABSTRACT: Several applications of PIC simulations for understanding basic physics phenomena in low-temperature plasmas are presented: capacitive radiofrequency discharges in Oxygen, dusty plasmas and negative ion sources for heating of fusion plasmas. The analysis of these systems based on their microscopic properties as accessible with PIC gives improved insight into their complex behavior. These studies are results of joint efforts over about one decade of research groups from Greifswald University, Germany; Bari University, Italy; Keio University, Japan and Innsbruck University, Austria. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Beiträge aus der Plasmaphysik 11/2007; 47(8‐9):595 - 634.
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ABSTRACT: We present results of 1d3v particle-in-cell Monte Carlo collisions simulations of a capacitive RF discharge in oxygen. Several direct comparisons between experiment and modelling are presented. The calculated ion energy distributions show good agreement with the experimentally measured ones for different discharge parameters. A plausible explanation of a double emissive layer near the powered electrode recently discovered in experiments is suggested. Heavy particle dissociative excitation collisions seem to be responsible for the formation of a second emissive layer close to the electrode. Introducing this process into the simulation a rather good agreement of the simulated axial emission profile with the experimentally observed one can be achieved. This delivers an estimate for the cross section of this collision.
Journal of Physics D Applied Physics 11/2007; 40(21):6601. · 2.54 Impact Factor
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ABSTRACT: We present an one-dimensional particle-in-cell Monte-Carlo model for capacitively coupled radio-frequency discharges in oxygen. The model quantitatively describes the central part of the discharge. For a given voltage and pressure, it self-consistently determines the electric potential and the distribution functions for electrons, negatively charged atomic oxygen, and positively charged molecular oxygen. Previously used collision cross sections are critically assessed and in some cases modified. Provided associative detachment due to metastable oxygen molecules is included in the model, the electro-negativities in the center of the discharge are in excellent agreement with experiments. Due to lack of empirical data for the cross section of this process, we propose a simple model and discuss its limitations.
06/2007;
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ABSTRACT: In this series of three papers we present results from a combined experimental and theoretical effort to quantitatively describe capacitively coupled radio-frequency discharges in oxygen. The particle-in-cell Monte-Carlo model on which the theoretical description is based will be described in the present 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. Comment: 11 pages, 10 figures
05/2007;
