Browse ORBi

- What it is and what it isn't
- Green Road / Gold Road?
- Ready to Publish. Now What?
- How can I support the OA movement?
- Where can I learn more?

ORBi

Pseudopath semiclassical approximation to transport through open quantum billiards: Dyson equation for diffractive scattering ; ; et al in PHYSICAL REVIEW E (2005), 72(3), We present a semiclassical theory for transport through open billiards of arbitrary convex shape that includes diffractively scattered paths at the lead openings. Starting from a Dyson equation for the ... [more ▼] We present a semiclassical theory for transport through open billiards of arbitrary convex shape that includes diffractively scattered paths at the lead openings. Starting from a Dyson equation for the semiclassical Green's function we develop a diagrammatic expansion that allows a systematic summation over classical and pseudopaths, the latter consisting of classical paths joined by diffractive scatterings ("kinks"). This renders the inclusion of an exponentially proliferating number of pseudopath combinations numerically tractable for both regular and chaotic billiards. For a circular billiard and the Bunimovich stadium the path sum leads to a good agreement with the quantum path length power spectrum up to long path length. Furthermore, we find excellent numerical agreement with experimental studies of quantum scattering in microwave billiards where pseudopaths provide a significant contribution. [less ▲] Detailed reference viewed: 59 (0 UL)Multi-electron dynamics for neutralization of highly charged ions near surfaces ; Wirtz, Ludger ; et al in Vacuum (2004), 73(1), 3-7 We present a simulation of the neutralization of highly charged ions in front of a lithium fluoride surface including the close-collision regime above the surface. The present approach employs a Monte ... [more ▼] We present a simulation of the neutralization of highly charged ions in front of a lithium fluoride surface including the close-collision regime above the surface. The present approach employs a Monte-Carlo solution of the Liouville master equation for the joint probability density of the ionic motion and the electronic population of the projectile and the target surface. It includes single as well as double particle-hole (de)excitation processes and incorporates electron correlation effects through the conditional dynamics of population strings. For slow projectiles and normal incidence, the ionic motion depends sensitively on the interplay between image acceleration towards the surface and repulsion by an ensemble of positive hole charges in the surface ("trampoline effect"). For Ne10+ we find that image acceleration is dominant and no collective backscattering high above the surface takes place. (C) 2004 Elsevier Ltd. All rights reserved. [less ▲] Detailed reference viewed: 42 (0 UL)Potential-energy surfaces for charge exchange between singly charged ions and a LiF surface Wirtz, Ludger ; ; et al in Physical Review. A (2003), 68(3), We analyze the adiabatic potential-energy surfaces relevant for neutralization of singly charged ions in slow vertical incidence onto a lithium fluoride surface. The surface is represented by a cluster of ... [more ▼] We analyze the adiabatic potential-energy surfaces relevant for neutralization of singly charged ions in slow vertical incidence onto a lithium fluoride surface. The surface is represented by a cluster of varying size augmented by point charges of alternating sign in order to include the proper Madelung potential of the ionic crystal. Our calculation proceeds on the multiconfiguration self-consistent-field and multireference configuration-interaction levels. Size-consistency corrections based on the Davidson correction and multireference averaged quadratic coupled cluster methods are included as well. We emphasize the importance of a proper treatment of electron correlation signifying the polarization of the surrounding cluster environment in ab initio calculations of charge transfer at surfaces. From the topology of the surfaces, in particular the existence or absence of avoided crossings (or, more generally, conical intersections), qualitative predictions for the neutralization process can be made. The comparative analysis of potential curves for H+, C+, S+, and Ne+ projectiles provides an explanation for the recently observed threshold behavior for potential sputtering. [less ▲] Detailed reference viewed: 44 (0 UL)Semiclassical theory for transmission through open billiards: Convergence towards quantum transport Wirtz, Ludger ; ; et al in PHYSICAL REVIEW E (2003), 67(1), We present a semiclassical theory for transmission through open quantum billiards which converges towards quantum transport. The transmission amplitude can be expressed as a sum over all classical paths ... [more ▼] We present a semiclassical theory for transmission through open quantum billiards which converges towards quantum transport. The transmission amplitude can be expressed as a sum over all classical paths and pseudopaths which consist of classical path segments joined by "kinks," i.e., diffractive scattering at lead mouths. For a rectangular billiard we show numerically that the sum over all such paths with a given number of kinks K converges to the quantum transmission amplitude as K-->infinity. Unitarity of the semiclassical theory is restored as K approaches infinity. Moreover, we find excellent agreement with the quantum path-length power spectrum up to very long path length. [less ▲] Detailed reference viewed: 32 (0 UL)Angular distribution of highly charged ions transmitted through metallic microcapillaries ; Wirtz, Ludger ; et al in Journal of Electron Spectroscopy & Related Phenomena (2003), 129(2-3), 195-200 The angular distribution of Xe6+ ions with an energy of 800 eV/q transmitted through Ni microcapillaries is studied by a classical trajectory simulation. The results for clean and contaminated by layers ... [more ▼] The angular distribution of Xe6+ ions with an energy of 800 eV/q transmitted through Ni microcapillaries is studied by a classical trajectory simulation. The results for clean and contaminated by layers of insulating materials of the internal surface of capillary are presented. We show that the angular distributions can be used to identify different phases of hollow ion formation for clean metal surface. We also show that the angular distribution changes dramatically for the case of contaminated surface, i.e. when the image acceleration is depressed. (C) 2003 Elsevier Science B.V. All rights reserved. [less ▲] Detailed reference viewed: 40 (0 UL)Liouville master equation for multielectron dynamics: Neutralization of highly charged ions near a LiF surface Wirtz, Ludger ; ; et al in Physical Review. A (2003), 67(1), We present a simulation of the neutralization of highly charged ions in front of a lithium fluoride surface including the close-collision regime above the surface. The present approach employs a Monte ... [more ▼] We present a simulation of the neutralization of highly charged ions in front of a lithium fluoride surface including the close-collision regime above the surface. The present approach employs a Monte Carlo solution of the Liouville master equation for the joint probability density of the ionic motion and the electronic population of the projectile and the target surface. It includes single as well as double particle-hole (de)excitation processes and incorporates electron correlation effects through the conditional dynamics of population strings. The input in terms of elementary one- and two-electron transfer rates is determined from classical trajectory Monte Carlo calculations as well as quantum-mechanical Auger calculations. For slow projectiles and normal incidence, the ionic motion depends sensitively on the interplay between image acceleration towards the surface and repulsion by an ensemble of positive hole charges in the surface ("trampoline effect"). For Ne10+ we find that image acceleration is dominant and no collective backscattering high above the surface takes place. For grazing incidence, our simulation delineates the pathways to complete neutralization. In accordance with recent experimental observations, most ions are reflected as neutral or even as singly charged negative particles, irrespective of the charge state of the incoming ions. [less ▲] Detailed reference viewed: 90 (0 UL)Hollow-ion formation in microcapillaries ; Wirtz, Ludger ; et al in Physical Review. A (2001), 64(4), Transmission of highly charged ions through microcapillaries is studied theoretically by a classical trajectory simulation. The interaction of highly charged ions with the internal surface of the ... [more ▼] Transmission of highly charged ions through microcapillaries is studied theoretically by a classical trajectory simulation. The interaction of highly charged ions with the internal surface of the capillary is treated within the framework of dielectric-response theory. The simulation is based on the classical over-the-barrier model modified for open cylindrical surfaces. The multielectron evolution and relaxation is taken into account as a stochastic event sequence. We consider N6+ and Ne10+ with an energy of 2.1 keV/amu passing through a metallic microcapillary of Ni. We analyze the distance of closest approach. the angular distribution, and the distribution of the mean occupation numbers of n shells of highly charged ions. We find the resulting charge state distribution of transmitted projectiles in good agreement with recent measurements. Implications for nanotube targets will be discussed. [less ▲] Detailed reference viewed: 51 (0 UL)Kinetically assisted potential sputtering of insulators by highly charged ions ; ; et al in Physical Review Letters (2001), 86(16), 3530-3533 A new form of potential sputtering has been found for impact of slow (less than or equal to 1500 eV) multiply charged Xe ions (charge states up to q = 25) on MgOx. In contrast to alkali-halide or SiO2 ... [more ▼] A new form of potential sputtering has been found for impact of slow (less than or equal to 1500 eV) multiply charged Xe ions (charge states up to q = 25) on MgOx. In contrast to alkali-halide or SiO2 surfaces this mechanism requires the simultaneous presence of electronic excitation of the target material and of a kinetically formed collision cascade within the target in order to initiate the sputtering process. This kinetically assisted potential sputtering mechanism has been identified to be present for ether insulating surfaces as well. [less ▲] Detailed reference viewed: 52 (0 UL)Vertical incidence of slow Ne10+ ions on an LiF surface: Suppression of the trampoline effect Wirtz, Ludger ; ; et al in Nuclear Instruments & Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms (2001), 182 We present a Monte Carlo simulation of the neutralization of a slow Ne10+ ion in vertical incidence on an LiF(100) surface. The rates for resonant electron transfer between surface F- ions and the ... [more ▼] We present a Monte Carlo simulation of the neutralization of a slow Ne10+ ion in vertical incidence on an LiF(100) surface. The rates for resonant electron transfer between surface F- ions and the projectile are calculated using a classical trajectory Monte Carlo simulation, We investigate the influence of the hole mobility on the neutralization sequence. It is shown that backscattering above the surface due to the local positive charge up of the surface ("trampoline effect") does not take place. (C) 2001 Elsevier Science B.V. All rights reserved. [less ▲] Detailed reference viewed: 52 (0 UL)Scattering of highly charged ions at microcapillaries ; Wirtz, Ludger ; et al Scientific Conference (2001) Transmission of highly charged ions through microcapillaries is studied theoretically by a classical trajectory simulation based on the classical-over-the-barrier model. The interaction of highly charged ... [more ▼] Transmission of highly charged ions through microcapillaries is studied theoretically by a classical trajectory simulation based on the classical-over-the-barrier model. The interaction of highly charged ions with the internal surface of the capillary is treated within the framework of dielectric response theory. We find the resulting charge state distribution of transmitted projectiles in good agreement with measurements. We analyze the angular distributions and the distribution of the mean occupation numbers of n shells of highly charged ions. We also present theoretical results of the energy loss of the highly charged ions undergoing distant collisions at grazing incidence angles with the internal surface of the microcapillary target. [less ▲] Detailed reference viewed: 33 (0 UL)Observation of a threshold in potential sputtering of LiF surfaces ; ; Wirtz, Ludger et al in Nuclear Instruments & Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms (2000), 164 A quartz-crystal microbalance technique is used for measuring total sputtering yields for LiF under impact of slow (20 eV, 100 eV, 500 eV and 1000 eV kinetic energy) singly and doubly charged ions. At low ... [more ▼] A quartz-crystal microbalance technique is used for measuring total sputtering yields for LiF under impact of slow (20 eV, 100 eV, 500 eV and 1000 eV kinetic energy) singly and doubly charged ions. At low kinetic energies (less than or equal to 100 eV) potential sputtering (PS) (i.e., sputtering due to the projectiles potential energy) clearly dominates over kinetically induced sputtering. New insight into the mechanisms for PS is gained by determining the minimum potential energy necessary to induce PS. The measured potential energy threshold at around 10 eV provides evidence that PS can already be induced by the production of cold holes in the valence band of LiF via resonant neutralisation. (C) 2000 Elsevier Science B.V. All rights reserved. [less ▲] Detailed reference viewed: 56 (0 UL)Modular recursive Green's function method for ballistic quantum transport ; ; Wirtz, Ludger et al in Physical Review B (2000), 62(3), 1950-1960 A modification of the standard recursive Green's function method for quantum transport through microstructures is presented which is based on the decomposition into separable substructures. The Green's ... [more ▼] A modification of the standard recursive Green's function method for quantum transport through microstructures is presented which is based on the decomposition into separable substructures. The Green's functions for these modules are joined by discretized Dyson equations. Nonseparable structures can thereby be calculated with the help of a few recursions with high accuracy. We apply this method to the calculation of ballistic quantum transport through a circular and stadium-shaped quantum dot for high mode numbers to test semi-classical predictions in detail. Among other results we find the breakdown of the semiclassical approximation for long path lengths which is due to the spreading of wave packets in the cavity. [less ▲] Detailed reference viewed: 69 (0 UL)Charge-state evolution of highly charged ions transmitted through microcapillaries ; Wirtz, Ludger ; et al in Physical Review. A (2000), 61(2), The charge-state evolution of highly charged ions transmitted through microcapillaries is studied theoretically by a classical trajectory Monte Carlo simulation.: The interaction of highly charged ions ... [more ▼] The charge-state evolution of highly charged ions transmitted through microcapillaries is studied theoretically by a classical trajectory Monte Carlo simulation.: The interaction of highly charged ions with the internal surface of the capillary is treated within the framework of dielectric response theory. We analyze the distance of closest approach and the angular distributions of the highly charged ions at the exit of the microcapillary. We find the charge-state fraction of transmitted N6+ projectiles, in: good agreement with first measurements. Moreover, our calculations indicate that grazing collisions with the microcapillary surface hold the promise of direct observation of charge transfer and hollow-atom formation at a large distance from the surface. [less ▲] Detailed reference viewed: 64 (8 UL)Curve-crossing analysis for potential sputtering of insulators Wirtz, Ludger ; ; et al in Surface Science (2000), 451(1-3), 197-202 We develop a theoretical model for the recently observed threshold for potential sputtering of LiF by slow singly and doubly charged ions. The threshold coincides with the potential energy to create a ... [more ▼] We develop a theoretical model for the recently observed threshold for potential sputtering of LiF by slow singly and doubly charged ions. The threshold coincides with the potential energy to create a cold hole in the valence band of LiF by resonant neutralization. We calculate the level shift of the incident ion and the deformation of the valence band under the influence of the projectile. Resonant neutralization becomes possible for ions with recombination energies larger than 10 eV in agreement with the experimental findings. (C) 2000 Elsevier Science B.V. All rights reserved. [less ▲] Detailed reference viewed: 46 (0 UL)Interaction of highly charged ions with microcapillaries ; Wirtz, Ludger ; in PHYSICA SCRIPTA (1999), T80B The transmission of highly charged ions (HCI) through microcapillaries is investigated. The interaction of an HCI with the internal surface of the capillary is described in terms of a classical image ... [more ▼] The transmission of highly charged ions (HCI) through microcapillaries is investigated. The interaction of an HCI with the internal surface of the capillary is described in terms of a classical image potential. For Neq+ projectiles transmitted through a Ni capillary the impact parameter distribution and angular distribution of ions leaving the microcapillary is determined. We consider both a narrowly focussed and a dispersive incident beam relative to the acceptance angle of the capillary. Our results are compared with recent experiments. [less ▲] Detailed reference viewed: 55 (5 UL)Interaction of highly charged ions with microcapillaries ; Wirtz, Ludger ; in Nuclear Instruments & Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms (1999), 154(1-4), 307-311 We analyze the transmission of highly charged ions (HCI) through microcapillaries. The interaction of an HCI with the interval surface of the capillary is treated within the framework of linear response ... [more ▼] We analyze the transmission of highly charged ions (HCI) through microcapillaries. The interaction of an HCI with the interval surface of the capillary is treated within the framework of linear response. In the limiting case of an ideal conducting cylindrical surface, the interaction reduces to a classical image potential. For Neq+ projectiles transmitting through a Ni capillary calculations for the impact parameter distribution and angular distribution of hollow atoms are presented. We find the fraction of transmitted projectiles which are partially neutralized in good agreement with recent experiments. (C) 1999 Elsevier Science B.V. All rights reserved. [less ▲] Detailed reference viewed: 53 (0 UL)Gauge-invariant theory for semiclassical magnetotransport through ballistic microstructures Wirtz, Ludger ; ; in Physical Review B (1999), 59(4), 2956-2967 Within the semiclassical theory of magnetotransport through ballistic cavities, fluctuations in the transmission amplitude and in the conductance originate from the Aharonov-Bohm phase of directed areas ... [more ▼] Within the semiclassical theory of magnetotransport through ballistic cavities, fluctuations in the transmission amplitude and in the conductance originate from the Aharonov-Bohm phase of directed areas. We formulate the semiclassical transmission amplitude in gauge-invariant form. The gauge invariant phases can be visualized in terms of areas enclosed by classical paths, which consist of the real path connecting the entrance point to the exit point and a virtual path leading back to the entrance point. We implement this method on different levels of a semiclassical description of magnetotransport with applications to magnetoconductance fluctuations and correlations. The validity of the semiclassical theories is analyzed. [S0163-1829(99)10903-2]. [less ▲] Detailed reference viewed: 52 (0 UL)Threshold for potential sputtering of LiF ; ; et al in Physical Review Letters (1999), 83(19), 3948-3951 We have measured total sputtering yields for impact of slow (less than or equal to 100 eV) singly and doubly charged ions on LiF. The minimum potential energy necessary to induce potential sputtering (PS ... [more ▼] We have measured total sputtering yields for impact of slow (less than or equal to 100 eV) singly and doubly charged ions on LiF. The minimum potential energy necessary to induce potential sputtering (PS) from UF was determined to be about 10 eV. This threshold coincides with the energy necessary to produce a cold hole in the valence band of LiF by resonant neutralization. This allows the first unambiguous identification of PS induced by cold holes. Further stepwise increase of the sputtering yield with higher projectile potential energies provides evidence for additional defect-mediated sputtering mechanisms operative in alkali halides. [less ▲] Detailed reference viewed: 39 (0 UL)Oscillations in the magnetoconductance autocorrelation function for ballistic microstructures ; Wirtz, Ludger ; et al in Physical Review. B : Condensed Matter (1998), 57(16), 9875-9878 We present a comparison between experiment and theory for the magnetoconductance autocorrelation function C(Delta B) for transport through a stadium-shaped ballistic microstructure. The correlation ... [more ▼] We present a comparison between experiment and theory for the magnetoconductance autocorrelation function C(Delta B) for transport through a stadium-shaped ballistic microstructure. The correlation function displays damped oscillations which can be traced to the quantum interference between bundles of short trajectories. We present two different semiclassical calculations applicable for large and small mode numbers of the quantum wire, respectively. Good agreement is found with experimental data taken at relatively low mode numbers. [S0163-1829(98)03316-5]. [less ▲] Detailed reference viewed: 59 (0 UL)Geometry-dependent scattering through ballistic microstructures: Semiclassical theory beyond the stationary-phase approximation Wirtz, Ludger ; ; in Physical Review B (1997), 56(12), 7589-7597 The conductance of a ballistic microstructure shows strong fluctuations as a function of the Fermi wave number. We present a semiclassical-theory that describes these fluctuations in terms of bundles of ... [more ▼] The conductance of a ballistic microstructure shows strong fluctuations as a function of the Fermi wave number. We present a semiclassical-theory that describes these fluctuations in terms of bundles of short trajectories. These trajectories provide the dominant contribution to electron transport through a weakly open microstructure. For the coupling between the quantum wires and the cavity, contributions beyond the stationary phase approximation are taken into account giving rise to diffraction effects. A comparison with full quantum calculations for st rectangular billiard is made. The peak positions of the power spectrum agree very well between the quantum and semiclassical theories. Numerical evidence is found for the breakdown of the semiclassical approximation for long paths. A simple explanation in terms of the dispersion of the semiclassical wave packet in the interior of the cavity is proposed. [less ▲] Detailed reference viewed: 71 (2 UL) |
||