Tunable sub-luminal propagation of narrowband x-ray pulses

Group velocity control is demonstrated for x-ray photons of 14.4 keV energy via a direct measurement of the temporal delay imposed on spectrally narrow x-ray pulses. Sub-luminal light propagation is achieved by inducing a steep positive linear dispersion in the optical response of ${}^{57}$Fe M\"ossbauer nuclei embedded in a thin film planar x-ray cavity. The direct detection of the temporal pulse delay is enabled by generating frequency-tunable spectrally narrow x-ray pulses from broadband pulsed synchrotron radiation. Our theoretical model is in good agreement with the experimental data.Comment: 8 pages, 4 figure

Closed orbit correction at synchrotrons for symmetric and near-symmetric lattices

This contribution compiles the benefits of lattice symmetry in the context of closed orbit correction. A symmetric arrangement of BPMs and correctors results in structured orbit response matrices of Circulant or block Circulant type. These forms of matrices provide favorable properties in terms of computational complexity, information compression and interpretation of mathematical vector spaces of BPMs and correctors. For broken symmetries, a nearest-Circulant approximation is introduced and the practical advantages of symmetry exploitation are demonstrated with the help of simulations and experiments in the context of FAIR synchrotrons

One Dimensional Oxygen Ordering in YBa2Cu3O(7-delta)

A model consisting of oxygen-occupied and -vacant chains is considered, with repulsive first and second nearest-neighbor interactions V1 and V2, respectively. The statistical mechanics and the diffraction spectrum of the model is solved exactly and analytically with the only assumption V1 >> V2. At temperatures T ~ V1 only a broad maximum at (1/2,0,0) is present, while for ABS(delta - 1/2) > 1/14 at low enough T, the peak splits into two. The simple expression for the diffraction intensity obtained for T << V1 represents in a more compact form previous results of Khachaturyan and Morris[1],extends them to all delta and T/V2 and leads to a good agreement with experiment. [1] A.G.Khachaturyan and J.W.Morris, Jr., Phys.Rev.Lett. 64,76(1990)Comment: 13 pages,Revtex,3 figures available upon request but can be plotted using simple analytical functions,CNEA-CAB 92/04

Unified description of ballistic and diffusive carrier transport in semiconductor structures

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 thermo-ballistic 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.Comment: 23 pages, 10 figures, REVTEX

Zur inneren Mechanik des Radialreifens; Teil 1. Geschichtete Kompositschale mit globalem Verschiebungsansatz für das Gesamtlaminat

Diskutiert werden Theorie und numerischer Lösungsalgorithmus für Probleme der Statik vorgespannter mehrschichtiger anisotroper Rotationsschalen mit Berücksichtigung der quergerichteten Schubdeformationen und der geometrischen Nichtlinearität. Die Schichten der Schale sollen aus linear elastischen anisotropen Materialien bestehen. Als Lösungsfunktionen werden die Verschiebungen der äußeren und der inneren Schalenoberflächen gewählt, was besonders für Problemstellungen der Reifenmechanik zweckmäßig ist. Untersucht wird die Berechnung eines Radialreifens, der durch Innendruck und zusätzlich durch eine über einen Teil der Außenfläche verteilte Normalkraft belastet ist. Es wird gezeigt, daß der lokale Verlauf der quergerichteten Spannungskomponenten über die Dicke des Schichtpakets, welche die hauptsächliche Ursache für die Zerstörung des Radialreifens sind, hinreichend genau mit dem für das Laminat als Ganzes aufgestellten numerischen Algorithmus beschrieben werden kann

A spectral element reduced basis method in parametric CFD

We consider the Navier-Stokes equations in a channel with varying Reynolds numbers. The model is discretized with high-order spectral element ansatz functions, resulting in 14,259 degrees of freedom. The steady-state snapshot solutions define a reduced order space, which allows to accurately evaluate the steady-state solutions for varying Reynolds number with a reduced order model within a fixed-point iteration. In particular, we compare different aspects of implementing the reduced order model with respect to the use of a spectral element discretization. It is shown, how a multilevel static condensation (Karniadakis and Sherwin, Spectral/hp element methods for computational fluid dynamics, 2nd edn. Oxford University Press, Oxford, 2005) in the pressure and velocity boundary degrees of freedom can be combined with a reduced order modelling approach to enhance computational times in parametric many-query scenarios

Interface states in junctions of two semiconductors with intersecting dispersion curves

A novel type of shallow interface state in junctions of two semiconductors without band inversion is identified within the envelope function approximation, using the two-band model. It occurs in abrupt junctions when the interband velocity matrix elements of the two semiconductors differ and the bulk dispersion curves intersect. The in-plane dispersion of the interface state is found to be confined to a finite range of momenta centered around the point of intersection. These states turn out to exist also in graded junctions, with essentially the same properties as in the abrupt case.Comment: 1 figur

Amide neighbouring-group effects in peptides: phenylalanine as relay amino acid in long-distance electron transfer

In nature, proteins serve as media for long‐distance electron transfer (ET) to carry out redox reactions in distant compartments. This ET occurs either by a single‐step superexchange or through a multi‐step charge hopping process, which uses side chains of amino acids as stepping stones. In this study we demonstrate that Phe can act as a relay amino acid for long‐distance electron hole transfer through peptides. The considerably increased susceptibility of the aromatic ring to oxidation is caused by the lone pairs of neighbouring amide carbonyl groups, which stabilise the Phe radical cation. This neighbouring‐amide‐group effect helps improve understanding of the mechanism of extracellular electron transfer through conductive protein filaments (pili) of anaerobic bacteria during mineral respiration