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Search Results: 1 - 10 of 117876 matches for " T. Gradl "
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Optimization of the Multigrid-Convergence Rate on Semi-structured Meshes by Local Fourier Analysis
B. Gmeiner,T. Gradl,F. Gaspar,U. Rüde
Computer Science , 2014, DOI: 10.1016/j.camwa.2012.12.006
Abstract: In this paper a local Fourier analysis for multigrid methods on tetrahedral grids is presented. Different smoothers for the discretization of the Laplace operator by linear finite elements on such grids are analyzed. A four-color smoother is presented as an efficient choice for regular tetrahedral grids, whereas line and plane relaxations are needed for poorly shaped tetrahedra. A novel partitioning of the Fourier space is proposed to analyze the four-color smoother. Numerical test calculations validate the theoretical predictions. A multigrid method is constructed in a block-wise form, by using different smoothers and different numbers of pre- and post-smoothing steps in each tetrahedron of the coarsest grid of the domain. Some numerical experiments are presented to illustrate the efficiency of this multigrid algorithm.
Hole spin dynamics and hole $g$ factor anisotropy in coupled quantum well systems
C. Gradl,M. Kempf,D. Schuh,D. Bougeard,R. Winkler,C. Schüller,T. Korn
Physics , 2014, DOI: 10.1103/PhysRevB.90.165439
Abstract: Due to its p-like character, the valence band in GaAs-based heterostructures offers rich and complex spin-dependent phenomena. One manifestation is the large anisotropy of Zeeman spin splitting. Using undoped, coupled quantum wells (QWs), we examine this anisotropy by comparing the hole spin dynamics for high- and low-symmetry crystallographic orientations of the QWs. We directly measure the hole $g$ factor via time-resolved Kerr rotation, and for the low-symmetry crystallographic orientations (110) and (113a), we observe a large in-plane anisotropy of the hole $g$ factor, in good agreement with our theoretical calculations. Using resonant spin amplification, we also observe an anisotropy of the hole spin dephasing in the (110)-grown structure, indicating that crystal symmetry may be used to control hole spin dynamics.
Charmless B Decays
Wolfgang Gradl,for the BABAR Collaboration
Physics , 2007,
Abstract: We report several new and updated results from Belle and BABAR for rare hadronic B decays. These include many new results for B decays to eta'X and omegaX and new measurements of the longitudinal polarisation in the decays B -> rho K*.
Probing QCD with rare charmless B decays
Wolfgang Gradl,for the BABAR Collaboration
Physics , 2006,
Abstract: Rare charmless hadronic B decays are a good testing ground for QCD. In this paper we describe a selection of new measurements made by the BABAR and BELLE collaborations.
Decoherence-assisted initialization of a resident hole spin polarization in a two-dimensional hole gas
M. Kugler,K. Korzekwa,P. Machnikowski,C. Gradl,S. Furthmeier,M. Griesbeck,M. Hirmer,D. Schuh,W. Wegscheider,T. Kuhn,C. Schüller,T. Korn
Physics , 2011, DOI: 10.1103/PhysRevB.84.085327
Abstract: We investigate spin dynamics of resident holes in a p-modulation-doped GaAs/Al$_{0.3}$Ga$_{0.7}$As single quantum well. Time-resolved Faraday and Kerr rotation, as well as resonant spin amplification, are utilized in our study. We observe that nonresonant or high power optical pumping leads to a resident hole spin polarization with opposite sign with respect to the optically oriented carriers, while low power resonant optical pumping only leads to a resident hole spin polarization if a sufficient in-plane magnetic field is applied. The competition between two different processes of spin orientation strongly modifies the shape of resonant spin amplification traces. Calculations of the spin dynamics in the electron--hole system are in good agreement with the experimental Kerr rotation and resonant spin amplification traces and allow us to determine the hole spin polarization within the sample after optical orientation, as well as to extract quantitative information about spin dephasing processes at various stages of the evolution.
Spin dynamics in p-doped semiconductor nanostructures subject to a magnetic field tilted from the Voigt geometry
K. Korzekwa,C. Gradl,M. Kugler,S. Furthmeier,M. Griesbeck,M. Hirmer,D. Schuh,W. Wegscheider,T. Kuhn,C. Schüller,T. Korn,P. Machnikowski
Physics , 2013, DOI: 10.1103/PhysRevB.88.155303
Abstract: We develop a theoretical description of the spin dynamics of resident holes in a p-doped semiconductor quantum well (QW) subject to a magnetic field tilted from the Voigt geometry. We find the expressions for the signals measured in time-resolved Faraday rotation (TRFR) and resonant spin amplification (RSA) experiments and study their behavior for a range of system parameters. We find that an inversion of the RSA peaks can occur for long hole spin dephasing times and tilted magnetic fields. We verify the validity of our theoretical findings by performing a series of TRFR and RSA experiments on a p-modulation doped GaAs/Al_{0.3}Ga_{0.7}As single QW and showing that our model can reproduce experimentally observed signals.
Studies of aging and HV break down problems during development and operation of MSGC and GEM detectors for the Inner Tracking System of HERA-B
HERA-B Inner Tracker Collaboration,:,Y. Bagaturia,O. Baruth,H. B. Dreis,F. Eisele,I. Gorbunov,S. Gradl,W. Gradl,S. Hausmann,M. Hildebrandt,T. Hott,S. Keller,C. Krauss,B. Lomonosov,M. Negodaev,C. Richter,P. Robmann,B. Schmidt,U. Straumann,P. Truoel,S. Visbeck,T. Walter,C. Werner,U. Werthenbach,G. Zech,T. Zeuner,M. Ziegler
Physics , 2002, DOI: 10.1016/S0168-9002(02)01002-1
Abstract: The results of five years of development of the inner tracking system of the HERA-B experiment and first experience from the data taking period of the year 2000 are reported. The system contains 184 chambers, covering a sensitive area of about 20 * 20 cm2 each. The detector is based on microstrip gas counters (MSGCs) with diamond like coated (DLC) glass wafers and gas electron multipliers (GEMs). The main problems in the development phase were gas discharges in intense hadron beams and aging in a high radiation dose environment. The observation of gas discharges which damage the electrode structure of the MSGC led to the addition of the GEM as a first amplification step. Spurious sparking at the GEM cannot be avoided completely. It does not affect the GEM itself but can produce secondary damage of the MSGC if the electric field between the GEM and the MSGC is above a threshold depending on operation conditions. We observed that aging does not only depend on the dose but also on the spot size of the irradiated area. Ar-DME mixtures had to be abandoned whereas a mixture of 70% Ar and 30% CO2 showed no serious aging effects up to about 40 mC/cm deposited charge on the anodes. X-ray measurements indicate that the DLC of the MSGC is deteriorated by the gas amplification process. As a consequence, long term gain variations are expected. The Inner Tracker has successfully participated in the data taking at HERA-B during summer 2000.
Systems of PDEs obtained from factorization in loop groups
J. Dorfmeister,H. Gradl,J. Szmigielski
Physics , 1998,
Abstract: We propose a generalization of a Drinfeld-Sokolov scheme of attaching integrable systems of PDEs to affine Kac-Moody algebras. With every affine Kac-Moody algebra $\gg$ and a parabolic subalgebra $\gp$, we associate two hierarchies of PDEs. One, called positive, is a generalization of the KdV hierarchy, the other, called negative, generalizes the Toda hierarchy. We prove a coordinatization theorem, which establishes that the number of functions needed to express all PDEs of the the total hierarchy equals the rank of $\gg$. The choice of functions, however, is shown to depend in a noncanonical way on $\gp$. We employ a version of the Birkhoff decomposition and a ``2-loop'' formulation which allows us to incorporate geometrically meaningful solutions to those hierarchies. We illustrate our formalism for positive hierarchies with a generalization of the Boussinesq system and for the negative hierarchies with the stationary Bogoyavlenskii equation.
Issues and Opportunities in Exotic Hadrons
R. A. Briceno,T. D. Cohen,S. Coito,J. J. Dudek,E. Eichten,C. S. Fischer,M. Fritsch,W. Gradl,A. Jackura,M. Kornicer,G. Krein,R. F. Lebed,F. A. Machado,R. E. Mitchell,C. J. Morningstar,M. Peardon,M. R. Pennington,K. Peters,J. -M. Richard,C. -P. Shen,M. R. Shepherd,T. Skwarnicki,E. S. Swanson,A. P. Szczepaniak,C. -Z. Yuan
Physics , 2015,
Abstract: The last few years have been witness to a proliferation of new results concerning heavy exotic hadrons. Experimentally, many new signals have been discovered that could be pointing towards the existence of tetraquarks, pentaquarks, and other exotic configurations of quarks and gluons. Theoretically, advances in lattice field theory techniques place us at the cusp of understanding complex coupled-channel phenomena, modelling grows more sophisticated, and effective field theories are being applied to an ever greater range of situations. It is thus an opportune time to evaluate the status of the field. In the following, a series of high priority experimental and theoretical issues concerning heavy exotic hadrons is presented.
Cold-inducible RNA binding protein (CIRP), a novel XTcf-3 specific target gene regulates neural development in Xenopus
Stephanie van Venrooy, Dagmar Fichtner, Martin Kunz, Doris Wedlich, Dietmar Gradl
BMC Developmental Biology , 2008, DOI: 10.1186/1471-213x-8-77
Abstract: We identified cold-inducible RNA binding protein (CIRP) as novel XTcf-3 specific target gene. Furthermore, we show that knockdown of XTcf-3 by injection of an antisense morpholino oligonucleotide results in a general broadening of the anterior neural tissue. Depletion of XCIRP by antisense morpholino oligonucleotide injection leads to a reduced stability of mRNA and an enlargement of the anterior neural plate similar to the depletion of XTcf-3.Distinct steps in neural development are differentially regulated by individual Lef/Tcfs. For proper development of the anterior brain XTcf-3 and the Tcf-subtype specific target XCIRP appear indispensable. Thus, regulation of anterior neural development, at least in part, depends on mRNA stabilization by the novel XTcf-3 target gene XCIRP.The most prominent nuclear transducer of wnt/β-catenin signaling in the nucleus belong to the Lef/Tcf family of sequence specific HMG-box transcription factors. Higher vertebrates express four distinct members of this family, Tcf-1 (or Tcf-7), Tcf-3, Tcf-4 (or Tcf-7 like 2) and Lef-1 [1,2]. In Xenopus, XTcf-1, XTcf-3 and XTcf-4 are maternally provided, while the expression of XLef-1 starts only after mid-blastula transition [3]. The expression of XTcf-1, XTcf-3 and XLef-1 is widespread and overlapping in many tissues, with highest levels in the head region including brain, head mesoderm and branchial arches [3,4]. Zygotic expression of XTcf-4 is restricted in neurula and tailbud stages to the anterior midbrain [5] and in tadpoles to the entire midbrain [6].A partial functional redundancy of Lef/Tcf family members has been shown in mice, where the Tcf-1/Lef-1 and Tcf-1/Tcf-4 double knock-out mice revealed more than additive effects compared to the corresponding single knock-out mice [7,8]. In Xenopus, however, the individual Lef/Tcfs appear to have more non-redundant roles. Although XTcf-1, XTcf-3 and XTcf-4 are maternally provided, it has been shown by rescue experiments of Lef/Tcf depleted e
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