372 research outputs found
Creep, Relaxation and Viscosity Properties for Basic Fractional Models in Rheology
The purpose of this paper is twofold: from one side we provide a general
survey to the viscoelastic models constructed via fractional calculus and from
the other side we intend to analyze the basic fractional models as far as their
creep, relaxation and viscosity properties are considered. The basic models are
those that generalize via derivatives of fractional order the classical
mechanical models characterized by two, three and four parameters, that we
refer to as Kelvin-Voigt, Maxwell, Zener, anti-Zener and Burgers. For each
fractional model we provide plots of the creep compliance, relaxation modulus
and effective viscosity in non dimensional form in terms of a suitable time
scale for different values of the order of fractional derivative. We also
discuss the role of the order of fractional derivative in modifying the
properties of the classical models.Comment: 41 pages, 8 figure
Auxiliary particle theory of threshold singularities in photoemission and X-ray absorption spectra: Test of a conserving T-matrix approximation
We calculate the exponents of the threshold singularities in the
photoemission spectrum of a deep core hole and its X-ray absorption spectrum in
the framework of a systematic many-body theory of slave bosons and
pseudofermions (for the empty and occupied core level). In this representation,
photoemission and X-ray absorption can be understood on the same footing; no
distinction between orthogonality catastrophe and excitonic effects is
necessary. We apply the conserving slave particle T-matrix approximation
(CTMA), recently developed to describe both Fermi and non-Fermi liquid behavior
systems with strong local correlations, to the X-ray problem as a test case.
The numerical results for both photoemission and X-ray absorption are found to
be in agreement with the exact infrared powerlaw behavior in the weak as well
as in the strong coupling regions. We point out a close relation of the CTMA
with the parquet equation approach of Nozi{\`e}res et al.Comment: 10 pages, 9 figures, published versio
The excitation spectrum of mesoscopic proximity structures
We investigate one aspect of the proximity effect, viz., the local density of
states of a superconductor-normal metal sandwich. In contrast to earlier work,
we allow for the presence of an arbitrary concentration of impurities in the
structure. The superconductor induces a gap in the normal metal spectrum that
is proportional to the inverse of the elastic mean free path l_N for rather
clean systems. For a mean free path much shorter than the thickness of the
normal metal, we find a gap size proportional to l_N that approaches the
behavior predicted by the Usadel equation (diffusive limit). We also discuss
the influence of interface and surface roughness, the consequences of a
non-ideal transmittivity of the interface, and the dependence of our results on
the choice of the model of impurity scattering.Comment: 7 pages, 8 figures (included), submitted to PR
Digital Quantum Simulation with Rydberg Atoms
We discuss in detail the implementation of an open-system quantum simulator
with Rydberg states of neutral atoms held in an optical lattice. Our scheme
allows one to realize both coherent as well as dissipative dynamics of complex
spin models involving many-body interactions and constraints. The central
building block of the simulation scheme is constituted by a mesoscopic Rydberg
gate that permits the entanglement of several atoms in an efficient, robust and
quick protocol. In addition, optical pumping on ancillary atoms provides the
dissipative ingredient for engineering the coupling between the system and a
tailored environment. As an illustration, we discuss how the simulator enables
the simulation of coherent evolution of quantum spin models such as the
two-dimensional Heisenberg model and Kitaev's toric code, which involves
four-body spin interactions. We moreover show that in principle also the
simulation of lattice fermions can be achieved. As an example for controlled
dissipative dynamics, we discuss ground state cooling of frustration-free spin
Hamiltonians.Comment: submitted to special issue "Quantum Information with Neutral
Particles" of "Quantum Information Processing
Questioning Glutamate Excitotoxicity in Acute Brain Damage: The Importance of Spreading Depolarization
Background: Within 2 min of severe ischemia, spreading depolarization (SD) propagates like a wave through compromised gray matter of the higher brain. More SDs arise over hours in adjacent tissue, expanding the neuronal damage. This period represents a therapeutic window to inhibit SD and so reduce impending tissue injury. Yet most neuroscientists assume that the course of early brain injury can be explained by glutamate excitotoxicity, the concept that immediate glutamate release promotes early and downstream brain injury. There are many problems with glutamate release being the unseen culprit, the most practical being that the concept has yielded zero therapeutics over the past 30 years. But the basic science is also flawed, arising from dubious foundational observations beginning in the 1950s Methods: Literature pertaining to excitotoxicity and to SD over the past 60 years is critiqued. Results: Excitotoxicity theory centers on the immediate and excessive release of glutamate with resulting neuronal hyperexcitation. This instigates poststroke cascades with subsequent secondary neuronal injury. By contrast, SD theory argues that although SD evokes some brief glutamate release, acute neuronal damage and the subsequent cascade of injury to neurons are elicited by the metabolic stress of SD, not by excessive glutamate release. The challenge we present here is to find new clinical targets based on more informed basic science. This is motivated by the continuing failure by neuroscientists and by industry to develop drugs that can reduce brain injury following ischemic stroke, traumatic brain injury, or sudden cardiac arrest. One important step is to recognize that SD plays a central role in promoting early neuronal damage. We argue that uncovering the molecular biology of SD initiation and propagation is essential because ischemic neurons are usually not acutely injured unless SD propagates through them. The role of glutamate excitotoxicity theory and how it has shaped SD research is then addressed, followed by a critique of its fading relevance to the study of brain injury. Conclusions: Spreading depolarizations better account for the acute neuronal injury arising from brain ischemia than does the early and excessive release of glutamate.Grants to RDA from the Canadian Heart & Stroke Foundation, National Science Engineering and Research Council and the New Frontiers in Research
Fund, to E.F from the National Research, Development and Innovation Office of Hungary, grant no. K134377; and the EU’s Horizon 2020 research and
innovation program under grant agreement No. 739593, and to JPD from the DFG (German research Council) (DFG DR323/5-1,DFG DR 323/10-1) BMBF Bundesministerium fuer Bildung und Forschung (Era-Net Neuron EBio2, with funds from BMBF 01EW2004)
The Critical Role of Spreading Depolarizations in Early Brain Injury: Consensus and Contention
Background: When a patient arrives in the emergency department following a stroke, a traumatic brain injury, or sudden cardiac arrest, there is no therapeutic drug available to help protect their jeopardized neurons. One crucial reason is that we have not identified the molecular mechanisms leading to electrical failure, neuronal swelling, and blood vessel constriction in newly injured gray matter. All three result from a process termed spreading depolarization (SD). Because we only partially understand SD, we lack molecular targets and biomarkers to help neurons survive after losing their blood flow and then undergoing recurrent SD. Methods: In this review, we introduce SD as a single or recurring event, generated in gray matter following lost blood flow, which compromises the Na/K pump. Electrical recovery from each SD event requires so much energy that neurons often die over minutes and hours following initial injury, independent of extracellular glutamate. Results: We discuss how SD has been investigated with various pitfalls in numerous experimental preparations, how overtaxing the Na/K ATPase elicits SD. Elevated K or glutamate are unlikely natural activators of SD. We then turn to the properties of SD itself, focusing on its initiation and propagation as well as on computer modeling. Conclusions: Finally, we summarize points of consensus and contention among the authors as well as where SD research may be heading. In an accompanying review, we critique the role of the glutamate excitotoxicity theory, how it has shaped SD research, and its questionable importance to the study of early brain injury as compared with SD theory.This work was supported by grants from the Heart and Stroke Foundation of Canada and the National Science and Engineering Research Council of Canada to RDA, an NIH grant (NS106901) to CWS, a National Research, Development and Innovation Office of Hungary grant (K1343777) and EU Horizon 2020 research and innovation program (739953) to EF and from DFG Deutsche Forschungsgemeinschaft (German Research Council) (DFG DR 323/5-1), DFG DR 323/10-1, and BMBF Bundesministerium fuer Bildung und Forschung (EraNet Neuron EBio2, with funds from BMBF 01EW2004) to JPD
Magnetic phases and reorientation transitions in antiferromagnetically coupled multilayers
In antiferromagnetically coupled superlattices grown on (001) faces of cubic
substrates, e.g. based on materials combinations as Co/Cu, Fe/Si, Co/Cr, or
Fe/Cr, the magnetic states evolve under competing influence of bilinear and
biquadratic exchange interactions, surface-enhanced four-fold in-plane
anisotropy, and specific finite-size effects. Using phenomenological
(micromagnetic) theory, a comprehensive survey of the magnetic states and
reorientation transitions has been carried out for multilayer systems with even
number of ferromagnetic sub-layers and magnetizations in the plane. In
two-layer systems (N=2) the phase diagrams in dependence on components of the
applied field in the plane include ``swallow-tail'' type regions of
(metastable) multistate co-existence and a number of continuous and
discontinuous reorientation transitions induced by radial and transversal
components of the applied field. In multilayers (N \ge 4) noncollinear states
are spatially inhomogeneous with magnetization varying across the multilayer
stack. For weak four-fold anisotropy the magnetic states under influence of an
applied field evolve by a complex continuous reorientation into the saturated
state. At higher anisotropy they transform into various inhomogeneous and
asymmetric structures. The discontinuous transitions between the magnetic
states in these two-layers and multilayers are characterized by broad ranges of
multi-phase coexistence of the (metastable) states and give rise to specific
transitional domain structures.Comment: Manuscript 34 pages, 14 figures; submitted for publicatio
Teleparallel Energy-Momentum Distribution of Spatially Homogeneous Rotating Spacetimes
The energy-momentum distribution of spatially homogeneous rotating spacetimes
in the context of teleparallel theory of gravity is investigated. For this
purpose, we use the teleparallel version of Moller prescription. It is found
that the components of energy-momentum density are finite and well-defined but
are different from General Relativity. However, the energy-momentum density
components become the same in both theories under certain assumptions. We also
analyse these quantities for some special solutions of the spatially
homogeneous rotating spacetimes.Comment: 12 pages, accepted for publication in Int. J. Theor. Phy
Lymphocyte predominant cells detect Moraxella catarrhalis-derived antigens in nodular lymphocyte-predominant Hodgkin lymphoma.
Nodular lymphocyte-predominant Hodgkin lymphoma (NLPHL) is a rare lymphoma of B-cell origin with frequent expression of functional B-cell receptors (BCRs). Here we report that expression cloning followed by antigen screening identifies DNA-directed RNA polymerase beta' (RpoC) from Moraxella catarrhalis as frequent antigen of BCRs of IgD <sup>+</sup> LP cells. Patients show predominance of HLA-DRB1*04/07 and the IgVH genes encode extraordinarily long CDR3s. High-titer, light-chain-restricted anti-RpoC IgG1/κ-type serum-antibodies are additionally found in these patients. RpoC and MID/hag, a superantigen co-expressed by Moraxella catarrhalis that is known to activate IgD <sup>+</sup> B cells by binding to the Fc domain of IgD, have additive activation effects on the BCR, the NF-κB pathway and the proliferation of IgD <sup>+</sup> DEV cells expressing RpoC-specific BCRs. This suggests an additive antigenic and superantigenic stimulation of B cells with RpoC-specific IgD <sup>+</sup> BCRs under conditions of a permissive MHC-II haplotype as a model of NLPHL lymphomagenesis, implying future treatment strategies
Quantum computing implementations with neutral particles
We review quantum information processing with cold neutral particles, that
is, atoms or polar molecules. First, we analyze the best suited degrees of
freedom of these particles for storing quantum information, and then we discuss
both single- and two-qubit gate implementations. We focus our discussion mainly
on collisional quantum gates, which are best suited for atom-chip-like devices,
as well as on gate proposals conceived for optical lattices. Additionally, we
analyze schemes both for cold atoms confined in optical cavities and hybrid
approaches to entanglement generation, and we show how optimal control theory
might be a powerful tool to enhance the speed up of the gate operations as well
as to achieve high fidelities required for fault tolerant quantum computation.Comment: 19 pages, 12 figures; From the issue entitled "Special Issue on
Neutral Particles
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