924 research outputs found
Representations of solutions of the wave equation based on relativistic wavelets
A representation of solutions of the wave equation with two spatial
coordinates in terms of localized elementary ones is presented. Elementary
solutions are constructed from four solutions with the help of transformations
of the affine Poincar\'e group, i.e., with the help of translations, dilations
in space and time and Lorentz transformations. The representation can be
interpreted in terms of the initial-boundary value problem for the wave
equation in a half-plane. It gives the solution as an integral representation
of two types of solutions: propagating localized solutions running away from
the boundary under different angles and packet-like surface waves running along
the boundary and exponentially decreasing away from the boundary. Properties of
elementary solutions are discussed. A numerical investigation of coefficients
of the decomposition is carried out. An example of the field created by sources
moving along a line with different speeds is considered, and the dependence of
coefficients on speeds of sources is discussed.Comment: submitted to J. Phys. A: Math. Theor., 20 pages, 6 figure
Nutritional support for head-injured patients
Copyright John Wiley & Sons. This review is published as a Cochrane Review in the Cochrane Database of Systematic Reviews 2004, Issue 3. Cochrane Reviews are regularly updated as new evidence emerges and in response to comments and criticisms, and the Cochrane Database of Systematic Reviews should be consulted for the most recent version of the Review.’ Perel, P. , Yanagawa, T. , Bunn, F. , Roberts, I. , Wentz, R. and Pierro, A. Nutritional support for head-injured patients. Cochrane Database of Systematic Reviews 2004, Issue 3. Art. No.: CD001530. DOI: 10.1002/14651858.CD001530.pub2Background: Head injury increases the body's metabolic responses, and therefore nutritional demands. Provision of an adequate supply of nutrients is associated with improved outcome. The best route for administering nutrition (parenterally (TPN) or enterally (EN)), and the best timing of administration (for example, early versus late) of nutrients needs to be established. Objectives: To quantify the effect on mortality and morbidity of alternative strategies of providing nutritional support following head injury. Search strategy: Trials were identified by computerised searches of the Cochrane Injuries Group specialised register, Cochrane Central Register of Controlled Trials, MEDLINE, EMBASE, National Research Register, Web of Science and other electronic trials registers. Reference lists of trials and review articles were checked. The searches were last updated in July 2006. Selection criteria: Randomised controlled trials of timing or route of nutritional support following acute traumatic brain injury. Data collection and analysis: Two authors independently abstracted data and assessed trial quality. Information was collected on death, disability, and incidence of infection. If trial quality was unclear, or if there were missing outcome data, trialists were contacted in an attempt to get further information. Main results: A total of 11 trials were included. Seven trials addressed the timing of support (early versus delayed), data on mortality were obtained for all seven trials (284 participants). The relative risk (RR) for death with early nutritional support was 0.67 (95% CI 0.41 to 1.07). Data on disability were available for three trials. The RR for death or disability at the end of follow-up was 0.75 (95% CI 0.50 to 1.11). Seven trials compared parenteral versus enteral nutrition. Because early support often involves parenteral nutrition, three of the trials are also included in the previous analyses. Five trials (207 participants) reported mortality. The RR for mortality at the end of follow-up period was 0.66 (0.41 to 1.07). Two trials provided data on death and disability. The RR was 0.69 (95% Cl 0.40 to 1.19). One trial compared gastric versus jejunal enteral nutrition, there were no deaths and the RR was not estimable. Authors' conclusions: This review suggests that early feeding may be associated with a trend towards better outcomes in terms of survival and disability. Further trials are required. These trials should report not only nutritional outcomes but also the effect on death and disability.Peer reviewe
Transient quantum evolution of 2D electrons under photoexcitation of a deep center
We have considered the ballistic propagation of the 2D electron Wigner
distribution, which is excited by an ultrashort optical pulse from a
short-range impurity into the first quantized subband of a selectively-doped
heterostructure with high mobility. Transient ionization of a deep local state
into a continuum conduction c-band state is described. Since the quantum nature
of the photoexcitation, the Wigner distribution over 2D plane appears to be an
alternating-sign function. Due to a negative contribution to the Wigner
function, the mean values (concentration, energy, and flow) demonstrate an
oscillating transient evolution in contrast to the diffusive classical regime
of propagation.Comment: 8 pages, 6 figures, pape
Tunneling spin-galvanic effect
It has been shown that tunneling of spin-polarized electrons through a
semiconductor barrier is accompanied by generation of an electric current in
the plane of the interfaces. The direction of this interface current is
determined by the spin orientation of the electrons, in particular the current
changes its direction if the spin orientation changes the sign. Microscopic
origin of such a 'tunneling spin-galvanic' effect is the spin-orbit
coupling-induced dependence of the barrier transparency on the spin orientation
and the wavevector of electrons.Comment: 3 pages, 2 figure
Spatial structure of an individual Mn acceptor in GaAs
The wave function of a hole bound to an individual Mn acceptor in GaAs is
spatially mapped by scanning tunneling microscopy at room temperature and an
anisotropic, cross-like shape is observed. The spatial structure is compared
with that from an envelope-function, effective mass model, and from a
tight-binding model. This demonstrates that anisotropy arising from the cubic
symmetry of the GaAs crystal produces the cross-like shape for the hole
wave-function. Thus the coupling between Mn dopants in GaMnAs mediated by such
holes will be highly anisotropic.Comment: 3 figures, submitted to PR
Atrial fibrillation: the current epidemic.
Atrial fibrillation (AF) is the most common arrhythmia diagnosed in clinical practice. The consequences of AF have been clearly established in multiple large observational cohort studies and include increased stroke and systemic embolism rates if no oral anticoagulation is prescribed, with increased morbidity and mortality. With the worldwide aging of the population characterized by a large influx of "baby boomers" with or without risk factors for developing AF, an epidemic is forecasted within the next 10 to 20 years. Although not all studies support this evidence, it is clear that AF is on the rise and a significant amount of health resources are invested in detecting and managing AF. This review focuses on the worldwide burden of AF and reviews global health strategies focused on improving detection, prevention and risk stratification of AF, recently recommended by the World Heart Federation
Characterization of deep impurities in semiconductors by terahertz tunneling ionization
Tunneling ionization in high frequency fields as well as in static fields is suggested as a method for the characterization of deep impurities in semiconductors. It is shown that an analysis of the field and temperature dependences of the ionization probability allows to obtain defect parameters like the charge of the impurity, tunneling times, the Huang–Rhys parameter, the difference between optical and thermal binding energy, and the basic structure of the defect adiabatic potentials. Compared to static fields, high frequency electric fields in the terahertz-range offer various advantages, as they can be applied contactlessly and homogeneously even to bulk samples using the intense radiation of a high power pulsed far-infrared laser. Furthermore, impurity ionization with terahertz radiation can be detected as photoconductive signal with a very high sensitivity in a wide range of electric field strengths
Why and when the Minkowski's stress tensor can be used in the problem of Casimir force acting on bodies embedded in media
It is shown that the criticism by Raabe and Welsch of the
Dzyaloshinskii-Lifshitz-Pitaevskii theory of the van der Waals-Casimir forces
inside a medium is based on misunderstandings. It is explained why and at which
conditions one can use the ''Minkowski-like '' stress tensor for calculations
of the forces. The reason, why approach of Raabe and Welsch is incorrect, is
discussed.Comment: Comment, 2 pages. 2 misprints were correcte
Collective and static properties of model two-component plasmas
Classical MD data on the charge-charge dynamic structure factor of
two-component plasmas (TCP) modeled in Phys. Rev. A 23, 2041 (1981) are
analyzed using the sum rules and other exact relations. The convergent power
moments of the imaginary part of the model system dielectric function are
expressed in terms of its partial static structure factors, which are computed
by the method of hypernetted chains using the Deutsch effective potential.
High-frequency asymptotic behavior of the dielectric function is specified to
include the effects of inverse bremsstrahlung. The agreement with the MD data
is improved, and important statistical characteristics of the model TCP, such
as the probability to find both electron and ion at one point, are determined.Comment: 25 pages, 6 figures, 5 tables. Published in Physical Review E
http://link.aps.org/abstract/PRE/v76/e02640
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