7,567 research outputs found
Spectral Singularities of Complex Scattering Potentials and Infinite Reflection and Transmission Coefficients at real Energies
Spectral singularities are spectral points that spoil the completeness of the
eigenfunctions of certain non-Hermitian Hamiltonian operators. We identify
spectral singularities of complex scattering potentials with the real energies
at which the reflection and transmission coefficients tend to infinity, i.e.,
they correspond to resonances having a zero width. We show that a wave guide
modeled using such a potential operates like a resonator at the frequencies of
spectral singularities. As a concrete example, we explore the spectral
singularities of an imaginary PT-symmetric barrier potential and demonstrate
the above resonance phenomenon for a certain electromagnetic wave guide.Comment: Published versio
Selecting children for head CT following head injury
OBJECTIVE: Indicators for head CT scan defined by the 2007 National Institute for Health and Care Excellence (NICE) guidelines were analysed to identify CT uptake, influential variables and yield. DESIGN: Cross-sectional study. SETTING: Hospital inpatient units: England, Wales, Northern Ireland and the Channel Islands. PATIENTS: Children (3 years were much more likely to have CT than those <3 years (OR 2.35 (95% CI 2.08 to 2.65)). CONCLUSION: Compliance with guidelines and diagnostic yield was variable across age groups, the type of hospital and region where children were admitted. With this pattern of clinical practice the risks of both missing intracranial injury and overuse of CT are considerable
Laser-heated rocket studies
CW laser heated rocket propulsion was investigated in both the flowing core and stationary core configurations. The laser radiation considered was 10.6 micrometers, and the working gas was unseeded hydrogen. The areas investigated included initiation of a hydrogen plasma capable of absorbing laser radiation, the radiation emission properties of hot, ionized hydrogen, the flow of hot hydrogen while absorbing and radiating, the heat losses from the gas and the rocket performance. The stationary core configuration was investigated qualitatively and semi-quantitatively. It was found that the flowing core rockets can have specific impulses between 1,500 and 3,300 sec. They are small devices, whose heating zone is only a millimeter to a few centimeters long, and millimeters to centimeters in radius, for laser power levels varying from 10 to 5,000 kW, and pressure levels of 3 to 10 atm. Heat protection of the walls is a vital necessity, though the fraction of laser power lost to the walls can be as low as 10% for larger powers, making the rockets thermally efficient
The magnetoelectrochemical switch
In the field of spintronics, the archetype solid-state two-terminal device is the spin valve, where the resistance is controlled by the magnetization configuration. We show here how this concept of spin-dependent switch can be extended to magnetic electrodes in solution, by magnetic control of their chemical environment. Appropriate nanoscale design allows a huge enhancement of the magnetic force field experienced by paramagnetic molecular species in solutions, which changes between repulsive and attractive on changing the electrodes' magnetic orientations. Specifically, the field gradient force created within a sub-100-nm-sized nanogap separating two magnetic electrodes can be reversed by changing the orientation of the electrodes' magnetization relative to the current flowing between the electrodes. This can result in a breaking or making of an electric nanocontact, with a change of resistance by a factor of up to 103. The results reveal how an external field can impact chemical equilibrium in the vicinity of nanoscale magnetic circuits
XTE J2123-058: A New Neutron Star X-Ray Transient
We report on optical and RXTE observations of a new high-latitude bursting
X-ray transient, XTE J2123-058. We identified the optical counterpart, measured
the 5.9573 hr orbital period and constrained the binary inclination and the
source distance. The distance lower limit indicates that the source is at least
2.6 kpc from the Galactic plane, which is unusual for an LMXB. RXTE
observations were made between June and August 1998 during the first detected
outburst from this source. We describe correlations between X-ray timing and
spectral properties and discuss the possibility that the propeller mechanism
turned on during the decay of the outburst. During one of the RXTE
observations, we detect a pair of high frequency QPOs at 847.1 +/- 5.5 Hz and
1102 +/- 13 Hz simultaneously. According to the beat frequency model, the QPO
separation implies a neutron star spin period near 3.9 ms. As the X-ray flux
decreases at the end of the outburst, the amplitude of the optical modulation
increases significantly. This behavior can be explained if the size of the
accretion disk decreases during the decay of the outburst.Comment: 24 pages, 9 figures, accepted by Ap
Making the most of community energies:Three perspectives on grassroots innovation
Grassroots innovations for sustainability are attracting increasing policy attention. Drawing upon a wide range of empirical research into community energy in the UK, and taking recent support from national government as a case study, we apply three distinct analytical perspectives: strategic niche management; niche policy advocacy; and critical niches. Whilst the first and second perspectives appear to explain policy influence in grassroots innovation adequately, each also shuts out more transformational possibilities. We therefore argue that, if grassroots innovation is to realise its full potential, then we need to also pursue a third, critical niches perspective, and open up debate about more socially transformative pathways to sustainability
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