1,756 research outputs found
Open Heterotic Strings
We classify potential cosmic strings according to the topological charge
measurable outside the string core. We conjecture that in string theory it is
this charge that governs the stability of long strings. This would imply that
the SO(32) heterotic string can have endpoints, but not the E_8 x E_8 heterotic
string. We give various arguments in support of this conclusion.Comment: 15 pages. v.2: typos, references correcte
Electronic Properties of Ultra-Thin Aluminum Nanowires
We have carried out first principles electronic structure and total energy
calculations for a series of ultrathin aluminum nanowires, based on structures
obtained by relaxing the model wires of Gulseren et al. The number of
conducting channels is followed as the wires radius is increased. The results
suggest that pentagonal wires should be detectable, as the only ones who can
yield a channel number between 8 and 10.Comment: 9 pages + 3 figures, to appear on Surface Scienc
Effects of χ(3) nonlinearities in second-harmonic generation
We investigate the effects of higher-order, chi ((3)), nonlinearities on the process of second-harmonic generation. In the traveling-wave case we find substantive differences in the macroscopic behavior of the fields when the chi ((3)) components are present. In the intracavity cage, which has been investigated before using a Linearized analysis, we investigate regions where these analyses may not be valid, comparing and contrasting the full quantum simulations with previous results
Nonlinear self consistent high resolution beam halo algorithm in homomorphic and weakly chaotic systems
A technique is described which enables high resolution of halo in beam dynamic studies by direct simulation. The method consists in first solving the beam dynamics problem using coarse initial data. The regions of the initial data, which result in beam halo, or extremums in phase space, are identified. The dynamics are resolved by continuing the calculation using initial data points slightly offset from those that result in halo formation, thus filling in the halo structure. The solution is repeated with appropriate scaling of such things as charge per orbit etc. This process may be continued indefinitely. The method can also shed some light on the halo generation in weakly chaotic systems. The scheme is essentially different from the {Delta}f method in that no assumption is made about f{sub 0}. As an example, a bifurcation in a non-trivial space charge dominated homomorphic problem is resolved self-consistently using minor computational resources, rather than having to perform the calculation for 250 trillion effective particles
Recommendations for dietary calcium intake and bone health: the role of health literacy
Osteoporosis, a common disease of the skeleton, involves microarchitecturaldeterioration of the bone matrix and depletion of bonemineral; this results in an increased susceptibility to fracture [1]. Postfracture,there is a plethora of financial, personal and psychosocialoutcomes, including reduced mobility, impairment of daily activities,inability to work and loss of confidence [2,3]. A hip fracture has themost severe implications: one in five individuals die within the firstyear, while 60% of individuals who survive a hip fracture still requireassistance to walk one year later, and 33% are totally dependent or areadmitted to a nursing home [2,4]. Bone mass is an important predictorof osteoporosis, and future fracture risk [5], and calcium plays animportant role in normal growth, development and maintenance of theskeleton [6], including providing a dynamic store to maintain theintra- and extra-cellular calcium pools [7]. Calcium homeostasis isregulated by an integrated hormonal system that involves calcitonin,parathyroid hormone (PTH) and the PTH receptor, and 1,25-dihydroxyvitamin D and the vitamin D receptor [7,8], along withserum ionized calcium, and the calcium-sensing receptor [9]. Whenplasma concentrations of ionized calcium fall below optimal levels,bone resorption increases in order to restore the mineral equilibrium
European regulatory agenices should employ full time statisticians
No abstract available
Quantum superchemistry: Role of trapping profile and quantum statistics
The process of Raman photoassociation of a trapped atomic condensate to form condensed molecules has been labeled superchemistry because it can occur at 0 K and experiences coherent bosonic stimulation. We show here that the differences from ordinary chemical processes go even deeper, with the conversion rates depending on the quantum state of the reactants, as expressed by the Wigner function. We consider different initial quantum states of the trapped atomic condensate and different forms of the confining potentials, demonstrating the importance of the quantum statistics and the extra degrees of freedom which massive particles and trapping potentials make available over the analogous optical process of second-harmonic generation. We show that both mean-field analyses and quantum calculations using an inappropriate initial condition can make inaccurate predictions for a given system. This is possible whether using a spatially dependent analysis or a zero-dimensional approach as commonly used in quantum optics
Geometric K-Homology of Flat D-Branes
We use the Baum-Douglas construction of K-homology to explicitly describe
various aspects of D-branes in Type II superstring theory in the absence of
background supergravity form fields. We rigorously derive various stability
criteria for states of D-branes and show how standard bound state constructions
are naturally realized directly in terms of topological K-cycles. We formulate
the mechanism of flux stabilization in terms of the K-homology of non-trivial
fibre bundles. Along the way we derive a number of new mathematical results in
topological K-homology of independent interest.Comment: 45 pages; v2: References added; v3: Some substantial revision and
corrections, main results unchanged but presentation improved, references
added; to be published in Communications in Mathematical Physic
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