24,099 research outputs found
Snakes and ladders: localized solutions of plane Couette flow
We demonstrate the existence of a large number of exact solutions of plane
Couette flow, which share the topology of known periodic solutions but are
localized in space. Solutions of different size are organized in a
snakes-and-ladders structure strikingly similar to that observed for simpler
pattern-forming PDE systems. These new solutions are a step towards extending
the dynamical systems view of transitional turbulence to spatially extended
flows.Comment: submitted to Physics Review Letter
Excitations in time-dependent density-functional theory
An approximate solution to the time-dependent density functional theory
(TDDFT) response equations for finite systems is developed, yielding
corrections to the single-pole approximation. These explain why allowed
Kohn-Sham transition frequencies and oscillator strengths are usually good
approximations to the true values, and why sometimes they are not. The
approximation yields simple expressions for G\"orling-Levy perturbation theory
results, and a method for estimating expectation values of the unknown
exchange-correlation kernel.Comment: 4 pages, 1 tabl
Archean foreland basin tectonics in the Witwatersrand, South Africa
The Witwatersrand Basin of South Africa is the best-known of Archean sedimentary basins and contains some of the largest gold reserves in the world. Sediments in the basin include a lower flysch-type sequence and an upper molassic facies, both of which contain abundant silicic volcanic detritus. The strata are thicker and more proximal on the northwestern side of the basin which is, at least locally, bound by thrust faults. These features indicate that the Witwatersrand strata may have been deposited in a foreland basin and a regional geologic synthesis suggests that this basin developed initially on the cratonward side of an Andean-type arc. Remarkably similar Phanerozoic basins may be found in the southern Andes above zones of shallow subduction. It is suggested that the continental collision between the Kaapvaal and Zimbabwe Cratons at about 2.7 Ga caused further subsidence and deposition in the Witwatersrand Basin. Regional uplift during this later phase of development placed the basin on the cratonward edge of a collision-related plateau, now represented by the Limpopo Province. Similarities are seen between this phase of Witwatersrand Basin evolution and that of active basins north of the Tibetan Plateau. The geologic evidence does not agree with earlier suggestions that the Witwatersrand strata were deposited in a rift or half-graben
Analysis of band-gap formation in squashed arm-chair CNT
The electronic properties of squashed arm-chair carbon nanotubes are modeled
using constraint free density functional tight binding molecular dynamics
simulations. Independent from CNT diameter, squashing path can be divided into
{\it three} regimes. In the first regime, the nanotube deforms with negligible
force. In the second one, there is significantly more resistance to squashing
with the force being nN/per CNT unit cell. In the last regime,
the CNT looses its hexagonal structure resulting in force drop-off followed by
substantial force enhancement upon squashing. We compute the change in band-gap
as a function of squashing and our main results are: (i) A band-gap initially
opens due to interaction between atoms at the top and bottom sides of CNT. The
orbital approximation is successful in modeling the band-gap opening at
this stage. (ii) In the second regime of squashing, large
interaction at the edges becomes important, which can lead to band-gap
oscillation. (iii) Contrary to a common perception, nanotubes with broken
mirror symmetry can have {\it zero} band-gap. (iv) All armchair nanotubes
become metallic in the third regime of squashing. Finally, we discuss both
differences and similarities obtained from the tight binding and density
functional approaches.Comment: 16 pages and 6 figures, To appear in PR
Alloreactive cytotoxic T lymphocytes generated in the presence of viral- derived peptides show exquisite peptide and MHC specificity
The nature of alloreactivity to MHC molecules has been enigmatic, primarily because of the observation that allogeneic responses are considerably stronger than syngeneic responses. To better determine the specificity potential of allogeneic responses, we have generated alloreactive CTL specific for exogenous, viral-derived peptide ligands. This approach allowed us to critically evaluate both the peptide- and MHC-specificity of these alloreactive T cells. Exploiting the accessibility of the H-2Ld class I molecule for exogenous peptide ligands, alloreactive CTL were generated that are specific for either murine cytomegalovirus (MCMV) or lymphocytic choriomeningitis virus (LCMV) peptides bound by Ld alloantigens. Peptide specificity was initially observed in bulk cultures of alloreactive CTL only when tested on peptide-sensitized T2.Ld target cells that have defective presentation of endogenous peptides. Subsequent cloning of bulk alloreactive CTL lines generated to MCMV yielded CTL clones that had exquisitely specific MCMV peptide recognition requirement. All of the MCMV/Ld alloreactive CTL clones were also exquisitely MHC-specific in that none of the CTL clones lysed targets expressing MCMV/Lq complexes, even though Lq differs from Ld by only six amino acid residues and Lq also binds the MCMV peptide. This observation clearly demonstrates that alloreactive CTL are capable of the same degree of specificity for target cell recognition as are syngeneic CTL in MHC-restricted responses
Nutrition Strategies for Triathlon
Contemporary sports nutrition guidelines recommend that each athlete develop a personalised, periodised and practical approach to eating that allows him or her to train hard, recover and adapt optimally, stay free of illness and injury and compete at their best at peak races. Competitive triathletes undertake a heavy training programme to prepare for three different sports while undertaking races varying in duration from 20 min to 10 h. The everyday diet should be adequate in energy availability, provide CHO in varying amounts and timing around workouts according to the benefits of training with low or high CHO availability and spread high-quality protein over the day to maximise the adaptive response to each session. Race nutrition requires a targeted and well-practised plan that maintains fuel and hydration goals over the duration of the specific event, according to the opportunities provided by the race and other challenges, such as a hot environment. Supplements and sports foods can make a small contribution to a sports nutrition plan, when medical supplements are used under supervision to prevent/treat nutrient deficiencies (e.g. iron or vitamin D) or when sports foods provide a convenient source of nutrients when it is impractical to eat whole foods. Finally, a few evidence-based performance supplements may contribute to optimal race performance when used according to best practice protocols to suit the triathlete’s goals and individual responsiveness
Radiation Damping in the Photoionization of Fe^{14+}
A theoretical investigation of photoabsorption and photoionization of
Fe^{14+} extending beyond an earlier frame transformation R-matrix
implementation is performed using a fully-correlated, Breit-Pauli R-matrix
formulation including both fine-structure splitting of strongly-bound
resonances and radiation damping. The radiation damping of
resonances gives rise to a resonant photoionization cross section that is
significantly lower than the total photoabsorption cross section. Furthermore,
the radiation-damped photoionization cross section is found to be in good
agreement with recent experimental results once a global shift in energy of
eV is applied. These findings have important implications.
Firstly, the presently available synchrotron experimental data are applicable
only to photoionization processes and not to photoabsorption; the latter is
required in opacity calculations. Secondly, our computed cross section, for
which the L-shell ionization threshold is aligned with the NIST value, shows a
series of Rydberg resonances that are uniformly 3-4 eV
higher in energy than the corresponding experimental profiles, indicating that
the L-shell threshold energy values currently recommended by NIST are likely in
error.Comment: 4 pages, 1 figures, and 2 table
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