6,414 research outputs found
Power sums of Coxeter exponents
Consider an irreducible finite Coxeter system. We show that for any
nonnegative integer n the sum of the nth powers of the Coxeter exponents can be
written uniformly as a polynomial in four parameters: h (the Coxeter number), r
(the rank), and two further parameters.Comment: 14 page
Separation of quadrupolar and magnetic contributions to spin-lattice relaxation in the case of a single isotope
We present a NMR pulse double-irradiation method which allows one to separate
magnetic from quadrupolar contributions in the spin-lattice relaxation. The
pulse sequence fully saturates one transition while another is observed. In the
presence of a Delta m = 2 quadrupolar contribution, the intensity of the
observed line is altered compared to a standard spin-echo experiment. We
calculated analytically this intensity change for spins I=1, 3/2, 5/2, thus
providing a quantitative analysis of the experimental results. Since the pulse
sequence we used takes care of the absorbed radio-frequency power, no problems
due to heating arise. The method is especially suited when only one NMR
sensitive isotope is available. Different cross-checks were performed to prove
the reliability of the obtained results. The applicability of this method is
demonstrated by a study of the plane oxygen 17O (I = 5/2) in the
high-temperature superconductor YBa_2Cu_4O_8: the 17O spin-lattice relaxation
rate consists of magnetic as well as quadrupolar contributions.Comment: 7 pages, 6 figure
Concatenated dynamical decoupling with virtual pulses
The loss of quantum information due to interaction with external degrees of
freedom, which is known as decoherence, remains one of the main obstacles for
large-scale implementations of quantum computing. Accordingly, different
measures are being explored for reducing its effect. One of them is dynamical
decoupling (DD) which offers a practical solution because it only requires the
application of control pulses to the system qubits. Starting from basic DD
sequences, more sophisticated schemes were developed that eliminate
higher-order terms of the system-environment interaction and are also more
robust against experimental imperfections. A particularly successful scheme,
called concatenated DD (CDD), gives a recipe for generating higher order
sequences by inserting lower order sequences into the delays of a generating
sequence. Here, we show how this scheme can be improved further by converting
some of the pulses to virtual (and thus ideal) pulses. The resulting scheme,
called vCDD, has lower power deposition and is more robust against pulse
imperfections than the original CDD scheme.Comment: 9 pages, 9 figure
Alcohol, nutrition and health maintenance: selected aspects
In view of the developments in health care relating to the increased prevalence and incidence of chronic diseases and the continuing increase in health-care expenditure, more attention should be paid to health maintenance and disease prevention. Any strategy that can influence health maintenance is of interest, especially lifestyle factors such as nutrition, exercise or stress control. Alcohol has an important place in the daily life of many healthy as well as sick individuals. Alcohol has three major characteristics; it is a nutrient (energy source), a psycho-active drug and a toxin. Each consumer has the choice of which of the characteristics of alcohol he/she wants to utilise. Thus, alcohol represents one of the most important self-implemented disease modifiers in our modern society. The major determinants of the health effects of alcohol are the absolute amount consumed, the consumption frequency, associated lifestyle factors (e.g. smoking, nutrient intake, substrate composition, physical activity pattern) and last, but not least, the genetic background. There are few known disease conditions that have not already been associated positively or negatively with alcohol consumption. The list of diseases includes atherosclerosis, dementia, diabetes, obesity and conditions relating to Zn metabolism. Obesity represents the most important disease modifier in the world and the prevalence rates are increasing rapidly. Evidence suggests that alcohol represents a risk factor for overweight and obesity as a result of specific effects on energy metabolism and substrate metabolism. The potential role of alcohol as an important modulator for the postprandial lipidaemia and its role in the pathogenesis of modern diseases will be discusse
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