National Institute of Standards and Technology

About: National Institute of Standards and Technology is a government organization based out in Gaithersburg, Maryland, United States. It is known for research contribution in the topics: Laser & Scattering. The organization has 26667 authors who have published 60661 publications receiving 2215547 citations. The organization is also known as: National Bureau of Standards & NIST.

Stable, Tightly Confining Magnetic Trap for Evaporative Cooling of Neutral Atoms

Abstract: We describe a new type of magnetic trap whose time-averaged, orbiting potential (TOP) supplies tight and harmonic confinement of atoms. The TOP trap allows for long storage times even for cold atom samples by suppressing the loss due to nonadiabatic spin flips which limits the storage time in an ordinary magnetic quadrupole trap. In preliminary experiments on evaporative cooling of ${}^{87}\mathrm{Rb}$ atoms in the TOP trap, we obtain a phase-space density enhancement of up to 3 orders of magnitude and temperatures as low as 200 nK.

Parallel calculation of electron multiple scattering using Lanczos algorithms

Abstract: Real space multiple scattering calculations of the electronic density of states and x-ray spectra in solids typically scale as the cube of the system and basis set size, and hence are highly demanding computationally. For example, such x-ray absorption near edge structure (XANES) calculations typically require clusters of order ${N}_{R}$ atoms and s, p, and d states for convergence, with ${N}_{R}$ between about ${10}^{2}--{10}^{3};$ for this case about ${10}^{2}$ inversions of ${9N}_{R}\ifmmode\times\else\texttimes\fi{}{9N}_{R}$ matrices are needed, one for each energy point. We discuss here two ways to speed up these calculations: (1) message passing interface (MPI) parallel processing and (2) fast, Lanczos multiple scattering algorithms. Together these algorithms can reduce computation times typically by two orders of magnitude. These are both implemented in a generalization of the ab initio self-consistent FEFF8 code, which thus makes practical XANES calculations in complex systems with of order ${10}^{3}$ atoms. The Lanczos algorithm also yields a natural crossover between full and finite-order multiple scattering with increasing energy, thus differentiating the extended and near-edge regimes.

Spatial correlations of mobility and immobility in a glass-forming Lennard-Jones liquid

Abstract: Using extensive molecular dynamics simulations of an equilibrium, glass-forming Lennard-Jones mixture, we characterize in detail the local atomic motions. We show that spatial correlations exist among particles undergoing extremely large (``mobile'') or extremely small (``immobile'') displacements over a suitably chosen time interval. The immobile particles form the cores of relatively compact clusters, while the mobile particles move cooperatively and form quasi-one-dimensional, stringlike clusters. The strength and length scale of the correlations between mobile particles are found to grow strongly with decreasing temperature, and the mean cluster size appears to diverge near the mode-coupling critical temperature. We show that these correlations in the particle displacements are related to equilibrium fluctuations in the local potential energy and local composition.

CRC Handbook of Chemistry and Physics, 76th edition

Abstract: well as endogenous factors such as obesity or hormonal disorders. A conception closely associated in time with ovulation has been suggested to result in more boys.5 A high frequency of sexual intercourse should, according to this hypothesis, lead to a predominance of boys and a decline in the sex ratio according to age or parity could reflect a decline in sexual activity by age or a change in endogenous hormones over time. A low frequency could furthermore lead to low fecundity. A recent study showed that conception cycles with a short follicular phase produced more boys than girls.7 Exposures like dibromochloropropane (DBCP) have been shown to be associated with both low fecundity and a low sex ratio. De Cock et al found a difference in the time to pregnancy for boys and girls with the shortest time to pregnancy for the boys.8 These observations indicate a link between fecundity as measured by time to pregnancy and sex ratio. Based upon these observations we report data on sex ratio (measured as the proportion of boys among all newborns). It was expected that the sex ratio would decline with body mass index, age, parity, and low fecundity. The analyses are based upon data from a community trial named \"Healthy habits for two\" conducted between 1984 and 1987. All pregnant women in the Danish cities Odense and Aalborg were asked to give detailed information on lifestyle factors during pregnancy and obstetrical information on delivery was recorded from the medical files.9 The study subjects constituted 10 042 pregnant women (singletons only) after excluding 1808 women (15-26 %) with incomplete data. The participants delivered 5137 boys and 4905 girls (sex ratio = 0-51). Table 1 shows a weak but not significant trend towards a lower sex ratio with increasing age (except

A role-based access control model and reference implementation within a corporate intranet

Abstract: This paper describes NIST's enhanced RBAC model and our approach to designing and implementing RBAC features for networked Web servers. The RBAC model formalized in this paper is based on the properties that were first described in Ferraiolo and Kuhn [1992] and Ferraiolo et al. [1995], with adjustments resulting from experience gained by prototype implementations, market analysis, and observations made by Jansen [1988] and Hoffman [1996]. The implementation of RBAC for the Web (RBAC/Web) provides an alternative to the conventional means of administering and enforcing authorization policy on a server-by-server basis. RBAC/Web provides administrators with a means of managing authorization data at the enterprise level, in a manner consistent with the current set of laws, regulations, and practices.