Spontaneous macroscopic spin polarization in independent spinor Bose-Einstein condensates

We present experimental evidence for the spontaneous formation of a macroscopic spin polarization in overlapping regions of two independent Bose-Einstein condensates produced in different hyperfine states of 87Rb. The condensates are independent in the sense that we do not explicitly introduce a relative phase between them. A single "spin-tip" pulse maps the transverse spin polarization into longitudinal spin polarization, and the atomic density distributions are measured with a Stern-Gerlach imaging method. The resulting matter-wave interference patterns are anticorrelated.     

Chemical action: what is it,and why does it really matter?

Nanotechnology, as with many technologies before it, places a strain on existing legislation and poses a challenge to all administrative agencies tasked with regulating technology-based products. It is easy to see how statutory schemes become outdated, as our ability to understand and affect the world progresses. In this article, we address the regulatory problems that nanotechnology posses for the Food and Drug Administration’s (FDA) classification structure for “drugs” and “devices.” The last major modification to these terms was in 1976, with the enactment of the Medical Device Amendments. There are serious practical differences for a classification as a drug or device in terms of time to market and research. Drugs are classified, primarily, as acting by “chemical action.” We lay out some legal, philosophic, and scientific tools that serve to provide a useful, as well as legally and scientifically faithful, distinction between drugs and devices for the purpose of regulatory classification. These issues we raise are worth the consideration of anyone who is interested in the regulation of nano-products or other novel technologies.     

Robust Optimal Service Analysis of Single-Server Re-Entrant Queues

We generalize the analysis of J.A. Ball, M.V. Day, and P. Kachroo (Mathematics of Control, Signals, and Systems, vol. 12, pp. 307–345, 1999) to a fluid model of a single server re-entrant queue. The approach is to solve the Hamilton-Jacobi-Isaacs equation associated with optimal robust control of the system. The method of staged characteristics is generalized from Ball et al. (1999) to construct the solution explicitly. Formulas are developed allowing explicit calculations for the Skorokhod problem involved in the system equations. Such formulas are particularly important for numerical verification of conditions on the boundary of the nonnegative orthant. The optimal control (server) strategy is shown to be of linear-index type. Dai-type stability properties are discussed. A modification of the model in which new customers are allowed only at a specified entry queue is considered in 2 dimensions. The same optimal strategy is found in that case as well.     

How electron flow controls the thermochemistry of the addition of olefins to nickel dithiolenes: predictions by density functional theory

The reaction of a nickel dithiolene complex (1) and ethylene is a two-step process, in which the trans-product (2) forms first in the direct addition of the olefin to 1, while the more thermodynamically stable cis-product (3) involves isomerization of 2. The introduction of electron-withdrawing groups (cyano or trifluoromethyl) not only significantly lowers the activation energy (TS1) for the formation of trans-product, but it also strongly stabilizes the products (2, 3) such that they are favored by the free energy. However, these substituents leave the barrier for the conformational transformation step (TS2) nearly unchanged. On reduction, the previously favored adduct is now strongly disfavored.     

Modulation gap detection: effects of modulation rate, carrier separation, and mode of presentation.

Modulation gap detection (MGD) is a procedure that measures the sensitivity to an interruption in the modulation pattern imposed upon one or more carrier frequencies. The MGD task was developed to test conditions where a temporal event traverses frequency, but without a concomitant interruption in the spectral continuity of the stimulus. This contrasts with across-frequency gap detection where there is an inherent spectral discontinuity associated with the temporal gap, and where there is a marked decline in performance when the markers of the temporal gap are widely separated in frequency. The purpose of this study was to test the hypothesis that a wideband temporal analysis will be facilitated if there exists a spectral continuity throughout the temporal event. Experiment 1 established the procedure of MGD and indicated that a modulation rate of 8 Hz was optimal for the task. Experiment 2 showed that performance declined markedly when the carrier frequencies of the modulation markers were widely separated in frequency. This finding indicates that spectral continuity across the temporal event is not a sufficient prerequisite for the auditory system to undertake a wideband temporal analysis. Experiment 3 revealed that dichotic MGD also results in poor performance, similar to that seen for widely separated carrier frequencies in the monaural case. This supports the hypothesis that the "channels" across which temporal events are poorly processed do not necessarily correspond to peripheral frequency channels.     

Isotopy Stable Dynamics Relative to Compact Invariant Sets

Let f be an orientation-preserving homeomorphism of a compactorientable manifold. Sufficient conditions are given for thepersistence of a collection of periodic points under isotopyof f relative to a compact invariant set A. Two main applicationsare described. In the first, A is the closure of a single discreteorbit of f, and f has a Smale horseshoe, all of whose periodicorbits persist; in the second, A is a minimal invariant Cantorset obtained as the limit of a sequence of nested periodic orbits,all of which are shown to persist under isotopy relative toA. 1991 Mathematics Subject Classification: 58F20, 58F15.     

Magnetic resonance imaging in a model of atherosclerosis: use of a collar around the rabbit carotid artery

Transverse cardiac-cycle gated high resolution magnetic resonance images have been obtained from the neck of the New Zealand white rabbit both in normal animals and from those in which a collar had been earlier positioned around one carotid artery. The study included animals fed on normal and on high cholesterol diets with the surgical modification having been demonstrated previously to cause a rapid and reproducible lesion resembling early atherosclerosis. The aim of the work was to investigate the attainable spatial resolution and sensitivity at a field strength of 2 T using a large radiofrequency transmitter system and a surface coil receiver with which spin-echo images have been obtained. Visualization was enhanced using a three-dimensional interpolation technique. An image resolution of 200 microns was readily obtained but was shown to be insufficient for delineating pathological features within the artery wall such as intimal layer thickening. The results have been compared with histopathological findings which confirmed that any morphological changes were within the pixel resolution of the image. Extensions to the methodology are proposed which should be able to detect atherosclerotic changes with a resolution of 50 microns within a feasible imaging time. In addition, the MRI study of how the surgical intervention alters the artery shape and curvature was carried out and the MRI demonstrated that collar implantation in general does not occlude the artery and causes only a slight and gradual degree of curvature to the vessel.