Observations of attenuation at 20.6, 31.65 and 90.0 GHz: Preliminary results from Wallops Island, VA

Ground based radiometric observations of atmospheric attenuation at 20.6, 31.65, and 90.0 GHz were made at Wallops Island, Virginia during April and May 1989. Early results from the analysis of the data set are compared with previous observations from California and Colorado. The relative attenuation ratios observed at each frequency during clear, cloudy, and rainy conditions are shown. Plans for complete analysis of the data are described

DNA Identification Tests and the Courts

This Comment assesses the current state of forensic DNA tests and analyzes whether courts should admit the results of these tests as evidence. Section I provides a background discussion of how DNA tests work. This knowledge is essential for attorneys and courts seeking to evaluate expert testimony and analyze important issues concerning the reliability and admissibility of DNA test results. Section I also proposes safeguards and standards to facilitate the judicial acceptance of forensic DNA tests. Section II discusses judicial approaches to the admissibility of novel scientific techniques such as DNA tests, and concludes that courts should admit the results of DNA tests

DNA Identification Tests and the Courts

This Comment assesses the current state of forensic DNA tests and analyzes whether courts should admit the results of these tests as evidence. Section I provides a background discussion of how DNA tests work. This knowledge is essential for attorneys and courts seeking to evaluate expert testimony and analyze important issues concerning the reliability and admissibility of DNA test results. Section I also proposes safeguards and standards to facilitate the judicial acceptance of forensic DNA tests. Section II discusses judicial approaches to the admissibility of novel scientific techniques such as DNA tests, and concludes that courts should admit the results of DNA tests

Remodelling of the natural product fumagillol employing a reaction discovery approach

In the search for new biologically active molecules, diversity-oriented synthetic strategies break through the limitation of traditional library synthesis by sampling new chemical space. Many natural products can be regarded as intriguing starting points for diversity-oriented synthesis, wherein stereochemically rich core structures may be reorganized into chemotypes that are distinctly different from the parent structure. Ideally, to be suited to library applications, such transformations should be general and involve few steps. With this objective in mind, the highly oxygenated natural product fumagillol has been successfully remodelled in several ways using a reaction-discovery-based approach. In reactions with amines, excellent regiocontrol in a bis-epoxide opening/cyclization sequence can be obtained by size-dependent interaction of an appropriate catalyst with the parent molecule, forming either perhydroisoindole or perhydroisoquinoline products. Perhydroisoindoles can be further remodelled by cascade processes to afford either morpholinone or bridged 4,1-benzoxazepine-containing structures.P50 GM067041 - NIGMS NIH HHS; P50 GM067041-07 - NIGMS NIH HHS; P50 GM067041-08 - NIGMS NIH HHS; P50 GM067041-09 - NIGMS NIH HH

The Peierls substitution in an engineered lattice potential

Artificial gauge fields open new possibilities to realize quantum many-body systems with ultracold atoms, by engineering Hamiltonians usually associated with electronic systems. In the presence of a periodic potential, artificial gauge fields may bring ultracold atoms closer to the quantum Hall regime. Here, we describe a one-dimensional lattice derived purely from effective Zeeman-shifts resulting from a combination of Raman coupling and radiofrequency magnetic fields. In this lattice, the tunneling matrix element is generally complex. We control both the amplitude and the phase of this tunneling parameter, experimentally realizing the Peierls substitution for ultracold neutral atoms.Comment: 6 pages, 5 figure

Direct vs. indirect optical recombination in Ge films grown on Si substrates

The optical emission spectra from Ge films on Si are markedly different from their bulk Ge counterparts. Whereas bulk Ge emission is dominated by the material's indirect gap, the photoluminescence signal from Ge films is mainly associated with its direct band gap. Using a new class of Ge-on-Si films grown by a recently introduced CVD approach, we study the direct and indirect photoluminescence from intrinsic and doped samples and we conclude that the origin of the discrepancy is the lack of self-absorption in thin Ge films combined with a deviation from quasi-equilibrium conditions in the conduction band. The latter is confirmed by a simple model suggesting that the deviation from quasi-equilibrium is caused by the much shorter recombination lifetime in the films relative to bulk Ge

Number of Common Sites Visited by N Random Walkers

We compute analytically the mean number of common sites, W_N(t), visited by N independent random walkers each of length t and all starting at the origin at t=0 in d dimensions. We show that in the (N-d) plane, there are three distinct regimes for the asymptotic large t growth of W_N(t). These three regimes are separated by two critical lines d=2 and d=d_c(N)=2N/(N-1) in the (N-d) plane. For d<2, W_N(t)\sim t^{d/2} for large t (the N dependence is only in the prefactor). For 2<d<d_c(N), W_N(t)\sim t^{\nu} where the exponent \nu= N-d(N-1)/2 varies with N and d. For d>d_c(N), W_N(t) approaches a constant as t\to \infty. Exactly at the critical dimensions there are logaritmic corrections: for d=2, we get W_N(t)\sim t/[\ln t]^N, while for d=d_c(N), W_N(t)\sim \ln t for large t. Our analytical predictions are verified in numerical simulations.Comment: 5 pages, 3 .eps figures include