Search for a new gauge boson in electron-nucleus fixed-target scattering by the APEX experiment

We present a search at the Jefferson Laboratory for new forces mediated by sub-GeV vector bosons with weak coupling α' to electrons. Such a particle A' can be produced in electron-nucleus fixed-target scattering and then decay to an e + e- pair, producing a narrow resonance in the QED trident spectrum. Using APEX test run data, we searched in the mass range 175-250 MeV, found no evidence for an A'→ e+ e- reaction, and set an upper limit of α'/α ~/= 10(-6). Our findings demonstrate that fixed-target searches can explore a new, wide, and important range of masses and couplings for sub-GeV forces.     

Electrical measurements of nanoscale bismuth cluster films

A range of percolating atomic cluster films, with nanoscale overall dimensions, have been studied using a combination of in situ and ex situ electrical transport measurements, together with field emission electron microscopy and atomic force microscopy. Bismuth clusters with mean diameter 20 nm were deposited between electrical contacts defined by electron beam lithography. The morphology of the films can be understood within percolation theory, and the electrical measurements show complex behaviour characteristic of both percolation effects and modification of the cluster films by current flow and by oxidation.     

结构非线性分析中求解预定荷载水平的改进弧长法

本文对目前广泛应用于结构非线性分析之中的弧长法进行改进。改进后的弧长法除保持原有优点外，能够在自动跟踪结构非线性平衡路径的同时，进一步求得位于结构平衡睡径任一区段的任意预先指定的荷载水平及相应的变形。本文的方法可以推广应用于求解预先指定的应力或位移。数值算例表明了本文方法的计算精度、效率及可靠性。     

Morphological differences between Bi, Ag and Sb nano-particles and how they affect the percolation of current through nano-particle networks

Nano-particles of Bi, Ag and Sb have been produced in an inert gas aggregation source and deposited between lithographically defined electrical contacts on SiN. The morphology of these films have been examined by atomic force microscopy and scanning electron microscopy. The Bi nano-particles stick well to the SiN substrate and take on a flattened dome shape. The Ag nano-particles also stick well to the SiN surface; however they retain a more spherical shape. Whereas, many of the Sb nano-particles bounce off the SiN surface with only a small fraction of the Sb nano-particles aggregating at defects resulting in a non-random distribution of the clusters. These nano-scale differences in the film morphology influence the viability of applying percolation theory to in situ macroscopic measurements of the film conductivity, during the deposition process. For Bi and Ag nano-particles the increase in conductivity follows a power law. The power law exponent, t, was found to be 1.27 ±0.13 and 1.40 ±0.14, for Bi and Ag respectively, in agreement with theoretical predictions of t ≈1.3 for 2D random continuum percolation networks. Sb cluster networks do not follow this model and due to the majority of the Sb clusters bouncing off the surface. Differences in the current onset times and final conductance values of the films are also discussed.     

Seltene Erden

Ohne Zusammenfassung     

Construction and Application of a UHV Compatible Cluster Deposition System

The design and operation of a new UHV-compatible atomic cluster deposition system is described. The design is optimised for high cluster fluxes and for the production of cluster-assembled nano-devices. One key feature of the system is a high degree of flexibility, including interchangeable sputtering and inert gas aggregation sources, and two kinds of mass spectrometer, which allow both characterisation of the cluster size distribution and deposition of mass-selected clusters. Another key feature is that clusters are deposited onto electrically contacted lithographically defined devices mounted on an UHV-compatible cryostat cold finger, allowing deposition at room temperature as well as cryogenic and elevated temperatures. In-situ electrical characterisation of cluster-assembled devices can then be performed in the temperature range 1.2--75 K.     

Das Atomgewicht des Cadmiums

Ohne Zusammenfassung     

N-Nosyl Oxaziridines as Terminal Oxidants in Copper(II)-Catalyzed Olefin Oxyaminations

We report that N-4-nosyl-3-phenyloxaziridine is an effective terminal oxidant for copper(II)-catalyzed oxyamination recently developed in our labs. This oxaziridine can be prepared on multi-gram scale and is easily purified by recrystallization. The products of oxyamination using this oxaziridine bear protecting groups that can be readily removed in high yields under mild conditions.