A Tidally-Disrupted Asteroid Around the White Dwarf G29-38

The infrared excess around the white dwarf G29-38 can be explained by emission from an opaque flat ring of dust with an inner radius 0.14 of the radius of the Sun and an outer radius approximately equal to the Sun's. This ring lies within the Roche region of the white dwarf where an asteroid could have been tidally destroyed, producing a system reminiscent of Saturn's rings. Accretion onto the white dwarf from this circumstellar dust can explain the observed calcium abundance in the atmosphere of G29-38. Either as a bombardment by a series of asteroids or because of one large disruption, the total amount of matter accreted onto the white dwarf may have been comparable to the total mass of asteroids in the Solar System, or, equivalently, about 1% of the mass in the asteroid belt around the main sequence star zeta Lep.Comment: ApJ Letters, in pres

Large Magnetoresistance in Co/Ni/Co Ferromagnetic Single Electron Transistors

We report on magnetotransport investigations of nano-scaled ferromagnetic Co/Ni/Co single electron transistors. As a result of reduced size, the devices exhibit single electron transistor characteristics at 4.2K. Magnetotransport measurements carried out at 1.8K reveal tunneling magnetoresistance (TMR) traces with negative coercive fields, which we interpret in terms of a switching mechanism driven by the shape anisotropy of the central wire-like Ni island. A large TMR of about 18% is observed within a finite source-drain bias regime. The TMR decreases rapidly with increasing bias, which we tentatively attribute to excitation of magnons in the central island.Comment: 12 pages (including 4 figures). Accepted for publishing on AP

Theory of Tunneling Spectroscopy in a Mn12_{12} Single-Electron Transistor by Density-Functional Theory Methods

We consider tunneling transport through a Mn$_{12}$ molecular magnet using spin density functional theory. A tractable methodology for constructing many-body wavefunctions from Kohn-Sham orbitals allows for the determination of spin-dependent matrix elements for use in transport calculations. The tunneling conductance at finite bias is characterized by peaks representing transitions between spin multiplets, separated by an energy on the order of the magnetic anisotropy. The energy splitting of the spin multiplets and the spatial part of their many-body wave functions, describing the orbital degrees of freedom of the excess charge, strongly affect the electronic transport, and can lead to negative differential conductance.Comment: 4 pages, 3 figures, a revised version with minor change

High resolution measurements of kinetic energy release distributions of neon, argon, and krypton cluster ions using a three sector field mass spectrometer

Using a newly constructed three sector field mass spectrometer (resulting in a BE1E2 field configuration) we have measured the kinetic energy release distributions of neon, argon, and krypton cluster ions. In the present study we used the first two sectors, B and E1, constituting a high resolution mass spectrometer, to select the parent ions in terms of mass, charge, and energy, and studied the decay of those ions in the third field free region. Due to the improved mass resolution we were able to extend earlier studies carried out with a two sector field machine, where an upper size limit arose from the fact that several isotopomers contribute to a decaying parent ion beam when the cluster size exceeds a certain value. Furthermore we developed a new data analysis. It allows us to model also fragment ion peaks that are a superposition of different decay reactions and thus we can determine the average kinetic energy release for all decay reactions of a given cluster ion. In a further step we used these results to determine the binding energies of cluster ions Rg(n) (ngreater than or equal to10) by applying finite heat bath theory. The smaller sizes have not been included in this analysis, because the validity of finite heat bath theory becomes questionable below napproximate to10. The present average kinetic energy releases and binding energies are compared with other experiments and various calculations. (C) 2004 American Institute of Physics

Correlation Clustering Based Coalition Formation For Multi-Robot Task Allocation

In this paper, we study the multi-robot task allocation problem where a group of robots needs to be allocated to a set of tasks so that the tasks can be finished optimally. One task may need more than one robot to finish it. Therefore the robots need to form coalitions to complete these tasks. Multi-robot coalition formation for task allocation is a well-known NP-hard problem. To solve this problem, we use a linear-programming based graph partitioning approach along with a region growing strategy which allocates (near) optimal robot coalitions to tasks in a negligible amount of time. Our proposed algorithm is fast (only taking 230 secs. for 100 robots and 10 tasks) and it also finds a near-optimal solution (up to 97.66% of the optimal). We have empirically demonstrated that the proposed approach in this paper always finds a solution which is closer (up to 9.1 times) to the optimal solution than a theoretical worst-case bound proved in an earlier work

General method for extracting the quantum efficiency of dispersive qubit readout in circuit QED

We present and demonstrate a general three-step method for extracting the quantum efficiency of dispersive qubit readout in circuit QED. We use active depletion of post-measurement photons and optimal integration weight functions on two quadratures to maximize the signal-to-noise ratio of the non-steady-state homodyne measurement. We derive analytically and demonstrate experimentally that the method robustly extracts the quantum efficiency for arbitrary readout conditions in the linear regime. We use the proven method to optimally bias a Josephson traveling-wave parametric amplifier and to quantify different noise contributions in the readout amplification chain.Comment: 10 pages, 6 figure

Building Community, Fostering Collaboration, and Engaging Bridge Program Students with a college’s historical archives

Similar to smaller archives, this college’s archives have not been traditionally accessible online. Two instructors sought to teach summer bridge program (Boot Camp) students basic archival practices and quantitatively measure their information literacy skills through using the Information Literacy Skills (ILA) and Students’ Perceptions of their Information Skills-Questionnaire (SPIL-Q) instruments (cite). Boot Camp students’ average perceived confidence with IL skills as assessed by the SPIL-Q instrument increased from 4.00 to 4.77 (+19.2%) on the post-training SPIL-Q. By adding the ILA and SPIL-Q instruments to the course curriculum, combined with end of course reflection questions, the instructors were able to quantitatively determine if the students’ comprehension of evaluating information improved after handling, processing, and digitizing primary source documents. This study demonstrates the opportunities for community building and collaboration afforded by archivists and librarians engaging faculty and students with primary source exploration through college archives

Effect of the C-bridge length on the ultraviolet-resistance of oxycarbosilane low-k films

The ultra-violet (UV) and vacuum ultra-violet (VUV) resistance of bridging alkylene groups in organosilica films has been investigated. Similar to the Si-CH3 (methyl) bonds, the Si-CH2-Si (methylene) bonds are not affected by 5.6 eV irradiation. On the other hand, the concentration of the Si-CH2-CH2-Si (ethylene) groups decreases during such UV exposure. More significant difference in alkylene reduction is observed when the films are exposed to VUV (7.2 eV). The ethylene groups are depleted by more than 75% while only about 40% methylene and methyl groups loss is observed. The different sensitivity of bridging groups to VUV light should be taken into account during the development of curing and plasma etch processes of low-k materials based on periodic mesoporous organosilicas and oxycarbosilanes. The experimental results are qualitatively supported by ab-initio quantum-chemical calculations

Decreased glutamate transporter (GLT-1) expression in frontal cortex of rats with acute liver failure

It has been suggested that reduced astrocytic uptake of neuronally released glutamate contributes to the pathogenesis of hepatic encephalopathy in acute liver failure. In order to further address this issue, the recently cloned and sequenced astrocytic glutamate transporter GLT-1 was studied in brain preparations from rats with ischemic liver failure induced by portacaval anastomosis followed 24 h later by hepatic artery ligation and from appropriate sham-operated controls. GLT-1 expression was studied using reverse transcriptase-polymerase chain reaction (RT-PCR). Expression of GLT-1 transcript was significantly decreased in frontal cortex at coma stages of acute liver failure. Western blotting using a polyclonal antibody to GLT-1 revealed a concomitant decrease in expression of transporter protein in the brains of rats with acute liver failure. Reduced capacity of astrocytes to reuptake neuronally released glutamate, resulting from a GLT-1 transporter deficit and the consequently compromised neuron-astrocytic trafficking of glutamate could contribute to the pathogenesis of hepatic encephalopathy and brain edema, two major complications of acute liver failure