Design and Performance of a Practical Variable-Temperature Scanning Tunneling Potentiometry System

We have constructed a scanning tunneling potentiometry system capable of simultaneously mapping the transport-related electrochemical potential of a biased sample along with its surface topography. Combining a novel sample biasing technique with a continuous current-nulling feedback scheme pushes the noise performance of the measurement to its fundamental limit - the Johnson noise of the STM tunnel junction. The resulting 130 nV voltage sensitivity allows us to spatially resolve local potentials at scales down to 2 nm, while maintaining angstrom scale STM imaging, all at scan sizes of up to 15 um. A mm-range two-dimensional coarse positioning stage and the ability to operate from liquid helium to room temperature with a fast turn-around time greatly expand the versatility of the instrument. By performing studies of several model systems, we discuss the implications of various types of surface morphology for potentiometric measurements.Comment: 16 pages, 17 figures, accepted to Review of Scientific Instruments v2 - minor changes: cleaned up figures/figure caption

Spectroscopy of Globular Clusters in the Sculptor Group Galaxies NGC 253 and NGC 55

We have obtained spectra for 103 published globular cluster candidates in the Sculptor Group galaxies NGC 253 and NGC 55. On the basis of radial velocities and digitized plate images, 14 globular clusters are identified in NGC 253 and one probable globular cluster is identified in NGC 55. The majority of the objects in the sample appear to be background galaxies. We have obtained and analysed COSMOS plate scans of NGC 253 and NGC 55 and use these along with the spectroscopically identified clusters to define new samples of globular cluster candidates in the two galaxies which should have reduced contamination.Comment: 11 pages, 9 postscript figures, accepted for publication in MNRA

Causes, Consequences and Prevention of Fires in Domestic Refrigeration Systems

This study was proposed to investigate and examine the causes, consequences and prevention of fires occurring in domestic refrigeration appliances. The aims were to analyse such incidents, examine their characteristics and understand the underlying ignition and fire spread mechanisms that have led to their occurrence and how they might be prevented. The reasons for the cause and spread of domestic refrigeration fires have been examined, using information obtained from the analysis of fire data sets available in Great Britain and on the basis of fire investigations carried out in London. Visits to refrigerator disposal sites and local authority amenity centres also provided information on changes to appliance construction techniques and component use over several decades. Analysis of the available data suggests that once ignition occurs, fires caused by fridge/freezers are more likely to exhibit a higher degree of fire spread and produce greater levels of damage than other types of white goods appliance (washing machine, dishwasher or tumble dryer). Nearly 80% of fires with fridge/freezers as the source of ignition, spread beyond the first item involved, whilst almost 40% spread beyond the room of origin. Fires involving fridge/freezers also displayed a far higher casualty rate per fire than was found for the other types of appliance. Based upon evidence obtained from fire investigations a number of common failure modes leading to ignition in domestic refrigeration fires have been identified: (i) starter relay failures; (ii) PTC switch failures; (iii) mechanical defrost switch failures; (iv) capacitor failures; (v) solenoid valve failures; (vi) cut-out switch failures in integrated appliances, and (vii) rodents. Specific fire escalation and spread mechanisms have also been identified: plastic drip trays, “twin-wall” backing materials and polyurethane foam insulation panels. There is also evidence to suggest that the severity of refrigeration fires in Great Britain is significantly higher than in the USA. Based upon information obtained from LFB fridge and freezer fire investigations, and a comparison between the design and construction of refrigeration appliances used in Great Britain and USA, a number of recommendations have been made which could be used to significantly reduce the risk of a serious fire e.g. avoiding the usage of plastics in appliance housings and in particular employing a metal/non-combustible covering at the back of fridge and freezer appliances and ensuring that insulating foam is separated from the spread of fire by fire resisting material

The central engines of radio-quiet quasars

Two rival hypotheses have been proposed for the origin of the compact radio flux observed in radio-quiet quasars (RQQs). It has been suggested that the radio emission in these objects, typically some two or three orders of magnitude less powerful than in radio-loud quasars (RLQs), represents either emission from a circumnuclear starburst or is produced by radio jets with bulk kinetic powers 10^3 times lower than those of RLQs with similar luminosity ratios in other wavebands. We describe the results of high resolution (parsec-scale) radio-imaging observations of a sample of 12 RQQs using the Very Long Baseline Array (VLBA). We find strong evidence for jet-producing central engines in 8 members of our sample.Comment: 7 pages, 8 figures. Accepted for publication in MNRA

Toric Duality Is Seiberg Duality

We study four N=1 SU(N)^6 gauge theories, with bi-fundamental chiral matter and a superpotential. In the infrared, these gauge theories all realize the low-energy world-volume description of N coincident D3-branes transverse to the complex cone over a del Pezzo surface dP_3 which is the blowup of P^2 at three generic points. Therefore, the four gauge theories are expected to fall into the same universality class--an example of a phenomenon that has been termed "toric duality." However, little independent evidence has been given that such theories are infrared-equivalent. In fact, we show that the four gauge theories are related by the N=1 duality of Seiberg, vindicating this expectation. We also study holographic aspects of these gauge theories. In particular we relate the spectrum of chiral operators in the gauge theories to wrapped D3-brane states in the AdS dual description. We finally demonstrate that the other known examples of toric duality are related by N=1 duality, a fact which we conjecture holds generally.Comment: 46 pages, 2 figures, harvma

The Atacama Large Millimeter/submillimeter Array: Overview & status

The Atacama Large Millimeter/submillimeter Array (ALMA) is an international radio telescope under construction in the Atacama Desert of northern Chile. ALMA will be situated on a high-altitude site at 5000 m elevation which provides excellent atmospheric transmission over the instrument wavelength range of 0.3 to 3 mm. ALMA will be comprised of two key observing components: an array of up to sixty-four 12-m diameter antennas arranged in a multiple configurations ranging in size from 0.15 to ~14 km, and a set of four 12-m and twelve 7-m antennas operating in closely-packed configurations ~50m in diameter (known as the Atacama Compact Array, or ACA), providing both interferometric and total-power astronomical information. High-sensitivity dual-polarization 8 GHz-bandwidth spectral-line and continuum measurements between all antennas will be available from two flexible digital correlators

Scheduling aircraft landings - the static case

This is the publisher version of the article, obtained from the link below.In this paper, we consider the problem of scheduling aircraft (plane) landings at an airport. This problem is one of deciding a landing time for each plane such that each plane lands within a predetermined time window and that separation criteria between the landing of a plane and the landing of all successive planes are respected. We present a mixed-integer zero–one formulation of the problem for the single runway case and extend it to the multiple runway case. We strengthen the linear programming relaxations of these formulations by introducing additional constraints. Throughout, we discuss how our formulations can be used to model a number of issues (choice of objective function, precedence restrictions, restricting the number of landings in a given time period, runway workload balancing) commonly encountered in practice. The problem is solved optimally using linear programming-based tree search. We also present an effective heuristic algorithm for the problem. Computational results for both the heuristic and the optimal algorithm are presented for a number of test problems involving up to 50 planes and four runways.J.E.Beasley. would like to acknowledge the financial support of the Commonwealth Scientific and Industrial Research Organization, Australia