Pattern formation with trapped ions

Ion traps are a versatile tool to study nonequilibrium statistical physics, due to the tunability of dissipation and nonlinearity. We propose an experiment with a chain of trapped ions, where dissipation is provided by laser heating and cooling, while nonlinearity is provided by trap anharmonicity and beam shaping. The collective dynamics are governed by an equation similar to the complex Ginzburg-Landau equation, except that the reactive nature of the coupling leads to qualitatively different behavior. The system has the unusual feature of being both oscillatory and excitable at the same time. We account for noise from spontaneous emission and find that the patterns are observable for realistic experimental parameters. Our scheme also allows controllable experiments with noise and quenched disorder.Comment: 4 pages + appendi

The impact of new signals on precise marine navigation - initial results from an experiment in Harwich Harbour

The General Lighthouse Authorities of the United Kingdom and Ireland (GLAs) are supporting a project at University College London (UCL) to study whether it is possible to meet the International Maritime Organisation’s (IMO) future requirements for port and harbour approach using future GNSS constellations, as detailed in IMO resolution A.915. This paper presents the results of a trial focusing on the accuracy, integrity, availability and continuity of port navigation, port approach, and docking. Abstract The required accuracy for docking is 0.1 m (95\%), which currently necessitates the use of Real Time Kinematic (RTK) processing. We consider the single-epoch geometry-based approach, which is robust against loss of lock and will fully benefit from the additional satellites. The trial was held at the beginning of May 2008 and saw THV Alert navigate into Harwich Harbour while satellite observation data were recorded from the vessel and from shore-based reference stations. Additional data were obtained from nearby Ordnance Survey reference stations, and two total stations were used to track the vessel’s passage to provide a truth model. Several modernised GPS satellites were tracked. The data were processed under different scenarios, using software developed at UCL, and the positioning performance analysed. Abstract Providing integrity for single-epoch RTK is particularly difficult. The identification of phase observation outliers is not possible before the integer ambiguities are resolved, but an undetected outlier could prevent successful ambiguity resolution. However, it will not always be necessary to fix every ambiguity to achieve the required precision, particularly with a multi-GNSS constellation. This paper introduces a new algorithm for partial ambiguity resolution in the presence of measurement bias that has been developed and tested at UCL.  This algorithm results in an improved ambiguity resolution success rate at the expense of computation time

A Dynamic Look at Subprime Loan Performance

Does delinquency have any predictive power for the future performance of a mortgage? Analysis of a sample of subprime mortgages from the Loanperformance database on securitized private-label pool collateral using a two-step estimation procedure to control for the endogeneity of delinquency reveals strong support for the distressed prepayment theory that very delinquent loans are more likely to prepay than to default and that prepayment rates increase substantially as delinquency intensity increases. While delinquency leads predominantly to termination of a loan through prepayment, negative equity leads to termination through default. Does delinquency have any predictive power for the future performance of a mortgage? Analysis of a sample of subprime mortgages from the Loanperformance database on securitized private-label pool collateral using a two-step estimation procedure to control for the endogeneity of delinquency reveals strong support for the distressed prepayment theory that very delinquent loans are more likely to prepay than to default and that prepayment rates increase substantially as delinquency intensity increases. While delinquency leads predominantly to termination of a loan through prepayment, negative equity leads to termination through default

The Delinquency of Subprime Mortgages

The lag between the time that a borrower stops making payments on a mortgage and the termination of the loan plays a critical role in the costs borne by both borrower and lender on defaulted loans. While the prior literature uses a multinomial logit approach, statistical tests indicate that we cannot accept the associated assumption of Independence of Irrelevant Alternatives (IIA). Using a nested logit specification our results suggest that the recipe for delinquency involves young loans to low credit score borrowers with low or no documentation in housing markets with moderately volatile and flat or declining nominal house prices

Amplitude-equation formalism for four-wave-mixing geometry with transmission gratings

An amplitude equation is derived for a four-wave-mixing geometry with nearly counterpropagating, mutually incoherent, nondiffracting pump beams, spatially overlapping in a photorefractive material with a nonlocal response. This equation extends the earlier linear two-dimensional theory to the weakly nonlinear regime. The analysis also starts from a more complete equation for the photorefractive effect, which leads to the prediction of novel effects especially apparent in the nonlinear regime. Precise predictions for the spatiotemporal behavior of the grating amplitude in the nonlinear regime are presented. The range of validity of the amplitude equation is studied. The characteristics of the instability in the nonlinear regime are analyzed through a front-selection analysis

Domain Coarsening in Systems Far from Equilibrium

The growth of domains of stripes evolving from random initial conditions is studied in numerical simulations of models of systems far from equilibrium such as Rayleigh-Benard convection. The scaling of the size of the domains deduced from the inverse width of the Fourier spectrum is studied for both potential and nonpotential models. The morphology of the domains and the defect structures are however quite different in the two cases, and evidence is presented for a second length scale in the nonpotential case.Comment: 11 pages, RevTeX; 3 uufiles encoded postscript figures appende

A vapor barrier for cold testing printed circuit cards

Cold testing method prevents formation of frost on printed circuit boards and part holders during testing at sub-zero temperatures. Freon permits rapid attainment of the required testing temperature

Hysteresis and Post Walrasian Economics

Macroeconomics, hysteresis The “new consensus” dsge (dynamic stochastic general equilibrium) macroeconomic model has microfoundations provided by a single representative agent. In this model shocks to the economic environment do not have any lasting effects. In reality adjustments at the micro level are made by heterogeneous agents, and the aggregation problem cannot be assumed away. In this paper we show that the discontinuous adjustments made by heterogeneous agents at the micro level mean that shocks have lasting effects, aggregate variables containing a selective, erasable memory of the shocks experienced. This hysteresis framework provides foundations for the post-Walrasian analysis of macroeconomic systems