On statistically stationary homogeneous shear turbulence

A statistically stationary turbulence with a mean shear gradient is realized in a flow driven by suitable body forces. The flow domain is periodic in downstream and spanwise directions and bounded by stress free surfaces in the normal direction. Except for small layers near the surfaces the flow is homogeneous. The fluctuations in turbulent energy are less violent than in the simulations using remeshing, but the anisotropy on small scales as measured by the skewness of derivatives is similar and decays weakly with increasing Reynolds number.Comment: 4 pages, 5 figures (Figs. 3 and 4 as external JPG-Files

Moving to Extremal Graph Parameters

Which graphs, in the class of all graphs with given numbers n and m of edges and vertices respectively, minimizes or maximizes the value of some graph parameter? In this paper we develop a technique which provides answers for several different parameters: the numbers of edges in the line graph, acyclic orientations, cliques, and forests. (We minimize the first two and maximize the third and fourth.) Our technique involves two moves on the class of graphs. A compression move converts any graph to a form we call fully compressed: the fully compressed graphs are split graphs in which the neighbourhoods of points in the independent set are nested. A second consolidation move takes each fully compressed graph to one particular graph which we call H(n,m). We show monotonicity of the parameters listed for these moves in many cases, which enables us to obtain our results fairly simply. The paper concludes with some open problems and future directions

Alternative Buffer-Layers for the Growth of SrBi2Ta2O9 on Silicon

In this work we investigate the influence of the use of YSZ and CeO2/YSZ as insulators for Metal- Ferroelectric-Insulator-Semiconductor (MFIS) structures made with SrBi2Ta2O9 (SBT). We show that by using YSZ only the a-axis oriented Pyrochlore phase could be obtained. On the other hand the use of a CeO2/YSZ double-buffer layer gave a c-axis oriented SBT with no amorphous SiO2 inter- diffusion layer. The characteristics of MFIS diodes were greatly improved by the use of the double buffer. Using the same deposition conditions the memory window could be increased from 0.3 V to 0.9 V. From the piezoelectric response, nano-meter scale ferroelectric domains could be clearly identified in SBT thin films.Comment: 5 pages, 9 figures, 13 refernece

Money and Goldstone modes

Why is ``worthless'' fiat money generally accepted as payment for goods and services? In equilibrium theory, the value of money is generally not determined: the number of equations is one less than the number of unknowns, so only relative prices are determined. In the language of mathematics, the equations are ``homogeneous of order one''. Using the language of physics, this represents a continuous ``Goldstone'' symmetry. However, the continuous symmetry is often broken by the dynamics of the system, thus fixing the value of the otherwise undetermined variable. In economics, the value of money is a strategic variable which each agent must determine at each transaction by estimating the effect of future interactions with other agents. This idea is illustrated by a simple network model of monopolistic vendors and buyers, with bounded rationality. We submit that dynamical, spontaneous symmetry breaking is the fundamental principle for fixing the value of money. Perhaps the continuous symmetry representing the lack of restoring force is also the fundamental reason for large fluctuations in stock markets.Comment: 7 pages, 3 figure

Moist turbulent Rayleigh-Benard convection with Neumann and Dirichlet boundary conditions

Turbulent Rayleigh-Benard convection with phase changes in an extended layer between two parallel impermeable planes is studied by means of three-dimensional direct numerical simulations for Rayleigh numbers between 10^4 and 1.5\times 10^7 and for Prandtl number Pr=0.7. Two different sets of boundary conditions of temperature and total water content are compared: imposed constant amplitudes which translate into Dirichlet boundary conditions for the scalar field fluctuations about the quiescent diffusive equilibrium and constant imposed flux boundary conditions that result in Neumann boundary conditions. Moist turbulent convection is in the conditionally unstable regime throughout this study for which unsaturated air parcels are stably and saturated air parcels unstably stratified. A direct comparison of both sets of boundary conditions with the same parameters requires to start the turbulence simulations out of differently saturated equilibrium states. Similar to dry Rayleigh-Benard convection the differences in the turbulent velocity fluctuations, the cloud cover and the convective buoyancy flux decrease across the layer with increasing Rayleigh number. At the highest Rayleigh numbers the system is found in a two-layer regime, a dry cloudless and stably stratified layer with low turbulence level below a fully saturated and cloudy turbulent one which equals classical Rayleigh-Benard convection layer. Both are separated by a strong inversion that gets increasingly narrower for growing Rayleigh number.Comment: 19 pages, 13 Postscript figures, Figures 10,11,12,13, in reduced qualit

The Production of Health and the Valuation of Medical Inputs in Wage-Amenity Models

Using a hedonic wage-amenity model, this paper examines the valuation of medical inputs into the production of health. The data used in this study include the incomes, demographics and measures of human capital for households in eastern North Carolina with county level medical input supply. These data allow an estimate of the marginal value of medical care inputs such as the physician to population ratio and the availability of specialized services in an area of the country where the lack of available medical care has been of particular concern to policy makers. Our results indicate that while health care inputs are not a significant determinant of earnings overall, they are important in counties that have been designated as medically underserved. In underserved counties each additional physician per 10,000 individuals in the county decreases earnings by about 11.6%. This suggests that physicians act as an amenity and workers are willing to accept lower wages to locate in counties with a higher physician to population ratio

Phase Coherent Precessional Magnetization Reversal in Micro-scopic Spin Valve Elements

We study the precessional switching of the magnetization in microscopic spin valve cells induced by ultra short in-plane hard axis magnetic field pulses. Stable and highly efficient switching is monitored following pulses as short as 140 ps with energies down to 15 pJ. Multiple application of identical pulses reversibly toggles the cell's magnetization be-tween the two easy directions. Variations of pulse duration and amplitude reveal alter-nating regimes of switching and non-switching corresponding to transitions from in-phase to out-of-phase excitations of the magnetic precession by the field pulse. In the low field limit damping becomes predominant and a relaxational reversal is found allowing switching by hard axis fields below the in-plane anisotropy field threshold.Comment: 17 pages, 4 figure

Quantum repeaters based on entanglement purification

We study the use of entanglement purification for quantum communication over long distances. For distances much longer than the coherence length of a corresponding noisy quantum channel, the fidelity of transmission is usually so low that standard purification methods are not applicable. It is however possible to divide the channel into shorter segments that are purified separately and then connected by the method of entanglement swapping. This method can be much more efficient than schemes based on quantum error correction, as it makes explicit use of two-way classical communication. An important question is how the noise, introduced by imperfect local operations (that constitute the protocols of purification and the entanglement swapping), accumulates in such a compound channel, and how it can be kept below a certain noise level. To treat this problem, we first study the applicability and the efficiency of entanglement purification protocols in the situation of imperfect local operations. We then present a scheme that allows entanglement purification over arbitrary long channels and tolerates errors on the per-cent level. It requires a polynomial overhead in time, and an overhead in local resources that grows only logarithmically with the length of the channel.Comment: 19 pages, 16 figure

Fast high-efficiency integrated waveguide photodetectors using novel hybrid vertical/butt coupling geometry

We report a novel coupling geometry for integrated waveguide photodetectors−a hybrid vertical coupling/butt coupling scheme that allows the integration of fast, efficient, photodetectors with conventional double heterostructure waveguides. It can be employed to yield a planar, or pseudo-planar, surface that supports further levels of integration. The approach is demonstrated with a 25-µm-long p-i-n detector integrated with an InP/InGaAsP/InP waveguide, which displays a high (~90%) efficiency and large (~15 GHz) bandwidth. This is the fastest high-efficiency integrated waveguide photodetector reported to date