Reflection high-energy electron diffraction studies of the growth of lnAs/Ga_(1-x)In_xSb strained-layer superlattices

We have used reflection high‐energy electron diffraction to study the surface periodicity of the growth front of InAs/GaInSb strained‐layer superlattices (SLSs). We found that the apparent surface lattice spacing reproducibly changed during layers which subsequent x‐ray measurements indicated were coherently strained. Abrupt changes in the measured streak spacings were found to be correlated to changes in the growth flux. The profile of the dynamic streak spacing was found to be reproducible when comparing consecutive periods of a SLSs or different SLSs employing the same shuttering scheme at the InAs/GaInSb interface. Finally, when the interface shuttering scheme was changed, it was found that the dynamic streak separation profile also changed. Large changes in the shuttering scheme led to dramatic differences in the streak separation profile, and small changes in the shuttering scheme led to minor changes in the profile. In both cases, the differences in the surface periodicity profile occurred during the parts of the growth where the incident fluxes differed

Negative Specific Heat in a Quasi-2D Generalized Vorticity Model

Negative specific heat is a dramatic phenomenon where processes decrease in temperature when adding energy. It has been observed in gravo-thermal collapse of globular clusters. We now report finding this phenomenon in bundles of nearly parallel, periodic, single-sign generalized vortex filaments in the electron magnetohydrodynamic (EMH) model for the unbounded plane under strong magnetic confinement. We derive the specific heat using a steepest descent method and a mean field property. Our derivations show that as temperature increases, the overall size of the system increases exponentially and the energy drops. The implication of negative specific heat is a runaway reaction, resulting in a collapsing inner core surrounded by an expanding halo of filaments.Comment: 12 pages, 3 figures; updated with revision

Simulations with Complex Measures

Towards a solution to the sign problem in the simulations of systems having indefinite or complex-valued measures, we propose a new approach which yields statistical errors smaller than the crude Monte Carlo using absolute values of the original measures. The 1D complex-coupling Ising model is employed as an illustration.Comment: 3 pages, postcript (95K), contribution to LAT93, UM-P-93/10

Transport characteristics of L-point and Г-point electrons through GaAs-Ga_(1-x)Ai_xAs-GaAs(111} double heterojunctions

We present here a study on the transport characteristics of L‐point and Γ‐point derived electrons through abrupt GaAs–Ga_(1−x)Al_xAs–GaAs(111) double heterojunctions. The use of complex‐k band structures in the tight‐binding approximation and transfer matrices provide a reasonably accurate description of the wave function at the GaAs–Ga_(1−x)Al_xAs interface. A representation of the wave function in terms of bulk complex‐k Bloch states is used in the GaAs regions where the potential is bulklike. A representation of the wave function in terms of planar orbitals is used in the central Ga_(1−x)Al_xAs region where the potential deviates from its bulk value (i.e., interfacial region). Within this theoretical framework, realistic band structure effects are taken into account and no artificial rules regarding the connection of the wave function across the interface are introduced. The ten‐band tight‐binding model includes admixture in the total wave function of states derived from different extrema of the GaAs conduction band. States derived from the same extremum of the conduction band appear to couple strongly to each other, whereas states derived from different extrema are found to couple weakly. Transport characteristics of incoming L‐point and Γ‐point Bloch states are examined as a function of the energy of the incoming state, thickness of the Ga_(1−x)Al_xAs barrier, and alloy composition x. Transmission through the Ga_(1−x)Al_xAs barrier is either tunneling or propagating depending on the nature of the Bloch states available for strong coupling in the alloy. Since Bloch states derived from different extrema of the conduction band appear to couple weakly to each other, it seems possible to reflect the low velocity L‐point component of the current while transmitting the high velocity Γ‐point component

Cooperative emission of a pulse train in an optically thick scattering medium

An optically thick cold atomic cloud emits a coherent flash of light in the forward direction when the phase of an incident probe field is abruptly changed. Because of cooperativity, the duration of this phenomena can be much shorter than the excited lifetime of a single atom. Repeating periodically the abrupt phase jump, we generate a train of pulses with short repetition time, high intensity contrast and high efficiency. In this regime, the emission is fully governed by cooperativity even if the cloud is dilute.Comment: 5 pages, 3 figure

Fatigue tester achieves true axial motion through flex plates and bars

Lever load-amplifying fatigue testing machine with a load cycle frequency of 100 to 900 cycles per minute applies the load through true axial motion. Pivot friction and bearing wear are eliminated by replacing these parts with flex plates and bars

Effect of Land Application of Poultry Waste on Pesticide Loss

The poultry industry in Arkansas is a large, concentrated, growing industry that produces a high volume of fecal waste. Most of this waste is surface applied as pasture fertilizer. Pesticides are commonly used in the poultry industry for fly and litter beetle contro land are often a component of the surface-applied poultry waste. No information exists in the scientific literature regarding the transport of this pesticide component to nearby water supplies.Our research focused on cyromazine, a feed-through larvicide used to control flies in caged-layer hen houses. Tetrachlorvinphos and carbaryl are also used in poultry waste, but these pesticides have a relatively low solubility in water and rapid decomposition rate. Cyromazine, however, is highly soluble and stable in water. Since it may be readily washed from the pasture by heavy rainfall and may persist in surface and soil water, cyromazine appears to be potentially a much greater long-term threat to water quality than either carbaryl or tetrachlorvinphos. Therefore, the objective of this investigation was to examine the extent of cyromazine loss as a result of land application of caged layer manure. To quantify cyromazine loss from pasture plots treated with caged layer manure,research was conducted at the University of Arkansas Agricultural Experiment Station at Fayetteville. Plots of uniform slope were bordered to isolate surface runoff, fitted with runoff collectors, and established in fescue pasture. Suction lysimeters were placed at the 60 cm depth to sample soil water in the unsaturated zone. Caged layer manure was analyzed for cyromazine concentration and applied to the plots at three different rates. Rainfall was applied by simulator at two intensities. Surface runoff and lysimeter samples were measured and analyzed for cyromazine concentration. A solid phase extraction procedure was used to separate the cyromazine from the water samples and analysis was done by high performance liquid chromatography (HPLC). Results showed that a heavier manure application increased both the runoff and cyromazine concentration. Higher rainfall intensity also increased total cyromazine loss in the runoff, but provided enough runoff volume to decrease the cyromazine concentration. Soil water from the unsaturated zone was monitored for a year following the manure application, but neither cyromazine nor its metabolite, melamine, were detected

Indirect search for supersymmetry in rare B decays

QCD corrections to the gluino induced contribution to b --> s gamma are shown to be important in order to extract reliable bounds on the off-diagonal elements of the squark mass matrices.Comment: 4 pages including 2 postscript figure