Spin lifetimes and strain-controlled spin precession of drifting electrons in zinc blende type semiconductors

We study the transport of spin polarized electrons in n-GaAs using spatially resolved continuous wave Faraday rotation. From the measured steady state distribution, we determine spin relaxation times under drift conditions and, in the presence of strain, the induced spin splitting from the observed spin precession. Controlled variation of strain along [110] allows us to deduce the deformation potential causing this effect, while strain along [100] has no effect. The electric field dependence of the spin lifetime is explained quantitatively in terms of an increase of the electron temperature.Comment: 5 pages, 6 figure

ESTIMATING SITE-SPECIFIC NITROGEN CROP RESPONSE FUNCTIONS: A CONCEPTUAL FRAMEWORK AND GEOSTATISTICAL MODEL

Confirming the precision agriculture hypothesis for variable rate nitrogen applications (VRA) is challenging. To confront this challenge, researchers have used increasingly sophisticated statistical models to estimate and compare site-specific crop response functions (SSCRFs). While progress has been made, it has been hampered by the lack of a conceptual framework to guide the development of appropriate statistical models. This paper provides such a framework and demonstrates its utility by developing a heteroscedastic, fixed and random effects, geostatistical model to test if VRA can increase nitrogen returns. The novelty of the model is the inclusion of site, spatial, treatment, and treatment strip heteroscedasticity and correlation. Applied to data collected in 1995 from two corn nitrogen response experiments in South Central Minnesota, results demonstrate the importance of including site, spatial, treatment, and treatment strip effects in the estimation of SSCRFs. Results also indicate a significant potential for VRA to increase nitrogen returns and that these potential returns increase as the area of the management unit decreases. At one location, there was greater than a 95% chance that VRA could have increased profitability if the cost of implementing VRA was less than 14.5 $ha-1. At the other location, if implementation costs were less than 48.3$ ha-1, there was greater than a 95% chance of increased profitability.Crop Production/Industries,

Evidence of Temporal Variation in Site-Specific Crop Response to Fertilizer Inputs

Replaced with revised version of paper 07/27/06.Crop Production/Industries,

Estimating the Potential Value of Variable Rate Nitrogen Applications: A Comparison of Spatial Econometric and Geostatistical Models

Site-specific crop response functions (SSCRFs) are useful for estimating the value of variable rate nitrogen applications (VRA), but appropriate statistical models are necessary. Problems estimating SSCRFs using experimental field data include region, spatial, treatment, and strip dependent heteroskedasticity and correlation. We develop a spatial autoregressive error (SARE) model for dealing with these problems and compare results with previous analysis based on a geostatistical (GEO) model. VRA value estimates for the two models differ notably for 1995 data from Southern Minnesota. Furthermore, findings show that the results of a comparison of model performance are location specific.geostatistics, precision agriculture, site-specific crop response functions, spatial autoregressive error, variable rate nitrogen application, Crop Production/Industries,

MANAGING PHOSPHOROUS SOIL DYNAMICS OVER SPACE AND TIME

Understanding the relationship between soil fertility dynamics and crop response is conceptually appealing. Even more appealing is comprehension of the spatial and temporal heterogeneity of these connections over a production surface and across seasons. Knowledge of these interactions is complicated because nutrient carryover dynamics and crop response to inputs are determined simultaneously on the one-hand, and sequentially on the other. A second problem enters when crops are rotated, for example, in the corn-soybean system commonly practiced in the Corn Belt. This paper examines the nutrient carryover-crop response nexus using data from a corn-soybean, variable-rate nitrogen (N) and phosphorous (P) experiment conducted over five years. Site-specific corn response to N and P and soybean response to P are simultaneously estimated with a P carryover equation. These estimates are used in a dynamic programming model to map site-specific optimal N and P fertilizer policies, soil P evolution, and profitability. The net present value of managing N and P site-specifically is compared to a strategy where these inputs are managed uniformly following extension guidelines. The results suggest that when P-carryover is managed, site-specific returns to the variable-rate strategies are higher than returns to a conventional, uniform strategy.Crop Production/Industries,

A SYSTEMS APPROACH FOR EVALUATING ON-FARM SITE-SPECIFIC MANAGEMENT TRIALS: A CASE STUDY WITH VARIABLE RATE MANURE AND CROP QUALITY RESPONSE TO INPUTS

Site-specific application of manure has the potential to improve crop production and environmental quality. If manure is applied where it is needed, in the quantity required by the crop, over application, with attendant runoff and leaching problems can be reduced. To implement this approach growers need site-specific crop response information. Increasing availability of site-specific yield information offers a way to estimate such crop responses. The objective of this study is to develop a methodology for estimating site-specific response of corn and soybeans to manure given soil test information, and to use that methodology to analyze an on-farm manure management trial conducted near Sleepy Eye, Minnesota. Both quantity and quality of the crop is considered.Research Methods/ Statistical Methods,

Absence and Disability Management Practices for an Aging Workforce

The goal of Disability and Absence Management programming is to limit absence, control costs, and retain workers to maintain a productive workforce. This can include the development of supportive policies (e.g. flexible work options), manager and employee education, supportive benefit programs, return to work programs, among others. Increasingly, older workers have become a group of interest among Absence and Disability Management professionals, in part because many baby boomers are forgoing retirement and working longer. Projections suggest that by 2020 those 55 and over could account for 25% of workers. This shift is especially important given that disability prevalence increases with age – as the workforce ages, organizations will increasingly need to ensure their programming supports older workers. During the fall and winter of 2012-13, Cornell University’s Employment and Disability Institute and the Disability Management Employer Coalition (DMEC) collaborated on a survey and key informant interviews with DMEC members and conference attendees to learn more about what organizations are doing to respond to and prepare for an aging workforce

THE VALUE OF INFORMATION FOR VARIABLE RATE NITROGEN APPLICATIONS: A COMPARISON OF SOIL TEST, TOPOGRAPHICAL, AND REMOTE SENSING INFORMATION

We explore the value of soil test, topographical, and remote sensing information for guiding variable rate fertilizer applications in corn. Results suggest combining topographical and remote sensing information is more valuable than conventional soil tests. Considered separately, topographical and remote sensing information is not always as valuable as soil tests.Crop Production/Industries,

On the Connection of Anisotropic Conductivity to Tip Induced Space Charge Layers in Scanning Tunneling Spectroscopy of p-doped GaAs

The electronic properties of shallow acceptors in p-doped GaAs{110} are investigated with scanning tunneling microscopy at low temperature. Shallow acceptors are known to exhibit distinct triangular contrasts in STM images for certain bias voltages. Spatially resolved I(V)-spectroscopy is performed to identify their energetic origin and behavior. A crucial parameter - the STM tip's work function - is determined experimentally. The voltage dependent potential configuration and band bending situation is derived. Ways to validate the calculations with the experiment are discussed. Differential conductivity maps reveal that the triangular contrasts are only observed with a depletion layer present under the STM tip. The tunnel process leading to the anisotropic contrasts calls for electrons to tunnel through vacuum gap and a finite region in the semiconductor.Comment: 11 pages, 8 figure

Excitonic photoluminescence in symmetric coupled double quantum wells subject to an external electric field

The effect of an external electric field F on the excitonic photoluminescence (PL) spectra of a symmetric coupled double quantum well (DQW) is investigated both theoretically and experimentally. We show that the variational method in a two-particle electron-hole wave function approximation gives a good agreement with measurements of PL on a narrow DQW in a wide interval of F including flat-band regime. The experimental data are presented for an MBE-grown DQW consisting of two 5 nm wide GaAs wells, separated by a 4 monolayers (MLs) wide pure AlAs central barrier, and sandwiched between Ga_{0.7}Al_{0.3}As layers. The bias voltage is applied along the growth direction. Spatially direct and indirect excitonic transitions are identified, and the radius of the exciton and squeezing of the exciton in the growth direction are evaluated variationally. The excitonic binding energies, recombination energies, oscillator strengths, and relative intensities of the transitions as functions of the applied field are calculated. Our analysis demonstrates that this simple model is applicable in case of narrow DQWs not just for a qualitative description of the PL peak positions but also for the estimation of their individual shapes and intensities.Comment: 5 pages, 4 figures (accepted in Phys. Rev. B