7,346 research outputs found

    Morphological instability, evolution, and scaling in strained epitaxial films: An amplitude equation analysis of the phase field crystal model

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    Morphological properties of strained epitaxial films are examined through a mesoscopic approach developed to incorporate both the film crystalline structure and standard continuum theory. Film surface profiles and properties, such as surface energy, liquid-solid miscibility gap and interface thickness, are determined as a function of misfit strains and film elastic modulus. We analyze the stress-driven instability of film surface morphology that leads to the formation of strained islands. We find a universal scaling relationship between the island size and misfit strain which shows a crossover from the well-known continuum elasticity result at the weak strain to a behavior governed by a "perfect" lattice relaxation condition. The strain at which the crossover occurs is shown to be a function of liquid-solid interfacial thickness, and an asymmetry between tensile and compressive strains is observed. The film instability is found to be accompanied by mode coupling of the complex amplitudes of the surface morphological profile, a factor associated with the crystalline nature of the strained film but absent in conventional continuum theory.Comment: 16 pages, 10 figures; to be published in Phys. Rev.

    Topographic Regions Maps

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    Snowex 2017 Community Snow Depth Measurements: A Quality-Controlled, Georeferenced Product

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    Snow depth was one of the core ground measurements required to validate remotely-sensed data collected during SnowEx Year 1, which occurred in Colorado. The use of a single, common protocol was fundamental to produce a community reference dataset of high quality. Most of the nearly 100 Grand Mesa and Senator Beck Basin SnowEx ground crew participants contributed to this crucial dataset during 6-25 February 2017. Snow depths were measured along ~300 m transects, whose locations were determined according to a random-stratified approach using snowfall and tree-density gradients. Two-person teams used snowmobiles, skis, or snowshoes to travel to staked transect locations and to conduct measurements. Depths were measured with a 1-cm incremented probe every 3 meters along transects. In shallow areas of Grand Mesa, depth measurements were also collected with GPS snow-depth probes (a.k.a. MagnaProbes) at ~1-m intervals. During summer 2017, all reference stake positions were surveyed with <10 cm accuracy to improve overall snow depth location accuracy. During the campaign, 193 transects were measured over three weeks at Grand Mesa and 40 were collected over two weeks in Senator Beck Basin, representing more than 27,000 depth values. Each day of the campaign depth measurements were written in waterproof field books and photographed by National Snow and Ice Data Center (NSIDC) participants. The data were later transcribed and prepared for extensive quality assessment and control. Common issues such as protocol errors (e.g., survey in reverse direction), notebook image issues (e.g., halo in the center of digitized picture), and data-entry errors (sloppy writing and transcription errors) were identified and fixed on a point-by-point basis. In addition, we strove to produce a georeferenced product of fine quality, so we calculated and interpolated coordinates for every depth measurement based on surveyed stakes and the number of measurements made per transect. The product has been submitted to NSIDC in csv format. To educate data users, we present the study design and processing steps that have improved the quality and usability of this product. Also, we will address measurement and design uncertainties, which are different in open vs. forest areas

    Arbitration Preparation: A Union Rep\u27s Perspective

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    Who Stays and Who Leaves? Predicting College Student Persistence Using Comprehensive Retention Models

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    The purpose of this study was to use a comprehensive framework to examine academic, psychosocial, noncognitive, and other background factors that are related to retention at a large, public four-year institution in the southeastern United States. Specifically, the study examined what factors are most important in predicting first-to-second year retention both before the student enrolls at the university and after completion of their first semester of coursework. Data were drawn from institutional records, a survey instrument designed to measure psychosocial constructs, the ACT student record, and the National Center for Education Statistics. The sample for the study consisted of 12,342 students. Hierarchical generalized linear models and ensemble tree-based methods were utilized to identify important predictors of retention, ascertain the nature of the significant relationships, and to build models for predicting retention outcomes. An initial model was built for prediction before students enrolled followed by a second model with first semester performance variables added. Predictive validity was assessed by splitting the sample into a training and test set. Findings from the study showed that nontraditional factors were significant predictors of retention along with traditional predictors such as high school GPA. The results showed that the influence of financial factors and high school characteristics were among the most significant predictors of retention. Moreover, the results showed that multiple psychosocial factors are influential variables in retention outcomes. This study demonstrated that considering a variety of factors when forecasting postsecondary retention outcomes is vital for more accurate predictions. The models in this study showed that pre-college predictive models have the potential to be nearly as effective as models incorporating college performance and activity. The results of this study have important implications for higher education policymakers, college administrators, and high schools. Several of the relationships revealed have significant policy implications related to budget concerns, university programming, and college preparatory initiatives at the high school level. The study also provides a useful model for identifying students at risk of not being retained that could be adapted for implementation at other institutions and points the importance of a holistic understanding of the total student

    Surveillance and Spies in the Civil War: Exposing Confederate Conspiracies in America\u27s Heartland

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    Review of: Surveillance and Spies in the Civil War: Exposing Confederate Conspiracies in America’s Heartland, by Stephen E. Town

    A Punishment on the Nation: An Iowa Soldier Endures the Civil War

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    Review of: "A Punishment on the Nation: An Iowa Soldier Endures the Civil War," edited by Brian Craig Miller, part of the "Civil War in the North" series
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