Analytical solution of generalized Burton--Cabrera--Frank equations for growth and post--growth equilibration on vicinal surfaces

We investigate growth on vicinal surfaces by molecular beam epitaxy making use of a generalized Burton--Cabrera--Frank model. Our primary aim is to propose and implement a novel analytical program based on a perturbative solution of the non--linear equations describing the coupled adatom and dimer kinetics. These equations are considered as originating from a fully microscopic description that allows the step boundary conditions to be directly formulated in terms of the sticking coefficients at each step. As an example, we study the importance of diffusion barriers for adatoms hopping down descending steps (Schwoebel effect) during growth and post-growth equilibration of the surface.Comment: 16 pages, REVTeX 3.0, IC-DDV-94-00

Efecto de dos fuentes y cinco dosis de potasio sobre la acumulacion de fitomasa y la absorcion de nutrientes en maiz para ensilaje

Resumen (Spanish, English)66 P.Se sembró un híbrido de maíz semitardío (Asgrow 899) el 20 de Noviembre de 1994 en la Estación Experimental de la Facultad de Ciencias Agrarias de la Universidad de Talca, para estudiar el efecto de cinco dosis de potasio: 0, 50, 100, 150 y 200 kg de K2O ha-1, proveniente de dos fuentes de fertilizante; nitrato y cloruro de potasio, sobre la acumulación de fitomasa y la absorción de N, K+, Mg++, Ca++ y Na+. Las combinaciones de dosis y fuentes de potasio se analizaron mediante un experimento factorial distribuido en bloques al azar, con cuatro repeticiones. Se realizaron tres evaluaciones a partir de emergencia de plantas, en los estados 2.5, 5 y 8 de la escala de Hanway, midiendo fitomasa seca acumulada y concentraciones (%) de N, K+, Mg++, Ca++ y Na+ en plantas. Los resultados mostraron una positiva respuesta a la aplicación de K para acumulación de fitomasa (BMS), indistintamente de la fuente fertilizante, alcanzando las mayores respuestas con la aplicación de 50 kg de K2O ha-1 y, a partir 100 K2O ha-1, rendimientos máximos. Dosis superiores a las señaladas sólo incrementaron el contenido de K+ en plantas. Las concentraciones de N, K+ y Mg++ en plantas disminuyeron a través de la temporada; Ca++ y Na+ aumentaron hacia la madurez. El contenido de N en plantas aumentó por la aplicación de K, pero los incrementos fueron significativos hasta dosis de 50 kg de K2O ha-1, existiendo una relación lineal entre la absorción total de N y la acumulación de fitomasa. El cloruro no afectó la absorción de N. La absorción total de K+ aumentó en forma sostenida con aumentos de la fertilización potásica. La concentración promedio de Mg++ y de Na+ en plantas disminuyó por la aplicación de K, específicamente; con dosis mayores a 50 kg de K2O ha-1; esto sugiere un antagonismo entre K+ y Mg++ y, entre K+ y Na+, por competencia de sitios de unión a los transportadores de la membrana plasmática durante la absorción. La concentración de Ca++ aumentó por la fertilización potásica y sólo con dosis de 200 kg de K2O ha-1, la concentración final de Ca++ tendió a bajar, siendo similar a la agregación de 50 kg de K2O ha-1. Se discute la existencia de un antagonismo entre K+ y Ca++. Pese a estos resultados, la composición promedio de las plantas no se vio afectada, con respecto a los estándares nutricionales

Subtidal macrozoobenthos communities from northern Chile during and post El Niño 1997–1998

Despite a large amount of climatic and oceanographic information dealing with the recurring climate phenomenon El Niño (EN) and its well known impact on diversity of marine benthic communities, most published data are rather descriptive and consequently our understanding of the underlying mechanisms and processes that drive community structure during EN are still very scarce. In this study, we address two questions on the effects of EN on macrozoobenthic communities: (1) how does EN affect species diversity of the communities in northern Chile? and (2) is EN a phenomenon that restarts community assembling processes by affecting species interactions in northern Chile? To answer these questions, we compared species diversity and co-occurrence patterns of soft-bottoms macrozoobenthos communities from the continental shelf off northern Chile during (March 1998) and after (September 1998) the strong EN event 1997–1998. The methods used varied from species diversity and species co-occurrence analyses to multivariate ordination methods. Our results indicate that EN positively affects diversity of macrozoobenthos communities in the study area, increasing the species richness and diversity and decreasing the species dominance. EN represents a strong disturbance that affects species interactions that rule the species assembling processes in shallow-water, sea-bottom environments

Effect of Workload History on Task Performance

Objective: This study investigated the effects of workload history (specifically, sudden shifts in workload) on performance. Background: In 1993 the National Research Council identified workload transition as an important concern for human factors researchers. The study of workload history suggests that what an individual has been doing prior to a point in time has an effect on subsequent performance. One trend emerging from workload history studies is that a general decrement in performance is most likely to occur following a decrease in task demand. Method: The 198 participants were randomly assigned to a high-to-low or low-to-high condition. Participants performed a version of the Bakan Vigilance Task while correct responses, response times, and total errors were recorded. Results: Results supported previous research suggesting a workload decrease results in a performance decrement. More importantly, this study reports that either a sudden increase or decrease could lead to a loss in accuracy and a slowing of response time in a longer time course. Conclusion: An explanation of the decrement is offered in terms of adaptation models. In addition, a follow-up study suggested that the decrement is a result of something inherent in the workload shift rather than an effect of fatigue. Application: Workload history (more specifically, a workload shift) has significant implications for many work environments. These implications are particularly salient in occupations where individuals are confronted with varying levels of workload demand, especially safety-sensitive occupations.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline

A new perspective of the climatological features of upper-level cut-off lows in the Southern Hemisphere

This study presents a detailed view of the seasonal variability of upper-level cut-off lows (COLs) in the Southern Hemisphere. The COLs are identified and tracked using data from a 36-year period of the European Centre for Medium Range Weather Forecast reanalysis (ERA-Interim). The objective identification of the COLs uses a new approach, which is based on 300 hPa relative vorticity minima, and three restrictive criteria of the presence of a cold-core, stratospheric potential vorticity intrusion, and cut-off cyclonic circulation. The highest COL activity is in agreement with previous studies, located near three main continental areas (Australia, South America, and Africa), with maximum frequencies usually observed in the austral autumn. The COL mean intensity values show a marked seasonal and spatial variation, with maximum (minimum) values during the austral winter (summer), a unique feature that has not been observed previously in studies based on the geopotential. The link between intensity and lysis is examined, and finds that weaker systems are more susceptible to lysis in the vicinity of the Andes Cordillera, associated with the topographic Rossby wave. Lysis and genesis regions are close to each other, confirming that COLs are quasi-stationary systems. Also, COLs tend to move eastward and are faster over the higher latitudes. The mean growth/decay rates coincide with the major genesis and lysis density regions, such as the significant decay values across the Andes all year. As a consequence of using vorticity for the tracking method a longer lifetime of COLs is detected than in other studies, but this does not affect the total frequency of occurrence. Comparisons with other studies suggest that the differences in seasonality are due to uncertainties in the reanalyses and the methods used to identify COLs

Broker Fixed: The Racialized Social Structure and the Subjugation of Indigenous Populations in the Andes

Responding to calls to return racial analysis to indigenous Latin America, this article moves beyond the prejudicial attitudes of dominant groups to specify how native subordination gets perpetuated as a normal outcome of the organization of society. I argue that a naturalized system of indirect rule racially subordinates native populations through creating the position of mestizo “authoritarian intermediary.” Natives must depend on these cultural brokers for their personhood, while maintaining this privileged position requires facilitating indigenous exploitation. Institutional structures combine with cultural practices to generate a vicious cycle in which increased village intermediary success increases native marginalization. This racialized social structure explains my ethnographic findings that indigenous villagers continued to support the same coterie of mestizos despite their regular and sometimes extreme acts of peculation. My findings about the primacy of race suggest new directions for research into indigenous studies, ethnic mobilizations, and the global dimensions of racial domination