116 research outputs found
General boundary conditions for the envelope function in multiband k.p model
We have derived general boundary conditions (BC) for the multiband envelope
functions (which do not contain spurious solutions) in semiconductor
heterostructures with abrupt heterointerfaces. These BC require the
conservation of the probability flux density normal to the interface and
guarantee that the multiband Hamiltonian be self--adjoint. The BC are energy
independent and are characteristic properties of the interface. Calculations
have been performed of the effect of the general BC on the electron energy
levels in a potential well with infinite potential barriers using a coupled two
band model. The connection with other approaches to determining BC for the
envelope function and to the spurious solution problem in the multiband k.p
model are discussed.Comment: 15 pages, 2 figures; to be published in Phys. Rev. B 65, March 15
issue 200
Comparison of hypoxia among four river-dominated ocean margins: The Changjiang (Yangtze), Mississippi, Pearl, and RhĂŽne rivers
We examined the occurrence of seasonal hypoxia (O2<2 mg l-1) in the bottom waters of four river-dominated ocean margins (off the Changjiang, Mississippi, Pearl and RhĂŽne Rivers) and compared the processes leading to the depletion of oxygen. Consumption of oxygen in bottom waters is linked to biological oxygen demand fueled by organic matter from primary production in the nutrient-rich river plume and perhaps terrigenous inputs. Hypoxia occurs when this consumption exceeds replenishment by diffusion, turbulent mixing or lateral advection of oxygenated water. The margins off the Mississippi and Changjiang are affected the most by summer hypoxia, while the margins off the RhĂŽne and the Pearl rivers systems are less affected, although nutrient concentrations in the river water are very similar in the four systems. Spring and summer primary production is high overall for the shelves adjacent to the Mississippi, Changjiang and Pearl (1-10 g C m-2 d-1), and lower off the RhĂŽne River (<1 g C m-2 d-1), which could be one of the reasons of the absence of hypoxia on the RhĂŽne shelf. The residence time of the bottom water is also related to the occurrence of hypoxia, with the Mississippi margin showing a long residence time and frequent occurrences of hypoxia during summer over very large spatial scales, whereas the East China Sea (ECS)/Changjiang displays hypoxia less regularly due to a shorter residence time of the bottom water. Physical stratification plays an important role with both the Changjiang and Mississippi shelf showing strong thermohaline stratification during summer over extended periods of time, whereas summer stratification is less prominent for the Pearl and RhĂŽne partly due to the wind effect on mixing. The shape of the shelf is the last important factor since hypoxia occurs at intermediate depths (between 5 and 50 m) on broad shelves (Gulf of Mexico and ECS). Shallow estuaries with low residence time such as the Pearl River estuary during the summer wet season when mixing and flushing are dominant features, or deeper shelves, such as the Gulf of Lion off the RhĂŽne show little or no hypoxia
Narrative inquiry into (re)imagining alternative schools: a case study of Kevin Gonzales.
Although there are many alternative schools that strive for the successful education for their students, negative images of alternative schools persist. While some alternative schools are viewed as âidealistic havens,â many are viewed as âdumping grounds,â or âjuvenile detention centers.â Employing narrative inquiry, this article interrogates how a student, Kevin Gonzales, experiences his alternative education and raises questions about the role of alternative schools. Kevin Gonzalesâs story is presented in a literary form of biographical journal to provide a âmetaphoric loftâ that helps us imagine other students like Kevin. This, in turn, provokes us to examine our current educational practice, and to (re)imagine ways in which alternative education can provide the best possible educational experiences for disenfranchised students who are increasingly underserved by the public education system
A review of the stable istotope bio-geochemistry of the global silicon cycle and its associated trace elements
Silicon (Si) is the second most abundant element in the Earth's crust and is an important nutrient in the ocean. The global Si cycle plays a critical role in regulating primary productivity and carbon cycling on the continents and in the oceans. Development of the analytical tools used to study the sources, sinks, and fluxes of the global Si cycle (e.g., elemental and stable isotope ratio data for Ge, Si, Zn, etc.) have recently led to major advances in our understanding of the mechanisms and processes that constrain the cycling of Si in the modern environment and in the past. Here, we provide background on the geochemical tools that are available for studying the Si cycle and highlight our current understanding of the marine, freshwater and terrestrial systems. We place emphasis on the geochemistry (e.g., Al/Si, Ge/Si, Zn/Si, ÎŽ13C, ÎŽ15N, ÎŽ18O, ÎŽ30Si) of dissolved and biogenic Si, present case studies, such as the Silicic Acid Leakage Hypothesis, and discuss challenges associated with the development of these environmental proxies for the global Si cycle. We also discuss how each system within the global Si cycle might change over time (i.e., sources, sinks, and processes) and the potential technical and conceptual limitations that need to be considered for future studies
Response of a Lake Michigan coastal lake to anthropogenic catchment disturbance
A paleolimnological investigation of post-European sediments in a Lake Michigan coastal lake was used to examine the response of Lower Herring Lake to anthropogenic impacts and its role as a processor of watershed inputs. We also compare the timing of this response with that of Lake Michigan to examine the role of marginal lakes as âearly warningâ indicators of potential changes in the larger connected system and their role in buffering Lake Michigan against anthropogenic changes through biotic interactions and material trapping. Sediment geochemistry, siliceous microfossils and nutrient-related morphological changes in diatoms, identified three major trophic periods in the recent history of the lake. During deforestation and early settlement (pre-1845â1920), lake response to catchment disturbances results in localized increases in diatom abundances with minor changes in existing communities. In this early phase of disturbance, Lower Herring Lake acts as a sediment sink and a biological processor of nutrient inputs. During low-lake levels of the 1930s, the lake goes through a transitional period characterized by increased primary productivity and a major shift in diatom communities. Post-World War II (late 1940sâ1989) anthropogenic disturbances push Lower Herring Lake to a new state and a permanent change in diatom community structure dominated by Cyclotella comensis . The dominance of planktonic summer diatom species associated with the deep chlorophyll maximum (DCM) is attributed to epilimnetic nutrient depletion. Declining Si:P ratios are inferred from increased sediment storage of biogenic silica and morphological changes in the silica content of Aulacoseira ambigua and Stephanodiscus niagarae . Beginning in the late 1940s, Lower Herring Lake functions as a biogeochemical processor of catchment inputs and a carbon, nutrient and silica sink. Microfossil response to increased nutrients and increased storage of biogenic silica in Lower Herring Lake and other regional embayments occur approximately 20â25Â years earlier than in a nearby Lake Michigan site. Results from this study provide evidence for the role of marginal lakes and bays as nutrient buffering systems, delaying the impact of anthropogenic activities on the larger Lake Michigan system.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43091/1/10933_2004_Article_1688.pd
Astronomical Distance Determination in the Space Age: Secondary Distance Indicators
The formal division of the distance indicators into primary and secondary leads to difficulties in description of methods which can actually be used in two ways: with, and without the support of the other methods for scaling. Thus instead of concentrating on the scaling requirement we concentrate on all methods of distance determination to extragalactic sources which are designated, at least formally, to use for individual sources. Among those, the Supernovae Ia is clearly the leader due to its enormous success in determination of the expansion rate of the Universe. However, new methods are rapidly developing, and there is also a progress in more traditional methods. We give a general overview of the methods but we mostly concentrate on the most recent developments in each field, and future expectations. © 2018, The Author(s)
ErosĂŁo hĂdrica pĂłs-plantio em florestas de eucalipto na bacia do rio ParanĂĄ, no leste do Mato Grosso do Sul
Nas regiĂ”es tropicais, o desgaste provocado no solo por ação das ĂĄguas da chuva, ou seja, a erosĂŁo hĂdrica Ă© a mais importante forma de degradação do solo. Visto que os plantios florestais de eucalipto estĂŁo inseridos em ecossistemas sensĂveis Ă s perturbaçÔes antrĂłpicas em razĂŁo de ocorrĂȘncia de plantaçÔes em solos com baixos teores de argila, com baixa fertilidade natural e grande parte das plantaçÔes estabelecidas em antigas ĂĄreas agrĂcolas e de pastagens degradadas, surge a necessidade do entendimento dos processos que regem a erosĂŁo hĂdrica e suas relaçÔes com as perdas de solo e ĂĄgua nos sistemas florestais. Objetivaram-se com este trabalho calcular os valores de erosividade da chuva (fator R - EI30), estimar a tolerĂąncia de perda de solo (T) para as classes representativas nas ĂĄreas de estudo, avaliar as perdas de solo e ĂĄgua por erosĂŁo hĂdrica e verificar a influĂȘncia, por meio de anĂĄlise de componentes principais (ACP), de atributos fĂsicos e matĂ©ria orgĂąnica do solo sobre a erosĂŁo hĂdrica em florestas de eucalipto no estĂĄdio de pĂłs-plantio. Os tratamentos constituĂram de diferentes sistemas de manejo dos resĂduos e da disposição de plantio (nĂvel e desnĂvel), em dois biomas distintos, Cerrado e Floresta, e solo descoberto. Os solos foram classificados como Latossolo Vermelho distrĂłfico tĂpico textura mĂ©dia-alta fase floresta (LVd1) e Latossolo Vermelho distrĂłfico tĂpico textura mĂ©dia-baixa fase cerrado (LVd2). O estudo foi realizado em ĂĄreas experimentais de plantio de eucalipto localizadas no municĂpio de TrĂȘs Lagoas, na bacia do Rio ParanĂĄ, no leste do Mato Grosso do Sul. O Ăndice de erosividade anual obtido foi de 6.792,7 MJ mm ha-1 h-1 ano-1. Os valores de T variaram de 9,0 a 11,0 Mg ha-1 ano-1, para o LVd2 e LVd1, respectivamente. As perdas de solo apresentaram valores em torno de 0 a 0,505 Mg ha-1 no LVd1 e de 0 a 0,853 Mg ha-1, no LVd2. A ACP evidenciou-se eficiente na discriminação dos sistemas de manejo em razĂŁo da interação entre os atributos fĂsicos e matĂ©ria orgĂąnica do solo e suas relaçÔes com a erosĂŁo hĂdrica, possibilitando visualizar de forma clara a influĂȘncia do manejo sobre esses atributos e a relação de ambos com as perdas de solo e ĂĄgua
Long-Term Changes in Water Quality and Productivity in the Patuxent River Estuary: 1985 to 2003
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