University of Arizona

About: University of Arizona is a education organization based out in Tucson, Arizona, United States. It is known for research contribution in the topics: Population & Galaxy. The organization has 63805 authors who have published 155998 publications receiving 6854915 citations. The organization is also known as: UA & U of A.

Surface-polariton-like waves guided by thin, lossy metal films.

Abstract: The dispersion relations are solved for waves guided by a thin, lossy metal film surrounded by media of dielectric constant ${\ensuremath{\epsilon}}_{1}$ and ${\ensuremath{\epsilon}}_{3}$. For symmetric structures (${\ensuremath{\epsilon}}_{1}$=${\ensuremath{\epsilon}}_{3}$), there are the usual two Fano modes whose velocity and attenuation vary with film thickness. For very thin films, one of these modes can attain multicentimeter propagation distances when \ensuremath{\lambda}g1 \ensuremath{\mu}m. In addition, there are two leaky waves which correspond to waves localized at the ${\ensuremath{\epsilon}}_{1}$ (or ${\ensuremath{\epsilon}}_{3}$) dielectric-metal interface whose fields decay exponentially across the metal film and radiate an angular spectrum of plane waves into ${\ensuremath{\epsilon}}_{3}$ (or ${\ensuremath{\epsilon}}_{1}$, respectively). Both radiative waves can be interpreted as spatial transients, which could have physical significance near a transverse plane. When ${\ensuremath{\epsilon}}_{1}$\ensuremath{ e}${\ensuremath{\epsilon}}_{3}$, there are still four distinct solutions for a given film thickness, two radiative and two nonradiative. For lossy films, there are always two nonradiative solutions for thick enough films. As the thickness goes to infinity, the four solutions reduce to two waves, each radiative and nonradiative pair becoming degenerate. The physical interpretation of these solutions and their dependence on dielectric constant and wavelength are discussed.

Ecological restoration of southwestern ponderosa pine ecosystems: a broad perspective

Abstract: The purpose of this paper is to promote a broad and flexible perspective on ecological restoration of Southwestern (U.S.) ponderosa pine forests. Ponderosa pine forests in the region have been radically altered by Euro-American land uses, including livestock grazing, fire suppression, and logging. Dense thickets of young trees now abound, old- growth and biodiversity have declined, and human and ecological communities are in- creasingly vulnerable to destructive crown fires. A consensus has emerged that it is urgent to restore more natural conditions to these forests. Efforts to restore Southwestern forests will require extensive projects employing varying combinations of young-tree thinning and reintroduction of low-intensity fires. Treatments must be flexible enough to recognize and accommodate: high levels of natural heterogeneity; dynamic ecosystems; wildlife and other biodiversity considerations; scientific uncertainty; and the challenges of on-the-ground im- plementation. Ecological restoration should reset ecosystem trends toward an envelope of ''natural variability,'' including the reestablishment of natural processes. Reconstructed historic reference conditions are best used as general guides rather than rigid restoration prescriptions. In the long term, the best way to align forest conditions to track ongoing climate changes is to restore fire, which naturally correlates with current climate. Some stands need substantial structural manipulation (thinning) before fire can safely be reintro- duced. In other areas, such as large wilderness and roadless areas, fire alone may suffice as the main tool of ecological restoration, recreating the natural interaction of structure and process. Impatience, overreaction to crown fire risks, extractive economics, or hubris could lead to widespread application of highly intrusive treatments that may further damage forest ecosystems. Investments in research and monitoring of restoration treatments are essential to refine restoration methods. We support the development and implementation of a diverse range of scientifically viable restoration approaches in these forests, suggest principles for ecologically sound restoration that immediately reduce crown fire risk and incrementally return natural variability and resilience to Southwestern forests, and present ecological perspectives on several forest restoration approaches.

Path integration and cognitive mapping in a continuous attractor neural network model

Abstract: A minimal synaptic architecture is proposed for how the brain might perform path integration by computing the next internal representation of self-location from the current representation and from the perceived velocity of motion. In the model, a place-cell assembly called a “chart” contains a two-dimensional attractor set called an “attractor map” that can be used to represent coordinates in any arbitrary environment, once associative binding has occurred between chart locations and sensory inputs. In hippocampus, there are different spatial relations among place fields in different environments and behavioral contexts. Thus, the same units may participate in many charts, and it is shown that the number of uncorrelated charts that can be encoded in the same recurrent network is potentially quite large. According to this theory, the firing of a given place cell is primarily a cooperative effect of the activity of its neighbors on the currently active chart. Therefore, it is not particularly useful to think of place cells as encoding any particular external object or event. Because of its recurrent connections, hippocampal field CA3 is proposed as a possible location for this “multichart” architecture; however, other implementations in anatomy would not invalidate the main concepts. The model is implemented numerically both as a network of integrate-and-fire units and as a “macroscopic” (with respect to the space of states) description of the system, based on a continuous approximation defined by a system of stochastic differential equations. It provides an explanation for a number of hitherto perplexing observations on hippocampal place fields, including doubling, vanishing, reshaping in distorted environments, acquiring directionality in a two-goal shuttling task, rapid formation in a novel environment, and slow rotation after disorientation. The model makes several new predictions about the expected properties of hippocampal place cells and other cells of the proposed network.