3,419 research outputs found
Electron-Phonon Driven Spin Frustration in Multi-Band Hubbard Models: MX Chains and Oxide Superconductors
We discuss the consequences of both electron-phonon and electron-electron
couplings in 1D and 2D multi-band (Peierls-Hubbard) models. After briefly
discussing various analytic limits, we focus on (Hartree-Fock and exact)
numerical studies in the intermediate regime for both couplings, where unusual
spin-Peierls as well as long-period, frustrated ground states are found. Doping
into such phases or near the phase boundaries can lead to further interesting
phenomena such as separation of spin and charge, a dopant-induced phase
transition of the global (parent) phase, or real-space (``bipolaronic'')
pairing. We discuss possible experimentally observable consequences of this
rich phase diagram for halogen-bridged, transition metal, linear chain
complexes (MX chains) in 1D and the oxide superconductors in 2D.Comment: 6 pages, four postscript figures (appended), in regular Te
Niobrara National Scenic River, 1985-2000: Old arguments, new compromises
In May 1991, President George H. Bush signed into law the Niobrara Scenic River Designation Act, which gave federal scenic-river designation to a 70-mile stretch of this northern-Nebraska river. The successful effort to protect this river was a protracted, often acrimonious battle, pitting Nebraska neighbors against each other. Interested parties found themselves on opposing sides of a seemingly insurmountable divide, either believing that this river resource should be given federal protection to preserve it unimpaired for future generations, or arguing that the local people should be allowed to determine the fate of “their” river without federal interference. The twentieth century West has seen this same battle waged many times before the Niobrara case. From Hetch-Hetchy to Echo Park to Glen Canyon; from Buffalo River to Auburn Dam to the Sagebrush Rebellion, the debate has been much the same. Those who favor federal protective legislation contend that only the federal government has the wherewithal and the power to assure that these fragile resources are protected from development and short-sighted exploitation. Those opposed to federal designation argue that any such preservation actions would compromise their freedoms and property rights. The impetus for federal designation came from a group of landowners along the river, who first organized in 1980 and lobbied U.S. Senator J. James Exon to introduce federal legislation protecting the river. He did so in 1985. This initiated a six-year process of meetings, discussions, editorializing, angry rhetoric, and finally compromise, involving Nebraska’s entire Congressional delegation, three governors, countless local officials, and a number of the state’s newspapers. This thesis will consider the federal, state and local efforts that led to the designation of the Niobrara as a federal scenic river, and the efforts at managing the park in its first decade of existence
Fluid Delivery System for a Cell Culture on a Microfluidic Chip
This project report provides a description of the progress made in the development of a fluid delivery system for a microfluidic cell culture on a chip. The system is intended to be used in a humidified incubator in a university laboratory and the fluid delivery system is required to exist and operate within that incubator for extended periods of time. Therefore, the system will be gravity-driven and contain no electronic components. The key specification of the system is to provide fluid flow at a constant velocity.
After manufacturing and testing the device, all specifications were met except for the fluid velocity remaining constant over extended periods of time. This report will go into detail on the results of the tests that passed and why this specification was not met during testing, as well as future recommendations for this system
Irradiance calibration with solar diffuser
The sun's energy is used in combination of movable and fixed diffuser plates, windows and apertures which are positioned in a series of test sequences (modes) for reflectance monitoring and calibration without the use of man-made sources. There are three embodiments, or implementations, of the invention--one embodiment uses two diffusers--a working diffuser and a secondary diffuser--the second embodiment uses three diffusers, a working diffuser, a secondary diffuser and a reference diffuser--and the third embodiment uses two diffusers--a working diffuser and a secondary diffuser, the latter also functioning as a cover for the working diffuser. The movable diffusers are mounted on rotatable cones and, in all embodiments, the sun is blocked from reaching the diffusers when not in use. Thus, the sun is used as a stable source for calibration and monitoring and the sun/diffuser combination is used in such a way that the response of all elements of the optical subsystem of the TOMS can be unambiguously and efficiently characterized with high accuracy and precision
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