Analytical ground state for the three-band Hubbard model

For the calculation of charge excitations as those observed in, e.g., photo-emission spectroscopy or in electron-energy loss spectroscopy, a correct description of ground-state charge properties is essential. In strongly correlated systems like the undoped cuprates this is a highly non-trivial problem. In this paper we derive a non-perturbative analytical approximation for the ground state of the three-band Hubbard model on an infinite, half filled CuO_2 plane. By comparison with Projector Quantum Monte Carlo calculations it is shown that the resulting expressions correctly describe the charge properties of the ground state. Relations to other approaches are discussed. The analytical ground state preserves size consistency and can be generalized for other geometries, while still being both easy to interpret and to evaluate.Comment: REVTeX, 8 pages, 6 figures, to appear in Phys. Rev.

Topological Aspects of Spin and Statistics in Nonlinear Sigma Models

We study the purely topological restrictions on allowed spin and statistics of topological solitons in nonlinear sigma models. Taking as space the connected $d$-manifold $X$, and considering nonlinear sigma models with the connected manifold $M$ as target space, topological solitons are given by elements of $pi_d(M)$. Any topological soliton $\alpha \in \pi_d(M)$ determines a quotient \Stat_n(X,\alpha) of the group of framed braids on $X$, such that choices of allowed statistics for solitons of type $\alpha$ are given by unitary representations of \Stat_n(X,\alpha) when $n$ solitons are present. In particular, when $M = S^2$, as in the $O(3)$ nonlinear sigma model with Hopf term, and $\alpha \in \pi_2(S^2)$ is a generator, we compute that \Stat_n(\R^2,\alpha) = \Z, while \Stat_n(S^2,\alpha) = \Z_{2n}. It follows that phase $\exp(i\theta)$ for interchanging two solitons of type $\alpha$ on $S^2$ must satisfy the constraint $\theta = k\pi/n$, $k \in \Z$, when $n$ such solitons are present.Comment: 14 page

Large magnetoresistance effect due to spin-injection into a non-magnetic semiconductor

A novel magnetoresistance effect, due to the injection of a spin-polarized electron current from a dilute magnetic into a non-magnetic semiconductor, is presented. The effect results from the suppression of a spin channel in the non-magnetic semiconductor and can theoretically yield a positive magnetoresistance of 100%, when the spin flip length in the non-magnetic semiconductor is sufficiently large. Experimentally, our devices exhibit up to 25% magnetoresistance.Comment: 3 figures, submitted for publicatio

A compact 90 kilowatt electric heat source for heating inert gases to 1700 F

Design and fabrication of compact electric heat source for heating inert gase

Design and fabrication of a low-specific-weight parabolic dish solar concentrator

A segmented design and fabrication and assembly techniques were developed for a 1.8 m (6 ft) diameter parabolic concentrator for space application. This design and these techniques were adaptable to a low cost, mass-produced concentrator. Minimal machining was required. Concentrator segments of formed magnesium were used. The concentrator weighed only 1.6 kg sq m (0.32 lbm/sq ft)

Theoretical investigation into the possibility of very large moments in Fe16N2

We examine the mystery of the disputed high-magnetization \alpha"-Fe16N2 phase, employing the Heyd-Scuseria-Ernzerhof screened hybrid functional method, perturbative many-body corrections through the GW approximation, and onsite Coulomb correlations through the GGA+U method. We present a first-principles computation of the effective on-site Coulomb interaction (Hubbard U) between localized 3d electrons employing the constrained random-phase approximation (cRPA), finding only somewhat stronger on-site correlations than in bcc Fe. We find that the hybrid functional method, the GW approximation, and the GGA+U method (using parameters computed from cRPA) yield an average spin moment of 2.9, 2.6 - 2.7, and 2.7 \mu_B per Fe, respectively.Comment: 8 pages, 3 figure

Identification of Sources of Ground Water Salinization Using Geochemical Techniques

This report deals with saltwater sources that commonly mix and deteriorate fresh groundwater. It reviews characteristics of saltwater sources and geochemical techniques that can be used to identify these sources after mixing has occurred. The report is designed to assist investigators of saltwater problems in a step-by-step fashion. Seven major sources of saltwater are distinguished: (1) Natural saline groundwater, (2) Halite solution, (3) Seawater intrusion, (4) Oil and gas field brines, (5) Agricultural effluents, (6) Saline seep, and (7) Road salting. The geographic distribution of these sources was mapped individually and together, illustrating which ones are potential sources at any given area in the United States. In separate sections, each potential source is then discussed in detail regarding physical and chemical characteristics, examples of known techniques for identification of mixtures between freshwater and that source, and known occurrences by state. Individual geochemical parameters that are used within these techniques are presented in a separate section, followed by a discussion concerning where and how to obtain them. Also provided is a description of basic graphical and statistical methods that are used frequently in saltwater studies. An extensive list of references for further study concludes this report.Bureau of Economic Geolog

Hydrochemical Characterization of Saline Aquifers of the Texas Gulf Coast Used for Disposal of Industrial Waste

Disposal of toxic chemical wastes into geologic formations in the deep subsurface and the number of disposal wells used have increased sharply during the last 30 years. In Texas, permits for more than 200 waste disposal wells, which accept approximately 16 percent of the chemical wastes generated in the United States, were issued by the Texas Department of Water Resources (TDWR) during this period (Knape, 1984). Most waste disposal wells use porous, saline sandstone aquifers along the Texas Gulf Coast (fig. 1). Simultaneously, fresh groundwater is recovered from shallower, updip sections of these aquifers. Therefore, integrity of disposal zones is of importance both on a statewide and on a nationwide level. Waste injected into subsurface formations may react with formation fluid or formation material, depending on the chemical and physical nature of the phases involved. Aquifer material may dissolve or mineral matter may precipitate, thus changing original compositions of disposal zones. It is desirable to predict these changes to assure successful operation of injection practices. To satisfactorily evaluate the suitability of deep-well injection into Gulf Coast formations, hydrochemical and hydrogeological parameters of these aquifers are currently under study by the Bureau of Economic Geology, The University of Texas at Austin. This report represents the first stage of the characterization of saline aquifers in the Gulf Coast that are used for industrial waste disposal and summarizes geochemical parameters of disposal zones.Bureau of Economic Geolog