Two-dimensional Hubbard-Holstein bipolaron

We present a diagrammatic Monte Carlo study of the properties of the Hubbard-Holstein bipolaron on a two-dimensional square lattice. With a small Coulomb repulsion, U, and with increasing electron-phonon interaction, and when reaching a value about two times smaller than the one corresponding to the transition of light polaron to heavy polaron, the system suffers a sharp transition from a state formed by two weakly bound light polarons to a heavy, strongly bound on-site bipolaron. Aside from this rather conventional bipolaron a new bipolaron state is found for large U at intermediate and large electron-phonon coupling, corresponding to two polarons bound on nearest-neighbor sites. We discuss both the properties of the different bipolaron states and the transition from one state to another. We present a phase diagram in parameter space defined by the electron-phonon coupling and U. Our numerical method does not use any artificial approximation and can be easily modified to other bipolaron models with longer range electron-phonon and/or electron-electron interaction.Comment: 14 pages, 12 figure

Density Variations in the NW Star Stream of M31

The Pan Andromeda Archeological Survey (PAndAS) CFHT Megaprime survey of the M31-M33 system has found a star stream which extends about 120 kpc NW from the center of M31. The great length of the stream, and the likelihood that it does not significantly intersect the disk of M31, means that it is unusually well suited for a measurement of stream gaps and clumps along its length as a test for the predicted thousands of dark matter sub-halos. The main result of this paper is that the density of the stream varies between zero and about three times the mean along its length on scales of 2 to 20 kpc. The probability that the variations are random fluctuations in the star density is less than 10^-5. As a control sample we search for density variations at precisely the same location in stars with metallicity higher than the stream, [Fe/H]=[0, -0.5] and find no variations above the expected shot noise. The lumpiness of the stream is not compatible with a low mass star stream in a smooth galactic potential, nor is it readily compatible with the disturbance caused by the visible M31 satellite galaxies. The stream's density variations appear to be consistent with the effects of a large population of steep mass function dark matter sub-halos, such as found in LCDM simulations, acting on an approximately 10Gyr old star stream. The effects of a single set of halo substructure realizations are shown for illustration, reserving a statistical comparison for another study.Comment: ApJ revised version submitte

The Full-sky Astrometric Mapping Explorer -- Astrometry for the New Millennium

FAME is designed to perform an all-sky, astrometric survey with unprecedented accuracy. It will create a rigid astrometric catalog of 4x10^7 stars with 5 < m_V < 15. For bright stars, 5 < m_V < 9, FAME will determine positions and parallaxes accurate to < 50 microarcseconds, with proper motion errors < 50 microarcseconds/year. For fainter stars, 9 < m_V < 15, FAME will determine positions and parallaxes accurate to < 500 microarcseconds, with proper motion errors < 500 microarcseconds/year. It will also collect photometric data on these 4 x 10^7 stars in four Sloan DSS colors.Comment: 6 pages, 4 figures, to appear in "Working on the Fringe

Consolidation of Geologic Studies of Geopressured Geothermal Resources in Texas

The objective of the current studies at the Pleasant Bayou geopressured geothermal reservoir in Brazoria County, Texas, was to evaluate the resource base and long-term performance. The approach was to develop an integrated understanding of the hydrogeology of the reservoir and the hydrochemistry of the produced brine. Such an understanding would allow determination of the extent of lateral and vertical hydrologic continuity of the target zone and to identify the sources of brine being produced from the geopressured reservoir. The current phase of long-term production testing of the Frio C-zone at Pleasant Bayou Well No. 2 began in May 1988. During the past 16 months of production, nearly 6.8 million barrels of brine and 162.2 million cubic feet of gas have been produced, and a relatively small (less than 300 psi) drop in bottom-hole pressure has been observed at sustained producing rates of between 15,000 and 20,000 barrels per day. Earlier geologic studies have estimated the effective pore volume of the C-zone in the neighborhood of 6.2 to 6.6 billion barrels. Analysis of pressure and production data from current testing indicates that the limits of the geopressured reservoir at Pleasant Bayou have not been reached; that is, either the size of the reservoir could be larger than anticipated, or there could be a continuous influx of waters from other geopressured sources that sustains the reservoir energy at Pleasant Bayou. Geochemical testing has proved inconclusive in identifying other sources of water partly because of the variability of chemical composition within the produced zone. Evaluation of reservoir performance at active oil and gas fields in the immediate vicinity of the Pleasant Bayou fault block has not provided evidence of direct hydrologic communication between the geopressured aquifer and the overlying hydrocarbon reservoirs. The pattern of depletion in these oil and gas fields reflects some characteristic features that may become evident in Pleasant Bayou over a long period of production. Moreover, additional refinement of the integrated hydrogeologic-hydrochemical model is possible either through prolonged testing at Pleasant Bayou No. 2 well or through drilling and testing of additional wells in the Pleasant Bayou fault block. Determining the nature of bounding faults around the test well will require additional seismic data as well as multiwell testing of the reservoir.Bureau of Economic Geolog

Shear Alignment and Instability of Smectic Phases

We consider the shear flow of well-aligned one-component smectic phases, such as thermotropic smectics and lamellar diblock copolymers, below the critical region. We show that, as a result of thermal fluctuations of the layers, parallel ($c$) alignment is generically unstable and perpendicular ($a$) alignment is stable against long-wavelength undulations. We also find, surprisingly, that both $a$ and $c$ are stable for a narrow window of values for the anisotropic viscosity.Comment: To appear in PRL. Revtex, 1 figure

The classical limit for a class of quantum baker's maps

We show that the class of quantum baker's maps defined by Schack and Caves have the proper classical limit provided the number of momentum bits approaches infinity. This is done by deriving a semi-classical approximation to the coherent-state propagator.Comment: 18 pages, 5 figure

Proton-pump inhibitor use is associated with low serum magnesium concentrations

Although case reports link proton-pump inhibitor (PPI) use and hypomagnesemia, no large-scale studies have been conducted. Here we examined the serum magnesium concentration and the likelihood of hypomagnesemia ( < 1.6 mg/dl) with a history of PPI or histamine-2 receptor antagonist used to reduce gastric acid, or use of neither among 11,490 consecutive adult admissions to an intensive care unit of a tertiary medical center. Of these, 2632 patients reported PPI use prior to admission, while 657 patients were using a histamine-2 receptor antagon ist. PPI use was associated with 0.012 mg/dl lower adjusted serum magnesium concentration compared to users of no acid-suppressive medications, but this effect was restricted to those patients taking diuretics. Among the 3286 patients concurrently on diuretics, PPI use was associated with a significant increase of hypomagnesemia (odds ratio 1.54) and 0.028 mg/dl lower serum magnesium concentration. Among those not using diuretics, PPI use was not associated with serum magnesium levels. Histamine-2 receptor antagonist use was not significantly associated with magnesium concentration without or with diuretic use. The use of PPI was not associated with serum phosphate concentration regardless of diuretic use. Thus, we verify case reports of the association between PPI use and hypomagnesemia in those concurrently taking diuretics. Hence, serum magnesium concentrations should be followed in susceptible individuals on chronic PPI therapy.National Institute of Biomedical Imaging and Bioengineering (U.S.) (Grant 2R01 EB001659

Evaluation Research and Institutional Pressures: Challenges in Public-Nonprofit Contracting

This article examines the connection between program evaluation research and decision-making by public managers. Drawing on neo-institutional theory, a framework is presented for diagnosing the pressures and conditions that lead alternatively toward or away the rational use of evaluation research. Three cases of public-nonprofit contracting for the delivery of major programs are presented to clarify the way coercive, mimetic, and normative pressures interfere with a sound connection being made between research and implementation. The article concludes by considering how public managers can respond to the isomorphic pressures in their environment that make it hard to act on data relating to program performance.This publication is Hauser Center Working Paper No. 23. The Hauser Center Working Paper Series was launched during the summer of 2000. The Series enables the Hauser Center to share with a broad audience important works-in-progress written by Hauser Center scholars and researchers

Prospectus, November 14, 1984

YOUR COURAGE, DEDICATION AND SACRIFICES ARE NOT FORGOTTEN; Parkland Veterans Honored; Women Veterans receive deserved recognition; Landslide of volunteers hoped for; Recognition needed all year; Blood Drive November; Best Sellers in the Library; PC Happenings; Original performance at Parkland; How to Live to be 1000 is retirees topic; Gallery hosts touring exhibit; EMT refresher workshop scheduled; Parkland, GM cooperate; Student job prospects look great; Man on Street...Question: Did you vote on Tuesday, and whatever you answer, why?; Creative Corner...especially for you!; Doom story...The cataclysm begins; Fluffy White Clouds; Nursing on a psychiatric floor; Ember of Empathy; With Feeling; Hurry Monday!; Contradiction versus deception; the eve of destruction; Homo Sapiens; Essence; Straub has not even begun to hit his stride; Good music sometimes underrated; Anothe Bowie hit; Classifieds; Britter\u27s Knights devastate USA Adidas 113-55 in opener; Lady Cobras finish spectacular season with 39-9-1 clipping; Knights down Nonames 100-30; Cobra cagers have flexibility and depth for \u2784-\u2785 campaign; Stater, Phillips supply one-two punch for Cobrashttps://spark.parkland.edu/prospectus_1984/1004/thumbnail.jp

Discovery of New Ultracool White Dwarfs in the Sloan Digital Sky Survey

We report the discovery of five very cool white dwarfs in the Sloan Digital Sky Survey (SDSS). Four are ultracool, exhibiting strong collision induced absorption (CIA) from molecular hydrogen and are similar in color to the three previously known coolest white dwarfs, SDSS J1337+00, LHS 3250 and LHS 1402. The fifth, an ultracool white dwarf candidate, shows milder CIA flux suppression and has a color and spectral shape similar to WD 0346+246. All five new white dwarfs are faint (g > 18.9) and have significant proper motions. One of the new ultracool white dwarfs, SDSS J0947, appears to be in a binary system with a slightly warmer (T_{eff} ~ 5000K) white dwarf companion.Comment: 15 pages, 3 figures, submitted to ApJL. Higher resolution versions of finding charts are available at http://astro.uchicago.edu/~gates/findingchart