4,691 research outputs found

    Exchange operator formalism for an infinite family of solvable and integrable quantum systems on a plane

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    The exchange operator formalism in polar coordinates, previously considered for the Calogero-Marchioro-Wolfes problem, is generalized to a recently introduced, infinite family of exactly solvable and integrable Hamiltonians HkH_k, k=1k=1, 2, 3,..., on a plane. The elements of the dihedral group D2kD_{2k} are realized as operators on this plane and used to define some differential-difference operators DrD_r and DφD_{\varphi}. The latter serve to construct D2kD_{2k}-extended and invariant Hamiltonians \chh_k, from which the starting Hamiltonians HkH_k can be retrieved by projection in the D2kD_{2k} identity representation space.Comment: 12 pages, no figure; minor changes; published versio

    A Management Strategy Evaluation of the Impacts of Interspecific Competition and Recreational Fishery Dynamics on Vermilion Snapper (Rhomboplites aurorubens) in the Gulf of Mexico

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    In the Gulf of Mexico (GOM), Vermilion Snapper (Rhomboplites auroruben), are believed to compete with Red Snapper directly for prey and habitat. The two species share similar diets and have significant spatial overlap in the Gulf. Red Snapper are thought to be the dominate competitor, forcing Vermilion Snapper to feed on less nutritious prey when local resources are depleted. In addition to ecological pressures, GOM Vermilion Snapper support substantial commercial and recreational fisheries. Over the past decade, recreational landings have steadily increased, reaching a historical high in 2018. One cause may be stricter regulations for similar target species such as Red Snapper and Gray Triggerfish. A better understanding of the impact of ecosystem and fishery dynamics is essential for successful, long-term management of the stock. In this study, I used management strategy evaluation to assess the effectiveness of current and alternative harvest control rules (HCR) for the stock when accounting for interspecific competition and increased recreational landings. I developed an operating model that simulates the underlying population and fishery dynamics of the Vermilion Snapper stock and includes an index of Red Snapper competition. The annual competition index values were the estimated annual abundance of Red Snapper relative to the total virgin or near-virgin abundances of Vermilion and Red Snapper combined. In the second chapter, I used a random utility model to estimate the probability of a recreational angler targeting Vermilion Snapper given past management for Red Snapper and Gray Triggerfish. I incorporated the predicted targeting probabilities into the operating model from chapter one and evaluated the outcomes of the simulation. In both simulations, catch limits were set using empirical or model-based approaches. I ran 100 trials for each scenario, projected over 50 years. I found that the GOM Vermilion Snapper stock is resilient to competition and increased recreational landings, and all HCR effectively managed the stock. This study provides a methodology to incorporate interspecific dynamics into a single-species assessment model

    Determination of exchange energies in the sawtooth spin ring {Mo75V20} by ESR

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    The magnetism of the polyoxometalate cluster {Mo75V20}, containing a sawtooth ring of 10 corner-sharing triangles located on the equator of the barrel-shaped molecule, has remained debatable since it is masked by contributions from impurities as well as temperature-independent paramagnetism. In this article we demonstrate the usefulness of ESR measurements since the temperature dependence of the ESR intensity can discriminate between impurity and molecular contributions. We determine the exchange parameters and therefore also the low-lying spectrum of {Mo75V20}, especially the low-lying singlet states which so far have been probed solely by specific heat measurements.Comment: 5 pages, 5 figures, submitted to PR

    Madelung potentials and covalency effect in strained La1x_{1-x}Srx_xMnO3_3 thin films studied by core-level photoemission spectroscopy

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    We have investigated the shifts of the core-level photoemission spectra of La0.6_{0.6}Sr0.4_{0.4}MnO3_3 thin films grown on three kinds of substrates, SrTiO3_3, (LaAlO3_3)0.3_{0.3}-(SrAl0.5_{0.5}Ta0.5_{0.5}O3_3)0.7_{0.7}, and LaAlO3_3. The experimental shifts of the La 4d and Sr 3d core levels are almost the same as the calculation, which we attribute to the absence of covalency effects on the Madelung potentials at these atomic sites due to the nearly ionic character of these atoms. On the other hand, the experimental shifts of the O 1s1s and Mn 2p2p core levels are negligibly small, in disagreement with the calculation. We consider that this is due to the strong covalent character of the Mn-O bonds.Comment: 4 pages, 5 figure

    Magnetic field-induced phase transitions in a weakly coupled s = 1/2 quantum spin dimer system Ba3_{3}Cr2_{2}O8_{8}

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    By using bulk magnetization, electron spin resonance (ESR), heat capacity, and neutron scattering techniques, we characterize the thermodynamic and quantum phase diagrams of Ba3_3Cr2_2O8_8. Our ESR measurements indicate that the low field paramagnetic ground state is a mixed state of the singlet and the Sz_z = 0 triplet for HcH \perp c. This suggests the presence of an intra-dimer Dzyaloshinsky-Moriya (DM) interaction with a DM vector perpendicular to the c-axis

    Rofecoxib and cardiovascular adverse events in adjuvant treatment of colorectal cancer

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    Background Selective cyclooxygenase inhibitors may retard the progression of cancer, but they have enhanced thrombotic potential. We report on cardiovascular adverse events in patients receiving rofecoxib to reduce rates of recurrence of colorectal cancer. Methods All serious adverse events that were cardiovascular thrombotic events were reviewed in 2434 patients with stage II or III colorectal cancer participating in a randomized, placebo-controlled trial of rofecoxib, 25 mg daily, started after potentially curative tumor resection and chemotherapy or radiotherapy as indicated. The trial was terminated prematurely owing to worldwide withdrawal of rofecoxib. To examine possible persistent risks, we examined cardiovascular thrombotic events reported up to 24 months after the trial was closed. Results The median duration of active treatment was 7.4 months. The 1167 patients receiving rofecoxib and the 1160 patients receiving placebo were well matched, with a median follow-up period of 33.0 months (interquartile range, 27.6 to 40.1) and 33.4 months (27.7 to 40.4), respectively. Of the 23 confirmed cardiovascular thrombotic events, 16 occurred in the rofecoxib group during or within 14 days after the treatment period, with an estimated relative risk of 2.66 (from the Cox proportional-hazards model; 95% confidence interval [CI], 1.03 to 6.86; P = 0.04). Analysis of the Antiplatelet Trialists’ Collaboration end point (the combined incidence of death from cardiovascular, hemorrhagic, and unknown causes; of nonfatal myocardial infarction; and of nonfatal ischemic and hemorrhagic stroke) gave an unadjusted relative risk of 1.60 (95% CI, 0.57 to 4.51; P = 0.37). Fourteen more cardiovascular thrombotic events, six in the rofecoxib group, were reported within the 2 years after trial closure, with an overall unadjusted relative risk of 1.50 (95% CI, 0.76 to 2.94; P = 0.24). Four patients in the rofecoxib group and two in the placebo group died from thrombotic causes during or within 14 days after the treatment period, and during the follow-up period, one patient in the rofecoxib group and five patients in the placebo group died from cardiovascular causes. Conclusions Rofecoxib therapy was associated with an increased frequency of adverse cardiovascular events among patients with a median study treatment of 7.4 months’ duration. (Current Controlled Trials number, ISRCTN98278138.

    Simulations of Spinodal Nucleation in Systems with Elastic Interactions

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    Systems with long-range interactions quenched into a metastable state near the pseudospinodal exhibit nucleation that is qualitatively different than the classical nucleation observed near the coexistence curve. We have observed nucleation droplets in our Langevin simulations of a two-dimensional model of martensitic transformations and have determined that the structure of the nucleating droplet differs from the stable martensite structure. Our results, together with experimental measurements of the phonon dispersion curve, allow us to predict the nature of the droplet. These results have implications for nucleation in many solid-solid transitions and the structure of the final state

    Nanoscale structuring of tungsten tip yields most coherent electron point-source

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    This report demonstrates the most spatially-coherent electron source ever reported. A coherence angle of 14.3 +/- 0.5 degrees was measured, indicating a virtual source size of 1.7 +/-0.6 Angstrom using an extraction voltage of 89.5 V. The nanotips under study were crafted using a spatially-confined, field-assisted nitrogen etch which removes material from the periphery of the tip apex resulting in a sharp, tungsten-nitride stabilized, high-aspect ratio source. The coherence properties are deduced from holographic measurements in a low-energy electron point source microscope with a carbon nanotube bundle as sample. Using the virtual source size and emission current the brightness normalized to 100 kV is found to be 7.9x10^8 A/sr cm^2

    Linear approach to the orbiting spacecraft thermal problem

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    We develop a linear method for solving the nonlinear differential equations of a lumped-parameter thermal model of a spacecraft moving in a closed orbit. Our method, based on perturbation theory, is compared with heuristic linearizations of the same equations. The essential feature of the linear approach is that it provides a decomposition in thermal modes, like the decomposition of mechanical vibrations in normal modes. The stationary periodic solution of the linear equations can be alternately expressed as an explicit integral or as a Fourier series. We apply our method to a minimal thermal model of a satellite with ten isothermal parts (nodes) and we compare the method with direct numerical integration of the nonlinear equations. We briefly study the computational complexity of our method for general thermal models of orbiting spacecraft and conclude that it is certainly useful for reduced models and conceptual design but it can also be more efficient than the direct integration of the equations for large models. The results of the Fourier series computations for the ten-node satellite model show that the periodic solution at the second perturbative order is sufficiently accurate.Comment: 20 pages, 11 figures, accepted in Journal of Thermophysics and Heat Transfe

    Algorithm for Linear Response Functions at Finite Temperatures: Application to ESR spectrum of s=1/2 Antiferromagnet Cu benzoate

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    We introduce an efficient and numerically stable method for calculating linear response functions χ(q,ω)\chi(\vec{q},\omega) of quantum systems at finite temperatures. The method is a combination of numerical solution of the time-dependent Schroedinger equation, random vector representation of trace, and Chebyshev polynomial expansion of Boltzmann operator. This method should be very useful for a wide range of strongly correlated quantum systems at finite temperatures. We present an application to the ESR spectrum of s=1/2 antiferromagnet Cu benzoate.Comment: 4 pages, 4 figure
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