29,193 research outputs found

    Are Cost Conscious Community Colleges Sacrificing Quality?

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    Compared to their four-year public counterparts, community colleges have been more successful in holding down the costs of educating students but current research, albeit limited, suggests that this may have come at the expense of quality or at least outcomes. This essay addresses the questions surrounding this issue, including the behavior of costs at public two-year colleges and what the research says about quality and outcomes issues. It is based on my study of community colleges over the past twenty-five years

    Effective inter-band coupling in MgB2 due to anharmonic phonons

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    We investigate the origin of the inter-band coupling in MgB2 by focusing on its unusual phononic features, namely, the strong anharmonicity of the phonons and the presence of both linear and quadratic electron-phonon interactions of the Su-Schrieffer-Heeger (SSH) type. The bare electronic Hamiltonian has two bands with intra- and inter-band hopping, which lead to two decoupled hybridized bands. The phonon Hamiltonian including the anharmonic terms is diagonalized approximately by a squeezing transformation, which causes the softening of the phonon frequency. The linear SSH coupling amplitude is reduced, consistently with the estimates from first-principle calculations. Additionally, the quadratic coupling generates an effective phonon-induced interaction between the hybridized bands, which is non-vanishing even in the limit of vanishing inter-bare-band hopping amplitude.Comment: 11 page

    A model Hamiltonian for MgB2 which takes into account its unusual phononic features

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    Taking as a starting point the results of LDA calculations, which show that in MgB2 the phonons have a strong quartic anharmonicity and that the bond-stretching electron-phonon interaction (EPI) has both a linear and a large quadratic component, we propose a model Hamiltonian which succesfully matches a number of experimental evidences. We relate the single critical temperature for both superconducting gaps to a phonon-induced inter-band coupling whose amplitude increases with temperature. We also obtain phonon frequencies and linewidths depending on the band filling, as well as band energies and hybridization amplitudes depending on the phonon number.Comment: 19 pages, no figures, accepted on The European Physical Journal

    Using the Community College to Control College Costs: How Much Cheaper Is It?

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    Data from NCES indicate that it is two to three times more expensive to educate a student at a public four-year college than at a community college. These figures exaggerate the difference between the two when you calculate the costs of the first two years of education for students working on a bachelor’s degree. Using modified IPEDS data from the recently released Delta Project, this study shows that the cost per FTE and the public subsidy per FTE are lower at public master’s level colleges than at the community college. Trend data from 1987-2005 are examined along with appropriate cautions for interpreting the results

    Resonant purification of mixed states for closed and open quantum systems

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    Pure states are fundamental for the implementation of quantum technologies, and several methods for the purification of the state of a quantum system S have been developed in the past years. In this letter we present a new approach, based on the interaction of S with an auxiliary system P, having a wide range of applicability. Considering two-level systems S and P and assuming a particular interaction between them, we prove that complete purifications can be obtained under suitable conditions on the parameters characterizing P. Using analytical and numerical tools, we show that the purification process exhibits a resonant behavior in both the cases of system isolated from the external environment or not.Comment: 4 pages, LaTe

    If Community College Students Are So Poor Why Do Only 16.9% Of Them Receive Pell Grants?

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    In this paper the authors attempt to address the discrepancy between the perception of income levels for community college students, and the seemingly low percentage of those students who receive Pell grants. The authors try to solve this paradox using data, published and unpublished, from the U. S. Department of Labor

    Directional Phonon Suppression Function as a Tool for the Identification of Ultralow Thermal Conductivity Materials

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    Boundary-engineering in nanostructures has the potential to dramatically impact the development of materials for high-efficiency conversion of thermal energy directly into electricity. In particular, nanostructuring of semiconductors can lead to strong suppression of heat transport with little degradation of electrical conductivity. Although this combination of material properties is promising for thermoelectric materials, it remains largely unexplored. In this work, we introduce a novel concept, the directional phonon suppression function, to unravel boundary-dominated heat transport in unprecedented detail. Using a combination of density functional theory and the Boltzmann transport equation, we compute this quantity for nanoporous silicon materials. We first compute the thermal conductivity for the case with aligned circular pores, confirming a significant thermal transport degradation with respect to the bulk. Then, by analyzing the information on the directionality of phonon suppression in this system, we identify a new structure of rectangular pores with the same porosity that enables a four-fold decrease in thermal transport with respect to the circular pores. Our results illustrate the utility of the directional phonon suppression function, enabling new avenues for systematic thermal conductivity minimization and potentially accelerating the engineering of next-generation thermoelectric devices

    On stepdown control of the false discovery proportion

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    Consider the problem of testing multiple null hypotheses. A classical approach to dealing with the multiplicity problem is to restrict attention to procedures that control the familywise error rate (FWERFWER), the probability of even one false rejection. However, if ss is large, control of the FWERFWER is so stringent that the ability of a procedure which controls the FWERFWER to detect false null hypotheses is limited. Consequently, it is desirable to consider other measures of error control. We will consider methods based on control of the false discovery proportion (FDPFDP) defined by the number of false rejections divided by the total number of rejections (defined to be 0 if there are no rejections). The false discovery rate proposed by Benjamini and Hochberg (1995) controls E(FDP)E(FDP). Here, we construct methods such that, for any γ\gamma and α\alpha, P{FDP>γ}≤αP\{FDP>\gamma \}\le \alpha. Based on pp-values of individual tests, we consider stepdown procedures that control the FDPFDP, without imposing dependence assumptions on the joint distribution of the pp-values. A greatly improved version of a method given in Lehmann and Romano \citer10 is derived and generalized to provide a means by which any sequence of nondecreasing constants can be rescaled to ensure control of the FDPFDP. We also provide a stepdown procedure that controls the FDRFDR under a dependence assumption.Comment: Published at http://dx.doi.org/10.1214/074921706000000383 in the IMS Lecture Notes--Monograph Series (http://www.imstat.org/publications/lecnotes.htm) by the Institute of Mathematical Statistics (http://www.imstat.org
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