Are Cost Conscious Community Colleges Sacrificing Quality?

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

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

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

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

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

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

Resonant purification of mixed states for closed and open quantum systems

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

Stepup procedures for control of generalizations of the familywise error rate

Consider the multiple testing problem of testing null hypotheses $H_1,...,H_s$. A classical approach to dealing with the multiplicity problem is to restrict attention to procedures that control the familywise error rate ($\mathit{FWER}$), the probability of even one false rejection. But if $s$ is large, control of the $\mathit{FWER}$ is so stringent that the ability of a procedure that controls the $\mathit{FWER}$ to detect false null hypotheses is limited. It is therefore desirable to consider other measures of error control. This article considers two generalizations of the $\mathit{FWER}$. The first is the $k-\mathit{FWER}$, in which one is willing to tolerate $k$ or more false rejections for some fixed $k\geq 1$. The second is based on the false discovery proportion ($\mathit{FDP}$), defined to be the number of false rejections divided by the total number of rejections (and defined to be 0 if there are no rejections). Benjamini and Hochberg [J. Roy. Statist. Soc. Ser. B 57 (1995) 289--300] proposed control of the false discovery rate ($\mathit{FDR}$), by which they meant that, for fixed $\alpha$, $E(\mathit{FDP})\leq\alpha$. Here, we consider control of the $\mathit{FDP}$ in the sense that, for fixed $\gamma$ and $\alpha$, $P\{\mathit{FDP}>\gamma\}\leq \alpha$. Beginning with any nondecreasing sequence of constants and $p$-values for the individual tests, we derive stepup procedures that control each of these two measures of error control without imposing any assumptions on the dependence structure of the $p$-values. We use our results to point out a few interesting connections with some closely related stepdown procedures. We then compare and contrast two $\mathit{FDP}$-controlling procedures obtained using our results with the stepup procedure for control of the $\mathit{FDR}$ of Benjamini and Yekutieli [Ann. Statist. 29 (2001) 1165--1188].Comment: Published at http://dx.doi.org/10.1214/009053606000000461 in the Annals of Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical Statistics (http://www.imstat.org

On stepdown control of the false discovery proportion

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 ($FWER$), the probability of even one false rejection. However, if $s$ is large, control of the $FWER$ is so stringent that the ability of a procedure which controls the $FWER$ 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 ($FDP$) 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 $E(FDP)$. Here, we construct methods such that, for any $\gamma$ and $\alpha$, $P\{FDP>\gamma \}\le \alpha$. Based on $p$-values of individual tests, we consider stepdown procedures that control the $FDP$, without imposing dependence assumptions on the joint distribution of the $p$-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 $FDP$. We also provide a stepdown procedure that controls the $FDR$ 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

Role of the particle's stepping cycle in an asymmetric exclusion process: A model of mRNA translation

Messenger RNA translation is often studied by means of statistical-mechanical models based on the Asymmetric Simple Exclusion Process (ASEP), which considers hopping particles (the ribosomes) on a lattice (the polynucleotide chain). In this work we extend this class of models and consider the two fundamental steps of the ribosome's biochemical cycle following a coarse-grained perspective. In order to achieve a better understanding of the underlying biological processes and compare the theoretical predictions with experimental results, we provide a description lying between the minimal ASEP-like models and the more detailed models, which are analytically hard to treat. We use a mean-field approach to study the dynamics of particles associated with an internal stepping cycle. In this framework it is possible to characterize analytically different phases of the system (high density, low density or maximal current phase). Crucially, we show that the transitions between these different phases occur at different parameter values than the equivalent transitions in a standard ASEP, indicating the importance of including the two fundamental steps of the ribosome's biochemical cycle into the model.Comment: 9 pages, 9 figure