29,871 research outputs found
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
A model Hamiltonian for MgB2 which takes into account its unusual phononic features
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?
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
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?
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
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
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 (), the probability of
even one false rejection. However, if is large, control of the is so
stringent that the ability of a procedure which controls the 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 () 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 . Here, we construct methods such that, for any
and , . Based on -values of
individual tests, we consider stepdown procedures that control the ,
without imposing dependence assumptions on the joint distribution of the
-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 . We also
provide a stepdown procedure that controls the 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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