504 research outputs found
I AM My Sister\u27s Keeper: Considering the Experiences of African American Women Educators Working in Predominantly White Public School Systems
The stories of African American women educators are often not dominant narratives in the field of education. Scholarly readings often overlook, discard, or omit the perspectives and voices of African American women educators (Bloom & Erlandson, 2003); Mabokela & Madsen, 2007; Patton & Catching, 2009; Revere, 1986). Furthermore, stereotypes, misrepresentations, and misgivings about African American women are abundant in popular press and media. This dissertation served as an outlet to present the authentic storytelling of ten African American women educators (N=10) in their voices and own words. The historical omission of the voices of African American women has had wide-reaching, explicit, and inadvertent consequences. The outcomes, impact, and meaning that I and other African American women educators ascribed to this treatment was explored using Critical Race Theory framework and methodology. This study sought to explore the experiences of African American women educators through their awareness of double consciousness and being in a crooked room and the impact thereof as they navigate their school environments. The results indicated the following four primary themes: 1) African American women educators encounter expressions of racism in their schools; 2) African American women adjust their behavior to fit within their school environments; 3) African American women face additional barriers when gaining access to and navigating their roles in predominantly white schools; and 4) African American women educators possess skills and traits that promote success
An Iterative Model Reduction Scheme for Quadratic-Bilinear Descriptor Systems with an Application to Navier-Stokes Equations
We discuss model reduction for a particular class of quadratic-bilinear (QB)
descriptor systems. The main goal of this article is to extend the recently
studied interpolation-based optimal model reduction framework for QBODEs
[Benner et al. '16] to a class of descriptor systems in an efficient and
reliable way. Recently, it has been shown in the case of linear or bilinear
systems that a direct extension of interpolation-based model reduction
techniques to descriptor systems, without any modifications, may lead to poor
reduced-order systems. Therefore, for the analysis, we aim at transforming the
considered QB descriptor system into an equivalent QBODE system by means of
projectors for which standard model reduction techniques for QBODEs can be
employed, including aforementioned interpolation scheme. Subsequently, we
discuss related computational issues, thus resulting in a modified algorithm
that allows us to construct \emph{near}--optimal reduced-order systems without
explicitly computing the projectors used in the analysis. The efficiency of the
proposed algorithm is illustrated by means of a numerical example, obtained via
semi-discretization of the Navier-Stokes equations
Low Energy Electron Point Projection Microscopy of Suspended Graphene, the Ultimate "Microscope Slide"
Point Projection Microscopy (PPM) is used to image suspended graphene using
low-energy electrons (100-200eV). Because of the low energies used, the
graphene is neither damaged or contaminated by the electron beam. The
transparency of graphene is measured to be 74%, equivalent to electron
transmission through a sheet as thick as twice the covalent radius of
sp^2-bonded carbon. Also observed is rippling in the structure of the suspended
graphene, with a wavelength of approximately 26 nm. The interference of the
electron beam due to the diffraction off the edge of a graphene knife edge is
observed and used to calculate a virtual source size of 4.7 +/- 0.6 Angstroms
for the electron emitter. It is demonstrated that graphene can be used as both
anode and substrate in PPM in order to avoid distortions due to strong field
gradients around nano-scale objects. Graphene can be used to image objects
suspended on the sheet using PPM, and in the future, electron holography
Buckling Thin Disks and Ribbons with Non-Euclidean Metrics
I consider the problem of a thin membrane on which a metric has been
prescribed, for example by lithographically controlling the local swelling
properties of a polymer thin film. While any amount of swelling can be
accommodated locally, geometry prohibits the existence of a global strain-free
configuration. To study this geometrical frustration, I introduce a
perturbative approach. I compute the optimal shape of an annular, thin ribbon
as a function of its width. The topological constraint of closing the ribbon
determines a relationship between the mean curvature and number of wrinkles
that prevents a complete relaxation of the compression strain induced by
swelling and buckles the ribbon out of the plane. These results are then
applied to thin, buckled disks, where the expansion works surprisingly well. I
identify a critical radius above which the disk in-plane strain cannot be
relaxed completely.Comment: 6 pages, 5 figures; lengthened to clarify previously confusing
issues. To appear in EP
Emission spectra and intrinsic optical bistability in a two-level medium
Scattering of resonant radiation in a dense two-level medium is studied
theoretically with account for local field effects and renormalization of the
resonance frequency. Intrinsic optical bistability is viewed as switching
between different spectral patterns of fluorescent light controlled by the
incident field strength. Response spectra are calculated analytically for the
entire hysteresis loop of atomic excitation. The equations to describe the
non-linear interaction of an atomic ensemble with light are derived from the
Bogolubov-Born-Green-Kirkwood-Yvon hierarchy for reduced single particle
density matrices of atoms and quantized field modes and their correlation
operators. The spectral power of scattered light with separated coherent and
incoherent constituents is obtained straightforwardly within the hierarchy. The
formula obtained for emission spectra can be used to distinguish between
possible mechanisms suggested to produce intrinsic bistability.Comment: 18 pages, 5 figure
Slope water, Gulf Stream, and seasonal influences on southern Mid-Atlantic Bight circulation during the fall-winter transition
Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 110 (2005): C02009, doi:10.1029/2004JC002311.Observations from autumn, 2000, near the shelfbreak front in the Middle Atlantic Bight
are used to describe the transition from stratified summer conditions to well-mixed winter
conditions over the shelf. During the observational period, the front differed dramatically
from climatological conditions, with buoyant Gulf Stream water found shoreward over
the sub-surface shelfbreak front. Water mass analysis shows a large number of separate
water masses with shelf, slope and Gulf Stream origins. The coolest shelf water was
located at the shelfbreak and may be related to “cold pool” water masses observed to the
north during summer. Shoreward of this shelfbreak water mass, a mid-shelf front was
present which intersected the bottom at the 50 m isobath. High volume transports were
associated with both the shelfbreak and mid-shelf fronts. Transport estimates from the
cross-shelf sections were approximately 1 Sverdrup, which is large relative to previous
estimates of shelf transport. The foot of the front was near the 130 m isobath, much deeper
than the climatological position near the 75 m isobath, however this is consistent with a
recent theory relating the magnitude of alongshelf transport to the depth at which the front
intersects the bottom.The authors gratefully acknowledge support from the Duke University
- University of North Carolina Oceanographic Consortium
for ship time aboard the R/V Cape Hatteras. MSL also gratefully
acknowledges support from the Office of Naval Research (N00014-01-1-0260)
Phase-locked indistinguishable photons with synthesized waveforms from a solid-state source
Resonance fluorescence in the Heitler regime provides access to single
photons with coherence well beyond the Fourier transform limit of the
transition, and holds the promise to circumvent environment-induced dephasing
common to all solid-state systems. Here we demonstrate that the coherently
generated single photons from a single self-assembled InAs quantum dot display
mutual coherence with the excitation laser on a timescale exceeding 3 seconds.
Exploiting this degree of mutual coherence we synthesize near-arbitrary
coherent photon waveforms by shaping the excitation laser field. In contrast to
post-emission filtering, our technique avoids both photon loss and degradation
of the single photon nature for all synthesized waveforms. By engineering
pulsed waveforms of single photons, we further demonstrate that separate
photons generated coherently by the same laser field are fundamentally
indistinguishable, lending themselves to creation of distant entanglement
through quantum interference.Comment: Additional data and analysis in PDF format is available for download
at the publications section of our website:
http://www.amop.phy.cam.ac.uk/amop-ma
Laboratory experiments on eddy generation by a buoyant coastal current flowing over variable bathymetry
Author Posting. © American Meteorological Society, 2006. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 36 (2006): 395-411, doi:10.1175/JPO2857.1.Irminger rings are warm-core eddies formed off the west coast of Greenland. Recent studies suggest that these eddies, which are implicated in the rapid springtime restratification of the Labrador Sea, are formed by an internal instability of the West Greenland Current (WGC), triggered by bathymetric variations. This study seeks to explore the effect of the magnitude and downstream length scale of bathymetric variations on the stability of a simple model of the WGC in a series of laboratory experiments in which a buoyant coastal current was allowed to flow over bathymetry consisting of piecewise constant slopes of varying magnitude. The currents did not form eddies over gently sloping bathymetry and only formed eddies over steep bathymetry if the current width exceeded the width of the sloping bathymetry. Eddying currents were immediately stabilized if they flowed onto gently sloping topography. Bathymetric variations that persisted only a short distance downstream perturbed the flow locally but did not lead to eddy formation. Eddies formed only once the downstream length of the bathymetric variations exceeded a critical scale of about 8 Rossby radii. These results are consistent with the observed behavior of the WGC, which begins to form Irminger rings after entering a region where the continental slope abruptly steepens and becomes narrower than the WGC itself in a region spanning about 20–80 Rossby radii of downstream distance.The authors gratefully acknowledge
the National Science Foundation (Grant OCE-
9810657) and the Office of Naval Research (Grant
N00014-97-1-0934) for their support of the 2003 WHOI
Geophysical Fluid Dynamics Summer School where
much of the research presented in this paper was performed.
CLW received additional support from the Office
of Naval Research Grant N00014-98-1-0813
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