175 research outputs found
Refracting Immigration Rhetoric: The Struggle to Define Identity, Place and Nation in Southern Arizona
This thesis examines the refraction of immigration rhetoric in a local context through a collection of letters to the editor of southern Arizona’s largest and only daily newspaper, the Arizona Daily Star, for the period 2006-2010. The purpose is to further insight into the process by which xenophobic nationalism is both contested and legitimated ‘on the ground,’ within a violent paradigm of nativist rhetoric and exclusion. Findings reveal essential disjunctures between and within letter-writers’ conceptions of moral proximity and the social contract—as delimiting those obligations and expectations that inhere between society, the self and the stranger—as well as competing notions of legitimacy based, on the one hand, on an overarching and at times homogenizing myth of nation and, on the other, in rootedness to the cultural and historic particularities of place. These disjunctures point to a profusion of contradictory ideations, the struggle over which exposes the efforts of community members to contest and redefine the boundaries of societal norms within the context of emergent nationalism
Discreteness-induced resonances and AC voltage amplitudes in long one-dimensional Josephson junction arrays
New resonance steps are found in the experimental current-voltage
characteristics of long, discrete, one-dimensional Josephson junction arrays
with open boundaries and in an external magnetic field. The junctions are
underdamped, connected in parallel, and DC biased. Numerical simulations based
on the discrete sine-Gordon model are carried out, and show that the solutions
on the steps are periodic trains of fluxons, phase-locked by a finite amplitude
radiation. Power spectra of the voltages consist of a small number of harmonic
peaks, which may be exploited for possible oscillator applications. The steps
form a family that can be numbered by the harmonic content of the radiation,
the first member corresponding to the Eck step. Discreteness of the arrays is
shown to be essential for appearance of the higher order steps. We use a
multi-mode extension of the harmonic balance analysis, and estimate the
resonance frequencies, the AC voltage amplitudes, and the theoretical limit on
the output power on the first two steps.Comment: REVTeX, 17 pages, 7 figures, psfig; to appear in J. Applied Physic
Underdamped vortex flow devices
Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1995.Includes bibliographical references (leaves 77-80).by Amy E. Duwel.M.S
Harmonic resonances in nonlinear Josephson junction circuits : experimental and analytical studies
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1999.Includes bibliographical references (p. 191-196).by Amy Elizabeth Duwel.Ph.D
An investigation on thermoelastic damping of high-Q ring resonators
For applications requiring high performance angular rate measurements it is important to be able to design MEMS rate sensors with high quality factors (Q). This paper considers ring resonator based rate sensors and investigates the influence of design changes to the ring and support legs on thermoelastic damping, which is the dominant dissipation mechanism. A computational method is used to quantify the thermoelastic damping and a detailed parameter study is conducted to understand the influence of ring geometry, support legs and micro-machined slots around the ring circumference. The results show that damping in the support legs can have significant influence on the total energy dissipated from the resonator, and the optimum leg geometry can be identified to achieve high Q. It is also observed that the addition of slots improves Q for resonators having higher energy loss. However, for high-Q, rings slots have a detrimental effect. The results presented are useful for designing ring resonators with reduced levels of damping
Modeling Dispersive Coupling and Losses of Localized Optical and Mechanical Modes in Optomechanical Crystals
Periodically structured materials can sustain both optical and mechanical
excitations which are tailored by the geometry. Here we analyze the properties
of dispersively coupled planar photonic and phononic crystals: optomechanical
crystals. In particular, the properties of co-resonant optical and mechanical
cavities in quasi-1D (patterned nanobeam) and quasi-2D (patterned membrane)
geometries are studied. It is shown that the mechanical Q and optomechanical
coupling in these structures can vary by many orders of magnitude with modest
changes in geometry. An intuitive picture is developed based upon a
perturbation theory for shifting material boundaries that allows the
optomechanical properties to be designed and optimized. Several designs are
presented with mechanical frequency ~ 1-10 GHz, optical Q-factor Qo > 10^7,
motional masses meff 100 femtograms, optomechanical coupling length LOM < 5
microns, and a radiation-limited mechanical Q-factor Qm > 10^7.Comment: 25 pages, 9 figure
Row-switched states in two-dimensional underdamped Josephson junction arrays
When magnetic flux moves across layered or granular superconductor
structures, the passage of vortices can take place along channels which develop
finite voltage, while the rest of the material remains in the zero-voltage
state. We present analytical studies of an example of such mixed dynamics: the
row-switched (RS) states in underdamped two-dimensional Josephson arrays,
driven by a uniform DC current under external magnetic field but neglecting
self-fields. The governing equations are cast into a compact
differential-algebraic system which describes the dynamics of an assembly of
Josephson oscillators coupled through the mesh current. We carry out a formal
perturbation expansion, and obtain the DC and AC spatial distributions of the
junction phases and induced circulating currents. We also estimate the interval
of the driving current in which a given RS state is stable. All these
analytical predictions compare well with our numerics. We then combine these
results to deduce the parameter region (in the damping coefficient versus
magnetic field plane) where RS states can exist.Comment: latex, 48 pages, 15 figs using psfi
Superconducting and Quantum-Effect Devices
Contains reports on six research projects and a list of publications.National Science Foundation Grant DMR 94-02020National Science Foundation Fellowship MIP 88-58764U.S. Air Force - Office of Scientific Research Grant F30602-96-1-0059 Rome LaboratoryDefense Advanced Research Projects Agency/Consortium for Superconducting Electronics Contract MDA 972-90-C-002
VEGF Induces More Severe Cerebrovascular Dysplasia in Eng+/− than in Alk1+/− Mice
Brain arteriovenous malformations (BAVMs) are an important cause of intracranial hemorrhage (ICH) in young adults. A small percent of BAVMs is due to hereditary hemorrhagic telangiectasia 1 and 2 (HHT1 and 2), which are caused by mutations in two genes involved in transforming growth factor-β signaling: endoglin (Eng), and activin-like kinase 1 (Alk1). The BAVM phenotype has incomplete penetrance in HHT patients, and the mechanism is unknown. We tested the hypothesis that a “response-to-injury” triggers abnormal vascular (dysplasia) development, using Eng and Alk1 haploinsufficient mice. Adeno-associated virus (AAV) expressing vascular endothelial growth factor (VEGF) was used to mimic the injury conditions. VEGF overexpression caused a similar degree of angiogenesis in the brain of all groups, except that the cortex of Alk1+/− mice had a 33% higher capillary density than other groups. There were different levels of cerebrovascular dysplasia observed in haploinsufficient mice (Eng+/− > Alk1+/−), which simulates the relative penetrance of BAVM in HHT patients (HHT1 > HHT2). Few dysplastic capillaries were observed in AAV-LacZ-injected mice. Our data indicate that both angiogenic stimulation and genetic alteration are necessary for the development of vascular dysplasia, suggesting that anti-angiogenic therapies might be adapted to slow the progression of the disease and decrease the risk of spontaneous ICH
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