36,881 research outputs found
Spin dynamics and coherent tunnelling in the molecular magnetic rings Fe_6 and Fe_8
We present detailed calculations of low-energy spin dynamics in the ``ferric
wheel'' systems Na:Fe_6 and Cs:Fe_8 in a magnetic field. We compute by exact
diagonalisation the low-energy spectra and matrix elements for total-spin and
N'eel-vector components, and thus the time-dependent correlation functions of
these operators. Comparison of our results with the semiclassical theory of
coherent quantum tunnelling of the N'eel vector demonstrates the validity of a
two-state description for the low-energy dynamics of ferric wheels. We discuss
the implications of our results for mesoscopic quantum coherent phenomena, and
for the experimental techniques to observe them, in molecular magnetic rings.Comment: 10 pages, 8 figures; considerably expanded discussion; to appear in
Eur. Phys. J.
Coddling Spies: Why the Law Doesnât Adequately Address Computer Spyware
Consumers and businesses have attempted to use the common law of torts as well as federal statutes like the Computer Fraud and Abuse Act, the Stored Wire and Electronic Communications and Transactional Records Act, and the Wiretap Act to address the expanding problem of spyware. Spyware, which consists of software applications inserted into another\u27s computer to report a user\u27s activity to an outsider, is as innocuous as tracking purchases or as sinister as stealing trade secrets or an individual\u27s identity. Existing law does not address spyware adequately because authorization language, buried in click-through boilerplate, renders much of current law useless. Congress must act to make spyware companies disclose their intentions with conspicuous and clearly-stated warnings
Addressing the challenges of modeling the scattering from bottlebrush polymers in solution
Smallâangle scattering measurements of complex macromolecules in solution are used to establish relationships between chemical structure and conformational properties. Interpretation of the scattering data requires an inverse approach where a model is chosen and the simulated scattering intensity from that model is iterated to match the experimental scattering intensity. This raises challenges in the case where the model is an imperfect approximation of the underlying structure, or where there are significant correlations between model parameters. We examine three bottlebrush polymers (consisting of polynorbornene backbone and polystyrene side chains) in a good solvent using a model commonly applied to this class of polymers: the flexible cylinder model. Applying a series of constrained MonteâCarlo Markov Chain analyses demonstrates the severity of the correlations between key parameters and the presence of multiple close minima in the goodness of fit space. We demonstrate that a shapeâagnostic model can fit the scattering with significantly reduced parameter correlations and less potential for complex, multimodal parameter spaces. We provide recommendations to improve the analysis of complex macromolecules in solution, highlighting the value of Bayesian methods. This approach provides richer information for understanding parameter sensitivity compared to methods which produce a single, best fit
Endoscopic endonasal management of recurrent petrous apex cholesterol granuloma.
Petrous apex cholesterol granulomas (PACG) are uncommon lesions. Recurrence following transcranial or endonasal approaches to aerate the cyst occurs in up to 60% of cases. We describe the technical nuances pertinent to the endonasal endoscopic management of a recurrent symptomatic PACG and review the literature. A 19-year-old woman presented with a recurrent right abducens nerve paresis. Four months prior, she underwent an endonasal transsphenoidal surgery (TSS) for drainage of a symptomatic PACG. Current imaging documented recurrence of the right PACG. Transsphenoidal and infrapetrous approaches were performed to obtain a wider bony opening along the petrous apex and drain the cyst. A Doyle splint was inserted into the cyst's cavity and extended out into the sphenoid, maintaining patency during the healing process. Three months after surgery, the splint was removed endoscopically, allowing visualization of a patent cylindrical communication between both aerated cavities. The patient remains symptom- and recurrence-free. Endoscopic endonasal surgery must be adapted to manage recurrent PACG. A TSS may not be sufficient. An infrapetrous approach with wider bony opening, extensive removal of the cyst's anterior wall, and use of a stent are indicated for the treatment of recurrent PACG and to prevent recurrences
Effective Viscous Damping Enables Morphological Computation in Legged Locomotion
Muscle models and animal observations suggest that physical damping is
beneficial for stabilization. Still, only a few implementations of mechanical
damping exist in compliant robotic legged locomotion. It remains unclear how
physical damping can be exploited for locomotion tasks, while its advantages as
sensor-free, adaptive force- and negative work-producing actuators are
promising. In a simplified numerical leg model, we studied the energy
dissipation from viscous and Coulomb damping during vertical drops with
ground-level perturbations. A parallel spring-damper is engaged between
touch-down and mid-stance, and its damper auto-disengages during mid-stance and
takeoff. Our simulations indicate that an adjustable and viscous damper is
desired. In hardware we explored effective viscous damping and adjustability
and quantified the dissipated energy. We tested two mechanical, leg-mounted
damping mechanisms; a commercial hydraulic damper, and a custom-made pneumatic
damper. The pneumatic damper exploits a rolling diaphragm with an adjustable
orifice, minimizing Coulomb damping effects while permitting adjustable
resistance. Experimental results show that the leg-mounted, hydraulic damper
exhibits the most effective viscous damping. Adjusting the orifice setting did
not result in substantial changes of dissipated energy per drop, unlike
adjusting damping parameters in the numerical model. Consequently, we also
emphasize the importance of characterizing physical dampers during real legged
impacts to evaluate their effectiveness for compliant legged locomotion
From Registration to Recounts Revisited: Developments in the Election Ecosystems of Five Midwestern States
As a follow-up to a study of problems during the 2006 elections, examines the states' continuing adjustments to institutional arrangements, voter registration databases, convenience voting, and post-election processes in the 2008 elections
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