6,723 research outputs found
Plasticity in current-driven vortex lattices
We present a theoretical analysis of recent experiments on current-driven
vortex dynamics in the Corbino disk geometry. This geometry introduces
controlled spatial gradients in the driving force and allows the study of the
onset of plasticity and tearing in clean vortex lattices. We describe plastic
slip in terms of the stress-driven unbinding of dislocation pairs, which in
turn contribute to the relaxation of the shear, yielding a nonlinear response.
The steady state density of free dislocations induced by the applied stress is
calculated as a function of the applied current and temperature. A criterion
for the onset of plasticity at a radial location in the disk yields a
temperature-dependent critical current that is in qualitative agreement with
experiments.Comment: 11 pages, 4 figure
Enhanced diffusion and ordering of self-propelled rods
Starting from a minimal physical model of self propelled hard rods on a
substrate in two dimensions, we derive a modified Smoluchowski equation for the
system. Self -propulsion enhances longitudinal diffusion and modifies the mean
field excluded volume interaction. From the Smoluchowski equation we obtain
hydrodynamic equations for rod concentration, polarization and nematic order
parameter. New results at large scales are a lowering of the density of the
isotropic-nematic transition and a strong enhancement of boundary effects in
confined self-propelled systems.Comment: 4 pages, 2 figure
Translational Correlations in the Vortex Array at the Surface of a Type-II Superconductor
We discuss the statistical mechanics of magnetic flux lines in a
finite-thickness slab of type-II superconductor. The long wavelength properties
of a flux-line liquid in a slab geometry are described by a hydrodynamic free
energy that incorporates the boundary conditions on the flux lines at the
sample's surface as a surface contribution to the free energy. Bulk and surface
weak disorder are modeled via Gaussian impurity potentials. This free energy is
used to evaluate the two-dimensional structure factor of the flux-line tips at
the sample surface. We find that surface interaction always dominates in
determining the decay of translational correlations in the asymptotic
long-wavelength limit. On the other hand, such large length scales have not
been probed by the decoration experiments. Our results indicate that the
translational correlations extracted from the analysis of the Bitter patterns
are indeed representative of behavior of flux lines in the bulk.Comment: 23 pages, 1 figure (not included), harvmac.tex macro needed (e-mail
requests to [email protected] SU-CM-92-01
Simulations on a potential hybrid and compact attosecond X-ray source based on RF and THz technologies
We investigate through beam dynamics simulations the potential of a hybrid
layout mixing RF and THz technologies to be a compact X-ray source based on
Inverse Compton Scattering (ICS), delivering few femtoseconds to
sub-femtosecond pulses. The layout consists of an S-band gun as electron source
and a dielectric-loaded circular waveguide driven by a multicycle THz pulse to
accelerate and longitudinally compress the bunch, which will then be used to
produce X-ray pulses via ICS with an infrared laser pulse. The beam dynamics
simulations we performed, from the photocathode up to the ICS point, allows to
have an insight in several important physical effects for the proposed scheme
and also in the influence on the achievable bunch properties of various
parameters of the accelerating and transverse focusing devices. The study
presented in this paper leads to a preliminary layout and set of parameters
able to deliver at the ICS point, according to our simulations, ultrashort
bunches (around 1 fs rms), at 15 MeV, with at least 1 pC charge and
transversely focused down to around 10 um rms or below while keeping a compact
beamline (less than 1.5 m), which has not yet been achieved using only
conventional RF technologies. Future studies will be devoted to the
investigation of several potential ways to improve the achieved bunch
properties, to overcome the limitations identified in the current study and to
the definition of the technical requirements. This will lead to an updated
layout and set of parameters.Comment: To be published in Nucl. Inst. Meth. A as proceedings of the EAAC17
conference 9 pages, 11 figure
Global Democracy: Normative and Empirical Perspectives
Democracy is increasingly seen as the only legitimate form of government, but few people would regard international relations as governed according to democratic principles. Can this lack of global democracy be justified? Which models of global politics should contemporary democrats endorse and which should they reject? What are the most promising pathways to global democratic change? To what extent does the extension of democracy from the national to the international level require a radical rethinking of what democratic institutions should be? This book answers these questions by providing a sustained dialogue between scholars of political theory, international law, and empirical social science. By presenting a broad range of views by prominent scholars, it offers an in-depth analysis of one of the key challenges of our century: globalizing democracy and democratizing globalization
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