1,798 research outputs found
Magnetization reversal and nonexponential relaxation via instabilities of internal spin waves in nanomagnets
A magnetic particle with atomic spins ordered in an unstable direction is an
example of a false vacuum that decays via excitation of internal spin waves.
Coupled evolution of the particle's magnetization (or the vacuum state) and
spin waves, considered in the time-dependent vacuum frame, leads to a peculiar
relaxation that is very fast at the beginning but slows down to a
nonexponential long tail at the end. The two main scenarios are linear and
exponential spin-wave instabilities. For the former, the longitudinal and
transverse relaxation rates have been obtained analytically. Numerical
simulations show that the particle's magnetization strongly decreases in the
middle of reversal and then recovers.Comment: 6 EPL pages, 4 figure
Casimir torque between corrugated metallic plates
We consider two parallel corrugated plates and show that a Casimir torque
arises when the corrugation directions are not aligned. We follow the
scattering approach and calculate the Casimir energy up to second order in the
corrugation amplitudes, taking into account nonspecular reflections,
polarization mixing and the finite conductivity of the metals. We compare our
results with the proximity force approximation, which overestimates the torque
by a factor 2 when taking the conditions that optimize the effect. We argue
that the Casimir torque could be measured for separation distances as large as
1 Comment: 7 pages, 3 figures, contribution to QFEXT07 proceeding
Strategies for protecting intellectual property when using CUDA applications on graphics processing units
Recent advances in the massively parallel computational abilities of graphical processing units (GPUs) have increased their use for general purpose computation, as companies look to take advantage of big data processing techniques. This has given rise to the potential for malicious software targeting GPUs, which is of interest to forensic investigators examining the operation of software. The ability to carry out reverse-engineering of software is of great importance within the security and forensics elds, particularly when investigating malicious software or carrying out forensic analysis following a successful security breach. Due to the complexity of the Nvidia CUDA (Compute Uni ed Device Architecture) framework, it is not clear how best to approach the reverse engineering of a piece of CUDA software. We carry out a review of the di erent binary output formats which may be encountered from the CUDA compiler, and their implications on reverse engineering. We then demonstrate the process of carrying out disassembly of an example CUDA application, to establish the various techniques available to forensic investigators carrying out black-box disassembly and reverse engineering of CUDA binaries. We show that the Nvidia compiler, using default settings, leaks useful information. Finally, we demonstrate techniques to better protect intellectual property in CUDA algorithm implementations from reverse engineering
Non-critically squeezed light via spontaneous rotational symmetry breaking
We theoretically address squeezed light generation through the spontaneous
breaking of the rotational invariance occuring in a type I degenerate optical
parametric oscillator (DOPO) pumped above threshold. We show that a DOPO with
spherical mirrors, in which the signal and idler fields correspond to first
order Laguerre-Gauss modes, produces a perfectly squeezed vacuum with the shape
of a Hermite-Gauss mode, within the linearized theory. This occurs at any
pumping level above threshold, hence the phenomenon is non-critical.
Imperfections of the rotational symmetry, due e.g. to cavity anisotropy, are
shown to have a small impact, hence the result is not singular.Comment: 4 pages, 1 figure, replaced with resubmitted versio
Casimir energies with finite-width mirrors
We use a functional approach to the Casimir effect in order to evaluate the
exact vacuum energy for a real scalar field in dimensions, in the
presence of backgrounds that, in a particular limit, impose Dirichlet boundary
conditions on one or two parallel surfaces. Outside of that limit, the
background may be thought of as describing finite-width mirrors with
frequency-dependent transmission and reflection coefficients. We provide new
explicit results for the Casimir energy in some particular backgroundsComment: 18 pages, no figures. Version to appear in Phys. Rev.
The proximity force approximation for the Casimir energy as a derivative expansion
The proximity force approximation (PFA) has been widely used as a tool to
evaluate the Casimir force between smooth objects at small distances. In spite
of being intuitively easy to grasp, it is generally believed to be an
uncontrolled approximation. Indeed, its validity has only been tested in
particular examples, by confronting its predictions with the next to leading
order (NTLO) correction extracted from numerical or analytical solutions
obtained without using the PFA. In this article we show that the PFA and its
NTLO correction may be derived within a single framework, as the first two
terms in a derivative expansion. To that effect, we consider the Casimir energy
for a vacuum scalar field with Dirichlet conditions on a smooth curved surface
described by a function in front of a plane. By regarding the Casimir
energy as a functional of , we show that the PFA is the leading term in a
derivative expansion of this functional. We also obtain the general form of
corresponding NTLO correction, which involves two derivatives of . We
show, by evaluating this correction term for particular geometries, that it
properly reproduces the known corrections to PFA obtained from exact
evaluations of the energy.Comment: Minor changes. Version to appear in Phys. Rev.
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