2,175 research outputs found
Lepton Flavor Violating Photoleptonic Effect
We study lepton flavor violating analogs of the photoelectric effect, with a
final or instead of an electron: and . On the basis of the general parametrization of the matrix element
of the electromagnetic current we estimate the upper limits for the cross
sections and event rates of these processes, imposed by the current
experimental bounds on and decays.Comment: 6 pages, 1 figure, RevTeX
Robust two-dimensional subrecoil Raman cooling by adiabatic transfer in a tripod atomic system
We demonstrate two-dimensional robust Raman cooling in a four-level tripod
system, in which velocity-selective population transfer is achieved by a STIRAP
pulse. In contrast to basic 2D Raman cooling with square envelope pulses [Phys.
Rev. A 83, 023407 (2011)], the technique presented here allows for a wide
variation in the pulse duration and amplitude once the adiabaticity criterion
is satisfied. An efficient population transfer together with attaining of a
narrow spread of the resonant-velocity group leads to the narrowing of the
velocity-distribution spread down to , corresponding to an
effective temperature equal to . This robust method opens
new possibilities for cooling of neutral atoms.Comment: 8 pages, 3 figure
Effective anisotropy of thin nanomagnets: beyond the surface anisotropy approach
We study the effective anisotropy induced in thin nanomagnets by the nonlocal
demagnetization field (dipole-dipole interaction). Assuming a magnetization
independent of the thickness coordinate, we reduce the energy to an
inhomogeneneous onsite anisotropy. Vortex solutions exist and are ground states
for this model. We illustrate our approach for a disk and a square geometry. In
particular, we obtain good agreement between spin-lattice simulations with this
effective anisotropy and micromagnetic simulations.Comment: ReVTeX, 14 pages, 6 figure
Vortex motion in a finite-size easy-plane ferromagnet and application to a nanodot
We study the motion of a non-planar vortex in a circular easy-plane
ferromagnet, which imitates a magnetic nanodot. Analysis was done using
numerical simulations and a new collective variable theory which includes the
coupling of Goldstone-like mode with the vortex center. Without magnetic field
the vortex follows a spiral orbit which we calculate. When a rotating in-plane
magnetic field is included, the vortex tends to a stable limit cycle which
exists in a significant range of field amplitude B and frequency for a
given system size L. For a fixed , the radius R of the orbital motion
is proportional to L while the orbital frequency varies as 1/L and is
significantly smaller than . Since the limit cycle is caused by the
interplay between the magnetization and the vortex motion, the internal mode is
essential in the collective variable theory which then gives the correct
estimate and dependency for the orbit radius . Using this
simple theory we indicate how an ac magnetic field can be used to control
vortices observed in real magnetic nanodots.Comment: 15 pages (RevTeX), 14 figures (eps
Switching between different vortex states in 2-dimensional easy-plane magnets due to an ac magnetic field
Using a discrete model of 2-dimensional easy-plane classical ferromagnets, we
propose that a rotating magnetic field in the easy plane can switch a vortex
from one polarization to the opposite one if the amplitude exceeds a threshold
value, but the backward process does not occur. Such switches are indeed
observed in computer simulations.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Vortex Polarity Switching in Magnets with Surface Anisotropy
Vortex core reversal in magnetic particle is essentially influenced by a
surface anisotropy. Under the action of a perpendicular static magnetic field
the vortex core undergoes a shape deformationof pillow- or barrel-shaped type,
depending on the type of the surface anisotropy. This deformation plays a key
point in the switching mechanism: We predict that the vortex polarity switching
is accompanied (i) by a linear singularity in case of Heisenberg magnet with
bulk anisotropy only and (ii) by a point singularities in case of surface
anisotropy or exchange anisotropy. We study in details the switching process
using spin-lattice simulations and propose a simple analytical description
using a wired core model, which provides an adequate description of the Bloch
point statics, its dynamics and the Bloch point mediated switching process. Our
analytical predictions are confirmed by spin-lattice simulations for Heisenberg
magnet and micromagnetic simulations for nanomagnet with account of a dipolar
interaction.Comment: 17 pages, 15 figure
The Closest Known Flyby of a Star to the Solar System
Passing stars can perturb the Oort Cloud, triggering comet showers and potentially extinction events on Earth. We combine velocity measurements for the recently discovered, nearby, low-mass binary system WISE J072003.20-084651.2 ("Scholz's star") to calculate its past trajectory. Integrating the Galactic orbits of this ~0.15 M_⊙ binary system and the Sun, we find that the binary passed within only 52^(+23)_(−14) kAU (0.25^(+0.11)_(−0.07) pc) of the Sun 70^(+15)_(−10) kya (1σ uncertainties), i.e., within the outer Oort Cloud. This is the closest known encounter of a star to our solar system with a well-constrained distance and velocity. Previous work suggests that flybys within 0.25 pc occur infrequently (~0.1 Myr^(−1)). We show that given the low mass and high velocity of the binary system, the encounter was dynamically weak. Using the best available astrometry, our simulations suggest that the probability that the star penetrated the outer Oort Cloud is ~98%, but the probability of penetrating the dynamically active inner Oort Cloud (<20 kAU) is ~10^(−4). While the flyby of this system likely caused negligible impact on the flux of long-period comets, the recent discovery of this binary highlights that dynamically important Oort Cloud perturbers may be lurking among nearby stars
Experimental Study of Crystal Channelling at CERN-SPS for Beam-Halo Cleaning
An efficient and robust collimation system is mandatory for any superconducting hadron collider, in particular for the LHC, which will store a beam of unprecedented high intensity and energy. The usage of highly efficient and short primary bent-crystal collimators might be a possibility for reaching nominal and ultimate LHC intensity. Over the last years, groups in Italy (Ferrara) and Russia (St. Petersburg) have developed crystal production methods, that considerably improve the crystal quality. These developments led, in turn, to a surprising increase in the channeling efficiency and to the recent observation of the âワvolume reflectionâ mechanism. The aim of the proposed experiment is the setup of a beam test facility, directing primary protons from the SPS onto a bent silicon crystal, and the quantitative study of single-pass efficiency for all involved processes. Final goal will be the extrapolation of important information on the feasibility of a crystal collimator for halo cleaning in the LHC. The experiment will be performed in the H8 beamline at the CERN SPS where a beam with very small divergence can be provided thus representing a unique facility for tests and characterization of crystals to be used for particle channeling studies
A comparative study of hydrophilic phosphine hexanuclear rhenium cluster complexes’ toxicity
Octahedral rhenium cluster compound Na2H8[{Re6Se8}(P(C2H4CONH2)(C2H4COO)2)6] has recently emerged as a very promising X-ray contrast agent for biomedical applications. However, the synthesis of this compound is rather challenging due to difficulty to control the hydrolysis of initial P(C2H4CN)3 ligand during the reaction process. Therefore, in this report we compare the in vitro and in vivo toxicity of Na2H8[{Re6Se8}(P(C2H4CONH2)(C2H4COO)2)6] with those of related compounds featuring fully hydrolysed form of the phosphine ligand, namely Na2H14[{Re6Q8}(P(C2H4COO)3)6] (Q = S or Se). Our results demonstrate that cytotoxicity and acute in vivo toxicity of the complex Na2H8[{Re6Se8}(P(C2H4CONH2)(C2H4COO)2)6] solutions were considerably lower than those of compounds with fully hydrolysed ligand P(C2H4COOH)3. Such behavior can be explained by the higher osmolality of Na2H14[{Re6Q8}(P(C2H4COO)3)6] versus Na2H8[{Re6Se8}(P(C2H4CONH2)(C2H4COO)2)6]
Casimir scaling as a test of QCD vacuum
Recent accurate measurements of static potentials between sources in various
representations of the gauge group SU(3) performed by G.Bali provide a crucial
test of the QCD vacuum models and different approaches to confinement. The
Casimir scaling of the potential observed for all measured distances implies
strong suppression of higher cumulant contributions. The consequences for the
instanton vacuum model and the spectrum of the QCD string are also discussed.Comment: LaTeX, 15 pages, 1 figur
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