8,251 research outputs found
On Flux Quantization in F-Theory II: Unitary and Symplectic Gauge Groups
We study the quantization of the M-theory G-flux on elliptically fibered
Calabi-Yau fourfolds with singularities giving rise to unitary and symplectic
gauge groups. We seek and find its relation to the Freed-Witten quantization of
worldvolume fluxes on 7-branes in type IIB orientifold compactifications on
Calabi-Yau threefolds. By explicitly constructing the appropriate four-cycles
on which to calculate the periods of the second Chern class of the fourfolds,
we find that there is a half-integral shift in the quantization of G-flux
whenever the corresponding dual 7-brane is wrapped on a non-spin submanifold.
This correspondence of quantizations holds for all unitary and symplectic gauge
groups, except for SU(3), which behaves mysteriously. We also perform our
analysis in the case where, in addition to the aforementioned gauge groups,
there is also a 'flavor' U(1)-gauge group.Comment: 33 pages, 4 figure
Structure of the solar photosphere studied from the radiation hydrodynamics code ANTARES
The ANTARES radiation hydrodynamics code is capable of simulating the solar
granulation in detail unequaled by direct observation. We introduce a
state-of-the-art numerical tool to the solar physics community and demonstrate
its applicability to model the solar granulation. The code is based on the
weighted essentially non-oscillatory finite volume method and by its
implementation of local mesh refinement is also capable of simulating turbulent
fluids. While the ANTARES code already provides promising insights into
small-scale dynamical processes occurring in the quiet-Sun photosphere, it will
soon be capable of modeling the latter in the scope of radiation
magnetohydrodynamics. In this first preliminary study we focus on the vertical
photospheric stratification by examining a 3-D model photosphere with an
evolution time much larger than the dynamical timescales of the solar
granulation and of particular large horizontal extent corresponding to on the solar surface to smooth out horizontal spatial
inhomogeneities separately for up- and downflows. The highly resolved Cartesian
grid thereby covers of the upper convection zone and the
adjacent photosphere. Correlation analysis, both local and two-point, provides
a suitable means to probe the photospheric structure and thereby to identify
several layers of characteristic dynamics: The thermal convection zone is found
to reach some ten kilometers above the solar surface, while convectively
overshooting gas penetrates even higher into the low photosphere. An wide transition layer separates the convective from the
oscillatory layers in the higher photosphere.Comment: Accepted for publication in Astrophysics and Space Science; 18 pages,
12 figures, 2 tables; typos correcte
Surface Properties of Aperiodic Ising Quantum Chains
We consider Ising quantum chains with quenched aperiodic disorder of the
coupling constants given through general substitution rules. The critical
scaling behaviour of several bulk and surface quantities is obtained by exact
real space renormalization.Comment: 4 pages, RevTex, reference update
Atom gratings produced by large angle atom beam splitters
An asymptotic theory of atom scattering by large amplitude periodic
potentials is developed in the Raman-Nath approximation. The atom grating
profile arising after scattering is evaluated in the Fresnel zone for
triangular, sinusoidal, magneto-optical, and bichromatic field potentials. It
is shown that, owing to the scattering in these potentials, two
\QTR{em}{groups} of momentum states are produced rather than two distinct
momentum components. The corresponding spatial density profile is calculated
and found to differ significantly from a pure sinusoid.Comment: 16 pages, 7 figure
A two-frequency acousto-optic modulator driver to improve the beam pointing stability during intensity ramps
We report on a scheme to improve the pointing stability of the first order beam diffracted by an acousto-optic modulator (AOM). Due to thermal effects inside the crystal, the angular position of the beam can change by as much as 1 mrad when the radio-frequency power in the AOM is reduced to decrease the first order beam intensity. This is done for example to perform forced evaporative cooling in ultracold atom experiments using far-off-resonant optical traps. We solve this problem by driving the AOM with two radio-frequencies and . The power of is adjusted relative to the power of to keep the total power constant. Using this, the beam displacement is decreased by a factor of twenty. The method is simple to implement in existing experimental setups, without any modification of the optics
Bose Einstein Condensate in a Box
Bose-Einstein condensates have been produced in an optical box trap. This
novel optical trap type has strong confinement in two directions comparable to
that which is possible in an optical lattice, yet produces individual
condensates rather than the thousands typical of a lattice. The box trap is
integrated with single atom detection capability, paving the way for studies of
quantum atom statistics.Comment: 4 pages, 5 figure
Ultracold atoms in radio-frequency-dressed potentials beyond the rotating wave approximation
We study dressed Bose-Einstein condensates in an atom chip radio-frequency
trap. We show that in this system sufficiently strong dressing can be achieved
to cause the widely used rotating wave approximation (RWA) to break down. We
present a full calculation of the atom - field coupling which shows that the
non-RWA contributions quantitatively alter the shape of the emerging dressed
adiabatic potentials. The non-RWA contributions furthermore lead to additional
allowed transitions between dressed levels. We use RF spectroscopy of
Bose-Einstein condensates trapped in the dressed state potentials to directly
observe the transition from the RWA to the beyond-RWA regime.Comment: 6 pages, 4 figure
c-Abl and Src-family kinases cross-talk in regulation of myeloid cell migration
AbstractCytoskeleton dynamics are regulated by Src-family tyrosine kinases (SFKs) and c-Abl. We found that the SFK members Hck and c-Fgr regulate tyrosine phosphorylation of c-Abl and c-Abl associates with ÎČ1 integrin-bound Hck or c-Fgr in murine macrophages. Studies with selective inhibitors and cells from SFK-deficient mice showed that c-Abl and SFK regulate migration and activation of the small GTPases Cdc42 and Rac in macrophages. Additionally, human neutrophil chemotactic activity was reduced by c-Abl inhibitors, and neutrophils from chronic myeloid leukaemia patients displayed an increased chemotactic ability. Hence, Src-family kinase and c-Abl cross-talk in the regulation of myeloid cell migration.Structured summaryMINT-7296608: Integrin beta-1 (uniprotkb:P09055) physically interacts (MI:0914) with Hck (uniprotkb:P08103), Abl (uniprotkb:P00520) and Fgr (uniprotkb:P14234) by anti bait coimmunoprecipitation (MI:0006) MINT-7296596: Integrin beta-1 (uniprotkb:P09055) physically interacts (MI:0914) with Fgr (uniprotkb:P14234) and Abl (uniprotkb:P00520) by anti bait coimmunoprecipitation (MI:0006
Experimental Evidence for Efimov Quantum States
Three interacting particles form a system which is well known for its complex
physical behavior. A landmark theoretical result in few-body quantum physics is
Efimov's prediction of a universal set of weakly bound trimer states appearing
for three identical bosons with a resonant two-body interaction. Surprisingly,
these states even exist in the absence of a corresponding two-body bound state
and their precise nature is largely independent of the particular type of the
two-body interaction potential. Efimov's scenario has attracted great interest
in many areas of physics; an experimental test however has not been achieved.
We report the observation of an Efimov resonance in an ultracold thermal gas of
cesium atoms. The resonance occurs in the range of large negative two-body
scattering lengths and arises from the coupling of three free atoms to an
Efimov trimer. We observe its signature as a giant three-body recombination
loss when the strength of the two-body interaction is varied near a Feshbach
resonance. This resonance develops into a continuum resonance at non-zero
collision energies, and we observe a shift of the resonance position as a
function of temperature. We also report on a minimum in the recombination loss
for positive scattering lengths, indicating destructive interference of decay
pathways. Our results confirm central theoretical predictions of Efimov physics
and represent a starting point from which to explore the universal properties
of resonantly interacting few-body systems.Comment: 8 pages, 4 figures, Proceedings of ICAP-2006 (Innsbruck
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