2,050 research outputs found
Pump-Probe Experiments on the Single-Molecule Magnet Fe8 : Measurement of Excited Level Lifetimes
We present magnetization measurements on the single molecule magnet Fe8 in
the presence of pulsed microwave radiation. A pump-probe technique is used with
two microwave pulses with frequencies of 107 GHz and 118 GHz and pulse lengths
of several nanoseconds to study the spin dynamics via time-resolved
magnetization measurements using a Hall probe magnetometer. We find evidence
for short spin-phonon relaxation times of the order of one microsecond. The
temperature dependence of the spin-phonon relaxation time in our experiments is
in good agreement with previously published theoretical results. We also
established the presence of very short energy diffusion times, that act on a
timescale of about 70 ns.Comment: submitted to Phys. Rev. Lett. (01 March 2007
Assessment of Unsteady Propagation Characteristics and Corrections in Aeroacoustic Wind Tunnels Using an Acoustic Pulse
Two types of aeroacoustic wind tunnel test section configurations have been tested in the NASA Langley Quiet Flow Facility. The first is a more traditional open-jet configuration, where test section flow passes unbounded through the facility anechoic chamber. The second is the more recent Kevlar wall configuration, where a tensioned Kevlar sheet bounds the test section flow from the facility anechoic chamber. For both configurations, acoustic instrumentation is in the surrounding quiescent space. Both configurations are evaluated with a laser-based pulsed acoustic source, which provides unique capability for assessing the facility unsteady acoustic propagation characteristics. Metrics based on the wander and spread of the pulses are evaluated and show that measurements using Kevlar walls experience dramatically reduced unsteady effects when compared to the open-jet configuration. This leads to a corresponding improvement in coherence between microphones with the Kevlar configuration. Corrections for magnitude and phase for propagation through Kevlar as compared to open-jet propagation are calculated. While limitations in the experimental setup make quantitative analysis difficult, qualitative analysis shows Kevlar magnitude corrections similar to those determined in previous literature. Directivity effects beyond those already present for open-jet configurations are minimal. Phase corrections relative to open-jet configurations are indeterminate within the limitations of the experiment, though data suggest such corrections are not extreme. The background noise produced by the Kevlar is found to be its one drawback when compared with the open-jet configuration, showing significantly greater levels at high frequencies
Significant contribution to total mass from very small glaciers
A single large glacier can contain tens of millions of times the mass of a small glacier. Nevertheless, very small glaciers (with area &le;1 km<sup>2</sup>) are so numerous that their contribution to the world's total ice volume is significant and may be a notable source of error if excluded. With current glacier inventories, total global volume errors on the order of 10% are possible. However, to reduce errors to below 1% requires the inclusion of glaciers that are smaller than those recorded in most inventories. At the global scale, 1% accuracy requires a list of all glaciers and ice caps (GIC, exclusive of the ice sheets) larger than 1 km<sup>2</sup>, and for regional estimates requires a complete list of all glaciers down to the smallest possible size. For this reason, sea-level rise estimates and other total mass and total volume analyses should not omit the world's smallest glaciers. In particular, upscaling GIC inventories has been common practice in sea level estimates, but downscaling may also be necessary to include the smallest glaciers
Influence of antisymmetric exchange interaction on quantum tunneling of magnetization in a dimeric molecular magnet Mn6
We present magnetization measurements on the single molecule magnet Mn6,
revealing various tunnel transitions inconsistent with a giant-spin
description. We propose a dimeric model of the molecule with two coupled spins
S=6, which involves crystal-field anisotropy, symmetric Heisenberg exchange
interaction, and antisymmetric Dzyaloshinskii-Moriya exchange interaction. We
show that this simplified model of the molecule explains the experimentally
observed tunnel transitions and that the antisymmetric exchange interaction
between the spins gives rise to tunneling processes between spin states
belonging to different spin multiplets.Comment: 5 pages, 4 figure
Improved and Perfect Actions in Discrete Gravity
We consider the notion of improved and perfect actions within Regge calculus.
These actions are constructed in such a way that they - although being defined
on a triangulation - reproduce the continuum dynamics exactly, and therefore
capture the gauge symmetries of General Relativity. We construct the perfect
action in three dimensions with cosmological constant, and in four dimensions
for one simplex. We conclude with a discussion about Regge Calculus with curved
simplices, which arises naturally in this context.Comment: 28 pages, 2 figure
Magnetization dynamics in the single-molecule magnet Fe8 under pulsed microwave irradiation
We present measurements on the single molecule magnet Fe8 in the presence of
pulsed microwave radiation at 118 GHz. The spin dynamics is studied via time
resolved magnetization experiments using a Hall probe magnetometer. We
investigate the relaxation behavior of magnetization after the microwave pulse.
The analysis of the experimental data is performed in terms of different
contributions to the magnetization after-pulse relaxation. We find that the
phonon bottleneck with a characteristic relaxation time of 10 to 100 ms
strongly affects the magnetization dynamics. In addition, the spatial effect of
spin diffusion is evidenced by using samples of different sizes and different
ways of the sample's irradiation with microwaves.Comment: 14 pages, 12 figure
On the Expansions in Spin Foam Cosmology
We discuss the expansions used in spin foam cosmology. We point out that
already at the one vertex level arbitrarily complicated amplitudes contribute,
and discuss the geometric asymptotics of the five simplest ones. We discuss
what type of consistency conditions would be required to control the expansion.
We show that the factorisation of the amplitude originally considered is best
interpreted in topological terms. We then consider the next higher term in the
graph expansion. We demonstrate the tension between the truncation to small
graphs and going to the homogeneous sector, and conclude that it is necessary
to truncate the dynamics as well.Comment: 17 pages, 4 figures, published versio
Extrapolation of Multiplicity distribution in p+p(\bar(p)) collisions to LHC energies
The multiplicity (N_ch) and pseudorapidity distribution (dN_ch/d\eta) of
primary charged particles in p+p collisions at Large Hadron Collider (LHC)
energies of \sqrt(s) = 10 and 14 TeV are obtained from extrapolation of
existing measurements at lower \sqrt(s). These distributions are then compared
to calculations from PYTHIA and PHOJET models. The existing \sqrt(s)
measurements are unable to distinguish between a logarithmic and power law
dependence of the average charged particle multiplicity () on \sqrt(s),
and their extrapolation to energies accessible at LHC give very different
values. Assuming a reasonably good description of inclusive charged particle
multiplicity distributions by Negative Binomial Distributions (NBD) at lower
\sqrt(s) to hold for LHC energies, we observe that the logarithmic \sqrt(s)
dependence of are favored by the models at midrapidity. The dN_ch/d\eta
versus \eta for the existing measurements are found to be reasonably well
described by a function with three parameters which accounts for the basic
features of the distribution, height at midrapidity, central rapidity plateau
and the higher rapidity fall-off. Extrapolation of these parameters as a
function of \sqrt(s) is used to predict the pseudorapidity distributions of
charged particles at LHC energies. dN_ch/d\eta calculations from PYTHIA and
PHOJET models are found to be lower compared to those obtained from the
extrapolated dN_ch/d\eta versus \eta distributions for a broad \eta range.Comment: 11 pages and 13 figures. Substantially revised and accepted for
publication in Journal of Physics
Towards computational insights into the large-scale structure of spin foams
Understanding the large-scale physics is crucial for the spin foam approach
to quantum gravity. We tackle this challenge from a statistical physics
perspective using simplified, yet feature-rich models. In particular, this
allows us to explicitly answer whether broken symmetries will be restored by
renormalization: We observe a weak phase transition in both Migdal-Kadanoff and
tensor network renormalization. In this work we give a concise presentation of
the concepts, results and promises of this new direction of research.Comment: 10 pages, 9 figures, to be published in proceedings of the Loops'11
Madrid international conference on quantum gravit
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