688 research outputs found
Independent magnon states on magnetic polytopes
For many spin systems with constant isotropic antiferromagnetic
next-neighbour Heisenberg coupling the minimal energies E_{min}(S) form a
rotational band, i.e. depend approximately quadratically on the total spin
quantum number S, a property which is also known as Lande interval rule.
However, we find that for certain coupling topologies, including recently
synthesised icosidodecahedral structures this rule is violated for high total
spins. Instead the minimal energies are a linear function of total spin. This
anomaly results in a corresponding jump of the magnetisation curve which
otherwise would be a regular staircase.Comment: 11 pages, 4 figures, submitted to Eur. Phys. J.
Installation Effects of a Propeller Mounted on a High-Lift Wing with a Coanda Flap. Part I: Aeroacoustic Experiments
In this contribution, we present aeroacoustic experiments concerning installation effects of propellers. Such installation effects are important as they can significantly alter the sound radiation as compared to an isolated propeller. For this purpose, detailed experiments have been conducted in the NWB aeroacoustic wind tunnel in Braunschweig, Germany. The considered geometry is a nine-bladed propeller installed in front of a high-lift
wing (employing a Coanda flap). The results illustrate the influence of propeller rotational speed, blade pitch angle, wind tunnel velocity, and angle of attack variations on the sound radiation. Furthermore, with a source localisation technique insight is gained in the dominant sound sources, and reveals the importance of periodic as well as broadband noise for the considered geometry
Proton Spin Relaxation Induced by Quantum Tunneling in Fe8 Molecular Nanomagnet
The spin-lattice relaxation rate and NMR spectra of H in
single crystal molecular magnets of Fe8 have been measured down to 15 mK. The
relaxation rate shows a strong temperature dependence down to 400
mK. The relaxation is well explained in terms of the thermal transition of the
iron state between the discreet energy levels of the total spin S=10. The
relaxation time becomes temperature independent below 300 mK and is
longer than 100 s. In this temperature region stepwise recovery of the
H-NMR signal after saturation was observed depending on the return field of
the sweep field. This phenomenon is attributed to resonant quantum tunneling at
the fields where levels cross and is discussed in terms of the Landau-Zener
transition.Comment: 13 pages, 5 figure
Performance of ePix10K, a high dynamic range, gain auto-ranging pixel detector for FELs
ePix10K is a hybrid pixel detector developed at SLAC for demanding
free-electron laser (FEL) applications, providing an ultrahigh dynamic range
(245 eV to 88 MeV) through gain auto-ranging. It has three gain modes (high,
medium and low) and two auto-ranging modes (high-to-low and medium-to-low). The
first ePix10K cameras are built around modules consisting of a sensor flip-chip
bonded to 4 ASICs, resulting in 352x384 pixels of 100 m x 100 m each.
We present results from extensive testing of three ePix10K cameras with FEL
beams at LCLS, resulting in a measured noise floor of 245 eV rms, or 67 e
equivalent noise charge (ENC), and a range of 11000 photons at 8 keV. We
demonstrate the linearity of the response in various gain combinations: fixed
high, fixed medium, fixed low, auto-ranging high to low, and auto-ranging
medium-to-low, while maintaining a low noise (well within the counting
statistics), a very low cross-talk, perfect saturation response at fluxes up to
900 times the maximum range, and acquisition rates of up to 480 Hz. Finally, we
present examples of high dynamic range x-ray imaging spanning more than 4
orders of magnitude dynamic range (from a single photon to 11000
photons/pixel/pulse at 8 keV). Achieving this high performance with only one
auto-ranging switch leads to relatively simple calibration and reconstruction
procedures. The low noise levels allow usage with long integration times at
non-FEL sources. ePix10K cameras leverage the advantages of hybrid pixel
detectors with high production yield and good availability, minimize
development complexity through sharing the hardware, software and DAQ
development with all other versions of ePix cameras, while providing an upgrade
path to 5 kHz, 25 kHz and 100 kHz in three steps over the next few years,
matching the LCLS-II requirements.Comment: 9 pages, 5 figure
Rotational modes in molecular magnets with antiferromagnetic Heisenberg exchange
In an effort to understand the low temperature behavior of recently
synthesized molecular magnets we present numerical evidence for the existence
of a rotational band in systems of quantum spins interacting with
nearest-neighbor antiferromagnetic Heisenberg exchange. While this result has
previously been noted for ring arrays with an even number of spin sites, we
find that it also applies for rings with an odd number of sites as well as for
all of the polytope configurations we have investigated (tetrahedron, cube,
octahedron, icosahedron, triangular prism, and axially truncated icosahedron).
It is demonstrated how the rotational band levels can in many cases be
accurately predicted using the underlying sublattice structure of the spin
array. We illustrate how the characteristics of the rotational band can provide
valuable estimates for the low temperature magnetic susceptibility.Comment: 14 pages, 7 figures, to be published in Phys. Rev.
Magnetic Anisotropy in the Molecular Complex V15
We apply degenerate perturbation theory to investigate the effects of
magnetic anisotropy in the magnetic molecule V15. Magnetic anisotropy is
introduced via Dzyaloshinskii-Moriya (DM) interaction in the full Hilbert space
of the system. Our model provides an explanation for the rounding of
transitions in the magnetization as a function of applied field at low
temperature, from which an estimate for the DM interaction is found. We find
that the calculated energy differences of the lowest energy states are
consistent with the available data. Our model also offers a novel explanation
for the hysteretic nature of the time-dependent magnetization data.Comment: Final versio
Investigating the lateral dose response functions of point detectors in proton beams
Objective Point detector measurements in proton fields are perturbed by the volume effect originating from geometrical volume-averaging within the extended detector's sensitive volume and density perturbations by non-water equivalent detector components. Detector specific lateral dose response functions K(x) can be used to characterize the volume effect within the framework of a mathematical convolution model, where K(x) is the convolution kernel transforming the true dose profile D(x) into the measured signal profile of a detector M(x). The aim of this work is to investigate K(x) for detectors in proton beams. Approach The K(x) for five detectors were determined by iterative deconvolution of measurements of D(x) and M(x) profiles at 2 cm water equivalent depth of a narrow 150 MeV proton beam. Monte Carlo simulations were carried out for two selected detectors to investigate a potential energy dependence, and to study the contribution of volume-averaging and density perturbation to the volume effect. Main results The Monte Carlo simulated and experimentally determined K(x) agree within 2.1% of the maximum value. Further simulations demonstrate that the main contribution to the volume effect is volume-averaging. The results indicate that an energy or depth dependence of K(x) is almost negligible in proton beams. While the signal reduction from a Semiflex 3D ionization chamber in the center of a gaussian shaped field with 2 mm sigma is 32% for photons, it is 15% for protons. When measuring the field with a microDiamond the trend is less pronounced and reversed with a signal reduction for protons of 3.9% and photons of 1.9%. Significance The determined K(x) can be applied to characterize the influence of the volume effect on detectors measured signal profiles at all clinical proton energies and measurement depths. The functions can be used to derive the actual dose distribution from point detector measurements
Nature and origin of fluids in granulite facies metamorphism
The various models for the nature and origin of fluids in granulite facies metamorphism were summarized. Field and petrologic evidence exists for both fluid-absent and fluid-present deep crustal metamorphism. The South Indian granulite province is often cited as a fluid-rich example. The fluids must have been low in H2O and thus high in CO2. Deep crustal and subcrustal sources of CO2 are as yet unproven possibilities. There is much recent discussion of the possible ways in which deep crustal melts and fluids could have interacted in granulite metamorphism. Possible explanations for the characteristically low activity of H2O associated with granulite terranes were discussed. Granulites of the Adirondacks, New York, show evidence for vapor-absent conditions, and thus appear different from those of South India, for which CO2 streaming was proposed. Several features, such as the presence of high-density CO2 fluid inclusions, that may be misleading as evidence for CO2-saturated conditions during metamorphism, were discussed
Opposing role of dopamine D1 and D2 receptors in modulation of rat nucleus accumbens noradrenaline release
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