15,410 research outputs found
Universality classes in anisotropic non-equilibrium growth models
We study the effect of generic spatial anisotropies on the scaling behavior
in the Kardar-Parisi-Zhang equation. In contrast to its "conserved" variants,
anisotropic perturbations are found to be relevant in d > 2 dimensions, leading
to rich phenomena that include novel universality classes and the possibility
of first-order phase transitions and multicritical behavior. These results
question the presumed scaling universality in the strong-coupling rough phase,
and shed further light on the connection with generalized driven diffusive
systems.Comment: 4 pages, revtex, 2 figures (eps files enclosed
Spin polarization of light atoms in jellium: Detailed electronic structures
We revisit the problem of the spontaneous magnetization of an {\em sp}
impurity atom in a simple metal host. The main features of interest are: (i)
Formation of the spherical spin density/charge density wave around the
impurity; (ii) Considerable decrease in the size of the pseudoatom in the
spin-polarized state as compared with the paramagnetic one, and (iii) Relevance
of the electron affinity of the isolated atom to this spin polarization, which
is clarified by tracing the transformation of the pseudoatom into an isolated
negative ion in the low-density limit of the enveloping electron gas.Comment: 4 pages, 4 figures, accepted to Phys. Rev.
Modelling confounding effects from extracerebral contamination and systemic factors on functional near-infrared spectroscopy
Haemodynamics-based neuroimaging is widely used to study brain function. Regional blood flow changes characteristic of neurovascular coupling provide an important marker of neuronal activation. However, changes in systemic physiological parameters such as blood pressure and concentration of CO2 can also affect regional blood flow and may confound haemodynamics-based neuroimaging. Measurements with functional near-infrared spectroscopy (fNIRS) may additionally be confounded by blood flow and oxygenation changes in extracerebral tissue layers. Here we investigate these confounds using an extended version of an existing computational model of cerebral physiology, 'BrainSignals'. Our results show that confounding from systemic physiological factors is able to produce misleading haemodynamic responses in both positive and negative directions. By applying the model to data from previous fNIRS studies, we demonstrate that such potentially deceptive responses can indeed occur in at least some experimental scenarios. It is therefore important to record the major potential confounders in the course of fNIRS experiments. Our model may then allow the observed behaviour to be attributed among the potential causes and hence reduce identification errors
Nb3Sn wire shape and cross sectional area inhomogeneity in Rutherford cables
During Rutherford cable production the wires are plastically deformed and
their initially round shape is distorted. Using X-ray absorption tomography we
have determined the 3D shape of an unreacted Nb3Sn 11 T dipole Rutherford
cable, and of a reacted and impregnated Nb3Sn cable double stack.
State-of-the-art image processing was applied to correct for tomographic
artefacts caused by the large cable aspect ratio, for the segmentation of the
individual wires and subelement bundles inside the wires, and for the
calculation of the wire cross sectional area and shape variations. The 11 T
dipole cable cross section oscillates by 2% with a frequency of 1.24 mm (1/80
of the transposition pitch length of the 40 wire cable). A comparatively
stronger cross sectional area variation is observed in the individual wires at
the thin edge of the keystoned cable where the wire aspect ratio is largest.Comment: 6 pages, 11 figures, presented at EUCAS 201
Kinetics of phase-separation in the critical spherical model and local scale-invariance
The scaling forms of the space- and time-dependent two-time correlation and
response functions are calculated for the kinetic spherical model with a
conserved order-parameter and quenched to its critical point from a completely
disordered initial state. The stochastic Langevin equation can be split into a
noise part and into a deterministic part which has local scale-transformations
with a dynamical exponent z=4 as a dynamical symmetry. An exact reduction
formula allows to express any physical average in terms of averages calculable
from the deterministic part alone. The exact spherical model results are shown
to agree with these predictions of local scale-invariance. The results also
include kinetic growth with mass conservation as described by the
Mullins-Herring equation.Comment: Latex2e with IOP macros, 28 pp, 2 figures, final for
Jahreszeitliche Schwankungen und Einfluss der Ernährung auf die makuläre Pigmentdichte
Zusammenfassung: Hintergrund: Wir haben gezeigt, dass die makuläre Pigmentdichte (MPD) mit einem modifizierten konfokalen Scanning-Laser-Ophthalmoskop (HRA, Fa. Heidelberg Engineering, Heidelberg, Deutschland) bestimmt werden kann. In dieser Studie untersuchten wir, ob Schwankungen der MPD bei gesunden Probanden im Verlauf eines Jahres auftraten und ob die MPD von wechselnden Ernährungsgewohnheiten abhängig ist. Methode: Die MPD wurde mithilfe von Autofluoreszenzbildern, die mit einem HRA aufgenommen wurden, dargestellt und in einem auf die Fovea zentrierten Messfeld von 2° Durchmesser bei den Probanden im Abstand von 2Monaten innerhalb eines Jahres bestimmt. Ergebnisse: Wir schlossen 30 gesunde Probanden im Alter von 19-34Jahren (Mittel: 23±2Jahre) in die Studie ein. Die mittlere MPD war an Untersuchungszeitpunkt1 0,215±0,056D.U., bei der 2.Untersuchung 0,235±0,051D.U., bei der 3. 0,218±0,055D.U., an Untersuchungszeitpunkt4 0,228±057D.U., bei der 5.Untersuchung 0,225±0,053D.U. und bei der letzten 0,203±0,050D.U.. Die statistische Analyse zeigte keine signifikanten Unterschiede der MPD im Verlauf eines Jahres. Zusammenfassung: Die Studie zeigt, dass bei jungen und gesunden Probanden die MPD keinen jahreszeitlichen und diätetischen Schwankungen unterlieg
The Lagrangian frequency spectrum as a diagnostic for magnetohydrodynamic turbulence dynamics
For the phenomenological description of magnetohydrodynamic turbulence
competing models exist, e.g. Boldyrev [Phys.Rev.Lett. \textbf{96}, 115002,
2006] and Gogoberidze [Phys.Plas. \textbf{14}, 022304, 2007], which predict the
same Eulerian inertial-range scaling of the turbulent energy spectrum although
they employ fundamentally different basic interaction mechanisms. {A relation
is found that links} the Lagrangian frequency spectrum {with} the
autocorrelation timescale of the turbulent fluctuations, ,
and the associated cascade timescale, . Thus, the
Lagrangian energy spectrum can serve to identify weak
() and strong
() interaction mechanisms providing
insight into the turbulent energy cascade. The new approach is illustrated by
results from direct numerical simulations of two- and three-dimensional
incompressible MHD turbulence.Comment: accepted for publication in PR
Oxygen superstructures throughout the phase diagram of
Short-range lattice superstructures have been studied with high-energy x-ray
diffuse scattering in underdoped, optimally doped, and overdoped . A new four-unit-cell superstructure was observed in
compounds with . Its temperature, doping, and material dependence
was used to attribute its origin to short-range oxygen vacancy ordering, rather
than electronic instabilities in the layers. No significant diffuse
scattering is observed in YBaCuO. The oxygen superstructures must
be taken into account when interpreting spectral anomalies in
Magnetoelastic and structural properties of azurite Cu3(CO3)2(OH)2 from neutron scattering and muon spin rotation
Azurite, Cu3(CO3)2(OH)2, has been considered an ideal example of a
one-dimensional (1D) diamond chain antiferromagnet. Early studies of this
material imply the presence of an ordered antiferromagnetic phase below K while magnetization measurements have revealed a 1/3 magnetization
plateau. Until now, no corroborating neutron scattering results have been
published to confirm the ordered magnetic moment structure. We present recent
neutron diffraction results which reveal the presence of commensurate magnetic
order in azurite which coexists with significant magnetoelastic strain. The
latter of these effects may indicate the presence of spin frustration in zero
applied magnetic field. Muon spin rotation, SR, reveals an onset of
short-range order below 3K and confirms long-range order below .Comment: 5 pages, 4 figures, PHYSICAL REVIEW B 81, 140406(R) (2010
Particle-Particle, Particle-Scaling function (P3S) algorithm for electrostatic problems in free boundary conditions
An algorithm for fast calculation of the Coulombic forces and energies of
point particles with free boundary conditions is proposed. Its calculation time
scales as N log N for N particles. This novel method has lower crossover point
with the full O(N^2) direct summation than the Fast Multipole Method. The
forces obtained by our algorithm are analytical derivatives of the energy which
guarantees energy conservation during a molecular dynamics simulation. Our
algorithm is very simple. An MPI parallelised version of the code can be
downloaded under the GNU General Public License from the website of our group.Comment: 19 pages, 11 figures, submitted to: Journal of Chemical Physic
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