95,167 research outputs found
Spin Correlations in the Two-Dimensional Spin-5/2 Heisenberg Antiferromagnet Rb2MnF4
We report a neutron scattering study of the instantaneous spin correlations
in the two-dimensional spin S=5/2 square-lattice Heisenberg antiferromagnet
Rb_2MnF_4. The measured correlation lengths are quantitatively described, with
no adjustable parameters, by high-temperature series expansion results and by a
theory based on the quantum self-consistent harmonic approximation. Conversely,
we find that the data, which cover the range from about 1 to 50 lattice
constants, are outside of the regime corresponding to renormalized classical
behavior of the quantum non-linear sigma model. In addition, we observe a
crossover from Heisenberg to Ising critical behavior near the Neel temperature;
this crossover is well described by a mean-field model with no adjustable
parameters.Comment: 8 pages, LaTeX, with 6 included EPS figures, submitted to EPJ
Report of the ultraviolet and visible sensors panel
In order to meet the science objectives of the Astrotech 21 mission set the Ultraviolet (UV) and Visible Sensors Panel made a number of recommendations. In the UV wavelength range of 0.01 to 0.3 micro-m the focus is on the need for large format high quantum efficiency, radiation hard 'solar-blind' detectors. Options recommended for support include Si and non-Si charge coupled devices (CCDs) as well as photocathodes with improved microchannel plate readouts. For the 0.3 to 0.9 micro-m range, it was felt that Si CCDs offer the best option for high quantum efficiencies at these wavelengths. In the 0.9 to 2.5 micro-m the panel recommended support for the investigation of monolithic arrays. Finally, the panel noted that the implementation of very large arrays will require new data transmission, data recording, and data handling technologies
The Lorentz force in atmospheres of CP stars: 56 Arietis
The presence of electric currents in the atmospheres of magnetic chemically
peculiar (mCP) stars could bring important theoretical constrains about the
nature and evolution of magnetic field in these stars. The Lorentz force, which
results from the interaction between the magnetic field and the induced
currents, modifies the atmospheric structure and induces characteristic
rotational variability of pressure-sensitive spectroscopic features, that can
be analysed using phase-resolved spectroscopic observations. In this work we
continue the presentation of results of the magnetic pressure studies in mCP
stars focusing on the high-resolution spectroscopic observations of Bp star
56Ari. We have detected a significant variability of the Halpha, Hbeta, and
Hgamma spectral lines during full rotation cycle of the star. Then these
observations are interpreted in the framework of the model atmosphere analysis,
which accounts for the Lorentz force effects. We used the LLmodels stellar
model atmosphere code for the calculation of the magnetic pressure effects in
the atmosphere of 56Ari taking into account realistic chemistry of the star and
accurate computations of the microscopic plasma properties. The Synth3 code was
employed to simulate phase-resolved variability of Balmer lines. We demonstrate
that the model with the outward-directed Lorentz force in the dipole+quadrupole
configuration is likely to reproduce the observed hydrogen lines variation.
These results present strong evidences for the presence of non-zero global
electric currents in the atmosphere of this early-type magnetic star.Comment: Accepted by A&A, 9 pages, 7 figures, 2 table
Complete trails of co-authorship network evolution
The rise and fall of a research field is the cumulative outcome of its
intrinsic scientific value and social coordination among scientists. The
structure of the social component is quantifiable by the social network of
researchers linked via co-authorship relations, which can be tracked through
digital records. Here, we use such co-authorship data in theoretical physics
and study their complete evolutionary trail since inception, with a particular
emphasis on the early transient stages. We find that the co-authorship networks
evolve through three common major processes in time: the nucleation of small
isolated components, the formation of a tree-like giant component through
cluster aggregation, and the entanglement of the network by large-scale loops.
The giant component is constantly changing yet robust upon link degradations,
forming the network's dynamic core. The observed patterns are successfully
reproducible through a new network model
Recommended from our members
Learning under Distributed Weak Supervision
The availability of training data for supervision is a frequently encountered bottleneck of medical image analysis methods. While typically established by a clinical expert rater, the increase in acquired imaging data renders traditional pixel-wise segmentations less feasible. In this paper, we examine the use of a crowdsourcing platform for the distribution of super-pixel weak annotation tasks and collect such annotations from a crowd of non-expert raters. The crowd annotations are subsequently used for training a fully convolutional neural network to address the problem of fetal brain segmentation in T2-weighted MR images. Using this approach we report encouraging results compared to highly targeted, fully supervised methods and potentially address a frequent problem impeding image analysis research
Degenerate Four Virtual Soliton Resonance for KP-II
By using disipative version of the second and the third members of AKNS
hierarchy, a new method to solve 2+1 dimensional Kadomtsev-Petviashvili (KP-II)
equation is proposed. We show that dissipative solitons (dissipatons) of those
members give rise to the real solitons of KP-II. From the Hirota bilinear form
of the SL(2,R) AKNS flows, we formulate a new bilinear representation for
KP-II, by which, one and two soliton solutions are constructed and the
resonance character of their mutual interactions is studied. By our bilinear
form, we first time created four virtual soliton resonance solution for KP-II
and established relations of it with degenerate four-soliton solution in the
Hirota-Satsuma bilinear form for KP-II.Comment: 10 pages, 5 figures, Talk on International Conference Nonlinear
Physics. Theory and Experiment. III, 24 June-3 July, 2004, Gallipoli(Lecce),
Ital
Parabolic negative magnetoresistance in p-Ge/Ge1-xSix heterostructures
Quantum corrections to the conductivity due to the weak localization (WL) and
the disorder-modified electron-electron interaction (EEI) are investigated for
the high-mobility multilayer p-Ge/Ge1-xSix heterostructures at T = (0.1 -
20.0)K in magnetic field B up to 1.5T. Negative magnetoresistance with
logarithmic dependence on T and linear in B^2 is observed for B >= 0.1T. Such a
behavior is attributed to the interplay of the classical cyclotron motion and
the EEI effect. The Hartree part of the interaction constant is estimated
(F_/sigma = 0.44) and the WL and EEI contributions to the total quantum
correction /Delta /sigma at B = 0 are separated (/Delta /sigma_{WL} ~ 0.3/Delta
/sigma; /Delta /sigma_{EEI} ~ 0.7/Delta /sigma).Comment: 3 pages, 4 figure
Heap Formation in Granular Media
Using molecular dynamics (MD) simulations, we find the formation of heaps in
a system of granular particles contained in a box with oscillating bottom and
fixed sidewalls. The simulation includes the effect of static friction, which
is found to be crucial in maintaining a stable heap. We also find another
mechanism for heap formation in systems under constant vertical shear. In both
systems, heaps are formed due to a net downward shear by the sidewalls. We
discuss the origin of net downward shear for the vibration induced heap.Comment: 11 pages, 4 figures available upon request, Plain TeX, HLRZ-101/9
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