214,491 research outputs found
A continuum model for entangled fibres
Motivated by the study of fibre dynamics in the carding machine, a continuum model for the motion of a medium composed of fibres is derived under the assumption that the dominant forces are due to fibre-fibre interactions and that the material is in tension. To characterise the material we include the averaged values of density and velocity and introduce variables to describe the mean direction, alignment and entanglement. We assume that the bulk stress of the material depends on the density, entanglement, degree of alignment, average direction and shear-rates. A kinematic equation for the average direction and two proposed heuristic laws for the evolution of entanglement and degree of alignment are given to close the system. Extensional and shearing simulations are in good qualitative agreement with experimental results
Antiferromagnetic phase transition in a nonequilibrium lattice of Rydberg atoms
We study a driven-dissipative system of atoms in the presence of laser
excitation to a Rydberg state and spontaneous emission. The atoms interact via
the blockade effect, whereby an atom in the Rydberg state shifts the Rydberg
level of neighboring atoms. We use mean-field theory to study how the Rydberg
population varies in space. As the laser frequency changes, there is a
continuous transition between the uniform and antiferromagnetic phases. The
nonequilibrium nature also leads to a novel oscillatory phase and bistability
between the uniform and antiferromagnetic phases.Comment: 4 pages + appendi
Relativistic Quantum Thermodynamics of Ideal Gases in 2 Dimensions
In this work we study the behavior of relativistic ideal Bose and Fermi gases
in two space dimensions. Making use of polylogarithm functions we derive a
closed and unified expression for their densities. It is shown that both type
of gases are essentially inequivalent, and only in the non-relativistic limit
the spinless and equal mass Bose and Fermi gases are equivalent as known in the
literature.Comment: 6 pages, 1 figur
A Theory of Gamma-Ray Bursts
We present a specific scenario for the link between GRB and hypernovae, based
on Blandford-Znajek extraction of black-hole spin energy. Such a mechanism
requires a high angular momentum in the progenitor object. The observed
association of gamma-ray bursts with type Ibc supernovae leads us to consider
massive helium stars that form black holes at the end of their lives as
progenitors. We combine the numerical work of MacFadyen & Woosley with analytic
calculations, to show that about 1E53 erg each are available to drive the fast
GRB ejecta and the supernova. The GRB ejecta are driven by the power output
through the open field lines, whereas the supernova is powered by closed filed
lines and jet shocks. We also present a much simplified approximate derivation
of these energetics.
Helium stars that leave massive black-hole remnants in special ways, namely
via soft X-ray transients or very massive WNL stars. Since binaries naturally
have high angular momentum, we propose a link between black-hole transients and
gamma-ray bursts. Recent observations of one such transient, GRO J1655-40/Nova
Scorpii 1994, explicitly support this connection: its high space velocity
indicates that substantial mass was ejected in the formation of the black hole,
and the overabundance of alpha-nuclei, especially sulphur, indicates that the
explosion energy was extreme, as in SN 1998bw/GRB 980425. (abstract shortened)Comment: 32 pages, 8 figures, accepted for publication in New Astronom
Noise and Equivalent Circuit of Double Injection
Measurements of the highâfrequency noise of a silicon doubleâinjection diode result in ăi^2ă = αâ
4kT(1/r)Îf with α=1.04 and in agreement with the literature. A new interpretation demands Nyquist noise with αâĄ1 in these devices at high frequencies. This is in accord with an equivalent circuit derived for the doubleâinjection process. Speculations are made on the general validity of Nyquist noise in nonlinear devices at high frequencies. In addition, generationârecombination noise is suggested as the prime source of the lowâfrequency noise
Observation of the spontaneous vortex phase in the weakly ferromagnetic superconductor ErNiBC: A penetration depth study
The coexistence of weak ferromagnetism and superconductivity in ErNiBC suggests the possibility of a spontaneous vortex phase (SVP) in which
vortices appear in the absence of an external field. We report evidence for the
long-sought SVP from the in-plane magnetic penetration depth of high-quality single crystals of ErNiBC. In addition to
expected features at the N\'{e}el temperature = 6.0 K and weak
ferromagnetic onset at K, rises to a maximum
at K before dropping sharply down to 0.1 K. We assign the
0.45 K-maximum to the proliferation and freezing of spontaneous vortices. A
model proposed by Koshelev and Vinokur explains the increasing as a consequence of increasing vortex density, and its subsequent decrease
below as defect pinning suppresses vortex hopping.Comment: 5 pages including figures; added inset to Figure 2; significant
revisions to tex
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