3,881 research outputs found
Area-Constrained Planar Elastica
We determine the equilibria of a rigid loop in the plane, subject to the
constraints of fixed length and fixed enclosed area. Rigidity is characterized
by an energy functional quadratic in the curvature of the loop. We find that
the area constraint gives rise to equilibria with remarkable geometrical
properties: not only can the Euler-Lagrange equation be integrated to provide a
quadrature for the curvature but, in addition, the embedding itself can be
expressed as a local function of the curvature. The configuration space is
shown to be essentially one-dimensional, with surprisingly rich structure.
Distinct branches of integer-indexed equilibria exhibit self-intersections and
bifurcations -- a gallery of plots is provided to highlight these findings.
Perturbations connecting equilibria are shown to satisfy a first order ODE
which is readily solved. We also obtain analytical expressions for the energy
as a function of the area in some limiting regimes.Comment: 23 pages, several figures. Version 2: New title. Changes in the
introduction, addition of a new section with conclusions. Figure 14 corrected
and one reference added. Version to appear in PR
The VLT-FLAMES Tarantula Survey XXIII. Two massive double-lined binaries in 30 Doradus
Aims. We investigate the characteristics of two newly discovered short-period, double-lined, massive binary systems in the Large
Magellanic Cloud, VFTS 450 (O9.7 II–Ib + O7::) and VFTS 652 (B1 Ib + O9: III:).
Methods. We perform model-atmosphere analyses to characterise the photospheric properties of both members of each binary (denoting the “primary” as the spectroscopically more conspicuous component). Radial velocities and optical photometry are used to estimate the binary-system parameters.
Results. We estimate Teff = 27 kK, log g = 2.9 (cgs) for the VFTS 450 primary spectrum (34 kK, 3.6: for the secondary spectrum); and Teff = 22 kK, log g = 2.8 for the VFTS 652 primary spectrum (35 kK, 3.7: for the secondary spectrum). Both primaries show surface nitrogen enrichments (of more than 1 dex for VFTS 652), and probable moderate oxygen depletions relative to reference LMC abundances. We determine orbital periods of 6.89 d and 8.59 d for VFTS 450 and VFTS 652, respectively, and argue that the primaries must be close to filling their Roche lobes. Supposing this to be the case, we estimate component masses in the range ∼20–50 M⊙.
Conclusions. The secondary spectra are associated with the more massive components, suggesting that both systems are high-mass analogues of classical Algol systems, undergoing case-A mass transfer. Difficulties in reconciling the spectroscopic analyses with the light-curves and with evolutionary considerations suggest that the secondary spectra are contaminated by (or arise in) accretion disks
Theory for Dynamical Short Range Order and Fermi Surface Volume in Strongly Correlated Systems
Using the fluctuation exchange approximation of the one band Hubbard model,
we discuss the origin of the changing Fermi surface volume in underdoped
cuprate systems due to the transfer of occupied states from the Fermi surface
to its shadow, resulting from the strong dynamical antiferromagnetic short
range correlations. The momentum and temperature dependence of the quasi
particle scattering rate shows unusual deviations from the conventional Fermi
liquid like behavior. Their consequences for the changing Fermi surface volume
are discussed. Here, we investigate in detail which scattering processes
might be responsible for a violation of the Luttinger theorem. Finally, we
discuss the formation of hole pockets near half filling.Comment: 5 pages, Revtex, 4 postscript figure
Evidence that exogenous but not endogenous norepinephrine activates the presynaptic alpha,-adrenoceptors on serotonergic nerve endings in the rat hypothalamus
ABSTRACT ABBREVIATiONS: NE, norepinephnne; 5-HT, 5-hydroxytryptamine; 6F-NE, 6-fluoronorepinephrine. 72
Phase-Field Model of Mode III Dynamic Fracture
We introduce a phenomenological continuum model for mode III dynamic fracture
that is based on the phase-field methodology used extensively to model
interfacial pattern formation. We couple a scalar field, which distinguishes
between ``broken'' and ``unbroken'' states of the system, to the displacement
field in a way that consistently includes both macroscopic elasticity and a
simple rotationally invariant short scale description of breaking. We report
two-dimensional simulations that yield steady-state crack motion in a strip
geometry above the Griffith threshold.Comment: submitted to PR
Mean-field analysis of a dynamical phase transition in a cellular automaton model for collective motion
A cellular automaton model is presented for random walkers with biologically
motivated interactions favoring local alignment and leading to collective
motion or swarming behavior. The degree of alignment is controlled by a
sensitivity parameter, and a dynamical phase transition exhibiting spontaneous
breaking of rotational symmetry occurs at a critical parameter value. The model
is analyzed using nonequilibrium mean field theory: Dispersion relations for
the critical modes are derived, and a phase diagram is constructed. Mean field
predictions for the two critical exponents describing the phase transition as a
function of sensitivity and density are obtained analytically.Comment: 4 pages, 4 figures, final version as publishe
Theory of Spin Fluctuation-Induced Superconductivity Based on a d-p Model. II. -Superconducting State-
The superconducting state of a two-dimensional d-p model is studied from the
spin fluctuation point of view by using a strong coupling theory. The
fluctuation exchange (FLEX) approximatoin is employed to calculate the spin
fluctuations and the superconducting gap functions self-consistently in the
optimal- and over-doped regions of hole concentration. The gap function has a
symmetry of d_{x^2 - y^2} type and develops below the transition temperature
T_c more rapidly than in the BCS model. Its saturation value at the maximum is
about 10 T_c. When the spin fluctuation-induced superconductivity is well
stabilized at low temperatures in the optimal regime, the imaginary part of the
antiferromagnetic spin susceptibility shows a very sharp resonance peak
reminiscent of the 41 meV peak observed in the neutron scattering experiment on
YBCO. The one-particle spectral density around k=(pi,0) shows sharp
quasi-particle peaks followed by dip and hump structures bearing resemblance to
the features observed in the angle-resolved photoemission experiment. With
increasing doping concentration these features gradually disappear.Comment: 13 pages(LaTeX), 20 eps figure
Localization-delocalization transition of a reaction-diffusion front near a semipermeable wall
The A+B --> C reaction-diffusion process is studied in a system where the
reagents are separated by a semipermeable wall. We use reaction-diffusion
equations to describe the process and to derive a scaling description for the
long-time behavior of the reaction front. Furthermore, we show that a critical
localization-delocalization transition takes place as a control parameter which
depends on the initial densities and on the diffusion constants is varied. The
transition is between a reaction front of finite width that is localized at the
wall and a front which is detached and moves away from the wall. At the
critical point, the reaction front remains at the wall but its width diverges
with time [as t^(1/6) in mean-field approximation].Comment: 7 pages, PS fil
Dynamics of Phase Transitions: The 3D 3-state Potts model
In studies of the QCD deconfining phase transition or cross-over by means of
heavy ion experiments, one ought to be concerned about non-equilibrium effects
due to heating and cooling of the system. In this paper we extend our previous
study of Glauber dynamics of 2D Potts models to the 3D 3-state Potts model,
which serves as an effective model for some QCD properties. We investigate the
linear theory of spinodal decomposition in some detail. It describes the early
time evolution of the 3D model under a quench from the disordered into the
ordered phase well, but fails in 2D. Further, the quench leads to competing
vacuum domains, which are difficult to equilibrate, even in the presence of a
small external magnetic field. From our hysteresis study we find, as before, a
dynamics dominated by spinodal decomposition. There is evidence that some
effects survive in the case of a cross-over. But the infinite volume
extrapolation is difficult to control, even with lattices as large as .Comment: 12 pages; added references, corrected typo
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