21,617 research outputs found
Vorton Formation
In this paper we present the first analytic model for vorton formation. We
start by deriving the microscopic string equations of motion in Witten's
superconducting model, and show that in the relevant chiral limit these
coincide with the ones obtained from the supersonic elastic models of Carter
and Peter. We then numerically study a number of solutions of these equations
of motion and thereby suggest criteria for deciding whether a given
superconducting loop configuration can form a vorton. Finally, using a recently
developed model for the evolution of currents in superconducting strings we
conjecture, by comparison with these criteria, that string networks formed at
the GUT phase transition should produce no vortons. On the other hand, a
network formed at the electroweak scale can produce vortons accounting for up
to 6% of the critical density. Some consequences of our results are discussed.Comment: 41 pages; color figures 3-6 not included, but available from authors.
To appear in Phys. Rev.
High-field Electron Spin Resonance of Cu_{1-x}Zn_{x}GeO_{3}
High-Field Electron Spin Resonance measurements were made on powder samples
of Cu_{1-x}Zn_{x}GeO_{3} (x=0.00, 0.01, 0.02, 0.03 and 0.05) at different
frequencies (95, 110, 190, 220, 330 and 440 GHz) at low temperatures. The
spectra of the doped samples show resonances whose positions are dependent on
Zn concentration, frequency and temperature. The analysis of intensity
variation of these lines with temperature allows us to identify them as
originating in transitions within states situated inside the Spin Peierls gap.
A qualitative explanation of the details of the spectra is possible if we
assume that these states in the gap are associated with "loose" spins created
near the Zn impurities, as recently theoreticaly predicted. A new phenomenon of
quenching of the ESR signal across the Dimerized to Incommensurate
phase-boundary is observed.Comment: 4 pages, 5 ps figures in the text, submitted to Phys. Rev. Let
Clustering Properties of Dynamical Dark Energy Models
We provide a generic but physically clear discussion of the clustering
properties of dark energy models. We explicitly show that in quintessence-type
models the dark energy fluctuations, on scales smaller than the Hubble radius,
are of the order of the perturbations to the Newtonian gravitational potential,
hence necessarily small on cosmological scales. Moreover, comparable
fluctuations are associated with different gauge choices. We also demonstrate
that the often used homogeneous approximation is unrealistic, and that the
so-called dark energy mutation is a trivial artifact of an effective, single
fluid description. Finally, we discuss the particular case where the dark
energy fluid is coupled to dark matter.Comment: 5 page
Electron Transport through a Molecular Conductor with Center-of-Mass Motion
The linear conductance of a molecular conductor oscillating between two
metallic leads is investigated numerically both for Hubbard interacting and
noninteracting electrons. The molecule-leads tunneling barriers depend on the
molecule displacement from its equilibrium position. The results present an
interesting interference which leads to a conductance dip at the electron-hole
symmetry point, that could be experimentally observable. It is shown that this
dip is caused by the destructive interference between the purely electronic and
phonon-assisted tunneling channels, which are found to carry opposite phases.
When an internal vibrational mode is also active, the electron-hole symmetry is
broken but a Fano-like interference is still observed
How transformational leadership influences museums’ performance: a contextual ambidexterity view
Although researchers and managers recognise the tensional nature characterising the dynamics of museums and show a major concern with museums’ performance, the lack of studies examining organizational antecedents of performance is potentially limiting the understanding of the process by which top managers promote this key outcome. Drawing on the literature on ambidexterity and transformational leadership, we suggest that the transformational leadership of museums’ top managers facilitates the emergence of contextual ambidexterity which, in turn, influences museums’ performance. Data collected from a sample of 38 museums, including their top managers and 256 employees, support our hypotheses.info:eu-repo/semantics/acceptedVersio
Dynamics and control of the expansion of finite-size plasmas produced in ultraintense laser-matter interactions
The strong influence of the electron dynamics provides the possibility of
controlling the expansion of laser-produced plasmas by appropriately shaping
the laser pulse. A simple irradiation scheme is proposed to tailor the
explosion of large deuterium clusters, inducing the formation of shock
structures, capable of driving nuclear fusion reactions. Such a scenario has
been thoroughly investigated, resorting to two- and three-dimensional
particle-in-cell simulations. Furthermore, the intricate dynamics of ions and
electrons during the collisionless expansion of spherical nanoplasmas has been
analyzed in detail using a self-consistent ergodic-kinetic model. This study
clarifies the transition from hydrodynamic-like to Coulomb-explosion regimes
On the terminal velocity of sedimenting particles in a flowing fluid
The influence of an underlying carrier flow on the terminal velocity of
sedimenting particles is investigated both analytically and numerically. Our
theoretical framework works for a general class of (laminar or turbulent)
velocity fields and, by means of an ordinary perturbation expansion at small
Stokes number, leads to closed partial differential equations (PDE) whose
solutions contain all relevant information on the sedimentation process. The
set of PDE's are solved by means of direct numerical simulations for a class of
2D cellular flows (static and time dependent) and the resulting phenomenology
is analysed and discussed.Comment: 13 pages, 2 figures, submitted to JP
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