12,315 research outputs found
Linear evolution of sandwave packets
We investigate how a local topographic disturbance of a flat seabed may become morphodynamically active, according to the linear instability mechanism which gives rise to sandwave formation. The seabed evolution follows from a Fourier integral, which can generally not be evaluated in closed form. As numerical integration is rather cumbersome and not transparent, we propose an analytical way to approximate the solution. This method, using properties of the fastest growing mode only, turns out to be quick, insightful, and to perform well. It shows how a local disturbance develops gradually into a sandwave packet, the area of which increases roughly linearly with time. The elevation at the packet¿s center ultimately tends to increase, but this may be preceded by an initial stage of decrease, depending on the spatial extent of the initial disturbance. In the case of tidal asymmetry, the individual sandwaves in the packet migrate at the migration speed of the fastest growing mode, whereas the envelope moves at the group speed. Finally, we apply the theory to trenches and pits and show where results differ from an earlier study in which sandwave dynamics have been ignored
Velocity bias in a LCDM model
We use N-body simulations to study the velocity bias of dark matter halos,
the difference in the velocity fields of dark matter and halos, in a flat low-
density LCDM model. The high force, 2kpc/h, and mass, 10^9Msun/h, resolution
allows dark matter halos to survive in very dense environments of groups and
clusters making it possible to use halos as galaxy tracers. We find that the
velocity bias pvb measured as a ratio of pairwise velocities of the halos to
that of the dark matter evolves with time and depends on scale. At high
redshifts (z ~5) halos move generally faster than the dark matter almost on all
scales: pvb(r)~1.2, r>0.5Mpc/h. At later moments the bias decreases and gets
below unity on scales less than r=5Mpc/h: pvb(r)~(0.6-0.8) at z=0. We find that
the evolution of the pairwise velocity bias follows and probably is defined by
the spatial antibias of the dark matter halos at small scales. One-point
velocity bias b_v, defined as the ratio of the rms velocities of halos and dark
matter, provides a more direct measure of the difference in velocities because
it is less sensitive to the spatial bias. We analyze b_v in clusters of
galaxies and find that halos are ``hotter'' than the dark matter: b_v=(1.2-1.3)
for r=(0.2-0.8)r_vir, where r_vir is the virial radius. At larger radii, b_v
decreases and approaches unity at r=(1-2)r_vir. We argue that dynamical
friction may be responsible for this small positive velocity bias b_v>1 found
in the central parts of clusters. We do not find significant difference in the
velocity anisotropy of halos and the dark matter. The dark matter the velocity
anisotropy can be approximated as beta(x)=0.15 +2x/(x^2+4), where x is measured
in units of the virial radius.Comment: 13 pages, Latex, AASTeXv5 and natbi
The bisymplectomorphism group of a bounded symmetric domain
An Hermitian bounded symmetric domain in a complex vector space, given in its
circled realization, is endowed with two natural symplectic forms: the flat
form and the hyperbolic form. In a similar way, the ambient vector space is
also endowed with two natural symplectic forms: the Fubini-Study form and the
flat form. It has been shown in arXiv:math.DG/0603141 that there exists a
diffeomorphism from the domain to the ambient vector space which puts in
correspondence the above pair of forms. This phenomenon is called symplectic
duality for Hermitian non compact symmetric spaces.
In this article, we first give a different and simpler proof of this fact.
Then, in order to measure the non uniqueness of this symplectic duality map, we
determine the group of bisymplectomorphisms of a bounded symmetric domain, that
is, the group of diffeomorphisms which preserve simultaneously the hyperbolic
and the flat symplectic form. This group is the direct product of the compact
Lie group of linear automorphisms with an infinite-dimensional Abelian group.
This result appears as a kind of Schwarz lemma.Comment: 19 pages. Version 2: minor correction
Elementos polucionantes en tintura y aprestos. Manifestaciones de la polución, procedimientos y productos menos polucionantes.
Manifestaciones de la polución, procedimientos y productos menos polucionantes.Peer Reviewe
Numerical design of vehicles with optimal straight line stability on undulating road surfaces
xiv+121hlm.;24c
Experimental quantum information processing with 43Ca+ ions
For quantum information processing (QIP) with trapped ions, the isotope 43Ca+
offers the combined advantages of a quantum memory with long coherence time, a
high fidelity read out and the possibility of performing two qubit gates on a
quadrupole transition with a narrow-band laser. Compared to other ions used for
quantum computing, 43Ca+ has a relatively complicated level structure. In this
paper we discuss how to meet the basic requirements for QIP and demonstrate
ground state cooling, robust state initialization and efficient read out for
the hyperfine qubit with a single 43Ca+ ion. A microwave field and a Raman
light field are used to drive qubit transitions, and the coherence times for
both fields are compared. Phase errors due to interferometric instabilities in
the Raman field generation do not limit the experiments on a time scale of 100
ms. We find a quantum information storage time of many seconds for the
hyperfine qubit.Comment: 9 pages, 10 figure
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