8,062 research outputs found
Boxfishes (Teleostei: Ostraciidae) as a model system for fishes swimming with many fins: kinematics
Swimming movements in boxfishes were much more
complex and varied than classical descriptions indicated.
At low to moderate rectilinear swimming speeds
(<5 TL s^(-1), where TL is total body length), they were
entirely median- and paired-fin swimmers, apparently
using their caudal fins for steering. The pectoral and
median paired fins generate both the thrust needed for
forward motion and the continuously varied, interacting
forces required for the maintenance of rectilinearity. It
was only at higher swimming speeds (above 5 TL s^(-1)), when
burst-and-coast swimming was used, that they became
primarily body and caudal-fin swimmers. Despite their
unwieldy appearance and often asynchronous fin beats,
boxfish swam in a stable manner. Swimming boxfish used
three gaits. Fin-beat asymmetry and a relatively nonlinear
swimming trajectory characterized the first gait
(0–1 TL s^(-1)). The beginning of the second gait (1–3 TL s^(-1))
was characterized by varying fin-beat frequencies and
amplitudes as well as synchrony in pectoral fin motions.
The remainder of the second gait (3–5 TL s^(-1)) was
characterized by constant fin-beat amplitudes, varying finbeat
frequencies and increasing pectoral fin-beat
asynchrony. The third gait (>5 TL s^(-1)) was characterized
by the use of a caudal burst-and-coast variant. Adduction
was always faster than abduction in the pectoral fins.
There were no measurable refractory periods between
successive phases of the fin movement cycles. Dorsal and
anal fin movements were synchronized at speeds greater
than 2.5 TL s^(-1), but were often out of phase with pectoral
fin movements
Self-induced decoherence approach: Strong limitations on its validity in a simple spin bath model and on its general physical relevance
The "self-induced decoherence" (SID) approach suggests that (1) the
expectation value of any observable becomes diagonal in the eigenstates of the
total Hamiltonian for systems endowed with a continuous energy spectrum, and
(2), that this process can be interpreted as decoherence. We evaluate the first
claim in the context of a simple spin bath model. We find that even for large
environments, corresponding to an approximately continuous energy spectrum,
diagonalization of the expectation value of random observables does in general
not occur. We explain this result and conjecture that SID is likely to fail
also in other systems composed of discrete subsystems. Regarding the second
claim, we emphasize that SID does not describe a physically meaningful
decoherence process for individual measurements, but only involves destructive
interference that occurs collectively within an ensemble of presupposed
"values" of measurements. This leads us to question the relevance of SID for
treating observed decoherence effects.Comment: 11 pages, 4 figures. Final published versio
Detection, Properties, and Frequency of Local Calcium Release from the Sarcoplasmic Reticulum in Teleost Cardiomyocytes
Calcium release from the sarcoplasmic reticulum (SR) plays a central role in the regulation of cardiac contraction and rhythm in mammals and humans but its role is controversial in teleosts. Since the zebrafish is an emerging model for studies of cardiovascular function and regeneration we here sought to determine if basic features of SR calcium release are phylogenetically conserved. Confocal calcium imaging was used to detect spontaneous calcium release (calcium sparks and waves) from the SR. Calcium sparks were detected in 16 of 38 trout atrial myocytes and 6 of 15 ventricular cells. The spark amplitude was 1.45±0.03 times the baseline fluorescence and the time to half maximal decay of sparks was 27±3 ms. Spark frequency was 0.88 sparks µm−1 min−1 while calcium waves were 8.5 times less frequent. Inhibition of SR calcium uptake reduced the calcium transient (F/F0) from 1.77±0.17 to 1.12±0.18 (p = 0.002) and abolished calcium sparks and waves. Moreover, elevation of extracellular calcium from 2 to 10 mM promoted early and delayed afterdepolarizations (from 0.6±0.3 min−1 to 8.1±2.0 min−1, p = 0.001), demonstrating the ability of SR calcium release to induce afterdepolarizations in the trout heart. Calcium sparks of similar width and duration were also observed in zebrafish ventricular myocytes. In conclusion, this is the first study to consistently report calcium sparks in teleosts and demonstrate that the basic features of calcium release through the ryanodine receptor are conserved, suggesting that teleost cardiac myocytes is a relevant model to study the functional impact of abnormal SR function
Hadronization corrections to helicity components of the fragmentation function
In the hadronic decays of Z, gluon emission leads to the appearance of the
longitudinal component of the fragmentation function, F_L. Measurement of F_L
and the transverse component, F_T, could thus provide an insight into the gluon
fragmentation function. However, hadronization corrections at low x can be
significant. Here we present a method of accounting for such corrections, using
the JETSET event generator as illustration.Comment: 11 pages, 5 figure
From Bloch model to the rate equations II: the case of almost degenerate energy levels
Bloch equations give a quantum description of the coupling between an atom
and a driving electric force. In this article, we address the asymptotics of
these equations for high frequency electric fields, in a weakly coupled regime.
We prove the convergence towards rate equations (i.e. linear Boltzmann
equations, describing the transitions between energy levels of the atom). We
give an explicit form for the transition rates. This has already been performed
in [BFCD03] in the case when the energy levels are fixed, and for different
classes of electric fields: quasi or almost periodic, KBM, or with continuous
spectrum. Here, we extend the study to the case when energy levels are possibly
almost degenerate. However, we need to restrict to quasiperiodic forcings. The
techniques used stem from manipulations on the density matrix and the averaging
theory for ordinary differential equations. Possibly perturbed small divisor
estimates play a key role in the analysis. In the case of a finite number of
energy levels, we also precisely analyze the initial time-layer in the rate
aquation, as well as the long-time convergence towards equilibrium. We give
hints and counterexamples in the infinite dimensional case
The order of the metal to superconductor transition
We present results from large-scale Monte Carlo simulations on the full
Ginzburg-Landau (GL) model, including fluctuations in the amplitude and the
phase of the matter-field, as well as fluctuations of the non-compact
gauge-field of the theory. {}From this we obtain a precise critical value of
the GL parameter \kct separating a first order metal to superconductor
transition from a second order one, \kct = (0.76\pm 0.04)/\sqrt{2}. This
agrees surprisingly well with earlier analytical results based on a disorder
theory of the superconductor to metal transition, where the value
\kct=0.798/\sqrt{2} was obtained. To achieve this, we have done careful
infinite volume and continuum limit extrapolations. In addition we offer a
novel interpretation of \kct, namely that it is also the value separating
\typeI and \typeII behaviour.<Comment: Minor corrections, present version accepted for publication in PR
Decoherence time in self-induced decoherence
A general method for obtaining the decoherence time in self-induced
decoherence is presented. In particular, it is shown that such a time can be
computed from the poles of the resolvent or of the initial conditions in the
complex extension of the Hamiltonian's spectrum. Several decoherence times are
estimated: for microscopic systems, and
for macroscopic bodies. For the particular case of a
thermal bath, our results agree with those obtained by the einselection
(environment-induced decoherence) approach.Comment: 11 page
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