7,179 research outputs found
Reproductive biology of carpenter seabream (Argyrozona argyrozona) (Pisces: Sparidae) in a marine protected area
The carpenter seabream (Argyrozona argyrozona) is an
endemic South African sparid that comprises an important part of the handline fishery. A three-year study (1998−2000) into its reproductive biology within the Tsitsikamma National Park revealed that these fishes are
serial spawning late gonochorists. The size at 50% maturity (L50) was estimated at 292 and 297 mm FL for both females and males, respectively. A likelihood ratio test revealed that there was no significant difference between male and female L50 (P>0.5). Both monthly gonadosomatic indices
and macroscopically determined ovarian stages strongly indicate that A. argyrozona within the Tsitsikamma
National Park spawn in the astral summer between November and April. The presence of postovulatory follicles (POFs) confirmed a six-month spawning season, and monthly proportions of early (0−6 hour old) POFs showed that spawning frequency was highest (once every 1−2 days) from December to March. Although spawning season was more highly correlated to photoperiod (r = 0.859) than temperature
(r = −0.161), the daily proportion of spawning fish was strongly correlated (r= 0.93) to ambient temperature over
the range 9−22oC. These results indicate that short-term upwelling events, a strong feature in the Tsitsikamma
National Park during summer, may negatively affect carpenter fecundity. Both spawning frequency and duration
(i.e., length of spawning season) increased with fish length. As a result of the allometric relationship between
annual fecundity and fish mass a 3-kg fish was calculated to produce fivefold more eggs per kilogram of body weight
than a fish of 1 kg. In addition to producing more eggs per unit of weight each year, larger fish also produce
significantly larger eggs
Rotating saddle trap as Foucault's pendulum
One of the many surprising results found in the mechanics of rotating systems
is the stabilization of a particle in a rapidly rotating planar saddle
potential. Besides the counterintuitive stabilization, an unexpected
precessional motion is observed. In this note we show that this precession is
due to a Coriolis-like force caused by the rotation of the potential. To our
knowledge this is the first example where such force arises in an inertial
reference frame. We also propose an idea of a simple mechanical demonstration
of this effect.Comment: 13 pages, 9 figure
Pumped current and voltage for an adiabatic quantum pump
We consider adiabatic pumping of electrons through a quantum dot. There are
two ways to operate the pump: to create a dc current or to create a
dc voltage . We demonstrate that, for very slow pumping,
and are not simply related via the dc conductance as . For the case of a chaotic quantum dot, we consider the statistical
distribution of . Results are presented for the limiting
cases of a dot with single channel and with multichannel point contacts.Comment: 6 pages, 4 figure
Current-induced transverse spin wave instability in a thin nanomagnet
We show that an unpolarized electric current incident perpendicular to the
plane of a thin ferromagnet can excite a spin-wave instability transverse to
the current direction if source and drain contacts are not symmetric. The
instability, which is driven by the current-induced ``spin-transfer torque'',
exists for one current direction only.Comment: 4 pages, 2 figure
Rectification of displacement currents in an adiabatic electron pump
Rectification of ac displacement currents generated by periodic variation of
two independent gate voltages of a quantum dot can lead to a dc voltage linear
in the frequency. The presence of this rectified displacement current could
account for the magnetic field symmetry observed in a recent measurement on an
adiabatic quantum electron pump by Switkes et al. [Science 283, 1905 (1999)].Comment: 2 pages, RevTeX; 1 figur
A spin pump turnstile: parametric pumping of a spin-polarized current through a nearly-closed quantum dot
We investigate parametric pumping of a spin-polarized current through a
nearly-closed quantum dot in a perpendicular magnetic field. Pumping is
achieved by tuning the tunnel couplings to the left and right lead - thereby
operating the quantum dot as a turnstile - and changing either the magnetic
field or a gate-voltage. We analyze the quantum dynamics of a pumping cycle and
the limiting time scales for operating the quantum dot turnstile as a pure spin
pump. The proposed device can be used as a fully controllable double-sided and
bipolar spin filter and to inject spins "on demand".Comment: 5 pages, 2 figures, one reference correcte
Weak Coulomb blockade effect in quantum dots
We develop the general non-equilibrium theory of transport through a quantum
dot, including Coulomb Blockade effects via a 1/N expansion, where N is the
number of scattering channels. At lowest order we recover the Landauer formula
for the current plus a self-consistent equation for the dot potential. We
obtain the leading corrections and compare with earlier approaches. Finally, we
show that to leading and next leading order in 1/N there is no interaction
correction to the weak localization, in contrast to previous theories, but
consistent with experiments by Huibers et al. [Phys. Rev. Lett. 81, 1917
(1998)], where N=4.Comment: 4 pages, 2 figures. Published versio
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