340 research outputs found
Nest-site selection in the Cape Sugarbird
Cape sugarbirds, Promerops cafer, preferred certain pro tea bushes as nest-sites. These bushes were characterized by relatively large leaves and dense foliage. Nests were usually placed in the central parts of bushes Nest-sites were shielded from the cold night sky, and were associated with relatively favourable air temperatures and protection from wind in a season of unfavourable weather. Data on microclimate of nest-sites, together with those on heat loss from an incubating sugarbird, were interpreted in relation to nest-site selection as adaptive behaviour promoting breeding success through conservation of energy. Sugarbirds nesting relatively high off the ground and in dense vegetation were considered to be decreasing die risk of exposure to predators
Activity budgets of male Maccoa ducks
The behaviour of breeding and non-breeding male maccoa ducks (Oxywa maccoa) was studied in South Africa. Individual ducks were observed continuously from sunrise to sunset in the wild, and all ritualized and non-ritualized behaviour was recorded. Diurnal time budgets of territorial and non-territorial males were translated into energy budgets, showing relative time and energy expenditures for resting, foraging, preening and bathing, territorial defence and courtship activities. A territorial male spent one-third of the daytime on active behaviour associated with reproduction. Territorial males averaged 297 displays per fifteen-hour day, or about one display every three minutes. Active defence of territory occupied 15 per cent of a male's time budget, adding an estimated 20 per cent to the energy a non-breeding male spent on self-maintenance. Active courtship of females occupied IS per cent of a male's time budget, adding 18 per cent to the energy a non-breeding male spent on self-maintenance. The increase in time spent on reproductive behaviour over that of a non-breeding male is achieved mainly through decrease of time allocated to resting in the budget of the non-breeder. Territorial males increased energy expenditure by a factor of 1,3, but devoted the same portion of time as non-breeders to actual feeding (21 per cent). Energy and time-saving strategies were associated with territorial defence and courtship, contributing towards optimization of time and energy budgeting
Solitons, solitonic vortices, and vortex rings in a confined Bose-Einstein condensate
Quasi-one-dimensional solitons that may occur in an elongated Bose-Einstein
condensate become unstable at high particle density. We study two basic modes
of instability and the corresponding bifurcations to genuinely
three-dimensional solitary waves such as axisymmetric vortex rings and
non-axisymmetric solitonic vortices. We calculate the profiles of the above
structures and examine their dependence on the velocity of propagation along a
cylindrical trap. At sufficiently high velocity, both the vortex ring and the
solitonic vortex transform into an axisymmetric soliton. We also calculate the
energy-momentum dispersions and show that a Lieb-type mode appears in the
excitation spectrum for all particle densities.Comment: RevTeX 9 pages, 9 figure
Quantum Depletion of an Excited Condensate
We analyze greying of the dark soliton in a Bose-Einstein condensate in the
limit of weak interaction between atoms. The condensate initially prepared in
the excited dark soliton state is loosing atoms because of spontaneous quantum
depletion. These atoms are depleted from the soliton state into single particle
states with nonzero density in the notch of the soliton. As a result the image
of the soliton is losing contrast. This quantum depletion mechanism is
efficient even at zero temperature when a thermal cloud is absent.Comment: 4 pages; version to appear in Phys.Rev.A; change in the title plus a
number of small changes in the tex
Growth of detector-grade CZT by Traveling Heater Method (THM): An advancement
In this present work we report the growth of Cd{sub 0.9}Zn{sub 0.1}Te doped with In by a modified THM technique. It has been demonstrated that by controlling the microscopically flat growth interface, the size distribution and concentration can be drastically reduced in the as-grown ingots. This results in as-grown detector-grade CZT by the THM technique. The three-dimensional size distribution and concentrations of Te inclusions/precipitations were studied. The size distributions of the Te precipitations/inclusions were observed to be below the 10-{micro}m range with the total concentration less than 10{sup 5} cm{sup -3}. The relatively low value of Te inclusions/precipitations results in excellent charge transport properties of our as-grown samples. The ({mu}{tau}){sub e} values for different as-grown samples varied between 6-20 x 10{sup -3} cm{sup 2}/V. The as-grown samples also showed fairly good detector response with resolution of {approx}1.5%, 2.7% and about 3.8% at 662 keV for quasi-hemispherical geometry for detector volumes of 0.18 cm{sup 3}, 1 cm{sup 3} and 4.2 cm{sup 3}, respectively
Quantum carpet interferometry for trapped atomic Bose-Einstein condensates
We propose an ``interferometric'' scheme for Bose-Einstein condensates using
near-field diffraction. The scheme is based on the phenomenon of intermode
traces or quantum carpets; we show how it may be used in the detection of weak
forces.Comment: 4 figures. Submitted to Phys. Rev.
Matter wave solitons at finite temperatures
We consider the dynamics of a dark soliton in an elongated harmonically
trapped Bose-Einstein condensate. A central question concerns the behavior at
finite temperatures, where dissipation arises due to the presence of a thermal
cloud. We study this problem using coupled Gross-Pitaevskii and -body
simulations, which include the mean field coupling between the condensate and
thermal cloud. We find that the soliton decays relatively quickly even at very
low temperatures, with the decay rate increasing with rising temperature.Comment: 6 pages, 2 figures, submitted to the Proceedings of QFS '0
Nonlinear waves in a cylindrical Bose-Einstein condensate
We present a complete calculation of solitary waves propagating in a steady
state with constant velocity v along a cigar-shaped Bose-Einstein trap
approximated as infinitely-long cylindrical. For sufficiently weak couplings
(densities) the main features of the calculated solitons could be captured by
effective one-dimensional (1D) models. However, for stronger couplings of
practical interest, the relevant solitary waves are found to be hybrids of
quasi-1D solitons and 3D vortex rings. An interesting hierarchy of vortex rings
occurs as the effective coupling constant is increased through a sequence of
critical values. The energy-momentum dispersion of the above structures is
shown to exhibit characteristics similar to a mode proposed sometime ago by
Lieb within a strictly 1D model, as well as some rotonlike features.Comment: 10 pages, 12 figure
Simple method for excitation of a Bose-Einstein condensate
An appropriate, time-dependent modification of the trapping potential may be
sufficient to create effectively collective excitations in a cold atom
Bose-Einstein condensate. The proposed method is complementary to earlier
suggestions and should allow the creation of both dark solitons and vortices.Comment: 8 pages, 7 figures, version accepted for publication in Phys. Rev.
Generation and evolution of vortex-antivortex pairs in Bose-Einstein condensates
We propose a method for generating and controlling a spatially separated
vortex--antivortex pair in a Bose-Einstein condensate trapped in a toroidal
potential. Our simulations of the time dependent Gross-Pitaevskii equation show
that in toroidal condensates vortex dynamics are different from the dynamics in
the homogeneous case. Our numerical results agree well with analytical
calculations using the image method. Our proposal offers an effective example
of coherent generation and control of vortex dynamics in atomic condensates.Comment: 4 pages, 2 figure
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