1,373 research outputs found
Enhanced heat flow in the hydrodynamic-collisionless regime
We study the heat conduction of a cold, thermal cloud in a highly asymmetric
trap. The cloud is axially hydrodynamic, but due to the asymmetric trap
radially collisionless. By locally heating the cloud we excite a thermal dipole
mode and measure its oscillation frequency and damping rate. We find an
unexpectedly large heat conduction compared to the homogeneous case. The
enhanced heat conduction in this regime is partially caused by atoms with a
high angular momentum spiraling in trajectories around the core of the cloud.
Since atoms in these trajectories are almost collisionless they strongly
contribute to the heat transfer. We observe a second, oscillating hydrodynamic
mode, which we identify as a standing wave sound mode.Comment: Sumitted to Phys. Rev. Letters, 4 pages, 4 figure
Reaching the hydrodynamic regime in a Bose-Einstein condensate by suppression of avalanche
We report the realization of a Bose-Einstein condensate (BEC) in the
hydrodynamic regime. The hydrodynamic regime is reached by evaporative cooling
at a relative low density suppressing the effect of avalanches. With the
suppression of avalanches a BEC containing 120.10^6 atoms is produced. The
collisional opacity can be tuned from the collisionless regime to a collisional
opacity of more than 3 by compressing the trap after condensation. In the
collisional opaque regime a significant heating of the cloud at time scales
shorter than half of the radial trap period is measured. This is direct proof
that the BEC is hydrodynamic.Comment: Article submitted for Phys. Rev. Letters, 6 figure
Reproductive value, sensitivity, and nonlinearity: Population-management heuristics derived from classical demography
In classical demographic theory, reproductive value and stable age distribution are proportional to the sensitivities of the asymptotic population size to changes in mortality and maternity, respectively. In this note we point out that analogous relationships hold if the maternity function is allowed to depend on the population density. The relevant formulae can essentially be obtained by replacing the growth rate ("Lotka'sr") with zero. These facts may be used to derive heuristics for population management (pest control)
Large atom number Bose-Einstein condensate of sodium
We describe the setup to create a large Bose-Einstein condensate containing
more than 120x10^6 atoms. In the experiment a thermal beam is slowed by a
Zeeman slower and captured in a dark-spot magneto-optical trap (MOT). A typical
dark-spot MOT in our experiments contains 2.0x10^10 atoms with a temperature of
320 microK and a density of about 1.0x10^11 atoms/cm^3. The sample is spin
polarized in a high magnetic field, before the atoms are loaded in the magnetic
trap. Spin polarizing in a high magnetic field results in an increase in the
transfer efficiency by a factor of 2 compared to experiments without spin
polarizing. In the magnetic trap the cloud is cooled to degeneracy in 50 s by
evaporative cooling. To suppress the 3-body losses at the end of the
evaporation the magnetic trap is decompressed in the axial direction.Comment: 11 pages, 12 figures, submitted to Review Of Scientific Instrument
Ethyl 6-methyl-4-[2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-3-yl]-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate
A new Biginelli compound, C18H25BN2O4S2, containing a boronate ester group was synthesized from a lithium bromide-catalysed reaction. The compound crystallizes with two independent molecules in the asymmetric unit that differ mainly in the conformation of the ester functionality. The crystal structure is stabilized by intermolecular N—H⋯O and N—H⋯S hydrogen bonds involving the 3,4-dihydropyrimidine-2(1H)-thione NH groups as donors and the carbonyl O and thiophene S atoms as acceptors
Branden als EGM-maatregel
Periodiek branden is een traditionele gebruiks- of beheermethode die in het verleden op meerdere plaatsen werd toegepast. Dit gebeurde met name in heideterreinen en soms in duingraslanden, om de vegetatie te verjongen en de opslag van struiken en bomen tegen te gaan. geleidelijk is de beheersmethode uit het zicht verdwenen. In dit rapport zijn de ervaringen met periodiek branden en de kennis die er in binnenland en buitenland is over de effecten op de beschikbaarheid van nutriënten, vegetatie en fauna op een rij gezet. Het rapport geeft aan in welke situaties branden een aantrekkelijke effectgerichte maatregel kan zijn. In dit onderzoek (in het kader van OBN) werkten samen: Stichting Bargerveen, Bware en Alterr
Repetition Probability Does Not Affect fMRI Repetition Suppression for Objects
Previously several functional magnetic resonance imaging (fMRI) studies point toward the role of perceptual expectations in determining adaptation or repetition suppression (RS) in humans. These studies showed that the probability of repetitions of faces within a block influences the magnitude of adaptation in face-related areas of the human brain (Summerfield et al., 2008). However, a current macaque single-cell/local field potential (LFP) recording study using objects as stimuli found no evidence for the modulation of the neural response by the repetition probability in the inferior temporal cortex (Kaliukhovich and Vogels, 2010). Here we examined whether stimulus repetition probability affects fMRI repetition suppression for nonface object stimuli in the human brain. Subjects were exposed to either two identical [repetition trials (RTs)] or two different [alternation trials (ATs)] object stimuli. Both types of trials were presented blocks consisting of either 75% [repetition blocks (RBs)] or 25% [alternation blocks (ABs)] of RTs. We found strong RS, i.e., a lower signal for RTs compared to ATs, in the object sensitive lateral occipital cortex as well as in the face-sensitive occipital and fusiform face areas. More importantly, however, there was no significant difference in the magnitude of RS between RBs and ABs in each of the areas. This is in agreement with the previous monkey single-unit/LFP findings and suggests that RS in the case of nonface visual objects is not modulated by the repetition probability in humans. Our results imply that perceptual expectation effects vary for different visual stimulus categories
Dispersion strengthening in vanadium microalloyed steels processed by simulated thin slab casting and direct charging. Part 2 - chemical characterisation of dispersion strengthening precipitates
The composition of the sub-15 nm particles in six related vanadium high strength low alloy steels, made by simulated thin slab direct charged casting, has been determined using electron energy loss spectroscopy (EELS). Such particles are considered to be responsible for dispersion hardening. For the first time, particles down to 4 nm in size have had their composition fully determined. In all the steels, the particles were nitrogen and vanadium rich and possibly slightly sub-stoichiometric carbonitrides. Equilibrium thermodynamics predicted much higher carbon to metal atomic ratios than observed in all cases so that kinetics and mechanical deformation clearly control the precipitation process. Thus it is important to formulate the steel with this in mind
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