291 research outputs found
Time-Resolved Studies of a Rolled-Up Semiconductor Microtube Laser
We report on lasing in rolled-up microtube resonators. Time-resolved studies
on these semiconductor lasers containing GaAs quantum wells as optical gain
material reveal particularly fast turn-on-times and short pulse emissions above
the threshold. We observe a strong red-shift of the laser mode during the pulse
emission which is compared to the time evolution of the charge-carrier density
calculated by rate equations
Rapid formation of large aggregates during the spring bloom of Kerguelen Island: observations and model comparisons
International audienceWhile production of aggregates and their subsequent sinking is known to be one pathway for the downward movement of organic matter from the euphotic zone, the rapid transition from non-aggregated to aggregated particles has not been reported previously. We made one vertical profile of particle size distributions (PSD; sizes ranging from 0.052 to several millimeters in equivalent spherical diameter) at pre-bloom stage and seven vertical profiles 3 weeks later over a 48 h period at early bloom stage using the Underwater Vision Profiler during the Kerguelen Ocean and Plateau Compared Study cruise 2 (KEOPS2, Octoberâ November 2011). In these naturally iron-fertilized waters southeast of Kerguelen Island (Southern Ocean), the total particle numerical abundance increased by more than four-fold within this time period. A massive total volume increase associated with particle size distribution changes was observed over the 48 h survey, showing the rapid formation of large particles and their accumulation at the base of the mixed layer. The results of a one-dimensional particle dynamics model support coagulation as the mechanism responsible for the rapid aggregate formation and the development of the V T subsurface maxima. The comparison of V T profiles between early bloom stage and pre-bloom stage indicates an increase of particulate export below 200 m when bloom has developed. These results highlight the role of coagulation in forming large particles and triggering carbon export at the early stage of a naturally iron-fertilized bloom, while zoo-plankton grazing may dominate later in the season. The rapid changes observed illustrate the critical need to measure carbon export flux with high sampling temporal resolution. Our results are the first published in situ observations of the rapid accumulation of marine aggregates and their export and the general agreement of this rapid event with a model of phyto-plankton growth and coagulation
Zero-field thermopower of a thin heterostructure membrane with a 2D electron gas
We study the low-temperature thermopower of micron sized, free-standing
membranes containing a two-dimensional electron system. Suspended membranes of
320 nm thickness including a high electron mobility structure in Hall bar
geometry of 34 {\mu}m length are prepared from GaAs/AlGaAs heterostructures
grown by molecular beam epitaxy. Joule heating on the central region of the
membrane generates a thermal gradient with respect to the suspension points
where the membrane is attached to cold reservoirs. Temperature measurements on
the membrane reveal strong thermal gradients due to the low thermal
conductivity. We measure the zero-field thermopower and find that the
phonon-drag contribution is suppressed at low temperatures up to 7 K.Comment: 5 page
Spin-wave spectrum in La2CuO4 -- double occupancy and competing interaction effects
The recently observed spin-wave energy dispersion along the AF zone boundary
in La2CuO4 is discussed in terms of double occupancy and competing interaction
effects in the Hubbard model on a square lattice.Comment: 4 pages, 2 figure
Stability of the doped antiferromagnetic state of the t-t'-Hubbard model
The next-nearest-neighbour hopping term t' is shown to stabilize the AF state
of the doped Hubbard model with respect to transverse perturbations in the
order- parameter by strongly suppressing the intraband particle-hole processes.
For a fixed sign of t', this stabilization is found to be significantly
different for electron and hole doping, which qualitatively explains the
observed difference in the degree of robustness of the AF state in the
electron-doped (Nd_{2-x}Ce_{x}CuO_{4}) and hole-doped (La_{2-x}Sr_{x}CuO_{4})
cuprates. The t'-U phase diagram is obtained for both signs of the t' term,
showing the different regions of stability and instability of the doped
antiferromagnet. Doping is shown to suppress the t'-induced frustration due to
the competing interaction J'. A study of transverse spin fluctuations in the
metallic AF state reveals that the decay of magnons into particle-hole
excitations yields an interesting low-energy result \Gamma \sim \omega for
magnon damping.Comment: 10 pages, 8 figure
Single-dot Spectroscopy of GaAs Quantum Dots Fabricated by Filling of Self-assembled Nanoholes
We study the optical emission of single GaAs quantum dots (QDs). The QDs are fabricated by filling of nanoholes in AlGaAs and AlAs which are generated in a self-assembled fashion by local droplet etching with Al droplets. Using suitable process parameters, we create either uniform QDs in partially filled deep holes or QDs with very broad size distribution in completely filled shallow holes. Micro photoluminescence measurements of single QDs of both types establish sharp excitonic peaks. We measure a fine-structure splitting in the range of 22â40ÎŒeV and no dependence on QD size. Furthermore, we find a decrease in excitonâbiexciton splitting with increasing QD size
The wineglass effect shapes particle export to the deep ocean in mesoscale eddies
Mesoscale eddies in the ocean strongly impact the distribution of planktonic particles, mediating carbon fluxes over ~1/3 of the world ocean. However, mechanisms controlling particle transport through eddies are complex and challenging to measure in situ. Here we show the subsurface distribution of eddy particles funneled into a wineglass shape down to 1000âm, leading to a sevenfold increase of vertical carbon flux in the eddy center versus the eddy flanks, the âwineglass effectâ. We show that the slope of the wineglass (R) is the ratio of particle sinking velocity to the radially inward velocity, such that R represents a tool to predict radial particle movement (here 0.05âmâsâ1). A simple model of eddy spindown predicts such an ageostrophic flow concentrating particles in the eddy center. We explore how size-specific particle flux toward the eddy center impacts eddies' biogeochemistry and export fluxes
Optical Properties of GaAs Quantum Dots Fabricated by Filling of Self-Assembled Nanoholes
Experimental results of the local droplet etching technique for the self-assembled formation of nanoholes and quantum rings on semiconductor surfaces are discussed. Dependent on the sample design and the process parameters, filling of nanoholes in AlGaAs generates strain-free GaAs quantum dots with either broadband optical emission or sharp photoluminescence (PL) lines. Broadband emission is found for samples with completely filled flat holes, which have a very broad depth distribution. On the other hand, partly filling of deep holes yield highly uniform quantum dots with very sharp PL lines
The Investigation of Intermediate Stage of Template Etching with Metal Droplets by Wetting Angle Analysis on (001) GaAs Surface
In this work, we study metal droplets on a semiconductor surface that are the initial stage for both droplet epitaxy and local droplet etching. The distributions of droplet geometrical parameters such as height, radius and volume help to understand the droplet formation that strongly influences subsequent nanohole etching. To investigate the etching and intermixing processes, we offer a new method of wetting angle analysis. The aspect ratio that is defined as the ratio of the height to radius was used as an estimation of wetting angle which depends on the droplet material. The investigation of the wetting angle and the estimation of indium content revealed significant materials intermixing during the deposition time. AFM measurements reveal the presence of two droplet groups that is in agreement with nanohole investigations. To explain this observation, we consider arsenic evaporation and consequent change in the initial substrate. On the basis of our analysis, we suggest the model of droplet evolution and the formation of two droplet groups
Cdk1 inactivation terminates mitotic checkpoint surveillance and stabilizes kinetochore attachments in anaphase
Two mechanisms safeguard the bipolar attachment of chromosomes in mitosis. A correction mechanism destabilizes erroneous attachments that do not generate tension across sister kinetochores [1]. In response to unattached kinetochores, the mitotic checkpoint delays anaphase onset by inhibiting the anaphase-promoting complex/cyclosome (APC/CCdc20) [2]. Upon satisfaction of both pathways, the APC/CCdc20 elicits the degradation of securin and cyclin B [3]. This liberates separase triggering sister chromatid disjunction and inactivates cyclin-dependent kinase 1 (Cdk1) causing mitotic exit. How eukaryotic cells avoid the engagement of attachment monitoring mechanisms when sister chromatids split and tension is lost at anaphase is poorly understood [4]. Here we show that Cdk1 inactivation disables mitotic checkpoint surveillance at anaphase onset in human cells. Preventing cyclin B1 proteolysis at the time of sister chromatid disjunction destabilizes kinetochore-microtubule attachments and triggers the engagement of the mitotic checkpoint. As a consequence, mitotic checkpoint proteins accumulate at anaphase kinetochores, the APC/CCdc20 is inhibited, and securin reaccumulates. Conversely, acute pharmacological inhibition of Cdk1 abrogates the engagement and maintenance of the mitotic checkpoint upon microtubule depolymerization. We propose that the simultaneous destruction of securin and cyclin B elicited by the APC/CCdc20 couples chromosome segregation to the dissolution of attachment monitoring mechanisms during mitotic exit
- âŠ