1,047 research outputs found
Picophytoplankton biomass distribution in the global ocean
The smallest marine phytoplankton, collectively termed picophytoplankton, have been routinely enumerated by flow cytometry since the late 1980s during cruises throughout most of the world ocean. We compiled a database of 40 946 data points, with separate abundance entries for Prochlorococcus, Synechococcus and picoeukaryotes. We use average conversion factors for each of the three groups to convert the abundance data to carbon biomass. After gridding with 1? spacing, the database covers 2.4% of the ocean surface area, with the best data coverage in the North Atlantic, the South Pacific and North Indian basins, and at least some data in all other basins. The average picophytoplankton biomass is 12 ± 22 µg Cl-1 or 1.9 g Cm-2. We estimate a total global picophytoplankton biomass of 0.53–1.32 Pg C (17–39% Prochlorococcus, 12–15% Synechococcus and 49–69% picoeukaryotes), with an intermediate/best estimate of 0.74 Pg C. Future efforts in this area of research should focus on reporting calibrated cell size and collecting data in undersampled regions
Quantum dynamics, dissipation, and asymmetry effects in quantum dot arrays
We study the role of dissipation and structural defects on the time evolution
of quantum dot arrays with mobile charges under external driving fields. These
structures, proposed as quantum dot cellular automata, exhibit interesting
quantum dynamics which we describe in terms of equations of motion for the
density matrix. Using an open system approach, we study the role of asymmetries
and the microscopic electron-phonon interaction on the general dynamical
behavior of the charge distribution (polarization) of such systems. We find
that the system response to the driving field is improved at low temperatures
(and/or weak phonon coupling), before deteriorating as temperature and
asymmetry increase. In addition to the study of the time evolution of
polarization, we explore the linear entropy of the system in order to gain
further insights into the competition between coherent evolution and
dissipative processes.Comment: 11pages,9 figures(eps), submitted to PR
Low X-Ray Luminosity Galaxy Clusters: Main goals, sample selection, photometric and spectroscopic observations
We present the study of nineteen low X-ray luminosity galaxy clusters (L 0.5--45 erg s), selected from the ROSAT
Position Sensitive Proportional Counters (PSPC) Pointed Observations (Vikhlinin
et al. 1998) and the revised version of Mullis et al. (2003) in the redshift
range of 0.16 to 0.7. This is the introductory paper of a series presenting the
sample selection, photometric and spectroscopic observations and data
reduction. Photometric data in different passbands were taken for eight galaxy
clusters at Las Campanas Observatory; three clusters at Cerro Tololo
Interamerican Observatory; and eight clusters at the Gemini Observatory.
Spectroscopic data were collected for only four galaxy clusters using Gemini
telescopes. With the photometry, the galaxies were defined based on the
star-galaxy separation taking into account photometric parameters. For each
galaxy cluster, the catalogues contain the PSF and aperture magnitudes of
galaxies within the 90\% completeness limit. They are used together with
structural parameters to study the galaxy morphology and to estimate
photometric redshifts. With the spectroscopy, the derived galaxy velocity
dispersion of our clusters ranged from 507 km~s for [VMF98]022 to 775
km~s for [VMF98]097 with signs of substructure. Cluster membership has
been extensively discussed taking into account spectroscopic and photometric
redshift estimates. In this sense, members are the galaxies within a projected
radius of 0.75 Mpc from the X-ray mission peak and with cluster centric
velocities smaller than the cluster velocity dispersion or 6000 km~s,
respectively. These results will be used in forthcoming papers to study, among
the main topics, the red cluster sequence, blue cloud and green populations;
the galaxy luminosity function and cluster dynamics.Comment: 13 pages, 6 tables, 9 figures. Uses emulateapj. Accepted for
publication in The Astronomical Journal. Some formatting errors fixe
Spin-orbit coupling and intrinsic spin mixing in quantum dots
Spin-orbit coupling effects are studied in quantum dots in InSb, a narrow-gap
material. Competition between different Rashba and Dresselhaus terms is shown
to produce wholesale changes in the spectrum. The large (and negative)
-factor and the Rashba field produce states where spin is no longer a good
quantum number and intrinsic flips occur at moderate magnetic fields. For dots
with two electrons, a singlet-triplet mixing occurs in the ground state, with
observable signatures in intraband FIR absorption, and possible importance in
quantum computation.Comment: REVTEX4 text with 3 figures (high resolution figs available by
request). Submitted to PR
Single and vertically coupled type II quantum dots in a perpendicular magnetic field: exciton groundstate properties
The properties of an exciton in a type II quantum dot are studied under the
influence of a perpendicular applied magnetic field. The dot is modelled by a
quantum disk with radius , thickness and the electron is confined in the
disk, whereas the hole is located in the barrier. The exciton energy and
wavefunctions are calculated using a Hartree-Fock mesh method. We distinguish
two different regimes, namely (the hole is located at the radial
boundary of the disk) and (the hole is located above and below the
disk), for which angular momentum transitions are predicted with
increasing magnetic field. We also considered a system of two vertically
coupled dots where now an extra parameter is introduced, namely the interdot
distance . For each and for a sufficient large magnetic field,
the ground state becomes spontaneous symmetry broken in which the electron and
the hole move towards one of the dots. This transition is induced by the
Coulomb interaction and leads to a magnetic field induced dipole moment. No
such symmetry broken ground states are found for a single dot (and for three
vertically coupled symmetric quantum disks). For a system of two vertically
coupled truncated cones, which is asymmetric from the start, we still find
angular momentum transitions. For a symmetric system of three vertically
coupled quantum disks, the system resembles for small the pillar-like
regime of a single dot, where the hole tends to stay at the radial boundary,
which induces angular momentum transitions with increasing magnetic field. For
larger the hole can sit between the disks and the state
remains the groundstate for the whole -region.Comment: 11 pages, 16 figure
Poly (lactic acid)/D-limonene/ZnO bio-nanocomposites with antimicrobial properties
Antimicrobial films of poly (lactic acid) (PLA)/D-limonene/zinc oxide (ZnO)-based bio-nanocomposites were prepared via melt compounding and subsequent thermocompression. D-limonene was incorporated at concentrations of 10 or 20 wt%, and ZnO pure nanoparticles and those organically modified with oleic acid (O-ZnO), with an average diameter of 13.5 nm, were included at concentrations of 3, 5, and 8 wt%. The plasticizing effect of D-Limonene was corroborated by a decrease in the glass transition temperature compared to pure PLA. The presence of ZnO and O-ZnO in the PLA matrix promoted a slight increase in the degree of crystallinity due to its nucleant performance. Although ZnO and O-ZnO induced lower thermal stability and slightly decreased microhardness in the composites, excellent antimicrobial performance was demonstrated. Both ZnO and O-ZnO nanocomposites reached 99.9% of effectiveness for nanoparticles content above 5 wt%, regardless of the source of irradiation, D-limonene concentration, and nanoparticle modification. Therefore, these bio-nanocomposites will allow for future advances in sustainable antimicrobial materials for the medical or food packaging fields.DICYT, Grant/Award Number: Project 022041ZR_POSTDOCT; Fondo Nacional de Desarrollo Científico y Tecnológico,Grant/Award Numbers: 1170226, 320029
Aharonov-Bohm signature for neutral excitons in type-II quantum dot ensembles
It is commonly believed that the Aharonov-Bohm (AB) effect is a typical
feature of the motion of a charged particle interacting with the
electromagnetic vector potential. Here we present a magnetophotoluminescence
study of type-II InP/GaAs self-assembled quantum dots, unambiguously revealing
the Aharonov-Bohm-type oscillations for neutral excitons when the hole ground
state changes its angular momentum from lh = 0 to lh = 1, 2, and 3. The hole
ring parameters derived from a simple model are in excellent agreement with the
structural parameters for this system.Comment: Revised version, 10 pages, 3 figure
Coulomb drag in mesoscopic rings
We develop a Luttinger liquid theory of the Coulomb drag of persistent
currents flowing in concentric mesoscopic rings, by incorporating non-linear
corrections to the electron dispersion relation. We demonstrate that at low
temperatures, interactions between electrons in different rings generate an
additional phase and thus alter the period of Aharonov-Bohm oscillations. The
resulting nondissipative drag depends strongly on the relative parity of the
electron numbers. We also show that interactions set a new temperature scale
below which the linear response theory does not apply at certain values of
external flux.Comment: Latex 10 pages + 2 Figure
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