13,623 research outputs found
90Sr uptake in the freshwater liverwort Riccia fluitans L.
90Sr is a radioactive isotope of strontium produced by nuclear fission, with a half-life of 28.8 years. This artificial radioisotope is present in natural ecosystems as the results of radioactive fallout from nuclear weapons or releases during nuclear power plants accidents; because of its similarity with Ca*2 is quickly incorporated into the biota. The high mobility of Sr+2 in aquatic compared with terrestrial ecosystems makes the uptake and accumulation of 90Sr+2 higher in aquatic than in terrestrial plants. Here we analyse the uptake rate, kinetics and retention, concentration factor (CF) of 90Sr in the freshwater liverwort Riccia fluiants. 90Sr uptake by R. fluitnas shows a bi-phasic kinetics that fits the Michaelis & Menten model in both micro and milimolar concentration ranges. Apparent semi-saturation constants (KM) were 15 µM and 2 mM for the high and low affinity ranges, respectively. The presence of the K+ channels blocker tetraethyamonium (10 mM TEA) inhibits 90Sr uptake by a 25%. However, the presence of 1 mM La+3 completely inhibits 90Sr uptake in this plant. Maximum incorporation rate occurs at alkaline external pH (8.3), either in plants grown in the presence of K+ or in the absence. Finally, gradual increases of the Ca2+ concentration in the medium progressively inhibits 90Sr uptake. CF values are higher in K+ -deficient plants, CF maximum of 1500, than in K+ -sufficient, (maximum CF of 600) and show similar responses to inhibitors, pH or Ca2+. CF values progressively decrease at increasing external Ca2+ concentrations, higher CF values are found at pH 8.3 but lower values are observed in the presence of TEA, being close to zero in the presence of La+3. The different 90Sr uptake rates in K+-sufficient and K+-deficient plants and TEA sensitivity indicate that one part of 90Sr would be transported through non-selective cation channels. Furthermore, Ca2+ and La3+ sensitivities suggest that 90Sr could be incorporated through Ca2+ channels.Universidad de Málaga. Campus de Excelencia Internacional AndalucÃa Tech.
Spanish MINECO: BFU2017-85117-R and BIO2016-81957-RED
Orphan penumbrae: Submerging horizontal fields
We investigate the properties of orphan penumbrae, which are photospheric
filamentary structures observed in active regions near polarity inversion lines
that resemble the penumbra of regular sunspots but are not connected to any
umbra. We use Hinode data from the Solar Optical Telescope to determine the
properties of orphan penumbrae. Spectropolarimetric data are employed to obtain
the vector magnetic field and line-of-sight velocities in the photosphere.
Magnetograms are used to study the overall evolution of these structures, and
G-band and Ca II H filtergrams are to investigate their brightness and apparent
horizontal motions. Orphan penumbrae form between regions of opposite polarity
in places with horizontal magnetic fields. Their magnetic configuration is that
of -shaped flux ropes. In the two cases studied here, the
opposite-polarity regions approach each other with time and the whole structure
submerges as the penumbral filaments disappear. Orphan penumbrae are very
similar to regular penumbrae, including the existence of strong gas flows.
Therefore, they could have a similar origin. The main difference between them
is the absence of a "background" magnetic field in orphan penumbrae. This could
explain most of the observed differences. The fast flows we detect in orphan
penumbrae may be caused by the siphon flow mechanism. Based on the similarities
between orphan and regular penumbrae, we propose that the Evershed flow is also
a manifestation of siphon flows.Comment: 15 pages, 15 figure
Effect of many modes on self-polarization and photochemical suppression in cavities
The standard description of cavity-modified molecular reactions typically involves a single (resonant) mode, while in reality, the quantum cavity supports a range of photon modes. Here, we demonstrate that as more photon modes are accounted for, physicochemical phenomena can dramatically change, as illustrated by the cavity-induced suppression of the important and ubiquitous process of proton-coupled electron-transfer. Using a multi-trajectory Ehrenfest treatment for the photon-modes, we find that self-polarization effects become essential, and we introduce the concept of self-polarization-modified Born–Oppenheimer surfaces as a new construct to analyze dynamics. As the number of cavity photon modes increases, the increasing deviation of these surfaces from the cavity-free Born–Oppenheimer surfaces, together with the interplay between photon emission and absorption inside the widening bands of these surfaces, leads to enhanced suppression. The present findings are general and will have implications for the description and control of cavity-driven physical processes of molecules, nanostructures, and solids embedded in cavities
Time and energy-resolved two photon-photoemission of the Cu(100) and Cu(111) metal surfaces
We present calculations on energy- and time-resolved two-photon photoemission
spectra of images states in Cu(100) and Cu(111) surfaces. The surface is
modeled by a 1D effective potential and the states are propagated within a
real-space, real-time method. To obtain the energy resolved spectra we employ a
geometrical approach based on a subdivision of space into two regions. We treat
electronic inelastic effects by taking into account the scattering rates
calculated within a GW scheme. To get further insight into the decaying
mechanism we have also studied the effect of the variation of the classical
Hartree potential during the excitation. This effect turns out to be small.Comment: 11 pages, 7 figure
Short-Time Critical Dynamics of Damage Spreading in the Two-Dimensional Ising Model
The short-time critical dynamics of propagation of damage in the Ising
ferromagnet in two dimensions is studied by means of Monte Carlo simulations.
Starting with equilibrium configurations at and magnetization
, an initial damage is created by flipping a small amount of spins in one
of the two replicas studied. In this way, the initial damage is proportional to
the initial magnetization in one of the configurations upon quenching the
system at , the Onsager critical temperature of the
ferromagnetic-paramagnetic transition. It is found that, at short times, the
damage increases with an exponent , which is much larger
than the exponent characteristic of the initial increase of the
magnetization . Also, an epidemic study was performed. It is found that
the average distance from the origin of the epidemic ()
grows with an exponent , which is the same,
within error bars, as the exponent . However, the survival
probability of the epidemics reaches a plateau so that . On the other
hand, by quenching the system to lower temperatures one observes the critical
spreading of the damage at , where all the measured
observables exhibit power laws with exponents , , and .Comment: 11 pages, 9 figures (included). Phys. Rev. E (2010), in press
Optical absorption in small BN and C nanotubes
We present a theoretical study of the optical absorption spectrum of small
boron-nitride and carbon nanotubes using time-dependent density-functional
theory and the random phase approximation. Both for C and BN tubes, the
absorption of light polarized perpendicular to the tube-axis is strongly
suppressed due to local field effects. Since BN-tubes are wide band-gap
insulators, they only absorb in the ultra-violet energy regime, independently
of chirality and diameter. In comparison with the spectra of the single C and
BN-sheets, the tubes display additional fine-structure which stems from the
(quasi-) one-dimensionality of the tubes and sensitively depends on the
chirality and tube diameter. This fine structure can provide additional
information for the assignment of tube indices in high resolution optical
absorption spectroscopy.Comment: 5 pages, 3 figure
Granular-Scale Elementary Flux Emergence Episodes in a Solar Active Region
We analyze data from Hinode spacecraft taken over two 54-minute periods
during the emergence of AR 11024. We focus on small-scale portions within the
observed solar active region and discover the appearance of very distinctive
small-scale and short-lived dark features in Ca II H chromospheric filtergrams
and Stokes I images. The features appear in regions with close-to-zero
longitudinal magnetic field, and are observed to increase in length before they
eventually disappear. Energy release in the low chromospheric line is detected
while the dark features are fading. In time series of magnetograms a diverging
bipolar configuration is observed accompanying the appearance of the dark
features and the brightenings. The observed phenomena are explained as
evidencing elementary flux emergence in the solar atmosphere, i.e small-scale
arch filament systems rising up from the photosphere to the lower chromosphere
with a length scale of a few solar granules. Brightenings are explained as
being the signatures of chromospheric heating triggered by reconnection of the
rising loops (once they reached chromospheric heights) with pre-existing
magnetic fields as well as to reconnection/cancellation events in U-loop
segments of emerging serpentine fields. We study the temporal evolution and
dynamics of the events and compare them with the emergence of magnetic loops
detected in quiet sun regions and serpentine flux emergence signatures in
active regions. Incorporating the novel features of granular-scale flux
emergence presented in this study we advance the scenario for serpentine flux
emergence.Comment: 24 pages, 9 figures. Accepted for publication in Solar Physic
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