20,433 research outputs found
Anomalous precursor diamagnetism at low reduced magnetic fields and the role of Tc inhomogeneities in the superconductors Pb55In45 and underdoped La1.9Sr0.1CuO4
The magnetic field dependence of the magnetization was measured above the
superconducting transition in a high-Tc underdoped cuprate La1.9Sr0.1CuO4 and
in a low-Tc alloy (Pb55In45). Near the superconducting transition [typically
for (T-Tc)/Tc<0.05] and under low applied magnetic field amplitudes [typically
for H/Hc2(0)<0.01, where Hc2(0) is the corresponding upper critical field
extrapolated to T=0 K] the magnetization of both samples presents a diamagnetic
contribution much larger than the one predicted by the Gaussian Ginzburg-Landau
(GGL) approach for superconducting fluctuations. These anomalies have been
already observed in cuprate compounds by various groups and attributed to
intrinsic effects associated with the own nature of these high-Tc
superconductors. However, we will see here that our results in both high and
low-Tc superconductors may be explained quantitatively, and consistently with
the GGL behavior observed at higher fields, by just taking into account the
presence in the samples of an uniform distribution of Tc inhomogeneities. These
Tc inhomogeneities, which may be in turn associated with stoichiometric
inhomogeneities, were estimated from independent measurements of the
temperature dependence of the field-cooled magnetic susceptibility under low
applied magnetic fields.Comment: 25 pages, including 6 figures and 1 table. Typos corrected. Compacte
Localization Effect in a 2D Superconducting Network without Disorder
The superconducting properties of a two-dimensional superconducting wire
network with a new geometry have been measured as a function of the external
magnetic field. The extreme localization effect recently predicted for this
periodic lattice is revealed as a suppression of the critical current when the
applied magnetic field corresponds to half a flux quantum per unit cell. For
this particular magnetic field, the observed vortex state configuration is
highly disordered.Comment: 6 pages, 2 eps figures, submitted to Physica C. Title change
Aharonov-Bohm cages in the GaAlAs/GaAs system
Aharonov-Bohm oscillations have been observed in a lattice formed by a two
dimensional rhombus tiling. This observation is in good agreement with a recent
theoretical calculation of the energy spectrum of this so-called T3 lattice. We
have investigated the low temperature magnetotransport of the T3 lattice
realized in the GaAlAs/GaAs system. Using an additional electrostatic gate, we
have studied the influence of the channel number on the oscillations amplitude.
Finally, the role of the disorder on the strength of the localization is
theoretically discussed.Comment: 6 pages, 11 EPS figure
Trabalho com plantas medicinais e aromåticas visam o controle alternativo de doenças e pragas no sistema de produção de hortaliças orgùnicas.
bitstream/item/71377/1/artigo-Mariane-Ricardo.pd
Radio detection of the young binary HD 160934
Precise determination of dynamical masses of pre-main-sequence (PMS) stars is
essential to calibrate stellar evolution models that are widely used to derive
theoretical masses of young low-mass objects. Binary stars in young, nearby
loose associations are particularly good candidates for this calibration since
all members share a common age. Interestingly, some of these young binaries
present a persistent and compact radio emission, which makes them excellent
targets for astrometric VLBI studies. We aim to monitor the orbital motion of
the binary system HD 160934, a member of the AB Doradus moving group. We
observed HD 160934 with the Very Large Array and the European VLBI Network at
8.4 and 5 GHz, respectively. The orbital information derived from these
observations was analyzed along with previously reported orbital measurements.
We show that the two components of the binary, HD 160934 A and HD 160934 c,
display compact radio emission at VLBI scales, providing precise information on
the relative orbit. Revised orbital elements were estimated. Future VLBI
monitoring of this pair should determine precise model-independent mass
estimates for the A and c components, which will serve as calibration tests for
PMS evolutionary models.Comment: 5 pages, 5 figures, accepted for publication in A&
Haffner 16: A Young Moving Group in the Making
The photometric properties of main sequence (MS) and pre-main sequence (PMS)
stars in the young cluster Haffner 16 are examined using images recorded with
the Gemini South Adaptive Optics Imager (GSAOI) and corrected for atmospheric
blurring by the Gemini Multi-Conjugate Adapative Optics System (GeMS). A rich
population of PMS stars is identified, and comparisons with isochrones suggest
an age < 10 Myr assuming a distance modulus of 13.5 (D = 5 kpc). When compared
with the solar neighborhood, Haffner 16 is roughly a factor of two deficient in
objects with sub-solar masses. PMS objects in the cluster are also more
uniformly distributed on the sky than bright MS stars. It is suggested that
Haffner 16 is dynamically evolved, and that it is shedding protostars with
sub-solar masses. Young low mass clusters like Haffner 16 are one possible
source of PMS stars in the field. The cluster will probably evolve on time
scales of ~ 100 - 1000 Myr into a diffuse moving group with a mass function
that is very different from that which prevailed early in its life.Comment: To appear in the Publications of the Astronomical Society of the
Pacifi
Detection, quantification, and characterization of polystyrene microplastics and adsorbed bisphenol A contaminant using electroanalytical techniques
The potential applications of electroanalytical techniques for the quantification and size characterization of nonelectroactive polystyrene microplastics is reported, in addition to characterizing the kinetics of adsorption of bisphenol A on these polystyrene microparticles. The individual adsorption events of very diluted polystyrene microparticles dispersions on glassy-carbon microelectrodes produce the blocking of the charge transfer of a mediator (ferrocene-methanol) thus decreasing the current of the recorded chronoamperogram in a stepwise manner. The magnitude of the current steps are in the order of pA values and can be related to the diameter of the plastic microparticles in the size range 0.1 to 10 ”m. The frequency of the current steps in the domain time used (120 s) allows to quantify the number concentration of these microparticles in the range 0.005 to 0.500 pM. Electrochemical impedance spectroscopy confirms the adsorption of the polystyrene microplastics on carbon microelectrodes (and to a lesser extent on platinum microelectrodes) under the same experimental conditions as above. On the other hand, the adsorbed microplastics become concentrators of other pollutants found in the environment. The sensitive differential-pulse voltammetry determination of bisphenol A (linear range 0.80â15.00 ”M; detection limit 0.24 ”M) was used together with a simple separation procedure for studying the adsorption of bisphenol A on polystyrene microparticles. The adsorption capacity (mg of bisphenol A retained per g of the polystyrene microplastics) decreased from approximately 5.7 to 0.8 mg gâ1 with increasing dosages of polystyrene microparticles from 0.2 to 1.6 g lâ1. The adsorption isotherms were modeled resulting in a monolayer of bisphenol A adsorbed on the microplastics (i.e., best fitted to a Langmuir model)
How to escape Aharonov-Bohm cages ?
We study the effect of disorder and interactions on a recently proposed
magnetic field induced localization mechanism. We show that both partially
destroy the extreme confinement of the excitations occuring in the pure case
and give rise to unusual behavior. We also point out the role of the edge
states that allows for a propagation of the electrons in these systems.Comment: 22 pages, 20 EPS figure
Entanglement cost of mixed states
We compute the entanglement cost of several families of bipartite mixed
states, including arbitrary mixtures of two Bell states. This is achieved by
developing a technique that allows us to ascertain the additivity of the
entanglement of formation for any state supported on specific subspaces. As a
side result, the proof of the irreversibility in asymptotic local manipulations
of entanglement is extended to two-qubit systems.Comment: 4 pages, no figures, (v4) new results, including a new method to
determine E_c for more general mixed states, presentation changed
significantl
Comparison of temporary or continuous immersion systems for micropropagation in liquid medium of axillary shoots of Salix viminalis.
In this study the response to culture in liquid medium of axillary shoots of two woody species, chestnut and willow, has been compared. Both species differ in their recalcitrance to micropropagation, especially in their rooting and acclimatization abilities. Parameters such as explant type, supporting material, bioreactor type, number of immersions and/or aerations were tested. Continuous and temporary immersion systems have been applied to both species and several differences were detected. Hyperhydricity was not observed in willow shoots cultured in liquid medium with aeration, but this disorder affected chestnut cultures unless supporting materials as rockwool cubes were used. Besides, willow shoots rooted spontaneously in liquid culture, whereas chestnut shoots required an auxin treatment. In spite of these differences, both species showed better performance in liquid culture than in semi-solid medium, and shoots from an adult willow genotype and from eight selected chestnut genotypes (hybrids of European and Asian chestnuts) were successfully rooted and acclimatized.Peer reviewe
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