1,962 research outputs found
The Rashba Hamiltonian and electron transport
The Rashba Hamiltonian describes the splitting of the conduction band as a
result of spin-orbit coupling in the presence of an external field and is
commonly used to model the electronic structure of confined narrow-gap
semiconductors. Due to the mixing of spin states some care has to be exercised
in the calculation of transport properties. We derive the velocity operator for
the Rashba-split conduction band and demonstrate that the transmission of an
interface between a ferromagnet and a Rashba-split semiconductor does not
depend on the magnetization direction, in contrast with previous assertions in
the literature.Comment: one tex file, two figures; paper to appear in this form in PRB (RC
Magnetic field effects of double-walled carbon nanotubes
A theoretical discussion of electronic and transport properties of a particular family of double-wall carbon nanotubes, named commensurate structures of the armchair type (n,n)@(2n,2n) is addressed. A single p-band tight binding hamiltonian is considered and the magnetic field is theoretically described by following the Peierls approximation into the hopping energies. Our emphasis is put on investigating the main effects of the geometrical aspects and relative positions of the tubes on the local density of states and on the conductance of the system. By considering intershell interactions between a set of neighboring atoms on the walls of the inner and outer tubes, we study the possibility of founding Aharonov-Bohm effects in the DWCNs when a magnetic field is applied along the axial direction
Cosmological Tracking Solutions
A substantial fraction of the energy density of the universe may consist of
quintessence in the form of a slowly-rolling scalar field. Since the energy
density of the scalar field generally decreases more slowly than the matter
energy density, it appears that the ratio of the two densities must be set to a
special, infinitesimal value in the early universe in order to have the two
densities nearly coincide today.
Recently, we introduced the notion of tracker fields to avoid this initial
conditions problem. In the paper, we address the following questions: What is
the general condition to have tracker fields? What is the relation between the
matter energy density and the equation-of-state of the universe imposed by
tracker solutions? And, can tracker solutions explain why quintessence is
becoming important today rather than during the early universe
Analytical strategies for determination of cadmium in Brazilian vinegar samples using ET AAS
AbstractThis paper proposes two methods for determination of cadmium in vinegar employing electrothermal atomic absorption spectrometry. The optimization step was performed using two-level full factorial and BoxâBehnken designs, being that a new multiple response function was established. Under experimental conditions of pyrolysis temperature of 640°C and atomization temperature of 2000°C, the direct method allows the analysis using the external calibration technique, with limit of quantification of 14ngLâ1 and characteristic mass of 1.2pg, having aluminium as chemical modifier. This method was applied in six samples of vinegar acquired from Salvador City, Brazil. The cadmium content varied from 20 to 890ngLâ1. Other method was also proposed by digestion using nitric acid and hydrogen peroxide in reflux system employing cold finger, being cadmium determined by ETAAS. The results obtained with the complete digestion procedure were in agreement with those found by the direct method proposed herein
Magnetic field dynamic strategies for the improved control of the angiogenic effect of mesenchymal stromal cells
project PTDC/EDM-EDM/30828/2017
SFRH/BD/114043/2015
co-financed by the ERDF under the PT2020 Partnership Agreement (POVI-01-0145-FEDER-007265), as well as from POR Lisboa 2020 grant PRECISE (Project N. 16394).
Publisher Copyright:
© 2021 by the authors. Licensee MDPI, Basel, Switzerland.This work shows the ability to remotely control the paracrine performance of mesenchymal stromal cells (MSCs) in producing an angiogenesis key molecule, vascular endothelial growth factor (VEGF-A), by modulation of an external magnetic field. This work compares for the first time the application of static and dynamic magnetic fields in angiogenesis in vitro model, exploring the effect of magnetic field intensity and dynamic regimes on the VEGF-A secretion potential of MSCs. Tissue scaffolds of gelatin doped with iron oxide nanoparticles (MNPs) were used as a platform for MSC proliferation. Dynamic magnetic field regimes were imposed by cyclic variation of the magnetic field intensity in different frequencies. The effect of the magnetic field intensity on cell behavior showed that higher intensity of 0.45 T was associated with increased cell death and a poor angiogenic effect. It was observed that static and dynamic magnetic stimulation with higher frequencies led to improved angiogenic performance on endothelial cells in comparison with a lower frequency regime. This work showed the possibility to control VEGF-A secretion by MSCs through modulation of the magnetic field, offering attractive perspectives of a non-invasive therapeutic option for several diseases by revascularizing damaged tissues or inhibiting metastasis formation during cancer progression.publishersversionpublishe
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Ultra-high sensitive strain sensor based on post-processed optical fiber Bragg grating
An ultra-high sensitive strain sensor is proposed. The sensing head, based on the post-processing of a fiber Bragg grating, is used to perform passive and active strain measurements. Both wavelength and full width half maximum dependences with the applied strain are studied for the passive sensor, where maximum sensitivities of 104.1 pm/ΌΔ and 61.6 pm/ΌΔ are respectively obtained. When combining the high performance of this sensor with a ring laser cavity configuration, the Bragg grating will act as a filter and high resolution measurements can be performed. With the proposed sensor, a resolution of 700 nΔ is achieved
The Tensor to Scalar Ratio of Phantom Dark Energy Models
We investigate the anisotropies in the cosmic microwave background in a class
of models which possess a positive cosmic energy density but negative pressure,
with a constant equation of state w = p/rho < -1. We calculate the temperature
and polarization anisotropy spectra for both scalar and tensor perturbations by
modifying the publicly available code CMBfast. For a constant initial curvature
perturbation or tensor normalization, we have calculated the final anisotropy
spectra as a function of the dark energy density and equation of state w and of
the scalar and tensor spectral indices. This allows us to calculate the
dependence of the tensor-to-scalar ratio on w in a model with phantom dark
energy, which may be important for interpreting any future detection of
long-wavelength gravitational waves.Comment: 5 pages, 4 figure
First Record Of Passiflora Pedata L. (passifloraceae) From MaranhĂŁo State, Northeastern Brazil
This study reports the first record of Passiflora pedata L. (Passifloraceae) from the state of Maranhão and from the Northeast Region of Brazil. The species was collected in the municipality of Buriticupu, which is located in a remnant of Amazon Forest in western Maranhão. This work adds to the knowledge of the flora of Maranhão and the distribution of P. pedata in the Brazilian Amazon. © 2016 Check List and Authors.12
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