173 research outputs found
Paramagnetic Meissner effect in superconductors from self-consistent solutions of Ginzburg-Landau equations
The paramagnetic Meissner effect (PME) is observed in small superconducting
samples, and a number of controversial explanations of this effect are
proposed, but there is as yet no clear understanding of its nature. In the
present paper PME is considered on the base of the Ginzburg-Landau theory (GL).
The one-dimensional solutions are obtained in a model case of a long
superconducting cylinder for different cylinder radii R, the GL-parameters
\kappa and vorticities m. Acording to GL-theory, PME is caused by the presence
of vortices inside the sample. The superconducting current flows around the
vortex to screeen the vortex own field from the bulk of the sample. Another
current flows at the boundary to screen the external field H from entering the
sample. These screening currents flow in opposite directions and contribute
with opposite signs to the total magnetic moment (or magnetization) of the
sample. Depending on H, the total magnetization M may be either negative
(diamagnetism), or positive (paramagnetism). A very complicated saw-like
dependence M(H) (and other characteristics), which are obtained on the base of
self-consistent solutions of the GL-equations, are discussed.Comment: 6 pages, 5 figures, RevTex, submitted to Phys. Rev.
Aging Effect in Ceramic Superconductors
A three-dimensional lattice of the Josephson junctions with a finite
self-conductance is employed to model ceramic superconductors. Using Monte
Carlo simulations it is shown that the aging disappears in the strong screening
limit. In the weeak screening regime aging is present even at low temperatures.
For intermediate values of the self-inductance aging occurs at intermediate
temperatures interval but is suppressed entirely at high and low temperatures.
Our results are in good agreement with experiments.Comment: 5 pages, 5 eps figures, to appear in Physical Review Letter
Second harmonics and compensation effect in ceramic superconductors
A three-dimensional lattice of the Josephson junctions with a finite
self-conductance is employed to model the ceramic superconductors. The
nonlinear ac susceptibility and the compensation effect are studied by Monte
Carlo simulations in this model. The compensation effect is shown to be due to
the existence of the chiral glass phase. We demonstrate, in agreement with
experiments, that this effect may be present in the ceramic superconductors
which show the paramagnetic Meissner effect.Comment: 6 pages, latex, 4 figures, Phys. Rev. B (accepted
The origin of paramagnetic magnetization in field-cooled YBa2Cu3O7 films
Temperature dependences of the magnetic moment have been measured in
YBa_2Cu_3O_{7-\delta} thin films over a wide magnetic field range (5 <= H <=
10^4 Oe). In these films a paramagnetic signal known as the paramagnetic
Meissner effect has been observed. The experimental data in the films, which
have strong pinning and high critical current densities (J_c ~ 2 \times 10^6
A/cm^2 at 77 K), are quantitatively shown to be highly consistent with the
theoretical model proposed by Koshelev and Larkin [Phys. Rev. B 52, 13559
(1995)]. This finding indicates that the origin of the paramagnetic effect is
ultimately associated with nucleation and inhomogeneous spatial redistribution
of magnetic vortices in a sample which is cooled down in a magnetic field. It
is also shown that the distribution of vortices is extremely sensitive to the
interplay of film properties and the real experimental conditions of the
measurements.Comment: RevTex, 8 figure
A comparative study of high-field diamagnetic fluctuations in deoxygenated YBa2Cu3O(7-x) and polycrystalline (Bi-Pb)2Sr2Ca3O(10)
We studied three single crystals of YBa2Cu3O{7-x} with Tc= 62.5, 52, and 41
K, and a textured specimen of (Bi-Pb)2Sr2Ca2Cu3O10 with Tc=108 K, for H//c
axis. The reversible data were interpreted in terms of 2D lowest-Landau-level
fluctuation theory. The data were fit well by the 2D LLL expression for
magnetization obtained by Tesanovic etal., producing reasonable values of kappa
but larger values of dHc2/dT. Universality was studied by obtaining a
simultaneous scaling of Y123 data and Bi2223. An expression for the 2D x-axis
LLL scaling factor used to obtain the simultaneous scaling was extracted from
theory, and compared with the experimental values. The comparison between the
values of the x-axis produced a deviation of 40% which suggests that the
hypothesis of universality of the 2D-LLL fluctuations is not supported by the
studied samples. We finaly observe that Y123 magnetization data for
temperatures above obbey a universal scaling obtained for the diamagnetic
fluctuation magnetization from a theory considering non-local field effects.
The same scaling was not obbeyed by the corresponding magnetization calculated
from the two-dimensional lowest-Landau-level theory.Comment: 7 pages 5 figures, accept in Journ. Low Temp. Phy
The Interface Region Imaging Spectrograph (IRIS)
The Interface Region Imaging Spectrograph (IRIS) small explorer spacecraft
provides simultaneous spectra and images of the photosphere, chromosphere,
transition region, and corona with 0.33-0.4 arcsec spatial resolution, 2 s
temporal resolution and 1 km/s velocity resolution over a field-of-view of up
to 175 arcsec x 175 arcsec. IRIS was launched into a Sun-synchronous orbit on
27 June 2013 using a Pegasus-XL rocket and consists of a 19-cm UV telescope
that feeds a slit-based dual-bandpass imaging spectrograph. IRIS obtains
spectra in passbands from 1332-1358, 1389-1407 and 2783-2834 Angstrom including
bright spectral lines formed in the chromosphere (Mg II h 2803 Angstrom and Mg
II k 2796 Angstrom) and transition region (C II 1334/1335 Angstrom and Si IV
1394/1403 Angstrom). Slit-jaw images in four different passbands (C II 1330, Si
IV 1400, Mg II k 2796 and Mg II wing 2830 Angstrom) can be taken simultaneously
with spectral rasters that sample regions up to 130 arcsec x 175 arcsec at a
variety of spatial samplings (from 0.33 arcsec and up). IRIS is sensitive to
emission from plasma at temperatures between 5000 K and 10 MK and will advance
our understanding of the flow of mass and energy through an interface region,
formed by the chromosphere and transition region, between the photosphere and
corona. This highly structured and dynamic region not only acts as the conduit
of all mass and energy feeding into the corona and solar wind, it also requires
an order of magnitude more energy to heat than the corona and solar wind
combined. The IRIS investigation includes a strong numerical modeling component
based on advanced radiative-MHD codes to facilitate interpretation of
observations of this complex region. Approximately eight Gbytes of data (after
compression) are acquired by IRIS each day and made available for unrestricted
use within a few days of the observation.Comment: 53 pages, 15 figure
Clathrin- and Dynamin-Independent Endocytosis of FGFR3 – Implications for Signalling
Endocytosis of tyrosine kinase receptors can influence both the duration and the specificity of the signal emitted. We have investigated the mechanisms of internalization of fibroblast growth factor receptor 3 (FGFR3) and compared it to that of FGFR1 which is internalized predominantly through clathrin-mediated endocytosis. Interestingly, we observed that FGFR3 was internalized at a slower rate than FGFR1 indicating that it may use a different endocytic mechanism than FGFR1. Indeed, after depletion of cells for clathrin, internalization of FGFR3 was only partly inhibited while endocytosis of FGFR1 was almost completely abolished. Similarly, expression of dominant negative mutants of dynamin resulted in partial inhibition of the endocytosis of FGFR3 whereas internalization of FGFR1 was blocked. Interfering with proposed regulators of clathrin-independent endocytosis such as Arf6, flotillin 1 and 2 and Cdc42 did not affect the endocytosis of FGFR1 or FGFR3. Furthermore, depletion of clathrin decreased the degradation of FGFR1 resulting in sustained signalling. In the case of FGFR3, both the degradation and the signalling were only slightly affected by clathrin depletion. The data indicate that clathrin-mediated endocytosis is required for efficient internalization and downregulation of FGFR1 while FGFR3, however, is internalized by both clathrin-dependent and clathrin-independent mechanisms
Perspectives on key principles of generalist medical practice in public service in sub-saharan africa: a qualitative study
The original publication is available at http://www.biomedcentral.comAbstract Background: The principles and practice of Family Medicine that arose in developed Western countries have been imported and adopted in African countries without adequate consideration of their relevance and appropriateness to the African context. In this study we attempted to elicit a priori principles of generalist medical practice from the experience of long-serving medical officers in a variety of African counties, through which we explored emergent principles of Family Medicine in our own context. Methods A descriptive study design was utilized, using qualitative methods. 16 respondents who were clinically active medical practitioners, working as generalists in the public services or non-profit sector for at least 5 years, and who had had no previous formal training or involvement in academic Family Medicine, were purposively selected in 8 different countries in southern, western and east Africa, and interviewed. Results The respondents highlighted a number of key issues with respect to the external environment within which they work, their collective roles, activities and behaviours, as well as the personal values and beliefs that motivate their behaviour. The context is characterized by resource constraints, high workload, traditional health beliefs, and the difficulty of referring patients to the next level of care. Generalist clinicians in sub-Saharan Africa need to be competent across a wide range of clinical disciplines and procedural skills at the level of the district hospital and clinic, in both chronic and emergency care. They need to understand the patient's perspective and context, empowering the patient and building an effective doctor-patient relationship. They are also managers, focused on coordinating and improving the quality of clinical care through teamwork, training and mentoring other health workers in the generalist setting, while being life-long learners themselves. However, their role in the community, was found to be more aspirational than real. Conclusions The study derived a set of principles for the practice of generalist doctors in sub-Saharan Africa based on the reported activities and approaches of the respondents. Patient-centred care using a biopsychosocial approach remains as a common core principle despite wide variations in context. Procedural and hospital care demands a higher level of skills particularly in rural areas, and a community orientation is desirable, but not widely practiced. The results have implications for the postgraduate training of family physicians in sub-Saharan Africa, and highlight questions regarding the realization of community-orientated primary care.Publishers' Versio
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