273 research outputs found
Chaos and plasticity in superconductor vortices: a low-dimensional dynamics
We present new results of numerical simulations for driven vortex lattices in
presence of random disorder at zero temperature. We show that the plastic
dynamics of vortices display dissipative chaos. Intermittency "routes to chaos"
have been clearly identified below the differential resistance peak. The peak
region is characterized by positive Lyapunov exponents characteristic of chaos,
and low frequency broad-band noise. Furthermore we find a low fractal dimension
of the strange attractor, which suggests that only a few dynamical variables
are sufficient to model the complex plastic dynamics of vortices.Comment: 8 pages, 6 figures, accepted for publication in The Physical Review
Analysis of tissue surrounding thyroid nodules by ultrasound digital images
Since US is not easily reproducible, the digital image analysis (IA) has been proposed so that the image evaluation is not subjective. In fact, IA meets the criteria of objectivity, accurateness, and reproducibility by a matrix of pixels whose value is displayed in a gray level. This study aims at evaluating via IA the tissue surrounding a thyroid nodule (backyard tissue, BT) from goitres with benign (b-BT) and malignant (m-BT) lesions. Sixty-nine US images of thyroid nodules surrounded by adequate thyroid tissue was classified as normoechoic and homogeneous were enrolled as study group. Forty-three US images from normal thyroid (NT) glands were included as controls. Digital images of 800 × 652 pixels were acquired at a resolution of eight bits with a 256 gray levels depth. By one-way ANOVA, the 43 NT glands were not statistically different (P = 0.91). Mean gray level of normal glands was significantly higher than b-BT (P = 0.026), and m-BT (P = 0.0001), while no difference was found between b-BT and m-BT (P = 0.321). NT tissue boundary external to the nodule was found at 6.0 ± 0.5 mm in cancers and 4.0 ± 0.5 mm in benignancies (P = 0.001). These data should indicate that the tissue surrounding a thyroid nodule may be damaged even when assessed as normal by US. This is of interest to investigate the extranodular effects of thyroid tumors
Electrochemical testing of an innovative dual membrane fuel cell design in reversible mode
Solid oxide fuel Cells (SOFC) are intrinsically reversible which makes them attractive for the development of reversible devices (rSOC). The main hurdles that have to be overcome are the higher degradation in electrolyzer (EL) mode and the slow and difficult switching form mode to mode. This work aims at the development and experimental validation of a concept for rSOC based on a new dual membrane fuel cell (dmFC) design which can overcome the existing problems of the classical SOFC. The kernel of the system is additional chamber - central membrane (CM) for water formation/evacuation in FC mode and injection in El mode. Its optimization in respect of microstructure and geometry in laboratory conditions is carried out on button cells. The electrochemical performance is evaluated based on volt-ampere characteristics (VACs) combined with impedance measurements in different working points. The influence of a catalyst in the water chamber is also examined. The VACs which give integral picture of the cell performance are in excellent agreement with the impedance studies which ensure deeper and quantitative information about the processes, including information about the rate limiting step. The results from the optimization of the water chamber show that the combination of design and material brings to important principle advantages in respect to the classical rSOC \u2013 better performance in electrolyzer mode combined with instantaneous switching
Bragg- and Moving-glasses: a theory of disordered vortex lattices
We study periodic lattices, such as vortex lattices in type II
superconductors in a random pinning potential.
For the static case we review the prediction that the phase diagram of such
systems consists of a topologically ordered Bragg glass phase, with quasi long
range translational order, at low fields. This Bragg glass phase undergoes a
transition at higher fields into another glassy phase, with dislocations, or a
liquid. This proposition is compatible with a large number of experimental
results on BSCCO or Thalium compounds. Further experimental consequences of our
results and relevance to other systems will be discussed.
When such vortex systems are driven by an external force, we show that, due
to periodicity in the direction transverse to motion, the effects of static
disorder persist even at large velocity. In , at weak disorder, or large
velocity the lattice forms a topologically ordered glass state, the ``moving
Bragg glass'', an anisotropic version of the static Bragg glass. The lattice
flows through well-defined, elastically coupled, static channels. We determine
the roughness of the manifold of channels and the positional correlation
functions. The channel structure also provides a natural starting point to
study the influence of topological defects such as dislocations. In or at
strong disorder the channels can decouple along the direction of motion leading
to a ``smectic'' like flow. We also show that such a structure exhibits an
effective transverse critical pinning force due to barriers to transverse
motion, and discuss the experimental consequences of this effect.Comment: Proceedings of M2S-HTSC-V conference (Beijing, Feb 97) to be
published in Physica C; 4 pages, 3 figures, uses espcrc2.st
Why pinning by surface irregularities can explain the peak effect in transport properties and neutron diffraction results in NbSe2 and Bi-2212 crystals?
The existence of a peak effect in transport properties (a maximum of the
critical current as function of magnetic field) is a well-known but still
intriguing feature of type II superconductors such as NbSe2 and Bi-2212. Using
a model of pinning by surface irregularities in anisotropic superconductors, we
have developed a calculation of the critical current which allows estimating
quantitatively the critical current in both the high critical current phase and
in the low critical current phase. The only adjustable parameter of this model
is the angle of the vortices at the surface. The agreement between the
measurements and the model is really very impressive. In this framework, the
anomalous dynamical properties close to the peak effect is due to co-existence
of two different vortex states with different critical currents. Recent neutron
diffraction data in NbSe2 crystals in presence of transport current support
this point of view
Structure of the flux lines lattice in NbSe2: Equilibrium state and influence of the magnetic history
We have performed small-angle neutron scattering (SANS) of the flux line
lattice (FLL) in a Fe doped NbSe_2 sample which presents a large peak effect in
the critical current. The scattered intensity and the width of the Bragg peaks
of the equilibrium FLL indicate an ordered structure in the peak effect zone.
The history dependence in the FLL structure has been studied using field cooled
and zero field cooled procedures, and each state shows the same intensity of
Bragg scattering and good orientational order. These results strongly suggest
that the peak effect is unrelated to a bulk disordering transition, and confirm
the role of a heterogeneous distribution of screening current.Comment: accepted in Phys. Rev.
rf-studies of vortex dynamics in isotropic type-II superconductors
We have measured the surface impedance of thick superconductors in the mixed
state over a broad 2 kHz - 20 MHz frequency range. The depinning cross-over is
observed; but it is much broader than expected from classical theories of
pinning. A striking result is the existence of size effects which invalidate
the common interpretation of the low-frequency surface inductance in terms of a
single penetration depth. Instead, a two-mode description of vortex dynamics,
assuming free vortex flow in the bulk and surface pinning, accounts
quantitatively for the spectrum of the complex apparent penetration depth.Comment: 20 pages, 6 figures, 28 reference
Moving glass phase of driven lattices
We study periodic lattices, such as vortex lattices, driven by an external
force in a random pinning potential. We show that effects of static disorder
persist even at large velocity. It results in a novel moving glass state with
topological order analogous to the static Bragg glass. The lattice flows
through well-defined, elastically coupled, {\it % static} channels. We predict
barriers to transverse motion resulting in finite transverse critical current.
Experimental tests of the theory are proposed.Comment: Revised version, shortened, 8 pages, REVTeX, no figure
Equilibration and Dynamic Phase Transitions of a Driven Vortex Lattice
We report on the observation of two types of current driven transitions in
metastable vortex lattices. The metastable states, which are missed in usual
slow transport measurements, are detected with a fast transport technique in
the vortex lattice of undoped
2H-NbSe. The transitions are seen by following the evolution of these
states when driven by a current. At low currents we observe an equilibration
transition from a metastable to a stable state, followed by a dynamic
crystallization transition at high currents.Comment: 5 pages, 4 figure
A Variant of GJD2, Encoding for Connexin 36, Alters the Function of Insulin Producing β-Cells.
Signalling through gap junctions contributes to control insulin secretion and, thus, blood glucose levels. Gap junctions of the insulin-producing β-cells are made of connexin 36 (Cx36), which is encoded by the GJD2 gene. Cx36-null mice feature alterations mimicking those observed in type 2 diabetes (T2D). GJD2 is also expressed in neurons, which share a number of common features with pancreatic β-cells. Given that a synonymous exonic single nucleotide polymorphism of human Cx36 (SNP rs3743123) associates with altered function of central neurons in a subset of epileptic patients, we investigated whether this SNP also caused alterations of β-cell function. Transfection of rs3743123 cDNA in connexin-lacking HeLa cells resulted in altered formation of gap junction plaques and cell coupling, as compared to those induced by wild type (WT) GJD2 cDNA. Transgenic mice expressing the very same cDNAs under an insulin promoter revealed that SNP rs3743123 expression consistently lead to a post-natal reduction of islet Cx36 levels and β-cell survival, resulting in hyperglycemia in selected lines. These changes were not observed in sex- and age-matched controls expressing WT hCx36. The variant GJD2 only marginally associated to heterogeneous populations of diabetic patients. The data document that a silent polymorphism of GJD2 is associated with altered β-cell function, presumably contributing to T2D pathogenesis
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