281 research outputs found
Magneto-optical imaging of magnetic flux patterns in superconducting films with antidots
Superconducting YBaCuO thin films were equipped with a special arrangement of
antidots (holes) of 1 micron radius in order to guide the stream of magnetic
flux moving in (or out of) the sample. The flux distribution and its dynamics
were visualized using real-time magneto-optical imaging. It is clearly
demonstrated that one-dimensional antidot arrays strongly facilitate
propagation of magnetic flux. We also demonstrate a possibility to alter the
direction of flux motion in a controlled way by special arrangement of
intercepting antidot arrays. Our resolution was sufficient for observation of
flux in particular antidots, which allows a more detailed dynamic analysis of
such systems.Comment: 4 pages, 5 figures, submitted to Physica C, Proc. of VORTEX-IV
Workshop on Crete-200
Mechanism for flux guidance by micrometric antidot arrays in superconducting films
A study of magnetic flux penetration in a superconducting film patterned with
arrays of micron sized antidots (microholes) is reported. Magneto-optical
imaging (MOI) of a YBCO film shaped as a long strip with perpendicular antidot
arrays revealed both strong guidance of flux, and at the same time large
perturbations of the overall flux penetration and flow of current. These
results are compared with a numerical flux creep simulation of a thin
superconductor with the same antidot pattern. To perform calculations on such a
complex geometry, an efficient numerical scheme for handling the boundary
conditions of the antidots and the nonlocal electrodynamics was developed. The
simulations reproduce essentially all features of the MOI results. In addition,
the numerical results give insight into all other key quantities, e.g., the
electrical field, which becomes extremely large in the narrow channels
connecting the antidots.Comment: 8 pages, 7 figure
Laser scanning microscopy of guided vortex flow in microstructured high-Tc films
We report the visualization of guidance of vortices by artificial microholes (antidots) in superconducting thin films using a low-temperature laser scanning microscope. Previously, guided motion of vortices via tilted rows of antidots in YBa2Cu3O7 films was detected indirectly by using resistive Hall-type measurements. Here we prove that vortices are steered between antidots into a priori chosen direction by imaging of resistive photoresponse with a spatial resolution down to about 1 mu m. We observe predominant paths for vortex motion. Vortices are nucleated and annihilated at antidots, i.e., antidots define starting and ending points of predominant vortex paths. Depending on the misorientation angle between rows of antidots and the current-driven direction of vortex motion, different channels dominate in antidot-guided vortex motion. Our experimental results can be explained by the n-channel model. Finally, we present direct measurements of the local critical currents. This technique can be used as a quantitative method for the analysis of vortex motion in micropatterned thin films. (c) 2006 American Institute of Physics
Oscillatory dynamics of a superconductor vortex lattice in high amplitude ac magnetic fields
In this work we study by ac susceptibility measurements the evolution of the
solid vortex lattice mobility under oscillating forces. Previous work had
already shown that in YBCO single crystals, below the melting transition, a
temporarily symmetric magnetic ac field (e.g. sinusoidal, square, triangular)
can heal the vortex lattice (VL) and increase its mobility, but a temporarily
asymmetric one (e.g. sawtooth) of the same amplitude can tear the lattice into
a more pinned disordered state. In this work we present evidence that the
mobility of the VL is reduced for large vortex displacements, in agreement with
predictions of recent simulations. We show that with large symmetric
oscillating fields both an initially ordered or an initially disordered VL
configuration evolve towards a less mobile lattice, supporting the scenario of
plastic flow.Comment: 5 pages, 4 figures. To appear in Phys. Rev.
History effect in inhomogeneous superconductors
A model was proposed to account for a new kind of history effect in the
transport measurement of a sample with inhomogeneous flux pinning coupled with
flux creep. The inhomogeneity of flux pinning was described in terms of
alternating weak pinning (lower jc) and strong pinning region (higher jc). The
flux creep was characterized by logarithmic barrier. Based on this model, we
numerically observed the same clockwise V-I loops as reported in references.
Moreover, we predicted behaviors of the V-I loop at different sweeping rates of
applied current dI/dt or magnetic fields Ba, etc. Electric transport
measurement was performed in Ag-sheathed Bi2-xPbxSr2Ca2Cu3Oy tapes immersed in
liquid nitrogen with and without magnetic fields. V-I loop at certain dI/dt and
Ba was observed. It is found that the area of the loop is more sensitive to
dI/dt than to Ba, which is in agreement well with our numerical results.Comment: To appear in Phys Rev B, October 1 Issu
Synthetic metabolic pathways for photobiological conversion of CO2 into hydrocarbon fuel
Liquid fuels sourced from fossil sources are the dominant energy form for mobile transport today. The consumption of fossil fuels is still increasing, resulting in a continued search for more sustainable methods to renew our supply of liquid fuel. Photosynthetic microorganisms naturally accumulate hydrocarbons that could serve as a replacement for fossil fuel, however productivities remain low. We report successful introduction of five synthetic metabolic pathways in two green cell factories, prokaryotic cyanobacteria and eukaryotic algae. Heterologous thioesterase expression enabled high-yield conversion of native fatty acyl-acyl carrier protein (ACP) into free fatty acids (FFA) in Synechocystis sp. PCC 6803 but not in Chlamydomonas reinhardtii where the polar lipid fraction instead was enhanced. Despite no increase in measurable FFA in Chlamydomonas, genetic recoding and over-production of the native fatty acid photodecarboxylase (FAP) resulted in increased accumulation of 7-heptadecene. Implementation of a carboxylic acid reductase (CAR) and aldehyde deformylating oxygenase (ADO) dependent synthetic pathway in Synechocystis resulted in the accumulation of fatty alcohols and a decrease in the native saturated alkanes. In contrast, the replacement of CAR and ADO with Pseudomonas mendocina UndB (so named as it is responsible for 1-undecene biosynthesis in Pseudomonas) or Chlorella variabilis FAP resulted in high-yield conversion of thioesterase-liberated FFAs into corresponding alkenes and alkanes, respectively. At best, the engineering resulted in an increase in hydrocarbon accumulation of 8- (from 1 to 8.5 mg/g cell dry weight) and 19-fold (from 4 to 77 mg/g cell dry weight) for Chlamydomonas and Synechocystis, respectively. In conclusion, reconstitution of the eukaryotic algae pathway in the prokaryotic cyanobacteria host generated the most effective system, highlighting opportunities for mix-and-match synthetic metabolism. These studies describe functioning synthetic metabolic pathways for hydrocarbon fuel synthesis in photosynthetic microorganisms for the first time, moving us closer to the commercial implementation of photobiocatalytic systems that directly convert CO2 into infrastructure-compatible fuels
Visualization of novel flux dynamics in YBa2Cu3O7-x thin films with antidots.
Using magneto-optical visualization, individual 2 μm diameter antidots (circular holes) in YB
Разработка способов десульфуризации трансформаторных масел
We report on combined dc and microwave electronic measurements of magnetic flux transport in micron and submicron-patterned high-T-c films. In a given temperature regime below the superconducting transition temperature T-c, the current-driven flux transport is restricted to flux motion guided by the submicron patterns. Via frequency-dependent measurements of the forward transmission coefficient S-21 it is demonstrated that the mechanism of the guided flux transport depends on the microwave frequency and the geometrical size of the superconducting structures. At low frequencies, flux is transported via Abrikosov vortices leading to additional microwave losses. Above a geometrically defined frequency, a different mechanism seems to be responsible for flux transport that does not contribute to the microwave losses and most likely represents a phase-slip type mechanism. The limiting vortex velocity obtained from the frequency dependence of the microwave properties agrees with the Larking-Ovchinnikov critical vortex velocity that is determined via dc pulse measurements. In spite of the change of mechanism, guidance of flux persists in these nanopatterns up to high frequencies of several GHz
Irreversibility, Mechanical Entanglement and Thermal Melting in Superconducting Vortex Crystals with Point Impurities
We discuss the onset of irreversibility and entanglement of vortex lines in
high Tc superconductors due to point disorder and thermal fluctuations using a
simplified cage model. A combination of Flory arguments, known results from
directed polymers in random media, and a Lindemann criterion are used to
estimate the field and temperature dependence of irreversibility, mechanical
entanglement and thermal melting. The qualitative features of this dependence,
including its nonmonotonicity when disorder is sufficiently strong, are in good
agreement with recent experiments.Comment: 7 pages, uses RevTeX, multicol.sty and epsf.sty, 5 EPS figures
include
Peak Effect in Superconductors: Absence of Phase Transition and Possibility of Jamming in Vortex Matter
The magnetic field dependence of the critical current for the
vortex phase of a disordered superconductor is studied numerically at zero
temperature. The increases rapidly near the upper critical field
similar to the peak effect (PE) phenomenon observed in many
superconductors. The real space configuration across the PE changes
continuously from a partially ordered domain (polycrystalline) state into an
amorphous state. The topological defect density
with for . There is no evidence of a phase transition in
the vicinity of the PE suggesting that an order-disorder transition is not
essential for the occurrence of the PE phenomenon. An alternative view is
presented wherein the vortex system with high dislocation density undergoes
jamming at the onset of the PE.Comment: 8 pages and 5 figure
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