3,075 research outputs found
Effects of Self-field and Low Magnetic Fields on the Normal-Superconducting Phase Transition
Researchers have studied the normal-superconducting phase transition in the
high- cuprates in a magnetic field (the vortex-glass or Bose-glass
transition) and in zero field. Often, transport measurements in "zero field"
are taken in the Earth's ambient field or in the remnant field of a magnet. We
show that fields as small as the Earth's field will alter the shape of the
current vs. voltage curves and will result in inaccurate values for the
critical temperature and the critical exponents and , and can
even destroy the phase transition. This indicates that without proper screening
of the magnetic field it is impossible to determine the true zero-field
critical parameters, making correct scaling and other data analysis impossible.
We also show, theoretically and experimentally, that the self-field generated
by the current flowing in the sample has no effect on the current vs. voltage
isotherms.Comment: 4 pages, 4 figure
Normal-Superconducting Phase Transition Mimicked by Current Noise
As a superconductor goes from the normal state into the superconducting
state, the voltage vs. current characteristics at low currents change from
linear to non-linear. We show theoretically and experimentally that the
addition of current noise to non-linear voltage vs. current curves will create
ohmic behavior. Ohmic response at low currents for temperatures below the
critical temperature mimics the phase transition and leads to incorrect
values for and the critical exponents and . The ohmic response
occurs at low currents, when the applied current is smaller than the
width of the probability distribution , and will occur in both the
zero-field transition and the vortex-glass transition. Our results indicate
that the transition temperature and critical exponents extracted from the
conventional scaling analysis are inaccurate if current noise is not filtered
out. This is a possible explanation for the wide range of critical exponents
found in the literature.Comment: 4 pages, 2 figure
U-Pb zircon dating of basement inliers within the Moine Supergroup, Scottish Caledonides: implications of Archaean protolith ages.
Basement gneiss inliers within the Scottish Caledonides have been conventionally correlated with the Archaean Lewisian Gneiss Complex of the Caledonian foreland. Alternatively, the inliers could represent allochthonous terranes accreted to Laurentia before or during the Caledonian orogeny. SIMS U-Pb zircon dating indicates that the Ribigill, Borgie, Farr and Western Glenelg basement inliers are characterized by late Archaean protolith ages, and a period of isotopic disturbance in the late Palaeoproterozoic. The data are broadly consistent with correlation between the inliers and components of the Lewisian Gneiss Complex of the Caledonian foreland. The c. 2900 Ma protolith ages support correlation of the Borgie and Farr inliers with the Assynt terrane, and a younger, c. 2800 Ma age for the Ribigill inlier supports correlation with the Rhiconich terrane.None of the studied inliers shows a complete match of protolith and early metamorphic histories with any of the Lewisian basement terranes, but differences between the inliers and the foreland are no greater than those recorded within the foreland basement terranes themselves. Therefore, it remains probable that the dated inlier gneisses formed a distal part of the Laurentian margin prior to final telescoping during the Caledonian orogeny
The dispersive self-dual Einstein equations and the Toda lattice
The Boyer-Finley equation, or -Toda equation is both a reduction
of the self-dual Einstein equations and the dispersionlesslimit of the
-Toda lattice equation. This suggests that there should be a dispersive
version of the self-dual Einstein equation which both contains the Toda lattice
equation and whose dispersionless limit is the familiar self-dual Einstein
equation. Such a system is studied in this paper. The results are achieved by
using a deformation, based on an associative -product, of the algebra
used in the study of the undeformed, or dispersionless,
equations.Comment: 11 pages, LaTeX. To appear: J. Phys.
Probing the limits of superconductivity
DC voltage versus current measurements of superconductors in a magnetic field
are widely interpreted to imply that a phase transition occurs into a state of
zero resistance. We show that the widely-used scaling function approach has a
problem: Good data collapse occurs for a wide range of critical exponents and
temperatures. This strongly suggests that agreement with scaling alone does not
prove the existence of the phase transition. We discuss a criterion to
determine if the scaling analysis is valid, and find that all of the data in
the literature that we have analyzed fail to meet this criterion. Our data on
YBCO films, and other data that we have analyzed, are more consistent with the
occurrence of small but non-zero resistance at low temperature.Comment: 13 page pdf file, figures included To be published in conference
proceedings of SPIE 200
Driven Morse Oscillator: Model for Multi-photon Dissociation of Nitrogen Oxide
Within a one-dimensional semi-classical model with a Morse potential the
possibility of infrared multi-photon dissociation of vibrationally excited
nitrogen oxide was studied. The dissociation thresholds of typical driving
forces and couplings were found to be similar, which indicates that the results
were robust to variations of the potential and of the definition of
dissociation rate.
PACS: 42.50.Hz, 33.80.WzComment: old paper, 8 pages 6 eps file
Limitation of energy deposition in classical N body dynamics
Energy transfers in collisions between classical clusters are studied with
Classical N Body Dynamics calculations for different entrance channels. It is
shown that the energy per particle transferred to thermalised classical
clusters does not exceed the energy of the least bound particle in the cluster
in its ``ground state''. This limitation is observed during the whole time of
the collision, except for the heaviest system.Comment: 13 pages, 15 figures, 1 tabl
The zero-field superconducting phase transition obscured by finite-size effects in thick films
We report on the normal-superconducting phase transition in thick
films in zero magnetic field. We find
significant finite-size effects at low currents even in our thickest films ( \AA). Using data at higher currents, we can unambiguously find
and , and show , as expected for the three-dimensional XY
model with diffusive dynamics. The crossover to two-dimensional behavior, seen
by other researchers in thinner films ( \AA), obscures the
three-dimensional transition in both zero field and the vortex-glass transition
in field, leading to incorrect values of (or ), , and . The
finite-size effects, usually ignored in thick films, are an explanation for the
wide range of critical exponents found in the literature.Comment: 5 pages, 4 figure
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