1,104 research outputs found
Magnetic relaxation of type II superconductors in a mixed state of entrapped and shielded flux
The magnetic relaxation has been investigated in type II superconductors when
the initial magnetic state is realized with entrapped and shielded flux (ESF)
contemporarily. This flux state is produced by an inversion in the magnetic
field ramp rate due to for example a magnetic field overshoot. The
investigation has been faced both numerically and by measuring the magnetic
relaxation in BSCCO tapes. Numerical computations have been performed in the
case of an infinite thick strip and of an infinite slab, showing a quickly
relaxing magnetization in the first seconds. As verified experimentally, the
effects of the overshoot cannot be neglected simply by cutting the first 10-100
seconds in the magnetic relaxation. On the other hand, at very long times, the
magnetic states relax toward those corresponding to field profiles with only
shielded flux or only entrapped flux, depending on the amplitude of the field
change with respect to the full penetration field of the considered
superconducting samples. In addition, we have performed numerical simulations
in order to reproduce the relaxation curves measured on the BSCCO(2223) tapes;
this allowed us to interpret correctly also the first seconds of the
curves.Comment: 9 pages, 12 figures submit to PR
Harmonics of the AC susceptibility as probes to differentiate the various creep models
We measured the temperature dependence of the 1st and the 3rd harmonics of
the AC magnetic susceptibility on some type II superconducting samples at
different AC field amplitudes, hAC. In order to interpret the measurements, we
computed the harmonics of the AC susceptibility as function of the temperature
T, by integrating the non-linear diffusion equation for the magnetic field with
different creep models, namely the vortex glass-collective creep
(single-vortex, small bundle and large bundle) and Kim-Anderson model. We also
computed them by using a non-linear phenomenological I-V characteristics,
including a power law dependence of the pinning potential on hAC. Our
experimental results were compared with the numerically computed ones, by the
analysis of the Cole-Cole plots. This method results more sensitive than the
separate component analysis, giving the possibility to obtain detailed
information about the contribution of the flux dynamic regimes in the magnetic
response of the analysed samples.Comment: 9 pages, 6 figures, submitted to Physica
BiOCuS: A new superconducting compound with oxypnictide - related structure
The discovery of about 50 K superconductivity in the tetragonal Fe-based
pnictides has stimulated the search for superconductivity in a wide class of
materials with similar structure. Copper forms compounds isostructural to
LaOFeAs. Single phase BiOCuS can be prepared by a solid state reaction at
temperature lower than 500 C from a mixture of Bi2O3, Bi2S3 and Cu2S. The
samples have been characterized by means of EDX analysis, X-ray diffraction,
magnetic and electrical measurements. The cell parameters are a = 3.8708 A, c =
8.565 A. Charge carrier doping can be realized either by F substitutions for O,
or by Cu off-stoichiometry. The latter doping route leads to the occurrence of
superconductivity below Tc = 5.8 K
Strong improvement of the transport characteristics of YBaCuO grain boundaries using ionic liquid gating
For more than 30 years, the remarkable superconducting properties of
REBaCuO (RE = rare earth) compounds
have triggered research studies across the world. Accordingly, significant
progresses have been made both from a basic understanding and a fabrication
processes perspective. Yet, today, the major technological bottleneck towards
the spread of their practical uses remains the exponential decay of their
critical current with grain misorientation in polycrystalline samples. In this
work, we used an ionic liquid to apply extremely high transverse electric
fields to YBaCuO thin films containing
a single well-defined low-angle grain boundary. Our study shows that this
technique is very effective to tune the IV characteristics of these weak-links.
In-magnetic field measurements allow us to discuss the type of the vortices
present at the grain boundary and to unveil a large variation of the local
depairing current density with gating. Comparing our results with the ones
obtained on chemically-doped grain boundaries, we discuss routes to evaluate
the role of local strain in the loss of transparency at cuprates low-angle
grain boundaries. In short, this study offers a new opportunity to discuss
scenarios leading to the reduced transport capabilities of grain boundaries in
cuprates
Assessing composition gradients in multifilamentary superconductors by means of magnetometry methods
We present two magnetometry-based methods suitable for assessing gradients in
the critical temperature and hence the composition of multifilamentary
superconductors: AC magnetometry and Scanning Hall Probe Microscopy. The
novelty of the former technique lies in the iterative evaluation procedure we
developed, whereas the strength of the latter is the direct visualization of
the temperature dependent penetration of a magnetic field into the
superconductor. Using the example of a PIT Nb3Sn wire, we demonstrate the
application of these techniques, and compare the respective results to each
other and to EDX measurements of the Sn distribution within the sub-elements of
the wire.Comment: 7 pages, 8 figures; broken hyperlinks are due to a problem with arXi
Spin Coulomb drag beyond the random phase approximation
We study the spin Coulomb drag in a quasi-two-dimensional electron gas beyond
the random phase approximation (RPA). We find that the finite transverse width
of the electron gas causes a significant reduction of the spin Coulomb drag.
This reduction, however, is largely compensated by the enhancement coming from
the inclusion of many-body local field effects beyond the RPA, thereby
restoring good agreement with the experimental observations by C. P. Weber
\textit{et al.}, Nature, \textbf{437}, 1330 (2005).Comment: 3 figures, accepted for publication in Phys. Rev. Let
Exchange and correlation effects on the plasmon dispersions and the Coulomb drag in low-density electron bilayers
We investigate the effect of exchange and correlation (xc) on the plasmon
spectrum and the Coulomb drag between spatially separated low-density
two-dimensional electron layers. We adopt a new approach, which employs dynamic
xc kernels in the calculation of the bi-layer plasmon spectra and of the
plasmon-mediated drag, and static many-body local field factors in the
calculation of the particle-hole contribution to the drag. The spectrum of
bi-layer plasmons and the drag resistivity are calculated in a broad range of
temperatures taking into account both intra- and inter-layer correlation
effects. We observe that both plasmon modes are strongly affected by xc
corrections. After the inclusion of the complex dynamic xc kernels, a decrease
of the electron density induces shifts of the plasmon branches in opposite
directions. And this is in stark contrast to the tendency obtained within the
RPA that both optical and acoustical plasmons move away from the boundary of
the particle-hole continuum with a decrease in the electron density. We find
that the introduction of xc corrections results in a significant enhancement of
the transresistivity and qualitative changes in its temperature dependence. In
particular, the large high-temperature plasmon peak that is present in the
random phase approximation is found to disappear when the xc corrections are
included. Our numerical results at low temperatures are in good agreement with
the results of recent experiments by M. Kellogg {\it et al.}, Solid State
Commun. \textbf{123}, 515 (2002).Comment: 28 pages, 15 figure
The consistency condition for the three-point function in dissipative single-clock inflation
We generalize the consistency condition for the three-point function in
single field inflation to the case of dissipative, multi-field, single-clock
models. We use the recently introduced extension of the effective field theory
of inflation that accounts for dissipative effects, to provide an explicit
proof to leading (non-trivial) order in the generalized slow roll parameters
and mixing with gravity scales. Our results illustrate the conditions necessary
for the validity of the consistency relation in situations with many degrees of
freedom relevant during inflation, namely that there is a preferred clock.
Departures from this condition in forthcoming experiments would rule out not
only single field but also a large class of multi-field models.Comment: 26+11 page
A Naturally Large Four-Point Function in Single Field Inflation
Non-Gaussianities of the primordial density perturbations have emerged as a
very powerful possible signal to test the dynamics that drove the period of
inflation. While in general the most sensitive observable is the three-point
function in this paper we show that there are technically natural inflationary
models where the leading source of non-Gaussianity is the four-point function.
Using the recently developed Effective Field Theory of Inflation, we are able
to show that it is possible to impose an approximate parity symmetry and an
approximate continuos shift symmetry on the inflaton fluctuations that allow,
when the dispersion relation is of the form , for a unique
quartic operator, while approximately forbidding all the cubic ones. The
resulting shape for the four-point function is unique. In the models where the
dispersion relation is of the form a similar construction
can be carried out and additional shapes are possible.Comment: 13 pages, 1 figure. v2: extended discussion on near-de-Sitter model
Galilean symmetry in the effective theory of inflation: new shapes of non-Gaussianity
We study the consequences of imposing an approximate Galilean symmetry on the
Effective Theory of Inflation, the theory of small perturbations around the
inflationary background. This approach allows us to study the effect of
operators with two derivatives on each field, which can be the leading
interactions due to non-renormalization properties of the Galilean Lagrangian.
In this case cubic non-Gaussianities are given by three independent operators,
containing up to six derivatives, two with a shape close to equilateral and one
peaking on flattened isosceles triangles. The four-point function is larger
than in models with small speed of sound and potentially observable with the
Planck satellite.Comment: 23 pages, 6 figures. v2: minor changes to match JCAP published
versio
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