67,747 research outputs found
Vortex state microwave resistivity in Tl-2212 thin films
We present measurements of the field induced changes in the 47 GHz complex
resistivity, , in TlBaCaCuO
(TBCCO) thin films with 105 K, prepared on CeO buffered
sapphire substrates. At low fields (10 mT) a very small irreversible
feature is present, suggesting a little role of intergranular phenomena. Above
that level exhibits a superlinear dependence with the
field, as opposed to the expected (at high frequencies) quasilinear behaviour.
We observe a crossover between predominantly imaginary to predominantly real
(dissipative) response with increasing temperature and/or field. In addition,
we find the clear scaling property , where the scaling field maps closely the melting
field measured in single crystals. We discuss our microwave results in terms of
loss of flux lines rigidity.Comment: 8 pages, 3 figures, proceedings of 9th HTSHFF, accepted for
publication on J. Supercon
Anisotropic renormalized fluctuations in the microwave resistivity in YBCO
We discuss the excess conductivity above Tc due to renormalized
order-parameter fluctuations in YBCO at microwave frequencies. We calculate the
effects of the uniaxial anisotropy on the renormalized fluctuations in the
Hartree approximation, extending the isotropic theory developed by Dorsey
[Phys. Rev. B 43, 7575 (1991)]. Measurements of the real part of the microwave
resistivity at 24 and 48 GHz and of the dc resistivity are performed on
different YBCO films. The onset of the superconducting transition and the
deviation from the linear temperature behavior above Tc can be fully accounted
for by the extended theory. According to the theoretical calculation here
presented, a departure from gaussian toward renormalized fluctuations is
observed. Very consistent values of the fundamental parameters (critical
temperature, coherence lenghts, penetration depth) of the superconducting state
are obtained.Comment: RevTex, 8 pages with 5 figures included, to be published in Physical
Review
Mixed-state microwave response in superconducting cuprates
We report measurements of the magnetic-field induced microwave complex
resistivity in REBaCuO thin films, with RE = Y, Sm.
Measurements are performed at 48 GHz by means of a resonant cavity in the
end-wall-replacement configuration. The magnetic field dependence is
investigated by applying a moderate (0.8 T) magnetic field along the c-axis.
The measured vortex state complex resistivity in
YBaCuO and SmBaCuO is
analyzed within the well-known models for vortex dynamics. It is shown that
attributing the observed response to vortex motion alone leads to
inconsistencies in the as-determined vortex parameters (such as the vortex
viscosity and the pinning constant). By contrast, attributing the entire
response to field-induced pair breaking leads to a nearly quantitative
description of the data.Comment: 6 pages, 4 figures, to be published in J. Supercond. as proceedings
of 8th HTSHFF (May 26th-29th, 2004, Begur, Spain
Predicted defect induced vortex core switching in thin magnetic nanodisks
We investigate the influence of artificial defects (small holes) inserted
into magnetic nanodisks on the vortex core dynamics. One and two holes
(antidots) are considered. In general, the core falls into the hole but, in
particular, we would like to remark an interesting phenomenon not yet observed,
which is the vortex core switching induced by the vortex-hole interactions. It
occurs for the case with only one hole and for very special conditions
involving the hole size and position as well as the disk size. Any small
deformation in the disk geometry such as the presence of a second antidot
changes completely the vortex dynamics and the vortex core eventually falls
into one of the defects. After trapped, the vortex center still oscillates with
a very high frequency and small amplitude around the defect center.Comment: 11pages, Revtex format, 17 figure
Strong Reduction of the Field-Dependent Microwave Surface Resistance in YBCO with BaZrO_3 Inclusions
We present measurements of the magnetic field dependent microwave surface
resistance in laser-ablated YBaCuO films on SrTiO
substrates. BaZrO crystallites were included in the films using composite
targets containing BaZrO inclusions with mean grain size smaller than 1
m. X-ray diffraction showed single epitaxial relationship between
BaZrO and YBaCuO. The effective surface resistance was
measured at 47.7 GHz for 6090 K and 00.8 T. The magnetic
field had a very different effect on pristine YBaCuO and
YBaCuO/BaZrO, while for 0 only a reduction of
in the YBaCuO/BaZrO film was observed,
consistent with dc measurements. At low enough , in moderate fields
YBaCuO/BaZrO exhibited an intrinsic thin film
resistance lower than the pure film. The results clearly indicate that
BaZrO inclusions determine a strong reduction of the field-dependent
surface resistance. From the analysis of the data in the framework of simple
models for the microwave surface impedance in the mixed state we argue that
BaZrO inclusions determine very steep pinning potentials.Comment: LaTeX, 6 pages, 4 figures, uses jpconf.cls and jpconf11.clo class
files, talk given at EUCAS 2007, submitted to J. Phys.: Conf. Serie
Universality class for bootstrap percolation with on the cubic lattice
We study the bootstrap percolation model on a cubic lattice, using
Monte Carlo simulation and finite-size scaling techniques. In bootstrap
percolation, sites on a lattice are considered occupied (present) or vacant
(absent) with probability or , respectively. Occupied sites with less
than occupied first-neighbours are then rendered unoccupied; this culling
process is repeated until a stable configuration is reached. We evaluate the
percolation critical probability, , and both scaling powers, and
, and, contrarily to previous calculations, our results indicate that the
model belongs to the same universality class as usual percolation (i.e.,
). The critical spanning probability, , is also numerically
studied, for systems with linear sizes ranging from L=32 up to L=480: the value
we found, , is the same as for usual percolation with
free boundary conditions.Comment: 11 pages; 4 figures; to appear in Int. J. Mod. Phys.
Strong reduction of field-dependent microwave surface resistance in YBaCuO with sub-micrometric BaZrO inclusions
We observe a strong reduction of the field induced thin film surface
resistance measured at high microwave frequency (47.7 GHz) in
YBaCuO thin films grown on SrTiO substrates, as a
consequence of the introduction of sub-micrometric BaZrO particles. The
field increase of the surface resistance is smaller by a factor of 3 in
the film with BaZrO inclusions, while the zero-field properties are not
much affected. Combining surface resistance and surface reactance data we
conclude (a) that BaZrO inclusions determine very deep and steep pinning
wells and (b) that the pinning changes nature with respect to the pure film.Comment: RevTeX; 4 pages, 3 figures; submitted to Applied Physics Letter
Reduction of the field-dependent microwave surface resistance in YBa_2Cu_3O_7 with sub-micrometric BaZrO_3 inclusions as a function of BaZrO_3 concentration
In order to study the vortex pinning determined by artificially introduced
pinning centers in the small-vortex displacement regime, we measured the
microwave surface impedance at 47.7 GHz in the mixed state of
YBaCuO thin films, where sub-micrometric BaZrO
particles have been incorporated. As a function of the BaZrO content, we
observe that the absolute losses slightly decrease up to a BaZrO content of
5%, and then increase. We found that the magnetic-field-induced losses behave
differently, in that they are not monotonic with increasing BaZrO
concentration: at small concentration (2.5%) the field-induced losses increase,
but large reduction of the losses themselves, by factors up to 3, is observed
upon further increasing the BaZrO concentration in the target up to 7%.
Using measurements of both surface resistance and surface reactance we estimate
vortex pinning-related parameters. We find that BaZrO inclusions introduce
deep and steep pinning wells. In particular, the minimum height of the energy
barrier for single vortices is raised. At larger BaZrO content (5% and 7%)
the phenomenon is at its maximum, but it is unclear whether it shows a
saturation or not, thus leaving room for further improvements.Comment: 7 pages, 7 figure
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