38,170 research outputs found
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
Directional pinning and anisotropy in YBa2Cu3O7-x with BaZrO3 nanorods: intrinsic and nanorods-induced anisotropy
We present a study of the anisotropic vortex parameters as obtained from
measurements of the microwave complex resistivity in the vortex state with a
tilted applied magnetic field in YBa2Cu3O7-x thin films with BaZrO3 nanorods.
We present the angular dependence of the vortex viscosity , the pinning
constant k_p and the upper limit for the creep factor \chi_M. We show that the
directional effect of the nanorods is absent in \eta, which is dictated by the
mass anisotropy \gamma. By contrast, pinning-mediated properties are strongly
affected by the nanorods. It is significant that the pinning and creep affected
by the nanorods is detectable also at our very high operating frequency, which
implies very short-range displacements of the vortices from their equilibrium
position.Comment: Proceedings of VORTEX VIII Conference, to be published in Physica
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
Effect of nanosize BaZrO3 inclusions on vortex parameters in YBaCuO
We report on the field dependence of the microwave complex resistivity data
in YBaCuO/BaZrO films grown by PLD at various BaZrO
content. The data, analyzed within a recently developed general framework for
the mixed-state microwave response of superconductors, yield the field
dependence of the fluxon parameters such as the vortex viscosity and the
pinning constant. We find that pinning undergoes a change of regime when the
BaZrO content in the target increases from 2.5 mol.% to 5 mol.%.
Simultaneously, the vortex viscosity becomes an increasing function of the
applied magnetic field. We propose a scenario in which flux lines are pinned as
bundles, and a crossover from dilute point pins to dense c-axis correlated
defects takes place between 2.5 and 5 mol.% in the BZO concentration. Our data
are inconsistent with vortices occupying mainly the BaZrO sites at low
fields, and suggest instead that vortices occupy both BaZrO sites and
interstitials in the YBaCuO matrix, even at low fields.Comment: Presented at EUCAS 2009, to be published in J. Phys.:Conf. Serie
Anisotropy and directional pinning in YBaCuO with BaZrO3 nanorods
Measurements of anisotropic transport properties (dc and high-frequency
regime) of driven vortex matter in YBaCuO with elongated
strong-pinning sites (c-axis aligned, self-assembled BaZrO nanorods) are
used to demonstrate that the effective-mass angular scaling takes place only in
intrinsic physical quantities (flux-flow resistivity), and not in
pinning-related Labusch parameter and critical currents. Comparison of the
dynamics at different time scales shows evidence for a transition of the vortex
matter toward a Mott phase, driven by the presence of nanorods. The strong
pinning in dc arises partially from a dynamic effect.Comment: 4 pages, 4 figures. Accepted for publication on Applied Physics
Letters. With respect to v1: changed title, slightly shortene
Quantum anisotropic Heisenberg chains with superlattice structure: a DMRG study
Using the density matrix renormalization group technique, we study spin
superlattices composed of a repeated pattern of two spin-1/2 XXZ chains with
different anisotropy parameters. The magnetization curve can exhibit two
plateaus, a non trivial plateau with the magnetization value given by the
relative sizes of the sub-chains and another trivial plateau with zero
magnetization. We find good agreement of the value and the width of the
plateaus with the analytical results obtained previously. In the gapless
regions away from the plateaus, we compare the finite-size spin gap with the
predictions based on bosonization and find reasonable agreement. These results
confirm the validity of the Tomonaga-Luttinger liquid superlattice description
of these systems.Comment: 6 pages, 6 figure
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