arXiv subjects

R. Rogai

Publications and source records attributed to R. Rogai.

7 recordsLinked to original sources

arXiv2011

Angular dependence of the high-frequency vortex response in YBa$_2$Cu$_3$O$_{7-x}$ thin film with self-assembled BaZrO$_3$ nanorods

We present a microwave study of the angular dependence of the flux-flow resistivity $ρ_{ff}$ and of the pinning constant $k_p$ in YBCO thin films containing BZO nanorods. We find that BZO nanorods are very efficient pinning centers, even in tilted fields. We find that $ρ_{ff}$ is a scaling function of a reduced field $H/f(θ)$. We extend a model for the anisotropic motion of vortices in uniaxially anisotropic superconductor, able to describe the experimental $f(θ)$ on the basis of only the intrinsic anisotropy of YBCO. The pinning constant $k_p$, by contrast, exhibits different field dependences in different angular ranges, consistent with pinning by BZO at angles as large as 60$^{\circ}$, and with pinning along the $a,b$ planes as originating from the same mechanism as in pure YBCO with the field along the c axis.

cond-mat.supr-con↗

arXiv2011

Microwave properties of DyBCO monodomain in the mixed state and comparison with other RE-BCO systems

We report on microwave measurements on DyBa$_2$Cu$_3$O$_{7-\rmδ}$ monodomains grown by the top-seeded melt-textured technique. We measured the field increase of the surface resistance $R_{\rm s}(H)$ in the a-b plane at 48.3 GHz. Measurements were performed at fixed temperatures in the range 70 K - $T_{\rm c}$ with a static magnetic field $μ_0H<0.8$ T parallel to the c-axis. Low field steep increase of the dissipation, typical signature of the presence of weak links, is absent, thus indicating the single-domain behaviour of the sample under study. The magnetic field dependence of $R_{\rm s}(H)$ is ascribed to the dissipation caused by vortex motion. The analysis of $X_{\rm s}(H)$ points to a free-flow regime, thus allowing to obtain the vortex viscosity as a function of temperature. We compare the results with those obtained on RE-BCO systems. In particular, we consider strongly pinned films of YBa$_2$Cu$_3$O$_{7-\rmδ}$ with nanometric BaZrO$_3$ inclusions.

cond-mat.supr-con↗

arXiv2009

Effect of nanosize BaZrO3 inclusions on vortex parameters in YBaCuO

We report on the field dependence of the microwave complex resistivity data in YBa$_2$Cu$_3$O$_{7-x}$/BaZrO$_3$ films grown by PLD at various BaZrO$_3$ 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$_3$ 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$_3$ sites at low fields, and suggest instead that vortices occupy both BaZrO$_3$ sites and interstitials in the YBa$_2$Cu$_3$O$_{7-x}$ matrix, even at low fields.

cond-mat.supr-con↗

arXiv2008

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 YBa$_{2}$Cu$_{3}$O$_{7-δ}$ thin films, where sub-micrometric BaZrO$_3$ particles have been incorporated. As a function of the BaZrO$_3$ content, we observe that the absolute losses slightly decrease up to a BaZrO$_3$ 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$_3$ 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$_3$ 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$_3$ inclusions introduce deep and steep pinning wells. In particular, the minimum height of the energy barrier for single vortices is raised. At larger BaZrO$_3$ 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.

cond-mat.supr-con↗

arXiv2007

Change of strength of vortex pinning in YBCO due to BaZrO_3 inclusions

We probe the short-range pinning properties with the application of microwave currents at very high driving frequencies (47.7 GHz) on YBa$_2$Cu$_3$O$_{7-δ}$ films with and without sub-micrometer BaZrO$_3$ inclusions. We explore the temperature and field ranges 60 K$<T<T_c$ and 0$<μ_0H<$0.8 T, with the field applied along the c-axis. The magnetic field induces a much smaller increase of the microwave resistivity, $Δρ_1(H)+\mathrm{i}Δρ_2(H)$, in YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$ with respect to pure YBa$_2$Cu$_3$O$_{7-δ}$. $Δρ_1(H)$ is slightly superlinear in pure YBa$_2$Cu$_3$O$_{7-δ}$ (suggesting a possible contribution of thermal activation), but linear or sublinear in YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$ (suggesting a possible suppression of thermal activation as a consequence of BaZrO$_3$ inclusions). These features persist up to close to $T_c$. We discuss our data in terms of the ratio $r=ΔX_s'(H)/ΔR_s'(H)$ in the framework of the models for the microwave surface impedance in the mixed state. Large $r$ are found in YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$, with little field dependence. By contrast, smaller values and stronger field dependences are found in pure YBa$_2$Cu$_3$O$_{7-δ}$. We discuss the different field dependence of the pinning constant.

cond-mat.supr-con↗

arXiv2007

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 YBa$_2$Cu$_3$O$_{7-δ}$ films on SrTiO$_3$ substrates. BaZrO$_3$ crystallites were included in the films using composite targets containing BaZrO$_3$ inclusions with mean grain size smaller than 1 $μ$m. X-ray diffraction showed single epitaxial relationship between BaZrO$_3$ and YBa$_2$Cu$_3$O$_{7-δ}$. The effective surface resistance was measured at 47.7 GHz for 60$< T <$90 K and 0$< μ_0H <$0.8 T. The magnetic field had a very different effect on pristine YBa$_2$Cu$_3$O$_{7-δ}$ and YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$, while for $μ_0H=$0 only a reduction of $T_c$ in the YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$ film was observed, consistent with dc measurements. At low enough $T$, in moderate fields YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$ exhibited an intrinsic thin film resistance lower than the pure film. The results clearly indicate that BaZrO$_3$ 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$_3$ inclusions determine very steep pinning potentials.

cond-mat.supr-con↗

arXiv2007

Strong reduction of field-dependent microwave surface resistance in YBa$_{2}$Cu$_{3}$O$_{7-δ}$ with sub-micrometric BaZrO$_3$ inclusions

We observe a strong reduction of the field induced thin film surface resistance measured at high microwave frequency ($ν=$47.7 GHz) in YBa$_{2}$Cu$_{3}$O$_{7-δ}$ thin films grown on SrTiO$_3$ substrates, as a consequence of the introduction of sub-micrometric BaZrO$_3$ particles. The field increase of the surface resistance is smaller by a factor of $\sim$3 in the film with BaZrO$_3$ inclusions, while the zero-field properties are not much affected. Combining surface resistance and surface reactance data we conclude (a) that BaZrO$_3$ inclusions determine very deep and steep pinning wells and (b) that the pinning changes nature with respect to the pure film.

cond-mat.supr-con↗