Fábio Teixeira Dias

FLUCTUATION CONDUCTIVITY IN MELT-TEXTURED YBa2Cu3O7-δ

Systematic measurements of the in-plane and out-of-plane fluctuations magnetoconductivity in a melt-textured sample of YBa2Cu3O7-δ are presented. The experiments were performed in fields ranging from 0.02 T to 5 T applied perpendicular to CuO2 atomic (ab) planes. The measurements of the in-plane fluctuations were performed in zero applied field. The method of analysis is based on the determination of the quantity , where Δσ = σ - σR is the fluctuation conductivity. In the normal phase, contributions from Gaussian regimes are clearly evidenced. These regimes are unstable regarding the application of magnetic fields perpendicular to the ab plane. Below TC, in approaching the zero resistance state, our results show a power-law behavior, that is indicative of a phase transition from paracoherent to a coherent state of the granular array.

Magnetic irreversibility: An important amendment in the zero-field-cooling and field-cooling method

This work was supported under the Brazilian grant MCTI= CNPq Universal 14=2012 (contract number 477506=2012-7). The experimental work at the HLD was supported by Euromagnet II (contract number 228043).

Paramagnetic moments and time effects in melt-textured NdBaCuO system with Nd422 inclusions

We have performed magnetic measurements in two melt-textured NdBa2Cu3O7-d samples with Nd422 inclusions under magnetic fields from 0.05 up to 14 T, applied parallel to the ab planes. The measurements were made with a superconducting quantum interference device (SQUID) and a vibrating sample magnetometer (VSM). Paramagnetic moments could be observed during FCC (field-cooled cooling) and FCW (field-cooled warming) experiments. This effect, known as Paramagnetic Meissner Effect (PME), persisted up to 14 T and strong irreversibilities were observed among FCC and FCW experiments, revealing the presence of time effects. These time effects were confirmed by specific magnetic relaxation experiments in different cooling rates and temperatures, showing an anomalous and curious paramagnetic behavior. We explain our results based on the flux-compressed state generated within nonsuperconducting regions of the sample, such as the Nd422 inclusions dispersed into the superconducting matrix. These inclusions may produce a strong vortex pinning that stabilize the paramagnetic state, allowing the admission of extra vortices into the sample responsible for the positive moments during the relaxation experiments.

FLUCTUATION CONDUCTIVITY OF POLYCRYSTALLINEYbBa2Cu3O7 -δSUPERCONDUCTORS

We studied the effect of superconducting fluctuations on the electrical conductivity of granular samples of Y1-xTbxBa2Cu3O7-δ superconductors. For all samples we observed that the transition proceeds in two stages. Above the critical temperature, the Gaussian and critical regimes were observed, independently of the terbium concentration. In the regime of approach to the zero resistance state, and for low concentrations of terbium, an occurrence of a coherence transition was observed. For high concentrations of Tb, the results show a strong disorder at mesoscopic levels.

Magnetic irreversibility and zero resistance in granular Y358 superconductor

We report on magnetization and magnetoresistance measurements of polycrystalline Y3Ba5Cu8O18 superconductor (Y358), prepared by solid state reaction in order to study the correlation between the magnetic irreversibility line and the zero electric resistance as a function of applied field. The magnetization measurements were performed using a MPMS-XL SQUID magnetometer and the magnetoresistance measurements were made using a PPMS, both from Quantum Design, up to 1 T. The granular microstructure was confirmed by scanning electron microscopy. In this case the grain junctions are weaker than in the Y123 system and the effect of the applied field on the grain couplings is much stronger. Consequently, in our sample the zero resistance line shifts away from the irreversibility line and is lower by more than 15 K at 1 T. We explain our results in terms of the superconducting glass model and in comparison with results obtained in other materials from the YBaCuO family.

Paramagnetic Meissner effect at high fields in YCaBaCuO single crystal

We report on systematic magnetization experiments in an Y1-xCaxBa2Cu3O7−δ (x = 0.25 at%) single crystal. The magnetization experiments were made using a superconducting quantum interference device magnetometer (SQUID). Magnetic moments were measured as functions of the temperature according to the zero-field cooling (ZFC), field-cooled cooling (FCC), and field-cooled warming (FCW) prescriptions. The time-dependence of the FC magnetization at fixed magnetic fields was studied. Magnetic fields up to 50 kOe were applied and a paramagnetic response related to the superconducting state was observed when strong enough fields were applied parallel to the c axis. The magnitude of the high field paramagnetic moment (HFPME) increases when the field is augmented. The effect shows strong and anomalous time dependence, such that the paramagnetic moment increases as a function of the time. An YBa2Cu3O7−δ single crystal exhibiting the same effect was used for comparison. We discuss our results in terms of the flux compression scenario into the sample modulated by Ca concentration.

Functional behavior of the anomalous magnetic relaxation observed in melt-textured YBa2Cu3O7-δ samples showing the paramagnetic Meissner effect

This work was financed by the Brazilian MCTI/CNPq Universal 14/2012 (contract number 477506/2012-7). We acknowledge the support of the HLD-HZDR, member of the European Magnetic Field Laboratory (EMFL).

Functional behavior of the melt-textured YBa2Cu3O7-δ Meissner effectanomalous magnetic relaxation observed in samples showing the paramagnetic

This work was financed by the Brazilian MCTI/CNPq Universal 14/2012 (contract number 477506/2012-7). We acknowledge the support of the HLD-HZDR, member of the European Magnetic Field Laboratory (EMFL).

Chemical introduced disorder effects on the critical current density and pinning mechanisms of YBa2-XSrXCu3O7-¿

We report on isotherm (T = 77.5 K) dc magnetization hysteresis loops of a series of YBa_2-xSr_xCu₃O_7-δ (x = 0, 0.02, 0.1, 0.25, and 0.37) single crystals with the purpose of studying the influence of chemically introduced site disorder on the YBa₂Cu₃O_7-δ critical current density J_c(B, T) and normalized pinning force density f(h). The Sr ion chemical disorder is inserted at YBa₂Cu₃O_7-δ structure by lattice distortion. Transmission electronic microscopy of the doped samples shows a structure constituted by a high density of twins, which is, probably, decorated by many small precipitates. The J_c(B, T) transported by the doped samples with x ≤ 0.10 is higher than that transported by the YBa₂Cu₃O_7-δ pure sample. In contrast, the J_c(B, T) transported by the doped samples with x > 0.10 is significant and systematically lower than that transported by the undoped sample. The analysis of the f(h) plots indicates, in the light of the Dew-Hughes model, that the preponderant pinning mechanism of YBa_2-xSr_xCu₃O_7-δ single crystals with x≤0.10 is the core normal point type. However, to the samples with x > 0.10, the f(h) analyses indicate that the preponderant pinning mechanism undergoes a crossover from the core normal point type to the core normal surface type. This crossover of pinning mechanism dynamics could be appointed as the possible reason of the decrease of the J_c(B, T) observed in the samples with x > 0.10.

Chemically Introduced Disorder Effects on the Critical Current Density and Pinning Mechanisms of

We report on isotherm (T = 77.5 K) dc magnetization hysteresis loops of a series of YBa_2-xSr_xCu₃O_7-δ (x = 0, 0.02, 0.1, 0.25, and 0.37) single crystals with the purpose of studying the influence of chemically introduced site disorder on the YBa₂Cu₃O_7-δ critical current density J_c(B, T) and normalized pinning force density f(h). The Sr ion chemical disorder is inserted at YBa₂Cu₃O_7-δ structure by lattice distortion. Transmission electronic microscopy of the doped samples shows a structure constituted by a high density of twins, which is, probably, decorated by many small precipitates. The J_c(B, T) transported by the doped samples with x ≤ 0.10 is higher than that transported by the YBa₂Cu₃O_7-δ pure sample. In contrast, the J_c(B, T) transported by the doped samples with x > 0.10 is significant and systematically lower than that transported by the undoped sample. The analysis of the f(h) plots indicates, in the light of the Dew-Hughes model, that the preponderant pinning mechanism of YBa_2-xSr_xCu₃O_7-δ single crystals with x≤0.10 is the core normal point type. However, to the samples with x > 0.10, the f(h) analyses indicate that the preponderant pinning mechanism undergoes a crossover from the core normal point type to the core normal surface type. This crossover of pinning mechanism dynamics could be appointed as the possible reason of the decrease of the J_c(B, T) observed in the samples with x > 0.10.