We investigated dynamics of the thermomagnetic avalanche development in melt-textured samples of the oxide YBaCuO high temperature superconductor. We found that the flux jumps, caused by the thermomagnetic instabilities, are accompanied by giant convergent oscillations of the surface induction. These results are compared with the dynamics of the flux jumps in other type-II superconductors, including conventional NbTi and oxide high temperature Bi _2Sr_2CaCu_2O_{8+δ} superconductor, and discussed in the framework of the available model of the thermomagnetic avalanche development.
We propose a semi-microscopic model of YBa_2Cu_3O_{6+y} to investigate the origin of the 60 K plateau in its phase diagram. Our model is a "phase only" approach to the high-temperature superconducting system in terms of collective variables. It is able to capture characteristic energy scales present in YBa_2Cu_3O_{6+y} by using adjustable parameters representing phase stiffnesses and allows for strong anisotropy within basal planes to simulate oxygen ordering. We solve the model calculated T_c for chosen system parameters investigating the influence of oxygen ordering and doping imbalance on the shape of YBa_2Cu_3O_{6+y} phase diagram. Our results suggest that the oxygen ordering alone does not seem to be responsible for the existence of the 60 K plateau. However, relying on experimental data unveiling that oxygen doping of YBa_2Cu_3O_{6+y} may introduce a significant charge imbalance between CuO_2 planes and other sites, we show that simultaneously the former are underdoped, while the latter - strongly overdoped almost in the whole region of oxygen doping in which YBa_2Cu_3O_{6+y} is superconducting. This provides two natural counter acting factors, which possibly lead to rise the 60 K plateau with increasing oxygen doping.
The influence of non-magnetic central node defect on magnetic hysteresis of regular square-shaped segment of magnetic dot array with perpendicular uniaxial anisotropy under the thermal activation was investigated via computer simulations based on stochastic Landau-Lifshitz-Gilbert equation. The aim of this study is to point out the simultaneous effect of anisotropy and thermal activation to the dynamical properties of magnetic dot arrays.
The influence of UV laser irradiation on the physical properties of epitaxial YBa_2Cu_3O_{7-δ} thin (<1μm) films fabricated by laser ablation was studied. The samples were irradiated by pulsed excimer laser beam at different incident energy densities E_{ir}≤130 mJ/cm^2, i.e. 2-3 times below the ablation threshold of YBaCuO. The analysis of the sample cross-section using transmission electron microscope shows that such irradiation forms a disordered layer on the top of the crystalline film. Sample surface amorphization was observed also by atomic force microscope. Due to strong UV irradiation absorption in YBaCuO this surface layer acts like a protective cover for the sample interior. Only a small reduction of superconducting critical temperature T_c was observed. Both transport measurements in normal state and magneto-optical studies in superconducting state indicate that between the disordered layer and the bottom non-modified part of the film there exists a wide transitional region with reduced carrier concentration.
We investigated the magnetostriction of ceramic YBa_2Cu_3O_{7-δ} sample both before and after fast neutron irradiation with the fluency of 7.55×10^{17} n/cm^2 and energy of above 0.5 MeV. We have found that after irradiation the widths of both transverse and longitudinal magnetostriction hysteresis loops at a given temperature and at a given external magnetic field increased several times. At 4.2 K and in 10 tesla the widths of the magnetostriction hysteresis loop of the irradiated sample are of an order of 10^{-5}. The results can be understood in the framework of the model of the magnetostriction induced by the pinning forces, assuming the stress in the sample to be induced mainly by the intragrain screening currents.
We study the physics of the mercurocuprate family from the structural point of view. We present a phenomenological approach determining the critical temperatures for homologous series HgBa_{2}Ca_{n-1}Cu_{n}O_{2n+2+δ} depending on the number of layers n. The model is based on the Lawrence- Doniach theory of layered superconductors modified for the case of inequivalent layers. The redistribution of charge was taken into account. This leads to observable nonmonotonic "bell"-shaped dependence of T_{c}(n) and provides a quantitative explanation of the experiments.
Complex perovskite oxides exhibit a rich spectrum of functional responses such as: superconductivity, magnetism etc. Combination of different oxides in multilayered structures increases the number of physical responses. Heterogeneous oxide structures represent a new class of nanostructures. They consist of ferromagnetic La_{0.67}Sr_{0.33}MnO_3 (F-LSMO) manganite and superconducting YBa_2Cu_3O_7 (Sc-YBCO) cuprate. The interaction between the two order parameters gives rise to new physical effects. In this review we will discuss various physical effects obtained in the bilayer and trilayer heterostructures. For example, the LSMO/YBCO bilayer structures are used to study the mechanism of magnetic pinning. The other possibility is the fabrication of spin valve-like structures LSMO/YBCO/LSMO. The spin dependent transport in trilayer structure was studied taking into account crossed Andreev reflection and electron co-tunneling processes.
A brief overview of the deposition and properties of YBa_{2}Cu_{3}O_{7} (YBCO) based heterostructures is presented. Emphasis is placed on the assessment of the deposition process and the properties of practical thin films and heterostructures of layered oxides. Besides a reference to the literature we present our recent results concerning deposition, structural and magnetic properties of YBa_{2}Cu_{3}O_{7} /RE_{1-x}A_{x}MnO_{3} (YBCO/REMO) heterostructures.
The early stages of growth and the evolution of surface microstructure of epitaxial c-axis YBa_{2}Cu_{3}O_{7-x} thin films were studied as a function of film thickness, growth temperature, and growth rate. The films were grown in situ on as-polished or annealed MgO(100) substrates by off-axis magnetron sputtering. Atomic force microscopy was used to observe the surface microstructure. From the results at early stages, we proposed a growth model of spirals for the films grown on as-polished substrate, i.e. spirals are formed around the surface roughness of the substrate. Growth temperature and growth rate dependence on the density of spirals support this model. That is, the density of spirals does not change according to the change of growth temperature (600-740°C) or growth rate (7-80 nm/h).
Flux pinning properties of single crystals and melt-textured samples of YBa_{2}Cu_{3}O_{7-x} were studied for external magnetic field oriented parallel and perpendicular to the CuO_{2} planes (ab-planes). For H ∥ ab vortex behaviour is mainly determined by a very effective intrinsic pinning mechanism. The irreversibility line is located in the region of higher fields and temperatures in comparison with the irreversibility line for H ⊥ ab. In a single crystal for H ⊥ ab, we observed an increase in the critical current density j_{c} with magnetic field - the so-called "fishtail effect" - in a very broad temperature range. For H ∥ ab, this effect is observed only at temperatures close to T_{c}, when the intrinsic pinning is much weaker.
In this paper we present the resistivity data for Pr and Zn codoped compound Y_{1-x}Pr_xBa_2[Cu_{1- y}Zn_y]_3O_{7-δ} with 0 & y & 0.1 and x=0.0, 0.1 and 0.2. The data is analysed in terms of the superconducting critical temperature T_c, residual resistivity ρ_0 and the resistivity slope dρ/dT corresponding to the linear ρ-T region. It is found that for x=0.1 Pr has a minimal influence on the in-plane processes for Zn impurity alone affecting slightly T_c and ρ_0. The slope dρ/dT becomes larger for 0.03 & y & 0.06 leading to larger depining effect and hence slower fall of T_c as a function of y. For x = 0.2 there is a drastic change, ρ_0 becomes abnormally large, dρ/dT becomes negative implying absence of depinning and a totally pinned charge stripes. Superconductivity vanishes at y=0.03. It is concluded that for x = 0.2 Pr converts the system from overdoped to underdoped region leading to the universal superconductor-insulator transition.
Specific heat of polycrystalline DyBa_{2}Cu_{3}O_{7} and Tl_{0.58}Pb _{0.42}Sr_{1.6}Ba_{0.4}Ca_{2}Cu_{3}O_{9} samples, as well as the single crystal of Bi_{2}Sr_{2}CaCu_{2}O_{8} have been measured within the temperature interval from 50 to 250 K. For Dy- and Tl-specimens the pronounced jump in specific heat and apparent contribution from Gaussian fluctuations of superconducting order parameter close to T_{c} have been observed. In contrary, for Bi-specimen only a rounded maximum within a broad interval around T_{c} has been detected. Magnetoresistance measurements as a function of temperature just below T_{c} have been carried out for Dy- and Tl-samples and the slopes of upper critical fields have been determined. The data have been analysed within a frame of Ginzburg-Landau-Abrikosov- Gorkov theory with additional Gaussian-like fluctuation term. The electronic specific heat coefficients γ, and the coherence length χ have been obtained.
The model for the cuprates based on the modified electron-phonon pairing mechanism has been tested. For this purpose, the superconductors with high value of the critical temperature have been taken into consideration. In particular: YBa_{2}Cu_{3}O_{7-y}, HgBa_{2}CuO_{4+y}, HgBa_{2}Cu_{1-x}Zn_{x}O_{4+y}, and HgBa_{2}Ca_{2}Cu_{3}O_{8+y}. It has been shown that the dependence of the ratio R_{1} ≡ 2Δ_{tot}^{(0)}/k_{B}T_{C} on the doping (p) can be properly predicted in the framework of the presented theory; the symbol Δ_{tot}^{(0)} denotes the energy gap amplitude at the temperature of zero kelvin, and T_{C} is the critical temperature. The numerical results have been supplemented by the formula which describes the function R_{1}(p).
The authors review recent studies of high temperature superconductors conducted with scanning tunneling and magnetic force microscopes. Emphasis is placed on the importance of surface and probe characterization, both of which are likely to affect the detailed nature of the observations.
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