Low-Field Phenomenology and Characteristic Lengths of Hydrogen- and Halogen-Doped YBa2Cu3O7−δ Compounds

We measure the diamagnetic response of YBa ₂ Cu ₃ O _y A _x (A=Cl, F, H) ceramics. We obtain low-field effective penetration depths of a few 10 m, first flux-penetration field H₁ distinguishable from the volume penetration field H_C1 and two characteristic temperatures T_s and T_c, T_cs, defined by the onset of the real and imaginary parts of the susceptibility, respectively. H ₁ (T) goes to zero as a power law H ₁ (T, x)=H ₁ (0, x) (T–T/T_c) ^2.7 near T_C. From the values of and H ₁ we infer the correlation length at T=4.2 K. A large range of values for (4.2) suggests that chemical disorder plays an essential role in the phenomenology of cuprates. We find (4.2)H ₁ (4.2) ^–1/3 . This, and the power law (4.2)H ₁ (4.2) ^–2/3 result from the scaling properties of a phase transition governed by disorder.     

Dip-Coating of YBa2Cu3O7?δ and YBa2Cu3O7?δ-Ag Films on Polycrystalline Rare-Earth Barium Niobate Substrates

Dip-coating and partial melting technique have been used to fabricate high quality superconducting YBa₂Cu₃O_7– and YBa₂Cu₃O_7–-Ag thick films with T _c(0)=92 K on polycrystalline REBa₂NbO₆ (RE=Pr, Nd, Sm, and Eu) substrates. The superconducting films showed excellent adhesion to the REBa₂NbO₆ substrate. The effect of Ag addition in YBa₂Cu₃O_7– on the current density, microstructure, and crystal orientation of the superconducting films developed on the above substrates have been discussed in detail. Dip-coating technique was found to be one of the easiest method for obtaining good quality superconducting YBa₂Cu₃O_7– thick films with thickness as low as 3 m even on polycrystalline substrates.     

Thermal annealing-induced ageing and structural effects in YBa2Cu3O7−δ superconductors

The YBa₂Cu₃O_7– superconductor loses part of its oxygen by thermal annealing, effectively at annealing temperaturesT _a 400 °C, under reduced air pressure. The release of the oxygen decreases monotonically as a function of time, at givenT _a, and presents two different slopes; fast at the beginning and slow later. The initial slopes (dM/dt) of the isothermal mass loss curves, measured at differentT _a, follow a straight line, as In (dM/dt) is plotted against theT _a ^–1 (K^–1), suggesting the release of the oxygen to be a kinetic process with an activation energyQ ₁=0.45 eV. The oxygen vacancies probably reside along the central-cage Cu-O chains, replacing O(4) ions, which break up spinal Cu-O linkages, and consequently the highT _c superconductivity properties suffer. A YBa₂Cu₃O₇ sample annealed for 30 min at 600 °C (at 10 mbar air pressure) thus gives a considerably reducedT _c at 50 K, compared to 91 K without annealing. However, annealing, peculiarly at a higherT _a 700 °C, for a short period of 10 min, has little effect on the highT _c appearing at 70 K. We believe that the oxygen vacancies possibly populate on O(1) sites along the side Cu-O chains, at this particular temperature, and become positively charged on trapping the hole, thus helping the superconductivity.     

Plasma-enhanced fluorination of YBa2Cu3O7−δ ceramics

Mechanisms of interaction between the reactive species of a (CF₄+O₂) plasma and YBa₂Cu₃O_7– ceramics have been proposed through detailed angle-resolved X-ray photoelectron spectroscopic analyses. At the surface of the outer grains, the plasma treatment removes (OH)^– and (CO₃)^2– species contained in the degradation layer and gives rise to a fluoride-rich layer, whereas in the bulk of the material the occurrence of metal-fluorine bonds in the superconducting phase has been assumed. An increase of the oxidation state of copper has been also detected, confirming the oxidizing effect of the plasma treatment. A comparison with the oxidation mechanisms obtained by fluorine gas treatment is proposed.     

Plasma-enhanced fluorination of YBa2Cu3O7−δ ceramics

The radiofrequency plasma technique involving mixtures of CF₄+O₂ gases has been applied to the surface treatment of high T_c superconducting oxides (YBa₂Cu₃O_7–). Investigation of the various experimental parameters of the process has shown that the improvement of the critical current density, J _c, mainly depends on the inlet precursor composition CF₄+%O₂, on the total pressure and on the reaction time. The presence of fluorine in the bulk of the ceramics has been observed from electron microprobe analysis, together with an increase of the Cu³⁺ content. The plasma-enhanced fluorination (PEF) treatment improves the superconducting properties of the materials: both values of the resistivity in the normal state and of the superconducting transition width are reduced and the critical transition temperature is improved by about 1 K.     

Novel vortex dynamics in untwinned YBa2Cu3O7−δ single crystals

Magnetotransport measurements on a clean, untwinned YBa₂Cu₃O_7– single crystal show that the vortex dynamics at temperatures just below the vortex lattice melting transition are highly dependent on the type of modulation of the probing current. While in the case of DC current the flux flow is disordered, the shaking of the vortex lattice by a square-wave current leads to a more uniform vortex motion. A small asymmetry (10%) in the durations of the positive and negative parts of the square-wave period induces periodical oscillations of the voltage response amplitude. The period of oscillations (100 s) is the same order of magnitude as the time needed for vortices to cross the sample (transit time). We relate the observed voltage oscillations to a periodically ordered vortex motion.On leave from Moscow Institute of Radioengineering, Electronics and Automation, 117454 Moscow, Russia     

Disordered Vortex Phases in YBa2Cu3O7?δ

The disordered vortex phases induced by line and point pinning in YBa₂Cu₃O_7– are explored. At high defect densities, only a single disordered solid separated from the liquid phase by a melting line is observed. At low defect densities the topology of the phase diagram changes dramatically, with a vortex lattice phase adjoining disordered phases at high or low field. Critical points at the termination of the first-order melting line separate the lattice and the disordered phases. The line defect disordered phases follow the expected Bose glass behavior, while the point defect disordered phases do not exhibit the expected vortex glass behavior.     

Anomalous Scattering Rate and Microwave Absorption in Bi2Sr2CaCu2O8+δ and YBa2Cu3O7?δ

We determine the scattering rate from microwave measurements for an optimally doped Bi-2212 single crystal, using a simple two-fluid model with a d-wave symmetry order parameter. In the superconducting state, the calculated scattering rate is three orders of magnitude smaller than that determined from ARPES experiments. A similar anomalously large decrease in the scattering rate is also required to explain the data within a gap-quasiparticle scenario for other HTS, such as YBa₂Cu₃O_7–. The results suggest that the assumption of normal excitations vanishing at low T is invalid and an additional charge mode is responsible for the microwave absorption.     

Oxygen Exchange in YBa2Cu3O7 – δ

The oxygen exchange in YBa₂Cu₃O_{7 –} materials of different densities was studied by thermogravimetry over wide ranges of temperatures and oxygen pressures. The results were correlated with x-ray diffraction (b – a), T _c (inductance method), and T _c data. According to polarized-light microscopy and coulometric titration results, the annealing conditions have a strong effect on the uniformity of oxygen distribution in the basal plane.     

Novel Asymmetric Critical State in YBa2Cu3O7−δ with Columnar Defects

We studied the critical state field profile using magneto-optical technique in YBa ₂ Cu ₃ O _7– with columnar defects (CDs) when the field is tilted away from CDs. We observed an asymmetric critical state field profile with the current discontinuity line (d-line) considerably away from the center of the sample. The shift of the d-line from the center (y) has symmetric hysteresis loop with respect to H = 0. When the field sweep direction is changed, the previous d-line disappears and a new d-line is generated at the opposite side. The shift of the d-line is parallel to the tilting plane of H and toward the direction of the misalignment of H from CDs. We interpret this asymmetry as caused by the in-plane magnetization, which is originated from the alignment of vortices to CDs. The field dependence of y is discussed in relation with the correlation length of vortices along CDs.     

Long range coupling in YBa2Cu3O7−δ/Y1−xPrxBa2Cu3O7−δ multilayers

YBa₂Cu₃O_7–/(Y_1–xPr_x)Ba₂Cu₃O_7– multilayers have been used to probe coupling through (Y_1–x:Pr_x)Ba₂Cu₃O_7– alloys. We observe that the coupling between ultrathin YBa₂Cu₃O_7– layers, 12 or 24 Å thick, survives through several hundred Å of (Y_1–xPr_x)Ba₂Cu₃O_7– with x=0.4 and 0.55. T_c versus the thickness of the spacer-alloy, and activation energies for flux motion, with fields parallel and perpendicular to the c-axis, have been used to probe this long range coupling. All these experiments point to an unusually large coupling length for these two alloy compositions. In the x=0.55 case this result is particularly surprising since the alloy material display a semiconducting behaviour for this composition. T_c measurements, activation energies, and a study of the vortex dynamics in these coupled multilayers is presented along with new results obtained on a series of multilayers built with a more insulating alloy, x=0.7.     

Unusual Charge Localization in Zn-doped and Heavily Underdoped YBa2Cu3O7−δ at Low Temperatures

The in-plane normal-state resistivity of Zn-doped YBa ₂ Cu ₃ O _7– and heavily underdoped pure YBCO single crystals is measured down to low temperatures under magnetic fields up to 18 T. We found that the temperature dependence of the normal-state _ab does not obey log(1/T) and tends not to diverge in the low temperature limit. The result suggests that the ground state of the normal state of YBCO is metallic.     

Mixed state odd Hall effect in YBa2Cu3O7−δ with unidirectional twins

We have studied the mixed state Hall effect in YBa₂Cu₃O_7– single crystals with unidirectional twins: _xy=f(H). An analysis of the Hall conductivity _xy in free flux flow regime reveals that _xy can be successfully described by two terms which are related to the quasiparticle excitations and the motion of free vortices respectively. We have discovered the strong twin dependence of the Hall conductivity _xy in the pinning (TAFF) regime and detected the sign reverse of the Hall conductivity _xy at changing angle (+45 °–45 °) between current and twin plane that indicated the presence of backflow of vortices. We have observed that _xy tends to - while approaching the melting line. These results provide the evidence of strong planar pinning influence on the Hall conductivity.     

Pore Structure and Transport Properties in Bulk YBa2Cu3O7?δ Doped with Sb2O3

In this work we have studied the pore structure and electrical transport properties of superconducting YBa₂Cu₃O_7?y polycrystalline samples doped by the addition of different Sb₂O₃ concentrations, i.e. resulting in (YBa₂Cu₃O_7?y )_1?x (Sb₂O₃) _x . The samples were prepared through the solid-state reaction method. Rietveld analyses of X-ray diffraction data were used to investigate how the lattice parameters are modified by doping. Specific superficial area measurements identified the principal characteristics of the pore structure of the samples and how these properties change with doping. The superconducting properties were studied by using zero field cooling magnetization and transport critical current measurements. The critical temperature of the samples does not depend on the doping level, but their transport critical current density strongly decreases as the Sb₂O₃ concentration is increased. Our experimental results suggest that for the samples studied here there is not a direct correlation between the modification by doping of both, the pore structure and the transport critical current density.     

A Comparative Study of Superconductivity and Thermally Activated Flux Flow of YBa2Cu3O7-δ and YBa2Cu3O7-δ/La1-x-yPrxCayMnO3 Bilayers

Journal of Superconductivity and Novel Magnetism - YBa2Cu3O7-δ/La1-x-yPrxCayMnO3 (YBCO/LPCMO) (x?~?0.31, y?~?0.35) bilayers with fixed ferromagnetic layer thickness...     

Cation interdiffusion between thick film of YBa2Cu3O7-δ and ceramic substrate

Thick films of superconducting oxides, YBa₂Cu₃O_7-, were successfully made by conventional screen-printing technology on Al₂O₃, MgO, and ZrO₂ substrates. Interdiffusion between the superconductive film and substrate was investigated using analytical electron microscopy. The results indicate that MgO and ZrO₂ are superior to Al₂O₃ for substrate materials.     

Oxygen mobility and charge transport in YBa2Cu3O7-δ

The out-diffusion of oxygen during heat treatment of fully oxygenated c-axis oriented epitaxial films of YBa₂Cu₃O_7- leads to large electric fields in the films. During heat treatment under oxygen pressure a transient electric resistance peak has been observed also.