Temperature dependence of oxygen content and phase transition in Y1Ba2Cu3O9−x compounds

Systematic TGA (Thermogravimetric analysis), magnetic susceptibility measurements and X-ray diffraction investigations have been made on the temperature dependence of oxygen content and phase transition in Y₁Ba₂Cu₃O_9–x compounds. The experimental results show that reversible oxygen relief and absorption appeared during heating and cooling in the temperature range of 400–950°C. A low cooling rate leads to higher oxygen content and quenching can suppress oxygen absorption. The deoxidation process is accompanied by a change of the cell constants,a andb increase whilec decreases with reduced oxygen content while the phase transition from the ortorhombic to the tetragonal system appeares. Nevertheless, this transition is a gradually changing process, no critical point of transition exists. Quenching stress and defects have a pronounced influence on the absorption and relief of oxygen. Maximum rates of oxygen relief and absorption were obtained at about 650°C. Oxygen absorption and desorption can be obtained only in an athermal process. Magnetic susceptibility measurements show that diamagnetic and paramagnetic suceptibilities increase obviously with increasing oxygen content. Deoxidation and oxygen absorption can be restrained by adding Fe to the Y₁Ba₂Cu₃O_9–x phase, which suggests that deoxidation and oxygen absorption are very sensitive to crystal structure and lattice distortions.     

Angular dependences of the peak effect in Yba2Cu3Ox single crystals

Angular dependences of the magnetization hysteresis loops have been studied at T=77 K on YBCO single crystals exhibiting the peak effect. The peak effect is shown to be related to the pinning of longitudinal vortices along the c axis at twin boundary-type ordered defects. The behavior of the peak effect at intermediate angles is explained by anisotropic magnetic field penetration into quasi-two-dimensional superconductors. In thin crystals with a dilute ordered-defect structure this can result in an enhanced peak effect due to formation of a vortex kink structure and to “internal” pinning of transverse vortex segments at Cu-O sheets. Fiz. Tverd. Tela (St. Petersburg) 39, 425–431 (March 1997)     

Kinetics of crystallization in Y Ba2Cu2.98Fe0.02Ox

The effect on the phase composition and range limits of existence of intermediate and superconducting phases in Y−Ba−Cu−O-system doped with 0.7 at.% of⁵⁷Fe are investigated and the obtained results are reported.     

Dynamics of fluxons in Y1−Ba2−Cu3−O7 single crystals

Summary Evidence is given that in Y₁−Ba₂−Cu₃−O₇ single crystals a critical state is developed on increasing (or decreasing) the applied fieldH ₀, only whenH ₀ is parallel to the crystalc-axis. An ?irreversibility? line in the (H, T)-plane can be drawn where the irreversibility of the second-harmonic response sets in. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Superconductivity in a new NdBa2Ca3Sr4Cu5Ox system

A new compound, NdBa₂Ca₃Sr₄Cu₅O_x, has been synthesised using the conventional solid state reaction technique. The material was characterised by powder XRD, electrical resistivity, ac susceptibility and dc magnetisation studies. The results of powder X-ray diffraction show that the structure is pseudo orthorhombic, having unit cell dimensions a = 5.47 Å, b = 5.46 Å and c = 14.58 Å. Magnetisation studies on a SQUID magnetometer showed a superconducting transition at 52 K. This was confirmed by the measurements of electrical resistance and ac susceptibility of this sample, which also showed superconductivity at 52 K. Details of preparation and characterisation are discussed.     

Kinetics of oxygen desorption in Y Ba2Cu3O7-δ

In this paper the kinetics of oxygen desorption from Y Ba₂Cu₃O_7-δ has been investigated and measurements carried out in the temperature range 350–750 C. It is shown that the oxygen desorption is diffusion-controlled (and not surface-controlled), and characteristic of diffusion from small grains in a loosely coupled porous matrix. At every temperature a number of time constants with well-defined ratios appear and have been identified and measured. These time constants can be related to the average bulk diffusion constant which shows an Arrhenius behaviour with deviations around 750°C which are ascribed to the well-known orthorhombic to tetragonal transition. In this paper we establish the basic underlying physical mechanism of oxygen desorption in Y Ba₂Cu₃O_7-δ.     

Anisotropy of the superconducting properties of YBa2Cu3O7−x single crystals with reduced oxygen content

In an investigation of the resistivity anisotropy of YBa₂Cu₃O_7−x single crystals with suboptimal oxygen content it is observed that the superconducting transition for the component ρc of the resistivity tensor is shifted to lower temperatures with respect to the transition for the component ρab. A similar shift is also observed for the transition in the temperature dependence of the dynamic magnetic susceptibility. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 309–313 (25 August 1998)     

Polarized Raman scattering in single crystals of Nd0.7Sr0.3MnO3

We report polarized Raman scattering in single crystals of Nd_0.7Sr_0.3MnO₃. The temperature dependence of the MnO₆ octahedral bending and stretching modes observed in the XX spectra points to the existence of local lattice distortions, possibly polarons. The XY spectra have been analyzed using a collision-dominated model, which allows the extraction of the carrier scattering rate.     

Band structure of YBa2Cu3Ox in relation with the oxygen vacancy distribution

The electronic band structure and the density of states of the YBa₂Cu₃O_x superconductor are computed for x = 7 using the extended Hückel scheme. Two ordered arrangements of the oxygen vacancies in the basal Cu-O planes have been considered. It is shown that the oxygen vacancy distribution plays a major role in the electronic structure of this compound and deeply influences the density of states at the Fermi level.     

Identification of surface oxygen vacancy-related phonon–plasmon coupling in TiO_2 single crystal

Oxygen vacancies(OVs) play a critical role in the physical properties and applications of titanium dioxide nanostructures, which are widely used in electrochemistry and photo catalysis nowadays. In this work, OVs were artificially introduced in the surface of a pure TiO_2 single crystal by pulsed laser irradiation. Raman spectra showed that the intensity of E_g mode was enhanced. Theoretical calculations disclose that this was caused by the strong coupling effect between the phonon vibration and plasmon induced by the OVs-related surface deformation, and good agreement was achieved between the experiments and theory.     

Remanent fields and critical currents in Tl2 Ba2Ca2Cu3Ox ceramic

The temperature and magnetic-field dependences of the remanent magnetization have been determined from far-field measurements for thallium ceramic samples. The current flow that gives rise to the observed magnetization is intragranular. The results are consistent with flux-creep-controlled transport with a distance between pinning centers of - 1/2/m. Using near-field measurements, the superconducting properties are found to be heterogeneous on a scale of 1 mm. Magnetic domains have been induced by pplying spatial variation in the applied field.