W G Zeng

Anelastic relaxation of oxygen vacancies and high-Tc superconductivity of YBa2Cu3O7-δ

It is suggested that the variation of the internal friction (IF) characteristics of a single-phase YBCO specimen resulting from an increase of oxygen deficiency δ is associated either with an order diffusion process between layers, or with an order/disorder diffusion process in the Cu(2) plane. The experimental results are explained in the light of these oxygen-vacancy migration processes. The essential connections between high-Tc superconductivity, electric behaviour and oxygen-vacancy ordering are discussed.

A study of internal friction phenomena at 300–420K in the Bi-based high Tc superconductor system

Abstract The self-induced ordering process and the superconductivity property of BiSrCaCu 2 Ox and Pb 0.34 Bi 1.72 Sr 2 Ca 1.2 Cu 2.5 Fe 0.22 Oy have been investigated based on internal friction (IF) experiment, x-ray diffraction and thermoanalysis. It is found that the IF peak at 300–420K is associated to the superconductivity property. We discuss the physical consequences arising from migration of oxygen atoms.

Internal friction study of high Tc BSCCO ceramic superconductor within the temperature range 88–623 K

Abstract Using internal friction and shear modulus measurements, we studied certain characteristics of relaxation and phase-like transitions which could accompany conductor-superconductor transition of BiSrCaCu2Oy when samples were sintered at different temperatures and went through thermocycles.

Very low-frequency anelastic study of YBa2Cu3O7-δ in thermal cycling

Internal friction (IF) measurement techniques are used to study high-Tc superconducting cuprates. IF peaks associated with oxygen deficiency are located over a temperature range from room temperature to 623 K at frequencies from 10-2 to 5 Hz. IF characteristics, i.e. activation energies, relaxation strength and ordering critical temperature, are investigated in relation to the orthorhombic-to-tetragonal transition, superconducting properties and electrical behaviour with progressively increasing oxygen deficiency, achieved through appropriate thermal cycling. Relationships between these properties are discussed and it is suggested that a fundamental connection exists between the migration of oxygen vacancies and the onset of superconductivity.

Studies of domain wall motion. I. Internal friction under magnetization

The internal friction (IF) measurement technique is used to study the magnetization of pure iron, nickel and Permalloy-42. The IF of samples undergoing a uniform magnetic field sweep is measured with a torsion pendulum. An IF peak is observed in the field range 0-10 Oe. The field sweeping increases the IF and hence the characteristic quantity is the difference between the IF measured in field sweeping and that in a static field, which is called dynamic internal friction. The dynamic IF shows a viscoelastic feature; it increases with increasing H/ omega where H is the field sweep rate and omega the angular frequency of the alternating strain. The behaviour of the dynamic IF shows that it is caused by the domain wall motion in field sweeping and thus is related to a magnetomechanical interaction.

Sensitivity of the phonon in YBCO to superconductivity by infrared studies on epitaxial film

Abstract Epitaxial films of YBCO superconductors whose C axes are primarily perpendicular to the film surface, were prepared. The samples, which were treated by means of a gettered annealing technique, were investigated by the X-ray diffraction pattern method (XRD) and infrared (IR) reflectance spectra in the 140–800 cm−1 spectral range. The strengths of the phonon structures are influenced greatly by the superconductivity of the sample, which can be determined by the annealing conditions. The relation between the resulting IR activity of phonons and the superconductivity is clear and the important role of phonons in superconductivity is demonstrated.

Studies of domain wall motion. II. Models of the dynamic internal friction

For pt.I see ibid., vol.2, p.9531 (1990). An interaction model of the motion equation of a domain wall (DW) is used to calculate the DW velocity and hence the internal friction (IF). A magnetomechanical interaction is proposed to explain the dynamic IF and the unique behaviour that the dynamic IF increases with H/ omega . The model shows that the dynamic IF is not caused by relaxation of DWs and is related to the average velocity of moving DWs. Furthermore, a phenomenological model is proposed and a two-term parametrization expression is used to fit the experimental data of the dynamic IF of Fe, and Ni and Permalloy satisfactorily. It is based on the assumption that the superposition of all contributions of magnetic objects (MOS) (a MO is a single equivalent object in terms of the dynamic properties due to correlation between a group of DWs) gives the field dependence of the dynamic IF. Preliminary discussion of the physical meaning of the parameters is given.

The charge transfer investigation: X-ray photoelectron spectroscopy (XPS) on superconducting Tl2Ca2Ba2Cu3O10 thin film

Abstract The single-phase superconducting thin film Tl 2 Ca 2 Ba 2 Cu 3 O 10 were prepared by dc-magnetron sputtering process on YSZ substrate and its X-ray photoelectron spectra at room and liquid nitrogen temperature were measured, respectively. From the relative intensities of the Ba3d δ/2 , Tl4f, Ca2p, O1s and Cu2p spectra taken at different take-off angles, we have tentatively assigned which core-shifts should be responsible for the superconducting phase transition and observed that there exist charge transfer between Ba, Ca and O atoms during the superconducting phase transition after ascertaining the spurious phase on the surface of the film. This picture might be helpful for our understanding the mechanism of superconductivity.

Infrared reflectance spectra and optical conductivity of Tl2CaBa2Cu2O8 and Tl2Ca2Ba2Cu3O10 superconducting thin film samples

Abstract Films of the Tl2CaBa2Cu2O8 (Tl2122 phase) and Tl2Ca2Ba2Cu3O10 (Tl2223 phase) superconductors were prepared by d.c. magnetron sputtering method. Their C-axes were perpendicular to the surface. We measured their infrared (IR) reflectivity spectra and assignments of the phonon oscillators were made. The optical conductivity of Tl2122 and Tl2223 phases in the normal state have been obtained from measurements of the reflectivity over a wide frequency range. The position of the plasma peak is tentatively determined.

A NEW METHOD FOR QUANTITATIVE STUDY OF FLUX PINNING IN TYPE II SUPERCONDUCTORS

The motion equation with an interaction force between flux and applied H is established and resolved for the moving flux in the process of magnetization of type II superconductors. The expressions of energy dissipation and the dynamic pinning force exerted on the moving flux can therefore be acquired when the low frequency internal friction, modulus and magnetization curve are measured simultaneously in the process of magnetization with a variety of applied magnetization rate H.