Y.Z. Zhang

Deviations from plastic barriers in Bi2Sr2CaCu2O8+delta thin films

The crystal and the magnetic structures as well as magnetostriction of the Laves-phase compound NdCo2 are investigated by means of temperature-dependent high-resolution neutron-powder diffraction. The compound crystallizes in the cubic Laves phase C15 structure at high temperature, undergoes a tetragonal distortion (space group I4l/amd) around T(C)approximate to 100 K and an orthorhombic distortion (space group Fddd) at T 42 K. The temperature dependence of lattice constants, magnetostriction constant, and magnetic moment indicate that the magnetic and structural transitions are second order in the vicinity of T-C and are first-order around 42 K. Refinements of magnetic structure reveal that the Nd moment distinctly exhibits an abrupt increase at the first-order transition and the easy magnetization direction of the compound changes from [001] in the tetragonal lattice to [011] in the orthorhombic lattice, indicating a strong coupling between crystal structure and magnetic properties at zero field. Analysis of the temperature dependence of bondlength suggests a strong magnetic exchange striction in the tetragonal structure and that the abrupt increase of the Nd moment is attributed essentially to a change in crystal electric field. Field-dependent neutron diffraction reveals a decoupling of the magnetic and structural transitions under relatively modest magnetic fields.

Thermally activated energy and critical magnetic fields of SmFeAsO0.9F0.1

Thermally activated flux flow and vortex glass transition of the recently discovered SmFeAsO0.9F0.1 superconductor are studied in magnetic fields up to 9.0 T. The thermally activated energy is analyzed in two analytic methods, of which one is conventional and generally used, while the other is closer to the theoretical description. The thermally activated energy values determined from both methods are discussed and compared. In addition, several critical magnetic fields determined from resistivity measurements are presented and discussed.

Rectifying properties of the YBa2Cu3O7−δ∕SrTiO3:Nb heterojunction

A heterojunction has been fabricated by growing a YBa2Cu3O7−δ (YBCO) film of the thickness of 1000A on a 0.5wt%Nb-doped SrTiO3 (STON) crystal, and its resistive behavior was experimentally studied. The strong asymmetry of the current-voltage (I–V) relation with respect to bias polarity indicates an excellent rectifying property of the junction in the whole temperature range studied. The superconducting transition of YBCO causes a visible reduction of diffusion potential, measured by the forward voltage corresponding to the current rush in the I–V curves, though the general rectifying behavior remains unchanged. This is possibly a result of the variation of the Fermi level of YBCO relative to that of STON, and suggests an alternative technique detecting the effects of superconducting transition in YBCO.

Positron annihilation study on the stress-field pinning mechanism in (Eu, Y)-123 superconductors

Abstract The simultaneous measurements of Doppler-broadened radiation spectra and lifetime spectra have been performed on (Y 1− x Eu x )Ba 2 Cu 3 O 7− y ( x =0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1) superconductors. The mean lifetime and S -parameter, respectively show a characteristic significantly symmetric to Eu-doping concentration. Since lifetime and Doppler broadening spectroscopies are rather sensitive to the variation of the lattice distortion and defects induced by the stress-field of lattice mismatch, such symmetric behaviors of the S -parameter and the mean lifetime suggest a striking characteristic of stress in Y 1− x Eu x -123 superconductor. When the doping concentration x reaches 30%, the density of defects corresponding to stress-field appears the maximum, and there is an optimum distribution of the stress-field so as to increase critical current J c in Eu-doped Y-123 superconductors. Our results of the critical current density obtained by means of measurements of susceptibility χ ′( T , H AC ) and χ ″( T , H AC ) also suggest that the enhancement of the critical current density to reach an optimum when Eu doping x is 30%. We also qualitatively discuss experimental results in the framework of the model of stress-field pinning. The model calculating results also show the stress-field induced by lattice mismatch is an effective pinning center, and further conform that the stress pinning mechanism is dominant in RE-doped Y-123 superconductors.

Synthesis and characterization of ZnO nanoflowers grown on AIN films by solution deposition

ZnO nanoflowers are synthesized on AlN films by solution method. The synthesized nanoflowers are composed of nanorods, which are pyramidal and grow from a central point, thus forming structures that are flower-shaped as a whole. The nanoflowers have two typical morphologies: plate-like and bush-like. The XRD spectrum corresponds to the side planes of the ZnO nanorods made up of the nanoflowers. The micro-Raman spectrum of the ZnO nanoflowers exhibits the E2 (high) mode and the second order multiple-phonon mode. The photoluminescence spectrum of the ZnO nanoflowers exhibits ultraviolet emission centred at 375 nm and a broad green emission centred at 526 nm.

A structural investigation of high-quality epitaxial thin films

The (PZT 53/47) and (PZT 20/80) films deposited on substrates by RF magnetron sputtering are single-phase epitaxial films. In order to prevent large thermal stresses being induced in the films during the preparation process, the PZT films were cooled slowly through the Curie temperature. Scanning electron microscopy (SEM) showed that the PZT 53/47 film had a pyramid structure on its surface, whereas the PZT 20/80 film had a smooth surface without any observable features. Transmission electron microscopy (TEM) observations showed similar features: the PZT 53/47 film had a coarse columnar structure, whereas the PZT 20/80 film had a fine crystalline structure; this is due to the better lattice matching in the latter.

Epitaxial growth of Bi2Sr2-xLaxCuO6+δ thin films by RF-magnetron sputtering

Superconducting Bi2Sr2-xLaxCuO6+delta thin films were successfully deposited on (100) LaAlO3 and (100) SrTiO3 substrates by in situ RF-magnetron sputtering. The highest zero resistance temperature T-c0, reached 29 K. The film morphology is shown to be related with the sputtering gas pressure. The microstructures. of the films show ab-twinning. Atomic force microscopy image showed the films grow as terrace islands with no screw dislocation. For obtaining high quality La-2201 superconducting thin films, the sputtering gas pressure should be higher, and the compositions of the targets have to be modified by trial and error

Analysis of Dynamic Response for HTS MagLev Vehicle Model

The paper deals with dynamic response of a HTS Maglev vehicle model. Evaluation of dynamic characteristics is required in mechanical design of Maglev vehicle system. Based on the mechanism of levitation and actual structure of the vehicle, a simplified vibration model is presented and analyzed with random vibration theory. The transient response at each DOF is measured. The analytical results show excellent agreement with the measurement results.

Superconductivity and ferroelectricity of Pb(Zr,Ti) O3/YBa2Cu3O7 integrated films

Abstract The superconductivity, ferroelectricity and dielectric properties of Pb(Zr,Ti)O 3 .YBa 2 Cu 3 O 7 integrated films fabricated by the DC/RF magnetron sputtering method have been investigated. The integrated films are homogeneous and epitaxially grown on the substrate. In a relatively broad band, from 120 to 1o kHz, the dielectric loss at 77 K was low. The PZT film exhibited saturation and remanence polarizations of 40 and 25 μC/cm 2 , respectively and a coercive field of 98 kV/cm. The zero resistance temperature T c0 did not decrease after an overlayer PZT was deposited onto the YBCO layer. But the T c0 was degraded when an ultrathin YBCO layer was used in the heterostructure and further degraded by ion milling.

Critical current and flux pinning near Tc of a YBa2Cu3O7 thin film

The voltage V versus current I of a high‐quality YBa2Cu3O7−x thin film with zero‐resistance temperature equal to 90.8 K was measured at temperatures near Tc (85, 87, and 90 K, respectively) under different magnetic fields (0–7 T). A significant result is that the critical‐current density of the film reached 1.37×104 A/cm2 (zero field) even at 90 K, implying that strong pinning centers exist in our sample. However, a small applied magnetic field will diminish the critical‐current densities remarkably. The pinning‐force densities are found to follow Kramer’s scaling law in both perpendicular and parallel directions of the magnetic fields to the c axis of the film. A possible influence of thermally activated flux creep on the pinning mechanism is confirmed.