A.A. Zhukov

The electric field within high-temperature superconductors: mapping the E-J-B surface

At a single temperature, the macroscopic irreversible magnetic behaviour of a superconductor can be described fully by the relationship between the electric field E, the current density J and the magnetic induction B. We show that this relationship can be readily visualized as a three-dimensional E-J-B surface. The utility of this surface is that it is model and parameter independent, and so obviates the need to define the critical current density, and the irreversibility field by arbitrarily imposed criteria. Furthermore, the flux creep rate corresponds directly to a gradient of this surface. Data from magnetic studies can be used to generate the surface, but some care with experimentation is needed to avoid distortion by artefacts, such as self-field effects and anisotropy. Transport studies map the surface too, but in a rather different (and limited) region of E-J-B space. The shape of the surface and its dependence on temperature reflect the underlying physics of vortex behaviour, and we discuss briefly the limitations on theoretical models imposed by its form.

The microwave surface impedance of MgB2 thin films

In this paper we present the results of measurements of the microwave surface impedance of a powder sample and two films of MgB2. The powder sample has a Tc = 39 K and the films have Tc = 29 K and 38 K. These samples show different temperature dependences of the field penetration depth. Over a period of six months, the film with Tc = 38 K degraded to a Tc of 35 K. We compare the results on all samples with data obtained elsewhere and discuss the implications as far as is possible at this stage.

Microwave surface resistance of MgB2

The microwave power and frequency dependence of the surface resistance of MgB2 films and powder samples were studied. Sample quality is relatively easy to identify by the breakdown in the ω2 law for poor-quality samples at all temperatures. The performance of MgB2 at 10 GHz and 21 K was compared directly with that of high-quality YBCO films. The surface resistance of MgB2 was found to be approximately three times higher at low microwave power and showed an onset of nonlinearity at microwave surface fields ten times lower than the YBCO film. It is clear that MgB2 films are not yet optimized for microwave applications.

Microstructural characterisation of high Jc, YBCO thick films grown at very high rates and high temperatures by PLD

Abstract In order to achieve scalability in processing of IBAD conductors for commercial applications, rapid YBCO film growth rates are required. In this work, scanning Raman spectroscopy, X-ray diffraction, scanning electron microscopy and, microwave surface resistance have been used to study high rate grown YBCO films. The films were grown by pulsed laser deposition on (1xa00xa00) SrTiO 3 using a high power industrial laser at growth temperatures from 750 to 870 °C and growth rates up to 4 μm/min.

Microwave surface resistance in MgB2

Measurements of the temperature dependence of the surface resistance at 3 GHz of 100 micron size fragments of MgB2 separated powder are presented and discussed. The microwave surface resistance data are compared to experimental results of Nb, Bi2Sr2CaCu2O8+δ (BSCCO) and theoretical predictions of s-wave weak coupling electron-phonon theory (BCS).

The influence of oxygen vacancies on flux pinning in (RE)Ba2Cu3O7- delta

Measurements of the irreversible magnetisation in single crystal and melt-textured (RE)Ba2Cu3O7- delta , with approximately 0.02< delta <0.2, show that at high fields, oxygen vacancies weaken the pinning, in contrast to earlier suggestions that vacancies are significant pinning centres in these materials. At low fields, the effects are small; the difference between the two field regimes may be related to the differences in flux line geometry.

Geometrical locking of the irreversible moment to the normal of a thin plate superconductor

Abstract We solve the Bean critical state model for the screening current distribution in an infinite superconducting slab in tilted magnetic field H , and generalise it in several respects. As the tilt angle increases, the irreversible magnetic moment m remains directed close to the plate normal n until H is almost perpendicular to n , then m rotates very rapidly. Vector magnetic moment measurements of YBa 2 Cu 3 O y single crystals and on samples of conventional superconductors, confirm the model calculations.

Vector magnetization studies of high purity YBa2Cu3Oy single crystal near the ab-plane

Abstract Using VSM with two perpendicular sense coils, we studied the vector magnetic moment of a high purity YBa 2 Cu 3 O y single crystal near the ab-plane. The results support the vortex lock-in to the CuO planes.

Anomalous behaviour of vortex creep in twinned YBCO crystals

Abstract We present magnetic flux creep measurements on a twinned YBCO crystal taken using a Vibrating Sample Magnetometer, in which the sample orientation may be varied with respect to the applied field. The measured flux creep rate is sometimes of negative sign, opposite to that which is usually observed. The data are discussed in terms of the inferred electrodynamics of the sample.

Effect of columnar defects on the vortex dynamics in a twinned YBCO 123 crystal

Abstract Vortex dynamics is investigated in a high quality twinned YBCO crystal irradiated to create columnar defects along the crystallographic c-axis with a 0.5T matching field. The dynamic creep rate Q has been studied as a function of field up to 8T between 10K and 91K. Due to competing correlated disorder from the twins and columns and also underlying point defect pinning, the dynamic behaviour is complicated at low fields. At fields above 2T, the dynamics resembles that of unirradiated twinned YBCO crystals at all temperatures.