K.-H. Müller

AC susceptibility of high temperature superconductors in a critical state model

Abstract A critical state model for a granular superconductor is employed to calculate the temperature and AC and DC magnetic field dependence of the complex susceptibility, χ = χ ′ + iχ ″, of a sintered bulk YBa 2 Cu 3 O 7- δ superconductor. Inter granular Josephson vortices are assumed to sweep in and out of the weak-link network while intragranular Abrikosov vortices move in and out of the superconducting grains, both causing bulk pinning hysteresis losses. The predictions of the model for χ′ and χ″ are consistent with experimental data and model parameters which characterize a high temperature granular superconductor can be determined. These parameters are the inter- and intragranular pinning force densities, the fraction of the superconducting grains, the grain size distribution and a London penetration depth which neglects grain anisotropy.

Josephson vortices and flux penetration in high temperature superconductors

Abstract We have investigated, experimentally and theoretically, the magnetic flux which threads a YBa 2 Cu 3 O 7− x cylinder sample at 77 K when a weak low frequency ac magnetic field is applied. We explain the highly nonlinear magnetic response in great detail quantitatively, using a critical state model where during each ac cycle, intergranular (Josephson) vortices sweep in and out of the cylinder. The Josephson vortex pinning force is found to be field independent and several orders of magnitude smaller than in conventional type II superconductors. This explains the weak shielding properties and low critical current density found in this material. At magnetic fields greater than about 50 Oe, vortices start to enter the superconducting grains.

Frequency dependence of AC susceptibility in high-temperature superconductors: Flux creep and critical state at grain boundaries

Abstract The loss peak of the AC susceptibility in polycrystalline high- T c superconductors shifts slightly to higher temperatures with increasing frequency of the applied AC magnetic field. It is shown that a flux creep term, added to the current density term in the critical state equation, can account for the observed frequency dependence. The magnitude of the peak shift is predicted to increase with decreasing average grain size and decreasing grain boundary junction current density. The model predictions are compared with the experimental data of Nikolo et al. Some of the parameters used in the calculation are determined by fitting data for χ′ and χ″ over the full temperature range using a recently developed model for granular superconductors. In addition, the relation between the intergranular pinning potential and the activation energy, which is extracted from log-frequency versus inverse χ″-peak temperature data, is clarified.

Role of incident kinetic energy of adatoms in thin film growth

Abstract Film growth as a function of incident kinetic energy of adatoms is studied by employing two-dimensional molecular dynamics simulations for Lennard-Jones particles. At thermal incident kinetic energies films exhibit a high void and defect content when grown on a zero temperature substrate. Films grown at adatom incident kinetic energies typical for sputter deposition show higher average density and improved epitaxy due to the larger impact mobility of condensing atoms.

Nonlinear magnetic flux response in high temperature superconductors

Abstract We have spectrum analysed the voltage induced in a pick-up coil wound around a cylindrical bulk sample of YBa 2 Cu 3 O 7−δ when driven by a weak low frequency AC magnetic field, superimposed on a weak DC magnetic field, at 77 K. The harmonics generated by the highly nonlinear response are explained quantitatively in great detail by employing a critical state model with a field independent pinning force density, where intergranular vortices sweep in and out of the Josephson weak link regions during each AC cycle. The symmetry relation for the local flux density, which relates the first and second half of an AC cycle, is broken in the presence of a DC magnetic field, causing even harmonics to appear. This suggests high-sensitivity applications in magnetometry.

Very strong intrinsic flux pinning and vortex avalanches in (Ba,K)Fe2As2 superconducting single crystals

We report that the (Ba,K)Fe2As2 crystal with Tc =3 2 K shows a pinning potential, U0, as high as 104 K, with U0 showing very little field dependence. The (Ba,K)Fe2As2 single crystals become isotropic at low temperatures and high magnetic fields, resulting in a very rigid vortex lattice, even in fields very close to Hc2. The isotropic rigid vortices observed in the two-dimensional (2D) (Ba,K)Fe2As2 distinguish this compound from 2D high-Tc cuprate superconductors with 2D vortices. The vortex avalanches were also observed at low temperatures in the (Ba,K)Fe2As2 crystal. It is proposed that it is the K substitution that induces both almost isotropic superconductivity and the very strong intrinsic pinning in the (Ba,K)Fe2As2 crystal.

Self-field hysteresis loss in periodically arranged superconducting strips

The magnetic field and current distributions inside superconducting strips arranged in a z-stack and an x-array are calculated analytically using the transformation method proposed by Mawatari for the case where a transport current is passed through the strips. The magnetic field distributions are used to calculate the self-field ac hysteresis losses of both the z-stack and the x-array. The self-field ac hysteresis loss of the z-stack is always larger than the loss of the x-array. q 1997 Elsevier Science B.V.

A model for the hysteretic critical current density in polycrystalline high-temperature superconductors

Abstract The transport critical current density, J c , of polycrystalline high-temperature superconductors has been found to be strongly hysteretic in an applied magnetic field. In order to explain the experimental data, a simple theoretical model is employed which is based on the demagnetization effect of grains in a granular superconductor. The demagnetization causes the magnetic field in the grain boundary Josephson junctions to become dependent on the grain magnetization which is strongly hysteretic resulting in an irreversible behaviour of J c . The model explains quantitatively the observed shift of the J c peak to positive fields in a decreasing applied field for both zero field cooling and field as well as the shift saturation for large maximal applied fields. The model further relates the observed broadening and height reduction of the J c peak to variations of the grain demagnetizing factor and the grain size in the sample.

Microstructure analysis of thin films deposited by reactive evaporation and by reactive ion plating

Electron microscopy, stylus‐type surface profilometry, and Raman microprobe characterization show distinct differences between thin films deposited by reactive evaporation and by reactive ion plating. Reactive evaporation yields thin films with the well‐known columnar microstructure with appreciable surface roughness and other deficiencies. Low‐voltage, high‐current reactive ion plating deposition produces thin films which are smooth and dense. Cross‐section electron micrographs of ion plated coatings reveal a densely packed polycrystalline structure for ZrO2 , while TiO2 appears to form vitreous films. Molecular dynamics computer simulation of the film formation process is in good qualitative agreement with the experiments. The results suggest the expansion of the Movchan–Demchishin structure zone model with an additional zone 4 for the vitreous phase, resulting from superthermal film formation conditions (thermal spiking).

Intergranular AC loss in high-temperature superconductors

Abstract The AC loss and the real part of the AC permeability at low frequency and low magnetic field were measured for a sintered polycrystalline YBaCuO sample. The dependence on the AC amplitude and a superimposed DC magnetic field is explained in terms of a critical state model which accounts for the intergranular and intragranular effects. The dependence of the intergranular pinning force density on the local magnetic field is derived from the pinning potential of a Josephson vortex in a granular superconductor. The large AC losses are mainly due to bulk pinning while losses caused by surface barriers to flux entry and exit as well as annihilation losses are negligible.