Uwe Bolz

Oxygen diffusion in YBa2Cu3O7−δ films with different microstructures

Abstract We have investigated the oxygen out- and in-diffusion in epitaxial c-axis oriented YBa2Cu3O7−δ films from isothermal four-point electric resistivity measurements. The films were grown by two different deposition methods: off-axis pulsed laser deposition (PLD) and hollow-cathode magnetron sputtering (HCMS). Chemical diffusion relaxation times have been determined for the out- and in-diffusion of oxygen at temperatures from 500 K up to 800 K. Both types of films yielded the same activation energy of about EA=1.1 eV for the diffusion processes. However, at the same temperature, differences in the diffusion relaxation times were observed. This difference results from the different morphologies and microstructures we have studied with scanning electron microscopy (SEM). Additionally, these results are compared to sputtered YBa2Cu3O7−δ films passivated by amorphous Si–C–H–O–N overlay films. The activation energy in these films was about 1 eV, and the diffusion relaxation times were much larger than in the samples without the overlay.

Dendritic and homogeneous regimes of flux penetration into YBCO films

Dendritic flux patterns in superconducting YBCO films are studied on a nanosecond time-scale. It is found that dendrites only develop for certain values of the external field and temperature.

Transient local resistivity maximum during temperature-dependent oxygen diffusion in YBa2Cu3O7-δ thin films

Abstract We have investigated the oxygen diffusion behavior by means of electric resistivity measurements on epitaxial, c -axis-oriented YBa 2 Cu 3 O 7− δ thin films in the temperature regime up to 800 K. A pronounced transient local resistivity maximum (TRM) was observed at about T =600 K during the first post-deposition heat treatment of the films in 900 mbar pure oxygen atmosphere. In the subsequent heating procedures this distinct maximum was not observed anymore. This temperature- and time-dependent effect appears to be associated with the dissociation of molecular oxygen on the sample surface representing an important step in the in-diffusion of oxygen. Changes in the surface microstructure and roughness during the first post-deposition heat treatment in molecular oxygen atmosphere appear to strongly facilitate the oxygen dissociation and to produce proper catalytic centers for this process. This model is supported by detailed experimental TRM studies investigating the influence of the following parameters: partial pressure of the molecular oxygen, film thickness, microstructure of off-axis pulsed laser deposited and hollow-cathode magnetron sputtered YBa 2 Cu 3 O 7− δ films, surface roughness, and passivation by means of a Si–C–H–O–N overlay film.

Ultrafast Magneto-Optical Study of Flux Avalanches in High-Tc Superconductors

An ultrafast magneto-optic pump-probe technique has been used to trigger and image a flux instability in high-temperature superconducting thin films. Snapshots of the dendritic flux avalanche spreading into the film could be obtained with a time resolution in the picosecond range.

MAGNETO-OPTIC INVESTIGATION OF MAGNETIC FLUX PENETRATION ON A NANOSECOND TIMESCALE

We have studied the dynamics of iron-garnet films using a pump-probe technique and were able to demonstrate that the response time of such a film is sufficiently short to investigate sub-nanosecond phenomena. Furthermore, we studied the penetration velocity of the dendritic instability in thin YBCO films. We found two regimes: An early stage with a penetration velocity of up to 17Okmls and a distinct later stage were the velocity is lower (= 18 km/s).

Investigations of the vortex matter in Bi2Sr2Ca1Cu2O8+δ single crystals

We report transverse-field muon spin rotation studies of the magnetic field distribution n(B) in the vortex state of the high temperature superconductor Bi2Sr2Ca1Cu2O8+δ (Bi-2212) and present data on three sets of overdoped, nearly optimized and underdoped single crystals which to our best knowledge provide the first evidence that a two-stage melting transition of the vortex matter occurs under equilibrium conditions. In addition, we compare the TF-μSR results with those from DC-magnetization measurements and magneto-optic experiments on the same crystals.