Influence of crystal defects on the irreversibility line of YBa2Cu3O7-x single crystals

High quality flux grown and defect enriched peritectically grown YBa₂Cu₃O_7–x single crystals have been investigated by an ac-susceptibility technique. This method allows to determine an irreversibility line from the temperature and field dependence of the peaked imaginary part of the susceptibility, which is due to magnetic losses. For magnetic fieldsH _ac perpendicular to thec-axis of the crystal, the irreversibility line of the defect enriched crystal shows a shift to higher field values as compared to the perfect crystal, a sign that crystal defects like Y₂BaCuO₅(211)-precipitates and microcracks act as strong pinning centers. ForH _ac parallel to thec-axis no clear evidence for a stronger pinning is found. From these results we conclude that different pinning mechanisms are dominating at different field orientations.     

Microwave magnetic-field effects in YBa2Cu3O7 single crystals near T c

Il Nuovo Cimento D - The non-linear response near T c and the complex electromagnetic conductivity of YBa2Cu3O7 single crystals at microwave frequencies are explained in the framework of the...     

Effects of excimer-laser irradiation of LaAlO3(100) single crystals: Influence on superconducting YBa2Cu3O7-x film growth

3 (100) single crystal substrates have been investigated. Poorly absorbed KrF irradiation leads to localized, deep, and unstable damage on the crystal surfaces. By contrast ArF has been found to induce well-localized roughness and microcracks on LaAlO₃(100) surfaces at fluences between 0.2 and 1.8 J/cm². The material emission threshold was estimated at around 0.5 J/cm² and cracks appeared above 1.4 J/cm². The substrate surface state determines YBa₂Cu₃O_7-x film growth, leading to morphological changes that have been related to a decrease in adatom mobility on the substrate. The most striking feature is the drastic reduction in YBa₂Cu₃O_7-x outgrowth density inside the irradiated areas. Irradiation has little effect on film crystalline properties. The main effect on the electrical properties can be restricted to a decrease of critical current density up to a factor of 10⁴, but such an important decrease is only observed in the most intensively irradiated substrates. Received: 18 November 1996/Accepted: 12 June 1997     

Temperature effects on the phonon spectrum in YBa2Cu3O7 single crystals and thin films

We have performed detailed investigations on the temperature dependence of the 335 cm^–1 phonon in single crystals and thin films of the YBa₂Cu₃O₇ superconductor. The frequency of this phonon exhibits a downshift of about 5 cm^–1 on passing the superconducting transition from above. The shift of the phonon in thin epitaxial films on MgO or SrTiO₃ substrates is only about 2.5 cm^–1. The width of the asymmetric phonon line displays a slight increase belowT _c due to the electron-phonon interaction in these systems.