We report on three different and complementary measurements, namely magnetisation measurements positron annihilation spectroscopy and NMR measurements, which give evidence that the formation of oxygen vacancy clusters is on the origin of the fishtail anomaly in YBa 2 Cu 3 O 7−δ . While in the case of YBa 2 Cu 3 O 7.0 the anomaly is intrinsically absent, it can be suppressed in the optimally doped state where vacancies are present. We therefore conclude that the single vacancies or point defects can not be responsible for this anomaly but that clusters of oxygen vacancies are on its origin.
Chemometric analysis employing reduced physicochemical parameters can provide an accurate prediction of chemical instability in peptides and proteins. The relative importance of these various factors could also be determined.
We have carried out 2D angular correlation of positron annihilation radiation measurements on oriented single crystals of the heavy-fermion superconductor CeCu 2 Si 2. Measurements have been made at 50 and 4 K which are above and below the Kondo temperature, T K ϳ15 K, for this compound. The measurements show a small temperature dependence. Experimental results have been compared with linear muffin-tin orbital bandstructure calculations for CeCu 2 Si 2 and the isostructural non-f-electron compound, LaCu 2 Si 2. The measured anisotropies agree well with the calculations. In addition, at both temperatures, the measured ͓001͔ projected k-space occupation densities reflect clearly the Fermi-surface breaks predicted by the calculations for CeCu 2 Si 2. This implies that local-density approximation calculations provide a good description of the electronic structure in CeCu 2 Si 2. We have also measured the positron lifetime in both superconducting (S-type͒ and magnetic (A-type͒ samples of CeCu 2 Si 2. The lifetime and intensity components in the Sand A-type polycrystals are different, suggesting that the nature of the defects that trap positrons is different in the two phases. ͓S0163-1829͑97͒16017-9͔
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