Positron annihilation in hydrogenated YBCO superconductor

The behaviour of hydrogenated YBa2Cu3Oy(YBCO) has been investigated by sputtering neutral-particle mass spectrometry (SNMS) and positron annihilation lifetime spectrum analysis. SNMS analysis shows that hydrogen in YBCO is in the covalent form. In order to interpret experimental results, a hydrogenation model is suggested in which hydrogen occupies sites H-1, H-2 and H-3 where H-1 is in the CuO chains and H-2 and H-3 are in the plane occupied by Y. To study the two positron annihilation lifetimes tau(1) and tau(2), the electronic densities in special "rooms" the unit cell of YBCO, which are relative to H-1, H-2 and H-3 were calculated. The results show that the process of tau(1) occurs in the "room" where barium is located, and that of tau(2) is decided by the hydrogen occupied sites H-2 and H-3 The variation of tau(2) following hydrogenation is considered to be a result of hydrogen occupying H-2 and then diffusing to H-1. (C) 1998 Kluwer Academic Publishers.