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 H1, H2 and H3 where H1 is in the CuO chains and H2 and H3 are in the plane occupied by Y. To study the two positron annihilation lifetimes τ1 and τ2, the electronic densities in special "rooms" the unit cell of YBCO, which are relative to H1, H2 and H3 were calculated. The results show that the process of τ1 occurs in the "room" where barium is located, and that of τ2 is decided by the hydrogen occupied sites H2 and H3. The variation of τ2 following hydrogenation is considered to be a result of hydrogen occupying H2 and then diffusing to H1.