The positron lifetime at various temperatures was measured, as a function of implantation energy, in epitaxial superconducting thin films. The coexistence of both shallow and deep positron trapping centers was detected. The shallow trapping centers included screw dislocations and twin boundaries. The binding energy of the shallow trapping centers was estimated to be 0.056eV. The deep trapping centers were assumed to be cation vacancies, especially Ba vacancies. On the surface of the sample, there were macroscopic free-volume holes in which positronium could form.

X.Y.Zhou, J.Störmer, R.L.Wang, J.Keimel, H.C.Li, G.Kögel, W.Triftshäuser: Physical Review B, 1996, 54[2], 1398-403