Positron-annihilation studies on the Bi-Sr-Ca-Cu-O superconductor

Abstract

The results of positron-lifetime measurements as a function of temperature, across T_c, and as a function of heat treatment are presented. The lifetime in Bi-Sr-Ca-Cu-O does not show any variation with temperature in the range of 10 to 300 K, a result that is in contrast with other cuprate superconductors. The absence of lifetime variation across T_c is understood in terms of the calculated positron-density distribution, which indicates that the maximum of the positron density is in the region between the Bi-O layers with no significant density in the superconducting CuO₂ layers. Positron-lifetime measurements as a function of heat treatment indicate a decrease in lifetime as the annealing temperature is lowered from 800° C to 100° C. The decrease in lifetime, which is correlated with the increase in the weight of the sample, is explained in terms of the intercalation of the excess Oxygen in the region between the Bi-O layers, which is the region probed by the positron.