POSITRON-LIFETIME STUDIES IN THE KONDO INSULATOR FESI

Positron lifetime measurements have been carried out in the Kondo insu lator FeSi in the temperature range of 5 K to 700 K. In the high tempe rature range of 500 K to 700 K, the lifetime is seen to increase rapid ly from similar to 133+/-2 ps to similar to 141+/-2 ps, and is associa ted with positron trapping at thermally generated vacancies. At temper atures below 200 K, the positron lifetime shows an unusual decrease to 122 ps at 5 K, and interestingly it is seen to correlate with the mea sured de conductivity. With the help of positron density distribution and lifetime calculations, the observed decrease in the positron lifet ime at low temperatures is inferred to arise due to the delocalization of the Fe d electron, an electronic structure change arising as a con sequence of the Kondo singlet formation.