Positron-lifetime studies in the Kondo insulator FeSi

Abstract

Positron lifetime measurements have been carried out in the Kondo insulator FeSi in the temperature range of 5 K to 700 K. In the high temperature range of 500 K to 700 K, the lifetime is seen to increase rapidly from ~133±2 ps to ~141±2 ps, and is associated with positron trapping at thermally generated vacancies. At temperatures 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 measured dc conductivity. With the help of positron density distribution and lifetime calculations, the observed decrease in the positron lifetime at low temperatures is inferred to arise due to the delocalization of the Fe d electron, an electronic structure change arising as a consequence of the Kondo singlet formation.