Positron lifetime studies in pure and chromium intercalated 2H–NbSe₂

Abstract

The layered 2H–NbSe₂ belongs to a class of very interesting low dimensional conductors which exhibits CDW instability at 32 K. In our earlier paper we have established the two dimensionality of the Fermi surface, in these layered compounds by positron angular correlation technique. The reduction in Fermi level along the basal plane due to CDW transition has been already demonstrated by the Japanese group. But the sensitivity of positron lifetime and Doppler broadening techniques for CDW transition is controversial. The Japanese group report drastic changes in Doppler broadening parameters whereas the American group do not find such a change. Positron lifetime measurements have been performed on pure and Cr intercalated 2H–NbSe₂ with Cr concentration 0.25, 0.33, and 0.5 with a view to assess the origin of the second lifetime component. The results show that there are two lifetime component τ₁~205 ps and τ₂~440 ps having intensity I₂~29%. On Chromium intercalation the intensity I₂ decreases systematically. The trapping rate of positrons shows a systematic decrease with increase of Cr content indicating that the second lifetime arises due to positron annihilation in van der Waal's gap. The defect free annihilation rate λ_f was found to increase on intercalation indicating a charge transfer from the guest intercalates to the host MCh₂ conduction bands.