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

Dhanalakshmi, S. R. ; Nagarajan, T. ; Ramasamy, S. ; Viswanath, R. N. ; Manivannan, A. ; Subba Rao, G. V. (1987) Positron lifetime studies in pure and chromium intercalated 2H–NbSe2 Crystal Research and Technology, 22 (12). pp. 1551-1555. ISSN 0232-1300

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Official URL: http://onlinelibrary.wiley.com/doi/10.1002/crat.21...

Related URL: http://dx.doi.org/10.1002/crat.2170221229


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 λf was found to increase on intercalation indicating a charge transfer from the guest intercalates to the host MCh2 conduction bands.

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