Positron Lifetimes in Smectic Liquid Crystals

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The mean lifetimes of positrons in 4,4'-n-octyloxycyanobiphenyl and p-decylox-ybenzylidene p'-amino 2-methyl butyl cinnamate were measured. The measurementsof magnetic susceptibility and differential scanning calorimetry were also carried outto study the influence of molecular order and phase transition on the mean lifetime ofpositrons. The poistron lifetime spectra consist of two components. The meanlifetimes of the shortlived and the longlived components in these liquid crystals in-crease abruptly at the solid-smectic transition. It seems that the change in the short-lived component at the mesomorphic phase transitions depends on the magnitude oftransition enthalpy. The longlived component shows a maximum in the smectic Aphase. The maximum mean lifetime of the longlived component in the smectic A maybe related to the size of defects in the well-defined fluid regions.
1991-02-15