Positron mobility in perylene

Based on Doppler-shift measurements we have determined the positron drift velocity [ital u] in a high-purity monoclinic [alpha]-perylene single crystal as a function of applied electric field [ital F] and temperature. The electric field is applied as a triangular wave with a maximum field [ital F][sub max]. At low fields the drift velocity displays a linear field dependence, while it assumes a sublinear field dependence above a characteristic velocity [ital v][sub [ital s]]=50 km/s and finally tends to saturate at 110 km/s, presumably due to optical-phonon generation above a certain threshold kinetic energy. Unlike in the case of diamond, [ital v][sub [ital s]] is much greater than the longitudinal sound velocity in the solid. By fitting the observed nonlinear electric-field dependence of [ital u] to a Shockley expression for acoustic deformation potential scattering of warm'' charge carriers we extract the zero-field limit of the positron mobility [mu][sub 0] along the crystallographic [ital c][prime] axis ([ital c][prime][parallel][ital a][times][ital b]). At 297 K [mu][sub 0]=(136[plus minus]3[plus minus]14) cm[sup 2] V[sup [minus]1] s[sup [minus]1], where the first error is statistical and the second is an estimated [plus minus]10% calibration uncertainty. Over the temperature range 100--350 K the mobility exhibits a [ital T][sup [italmore » n]] temperature dependence with [ital n]=[minus]1.04[plus minus]0.03, showing a clear departure from the [ital T][sup [minus]3/2] dependence one might expect. Below 100 K [mu][sub 0] still increases with decreasing temperature, but at a given temperature its value decreases as the maximum applied field [ital F][sub max] increases, possibly indicating interference caused by the presence of a field-enhanced accumulation of trapped carriers that cause scattering at low temperatures.« less