POSITRON MOBILITY IN PERYLENE

Based on Doppler-shift measurements we have determined the positron dr ift velocity u in a high-purity monoclinic alpha-perylene single cryst al as a function of applied electric field F and temperature. The elec tric field is applied as a triangular wave with a maximum field F(max) . At low fields the drift velocity displays a linear field dependence, while it assumes a sublinear field dependence above a characteristic velocity v(s) = 50 km/s and finally tends to saturate at 110 km/s, pre sumably due to optical-phonon generation above a certain threshold kin etic energy. Unlike in the case of diamond, v(s) is much greater than the longitudinal sound velocity in the solid. By fitting the observed nonlinear electric-field dependence of u to a Shockley expression for acoustic deformation potential scattering of ''warm'' charge carriers we extract the zero-field limit of the positron mobility mu0 along the crystallographic c' axis (c' parallel-to a X b). At 297 K mu0 = (136/-3+/-14)cm2 V-1 s-1, where the first error is statistical and the sec ond is an estimated +/-10% calibration uncertainty. Over the temperatu re range 100-350 K the mobility exhibits a T(n) temperature dependence with n = -1.04+/-0.03, showing a clear departure from the T-3/2 depen dence one might expect. Below 100 K mu0 still increases with decreasin g temperature, but at a given temperature its value decreases as the m aximum applied field F(max) increases, possibly indicating interferenc e caused by the presence of a field-enhanced accumulation of trapped c arriers that cause scattering at low temperatures. Below 50 K the limi t of mu0 as F(max) --> 0 does not further increase but reaches a maxim um value of almost-equal-to 1000 cm2 V-1 s-1, an upper limit which is presumably set by the presence of residual impurities. These data will be compared with positron mobility results obtained for anthracene.