Motion of Charge Carriers in NormalHe4

Abstract

Values for the mobilities of positive and negative charge carriers in normal ${\mathrm{He}}^4$ at the vapor pressure are presented. The interpretation of these measurements in terms of the usual microscopic models for the charge carriers is shown to require the correct hydrodynamic treatment of a modified Stokes-flow problem. Good agreement with experiment is found for both ${\mu }_{+}$ and ${\mu }_{-}$ below 4.2 °K, and new information is obtained about several features of the carrier structures. The behavior of ${\mu }_{+}$ and ${\mu }_{-}$ above 4.2 °K suggests the possibility that the viscosity of normal helium has a peculiar temperature dependence in this range. Sudden drops in ${\mu }_{+}$ and ${\mu }_{-}$ are observed near the critical temperature, and it is argued that this kind of effect may be due to critical-point fluctuations, rather than to changes in carrier structure as has been claimed by other authors.