Temperature dependence of positron diffusion in metals

Data are presented for the positron diffusion coefficient, $D_{+}$, in single crystals of Mo, Nb, Al, and Cd. The data clearly demonstrate that the temperature dependence of $D_{+}$ is much stronger than the $T^{\mathrm{-}0.5}$ that is theoretically predicted, ranging from about $T^{\mathrm{-}0.8}$ to a maximum of $T^{\mathrm{-}2.6}$. In addition, our results shown an anisotropy for the temperature dependences of $D_{+}$ in two principal directions of Cd ($T^{\mathrm{-}1.0}$ along an a axis versus $T^{\mathrm{-}2.6}$ along the c axis) which cannot be described by a simple diffusion model.