The mobility of ions trapped on vortex lines in pure⁴He and³He-⁴He solutions

We present measurements of the mobility of positive and negative ions trapped on vortex lines in pure⁴He and dilute³He-⁴He solutions over the temperature range 1.6>T>0.3 K. In pure⁴He below about 0.7 K, we observe several new effects not seen at higher temperatures and not easily explained with existing theories. Most notable are an enhanced broadening of the ion pulse and a rapid increase in the mobility with decreasing temperature. Measurements of the electric field dependence of the drift velocity in pure⁴He at low temperatures show a limiting velocity for sufficiently large fields. This behavior can be explained using a simple resonance theory. The inverse mobility data for solutions show sharp increases at certain critical temperatures, which are interpreted as being associated with the condensation of³He atoms onto the vortex cores. The dependence of the critical temperature on the bulk³He concentration is found to be in good agreement with a simple condensation theory. An extension of arguments used in this theory to lower temperatures leads to the picture of a³He-rich core growing with decreasing temperature, consistent with our lower temperature experimental data.