Study of Superfluidity in Liquid He by Ion Motion

Abstract

Ions produced in liquid helium by ionization with $\alpha$ particles have been exploited as microscopic probe particles to study the properties of the superfluid. A time-of-flight method was used to measure directly the drift velocity $u$ of the ions in the liquid for various values of the applied electric field $\mathcal{E}$ and at temperatures $T$ down to 0.5°K. The field independent mobility ${\mu }^{\mathrm{}\left(0\right)\mathrm{}}=\frac{u}{\mathcal{E}}$ obtained in the limit of sufficiently small fields increases very rapidly below the $\lambda$-point. Its temperature dependence over most of the temperature range below 2°K is of the form ${\mu }^{\mathrm{}\left(0\right)\mathrm{}}=\alpha \exp \left(\frac{{\Delta }^{\prime }}{\mathrm{kT}}\right)$ and can be explained by the scattering of the ions from the collective excitations (rotons) present in the fluid. The nonlinear dependence of $u$ on $\mathcal{E}$ at higher field strengths was also investigated and suggests the possibility of ions creating excitations in the quantum fluid as an important inelastic scattering process at sufficiently large fields. Some additional experiments are suggested.