Observation of Mass Flux through solid $^4$He

We have developed a novel apparatus and technique that allows us to maintain an interface between superfluid helium and hcp solid $^4$He at pressures greater than $\approx$ 25 bar, the low temperature solid-liquid coexistence pressure. We use this apparatus to inject helium into one side of the solid, creating a chemical potential difference across the solid, and we then look for a response in the pressure on the other side. We observe a flux of atoms through the solid[1] which tends to decrease with increasing solid pressure. There is also a complicated temperature dependence, which suggests hysteretic behavior. We will describe the experimental apparatus, and some of our results. \\ \\ $[1]$ M.W. Ray and R.B. Hallock Phys. Rev. Lett. \textbf{100} 235301 (2008)