Mobility of solid 4He

Recent torsional oscillator measurements [1,2] on solid $^{4}$He in the pores of Vycor and in bulk demonstrated non-classical rotational inertia and showed a decrease in period below about 200 mK, suggestive of a transition to a supersolid phase. It becomes interesting to see whether solid $^{4}$He exhibits any of the other unusual flow properties of a superfluid. We have performed capacitive measurements on the mobility of solid $^{4}$He in Vycor. By suddenly increasing the pressure in a cell containing a Vycor sample, we were able to monitor the pressure induced flow of solid $^{4}$He in the pores. Near its melting temperature the solid $^{4}$He does flow in/out of the pores, but the flow rate decreases rapidly with temperature, until no flow is observable below 700 mK. We do not see any flow at temperatures down to 25 mK, implying that either supersolid $^{4}$He in Vycor does not flow in response to a large pressure difference or that superflow occurs at a rate far slower than the critical velocities observed by Kim and Chan. We have also begun work on pressure induced flow of bulk solid $^{4}$He. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). 1. E. Kim and M.H.W. Chan, Nature 427, 225 (2004). 2. E. Kim and M.H.W. Chan, Science 305, 1941 (2004).