Supersolid Helium at High Pressure

Abstract

We have measured the pressure dependence of the supersolid fraction by a torsional oscillator technique. Superflow is found from 25.6 bar up to 136.9 bar. The supersolid fraction in the low temperature limit increases from 0.6 % at 25.6 bar near the melting boundary up to a maximum of 1.5% near 55 bar before showing a monotonic decrease with pressure extrapolating to zero near 170 bar.

Observation of Unusual Mass Transport in Solid hcpHe4

“…From that 20-hour data record we conclude that over the course of the measurement 1 × 10 −4 grams of 4 He must have moved through the cell from line 1 to line 2, and that about 4.5 × 10 −4 grams of 4 He must have joined the solid. If we write M/t = ξρvxy, as the mass flux from line 1 to line 2, where M is the mass that moved in time t, ρ is the density of helium, ξ is the fraction of the helium that can flow, v is the velocity of flow in the solid, and xy is the cross section that supports that flow, we find ξvxy = 8 × 10 −9 cm 3 /sec.…”

“…But, helium in the pores of Vycor, or other small pore geometries, is known to freeze at much higher pressures than does bulk helium [22,23,24]. Thus, the "sandwich" consists of solid helium held between two Vycor plugs, each containing superfluid 4 He.…”

“…Recent work by Day and Beamish [13] showed that the shear modulus of hcp solid 4 He increased at low temperature and demonstrated a temperature and 3 He impurity dependence very similar to that shown by the torsional oscillator results. The theoretical situation is also complex, with clear analytic predictions that a supersolid cannot exist without vacancies (or interstitials) [14], numerical predictions that no vacancies exist in the ground state of hcp solid 4 He [15,16,17], and ab initio simulations that predict that in the presence of disorder the solid can demonstrate superflow [15,18,19] along imperfections. But, there are alternate points of view [20].…”

“…We also have capacitive pressure gauges on the sample cell, C1 and C2, and can measure the pressure in situ. To conduct the experiment it is important that the helium in the Vycor, the liquid reservoirs atop the Vycor, and the lines that feed the Vycor contain 4 He that does not solidify. This is accomplished by imposing a temperature gradient between R and Shcp across the Vycor, a gradient which would present insurmountable difficulties if the Vycor were not present.…”

“…The motivation was to attempt to study hcp solid 4 He at pressures off the melting curve in a way that would allow a chemical potential gradient to be applied across the solid, but not by squeezing the hcp solid lattice directly. Rather, the idea is to inject helium atoms into the solid from the superfluid.…”