Torsional Oscillator and Synchrotron X-Ray Experiments on Solid ${}^4\mathrm{He}$ in Aerogel

X-ray diffraction experiments show that solid ${}^4\mathrm{He}$ grown in aerogel is highly polycrystalline, with an hcp crystal structure (as in bulk) and a crystallite size of approximately 100 nm. In contrast to the expectation that the highly disordered solid will have a large supersolid fraction, torsional oscillator measurements show a behavior that is strikingly similar to high purity crystals grown from the superfluid phase. The low temperature supersolid fraction is only $\sim 3\times {10}^{-4}$, and the onset temperature is $\sim 100\mathrm{mK}$.

Figure 1

The powder pattern for a solid helium sample in aerogel at 60 bar. The rings corresponding to the first 5 reflections of an hcp structure are indicated.

Figure 2

The diffraction peak for the (101) reflection. The width of the peak indicates a crystallite size of $\sim 1000\AA$.

Figure 3

The NCRIf from solid ${}^4\mathrm{He}$ grown in aerogel at pressures between 34 and 48 bar. The bulk solid data are taken from Ref. 8, Fig. 2(b). To emphasize its different temperature dependence, the BC data are scaled down by a factor of 3. All data were taken at low velocity (rim velocity $<10\mu \mathrm{m}/\mathrm{s}$). BC: blocked capillary; CP: constant pressure.