Torsion Pendulum energy dissipation due to $^3$He in aerogel. Dissipation signature of the A-phase

A torsion pendulum excited at acoustic frequencies was used to measure the dissipation $Q^{-1}$ and period shift of $^3$He confined in a 98\% open aerogel, compressed by 10\% along the axial direction. Data was taken in the range between 100mK and T$_c$, as well as below T$_c$ for a series of pressures. After accounting for bulk and empty cell contributions, $Q^{-1}$ is seen to be pressure and temperature independent in the normal state. The dissipation is larger than expected, which can be accounted for either by invoking a very long frictional relaxation time or by taking into account the internal friction in the aerogel that is affected by mass loading of $^3$He. In contrast, the dissipation in the superfluid state depends strongly on temperature and pressure. The A phase (observed on cooling) shows a higher dissipation than the B phase (observed on warming); the excess dissipation is greater at high pressures.