Dissipation signatures of the normal and superfluid phases in torsion pendulum experiments with He in aerogel

We present data for energy dissipation factor (Q−1) over a broad temperature range at various pressures of a torsion pendulum setup used to study He confined in a 98% open silica aerogel. Values for Q−1 above Tc are temperature independent and have weak pressure dependence. Below Tc, a deliberate axial compression of the aerogel by 10% widens the range of metastability for a superfluid Equal Spin Pairing (ESP) state; we observe this ESP phase on cooling and the B phase on warming over an extended temperature region. While the dissipation for the B phase tends to zero as T → 0, Q−1 exhibits a peak value greater than that at Tc at intermediate temperatures. Values for Q−1 in the ESP phase are consistently higher than in the B phase and are proportional to ρs/ρ until the ESP to B phase transition is attained. We apply a viscoelastic collision-drag model, which couples the motion of the helium and the aerogel through a frictional relaxation time τf . We conclude that unless τf is an order of magnitude larger than expected, an additional mechanism to dissipate energy not captured in the collision-drag model and related to the emergence of the superfluid order must exist. The extra dissipation below Tc is possibly associated with mutual friction between the superfluid phases and the clamped normal fluid. The pressure dependence of the measured dissipation in both superfluid phases is likely related to the pressure dependence of the gap structure of the “dirty” superfluid. The large dissipation in the ESP state is consistent with the phase being the A or the Polar with the order parameter nodes oriented in the plane of the cell and perpendicular to the aerogel anisotropy axis.