Study on the effects of anisotropic disorder on superfluid helium three in high porosity aerogel using longitudinal ultrasound.

摘要

Longitudinal sound attenuation measurements in superfluid 3He in 98% aerogel were conducted at pressures between 14 and 33 bar and in magnetic fields up to 4.44 kG. The temperature dependence of the ultrasound attenuation in the A-like phase was determined for the entire superfluid region by exploiting the field induced meta-stable A-like phase at the highest field. In lower fields, the A -- B transition in aerogel was identified by a smooth jump in attenuation on both cooling and warming. Based on the transitions observed on warming, a phase diagram as a function of pressure (P), temperature ( T) and magnetic field (B) is constructed. The transitions obtained by isothermal field sweeps are consistent with those by temperature sweeps at constant magnetic fields. The A -- B phase boundary in aerogel recedes to the corner of zero temperature and melting pressure in response to an increasing magnetic field, which is drastically different from the bulk.;The presence of elastic impurity scattering by aerogel limits the growth of the mean free path at low temperature. In this case, the dominance of temperature independent elastic scattering keeps the system from entering into collisionless limit on cooling. Therefore, it is expected that the sound attenuation obeys the o2-dependence. However, our result reveals that non-trivial frequency dependencies, departing from the o2-dependence appear as temperature lowers into the superfluid regime. This tendency is more evident at higher pressure and lower temperature. We attribute this property to the gapless behavior of superfluid 3He in aerogel.