Abstract
We have made ultrasonic measurements of the superfluid density and the dissipation in several porous alumina samples filled with 4 He. In one sample (formed by a sol-gel process) with high porosity (92%) and large pores (R=1600 Å), we found that the superfluid density disappeared at the transition with essentially the bulk exponent of 0.67, as expected. However, in a second material (slip-cast from fine powder) with lower porosity (52%) and much smaller pores (R=110 Å), the superfluid critical exponent was 0.83±.02. The difference in critical behavior in the two media was also reflected in the dissipation. The sound attenuation in the first sample was due to viscous motion of the normal fluid and did not show any unusual behavior at the superfluid transition. In the small pore sample, on the other hand, there was a clear critical attenuation peak centered at the transition temperature.
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