Studies of the Flow Properties of Superfluid Helium -3 and of the Superfluid Density of HELIUM-4 Films in Restricted Geometries.
The torsional oscillator technique provides a sensitive detector of normal fluid mass, (rho)(,n). We have used this technique to study the properties of several superfluid systems. One series of experiments involved the study of ('3)He-B, performing oscillatory flow through 5 and 2 micron orifices which were mounted across annular flow channels. Behavior similar to that of an rf-biased SQUID was expected. Instead, a single dissipation feature involving the destruction of superflow through the orifice was observed. We find that critical velocities calculated from our data scale as the inverse of the orifice diameter. Vortices are discussed as a possible dissipation mechanism. The addition of a dc flow through the orifice was found to have no observable effect. This result remains unexplained. A second experiment was designed to study (rho)(,n) in ('3)He-A, which is known to be anisotropic. We observed the effects of dc flows and magnetic fields on the anisotropy axis in a torroidal channel. From these experiments estimates can be made of (rho)(,n) parallel and perpendicular to the anisotropy axis. In addition, evidence was found for the presence of persistent currents lasting at least ten minutes. The third experiment described in this work, involves the study of (rho)(,n) in very thin ('4)He films adsorbed on Vycor glass. Due to the highly interconnected nature of the 70 (ANGSTROM) pores in this system, the temperature dependence of (rho)(,n) in the critical region was found previously to be similar to that observed in bulk. In our experiments, we have studied films with estimated superfluid densities as low as 3 x 10('18) atoms/cm('3). At densities below 10('19) atoms/cm('3) and transition temperatures in the vicinity of 10 mK, we observe a crossover to more mean-field like behavior. The evidence for this crossover is presented and discussed in relation to the dilute Bose gas. The final experiment discussed was designed to detect superfluidity in ('3)He confined in Vycor glass. Recent reports have been made of the observation of superfluidity in this system. Since the coherence lengths of the known phases of superfluid ('3)He are much larger than the pore sizes in the glass, this is a surprising result. No superfluid transition was observed in our experiments placing limits on the possible superfluid densities in the glass.
- Pub Date:
- Physics: Condensed Matter