C. J. Kennedy

The mechanical behavior of a vibrating wire in superfluid3He-B in the ballistic limit

An experimental procedure is described for the characterization of the response of very small vibrating wires to liquid3He at temperatures down to 120 µK. The relative scales of the mean free path in the liquid and the radius of the wire play a significant role in the interpretation of the results. It is shown that the same ideas concerning the transition from hydrodynamic to ballistic behavior as the temperature is reduced can be applied both to saturated3He-4He solutions and to superfluid3He-B.

A compact dilution refrigerator with vertical heat exchangers for operation to 2 mK

We describe a compactly designed dilution refrigerator with closely packed, vertical heat exchangers. The refrigerator reaches a temperature of 2 mK and is easily constructed, since the sintered heat exchangers are straight units. Vibrating wire resonators are employed in the mixing chamber as diagnostic tools, which may act as both thermometers and phase-boundary level indicators. There is a design problem in the vertical arrangement, namely, the sumps on the concentrated phase side that can slowly fill with dilute phase and degrade the performance. The problem is solved by draining the superfluid4He component in any collected dilute phase through superleaks into the mixing chamber.

A new NMR mode in the Landau field in superfluid3He-B

We report the experimental observation of the separate internal precession of the normal and superfluid magnetizations around the molecular Landau field in3He-B, the magnetic analog of second sound. The mode is detected by cross relaxation with conventional NMR precession when the external and Landau molecular fields have similar values. From NMR measurements down to 0.12Tc we conclude that the previously observed catastrophic relaxation phenomenon can be explained as a capture of the internal precession mode by the Larmor precession. This NMR mode provides an additional relation between the parameters F0a, F2a which taken with susceptibility data allows both parameters to be distinguished, giving for zero bar, F0a=−0.713 and F2a=0.4.

The mechanical properties of the condensate in 3He‐B in the quasiparticle‐free regime

We have made measurements of the damping and resonant frequency of a vibrating wire in superfluid 3He‐B at temperatures down to about 0.12Tc at 0.0 bar and at 7.3 bar pressure. At the lowest temperatures the wire is effectively in a quasiparticle vacuum, the contribution to the damping from the quasiparticles having disappeared. In this regime the behaviour of the wire becomes strongly non‐linear reflecting the mechanical response of the superfluid condensate.

Andreev exclusion of bulk state quasiparticles from a surface in 3He-A at the low temperature limit: implications for transport properties

Abstract We present preliminary measurements of the thermal boundary resistance between 3 He-A and sintered silver, and also of the thermal damping force on a vibrating wire. The A-phase is stabilised by a magnetic field in excess of about 0.35 T. Vibrating wire resonators in 3 He-B “contacts” serve as thermometers. The magnitudes of both the boundary resistance and the thermal damping force are found to be comparable to the corresponding B-phase values, and an exponential temperature dependence is evident, in spite of the T 3 dependence of the quasiparticle number density. We propose that Andreev scattering from a non-uniform A-phase texture is responsible.

Ballistic quasiparticle beam experiments in superfluid3He-B

In3He-B at temperatures of the order of 0.1Tc, where the meanfree path of the excitations measures several hundreds of metres, we are able to perform experiments on fully ballistic quasiparticle beams. We describe the first direct production and detection of a quasiparticle beam in superfluid3He with the use of a black-body radiator. The radiator may be used as a smoothly controllable continuous source and also as a bolometric detector, sensitive to a quasiparticle energy flux of order 100 fW mm−2.

The one-dimensional excitation gas in superfluid3He-A at very low temperatures

By setting up an experiment with a static A-B boundary stabilised by a magnetic field, we have been able to cool the liquid to the regime where the A-phase excitation gas is almost one-dimensional. We have measured the damping of a vibrating wire resonator in the A-phase while simultaneous measuring the damping in the B-phase. The response in the A-phase is found to follow an exponential temperature dependence and not a power law as might be naively expected. This effect is a result of the bending of the texture by the wire which excludes a large fraction of the bulk low energy excitations from the surface.

Mean free path effects and a search for superfluidity in 3He4He solutions near the solubility maximum from 0.4 to 4 mK

Abstract Saturated 3 He 4 He solutions have been cooled to temperatures lower than previously reported. Observations of the damping of a vibrating wire at pressures between 0 and 16 bar show no evidence for superfluidity down to 0.4 mK. At low temperatures a collisionless flow regime is reached in which mean free path effects become dominant.

The superfluid energy gap in 3He-B as a function of magnetic field in the low temperature limit

We have measured the change in the superfluid energy gap of 3He-B as a function of magnetic field in the zero temperature limit at zero pressure, by observing the pair breaking velocity of a vibrating wire resonator. The critical velocity as a function of magnetic field is in very good agreement with the values of the gap calculated by Ashida and Nagai. A clear transition to the A-phase is seen at the critical field.

Anomalous dynamic behaviour of a gas of quasiparticles in superfluid 3HeB

Abstract In 3 HeB it is possible to probe the unusual dynamics arising from the non-Newtonian dispersion relation of the excitations. Since the dispersion relation is tied to the rest frame of the condensate, it is distorted by condensate motion. A container of superfluid with a velocity gradient has a range of excitation dispersion curves, depending on velocity and thus the density of states available to the excitations varies with position. We show that, in consequence the behaviour of a moving object in the excitation gas shows unusual and often counter-intuitive behaviour. At low enough temperature, the force on a moving object becomes independent of velocity.