M. Bartkowiak

Forced oscillation of the A–B phase boundary in superfluid 3He

We present measurements conducted on the oscillating interface between superfluid He-3 A and B. The measurements, made at 150 mu K, are taken at frequencies between 100 mHz, where the boundary moves almost in thermal equilibrium with the fluid, and several kHz, where the boundary motion is highly non-linear and out of equilibrium. Driving the oscillations to higher amplitudes seems to produce different friction mechanisms dominant only after certain thresholds. As yet these phenomenon have defied any explanation, theoretical or otherwise.

The Unique Superfluid 3He A-B Interface: Surface Tension and Contact Angle

We have measured the A-B boundary wall contact angle and the phase interface surface tension in superfluid 3He. The measurements have been made in the ballistic temperature regime at zero pressure in magnetic fields up to 400 mT. We infer the surface energies from the behaviour of the phase boundary moving in and out of a stack of glass capillary tubes. We measure the wall contact angle from the observed capillary height and obtain the interphase surface tension from the level of over- or under-magnetisation when the interface “pops” out of the capillaries. This is the first measurement of both the surface tension and contact angle in high magnetic fields.

The thermodynamics of the A–B transition in superfluid 3He as a probe of the A-phase gap node geometry

We have used a magnetic field gradient to stabilise the phase boundary between the A- and B-phases of superfluid He-3 inside a quasiparticle black body radiator down to 146 mu K; The radiator technique enables us to measure very accurately the density of quasiparticle excitations and therefore the temperature of the B-phase inside the radiator. As we ramp the held through the transition we can observe the cooling (warming) effect when the volume of A-phase is increased (decreased). From these measurements we deduce the temperature dependence of the latent heat, which provides the lowest temperature verification yet of the existence of nodes in the A-phase order parameter.

The Stability of the Superfluid 3He AB Interface Pinned in an Aperture

We have been measuring the surface tension of the AB interface at zero pressure, in high magnetic fields and low temperatures below 0.2 Tc. We manipulate the phase boundary by controlling a magnetic field profile. We use the latent heat released/absorbed as the phase boundary moves to infer its position and velocity. We have observed that the motion of the interface through a small aperture is dependent on the magnetic field gradient. Here we extend numerical methods first used to calculate the shapes of liquid drops in a gravitational field to show that the gradient dependence can be accounted for by the deformation of the interface.

The Response of a Mechanical Oscillator at the Superfluid 3He AB Interface

From our long experience of using profiled magnetic fields to stabilize and manipulate the AB phase boundary in superfluid 3He in the zero-temperature limit, we have constructed a cell where we can place the AB interface in the vicinity of a pair of crossed vibrating wire resonators (VWRs). The VWRs provide sensitive mechanical probes of the A and B phase energy gaps and textures. Here we present preliminary resugts where we observe the dynamic response of the VWR at the AB interface.

Asymmetrical nucleation of the A–B transition in superfluid 3He

Using a magnetic field we can induce nucleation in superfluid He-3 both ways across the A-B phase boundary, at temperatures below 0.2T(c). The nucleation takes place in a quasiparticle black-body radiator in a variable magnetic field profile and is detected from the associated temperature jump. Before A-phase will nucleate, the parent B-phase must be overmagnetised by a history-dependent amount. Similarly, B-phase nucleation requires a degree of undermagnetisation. However, the transition is asymmetric and the history dependences are quite different