M. P. Enrico

Diffuse scattering model of the thermal damping of a wire moving through superfluid3He-B at very low temperatures

We present a microscopic model of the scattering of quasiparticles in superfluid3He-B by a moving solid surface. This is used to calculate the thermal damping of a wire resonator in the low temperature regime. The calculated damping force is in good agreement with experimental results when the quasiparticles are assumed to be scattered diffusely by the wire.

A highly sensitive nuclear recoil detector based on superfluid3He-B

The excitations in superfluid3He have a dispersion curve in which the energy minimum does not coincide with the momentum minimum. As a result, when a mechanical resonator moves through a gas of such excitations, normal and Andreev scattering processes introduce a large asymmetry into the momentum exchange and the mechanical resonator experiences a very large drag force. A gas of such excitations is thus very easy to detect even at very low densities. We have exploited this effect to monitor the increase in excitation density in a small volume caused by a particle interaction. The working volume is filled with superfluid3He-B at around 100 μK. A particle undergoing an interaction in the volume releases a shower of quasiparticle excitations which can be detected by the increase in damping on a vibrating wire resonator. A small hole in the container allows the excitations to leak out into the outside colder liquid to reset the working liquid to the resting state. Using an existing experiment we can detect nuclear recoil interactions depositing energies as low as 500 eV. Two simple modifications should allow us to detect interactions in the 10 eV range.

Specular and diffuse scattering of quasiparticles by a macroscopic object moving through superfluid3He-B

When a wire resonator is moving in superfluid3He-B at low temperature, there is a drag force due to scattering of thermally excited quasiparticles. The form of the graph of the drag force against the wire velocity depends strongly on the type of scattering at the wire surface. The force always rises linearly at zero velocity, and it tends to a limiting value at high wire velocity. However the graph is much more sharply curved if the scattering is diffuse than if it is specular. Here we present an analytic study of the form of the graph, by making estimates of the effect of Andreev reflection of scattered quasiparticles in the superflow round the wire. We show that the differences between diffuse and specular scattering can be understood intuitively, by considering the geometrical restrictions imposed by Andreev reflection on the scattering at the wire surface.

Andreev reflection of a beam of ballistic quasiparticle excitations incident on a static B-A phase interface in superfluid3He

We have attempted to observe the Andreev reflection of a beam of ballistic quasiparticle excitations as it impinges on a static B-A phase interface. The beam is created, and the Andreev reflected component measured, by the quasiparticle equivalent of a black-body radiator. The A-phase is stabilized at low temperature and pressure by a magnetic field which is generated by a miniature superconducting solenoid, situated next to the radiator orifice and which provides the necessary 0.34 T field in a small, localized region. Preliminary results indicate that we can observe Andreev reflection both from the high-field distorted B-phase energy gap and from the larger equatorial A-phase energy gap.

Superfluid 3He at very low temperatures: a very unusual excitation gas

Abstract The excitation gas in superfluid 3He at low temperatures shows a number of remarkable dynamical properties arising from the unusual dispersion curve. The existence of an energy gap leads to many of the observed properties varying rapidly with temperature, since the excitation density is dominated by the gap Boltzmann factor exp(—Δ/kT). But also, the fact that the minimum energy lies at finite momentum gives rise to Andreev scattering processes, in which the velocity of the excitation is reversed but the momentum left virtually unchanged. Since the dispersion curve looks different to a moving observer, there is the possibility of the free production of quasiparticle-quasihole pairs at a Landau critical velocity. At low temperatures the mean free path becomes much larger than any experimental size. Using vibrating wire resonators as universal probes, we can monitor the temperature, measure the Kapitza resistance, examine the nonlinear regime beyond the two-fluid model, observe the Landau velocity, create and detect thermal beams of excitation with black-body radiators, observe Andreev reflection directly and probe A-phase textures (in which the gas is one-dimensional). Future possibilities are discussed.

Texture dependence of the persistent NMR signal in superfluid3He-B

We have studied coherent magnetization precession in superfluid3He with pulsed NMR to temperatures of ≈0.1Tc. Persistent signals were observed with lifetimes up to 60s at a frequency of 0.8MHz. We have found that the behaviour is reproducible for small levels of NMR excitation, whilst becoming irreproducible for higher excitations. We infer from this that textures play an essential role in the formation of persistent signals.

Concentration dependence of the thermal boundary resistance between dilute3He−4He solutions and sintered silver powder

We have measured the3He concentration dependence of the thermal boundary resistanceRB between3He−4He dilute mixtures and submicron sintered silver between 10 and 150 mK. For concentrations greater than one percent, the results for the boundary resistance per inverse volume are insensitive to the concentration and have a magnitude similar to that predicted by the acoustic mismatch theory. For concentrations less than one percent, we observe an increase ofRB for decreasing concentrations.

Quasiparticle molasses: the giant force on an object moving through a beam of thermal excitations in superfluid3He

Owing to the unusual dispersion curve for excitations in superfluid3He an object moving through the excitation gas experiences a very much higher drag force than expected for a similar ‘conventional’ gas. We have made investigations of the force on a body moving through a unidirectional beam of thermal excitations in3He-B at very low temperatures which shows that owing to the nature of Andreev reflection by the surrounding flow field the moving body (in this case a vibrating wire resonator) experiences a giant drag forceindependently of the relative direction of motion between object and beam.

Direct measurement of the Andreev reflection of a beam of excitations in superfluid3He-B

Abstract We present a direct quantitative measurement of the Andreev reflection of a beam of quasiparticle excitations in the B-phase of superfluid3He in the ballistic regime. The beam, generated by a black-body radiator (which also acts as the detector), is partially Andreev reflected by a superflow gradient produced by a vibrating paddle located in front of the radiator hole. The retroreflective nature of the Andreev process ensures that reflected quasiparticles are returned to the radiator whereas normally scattered ones are not. The Andreev reflected component of the beam is detected as an increase in the quasiparticle density within the radiator over the value measured with no superflow.

Quasiparticle beams in superfluid3He-B at very low temperatures

Abstract We have been interested in developing source-experiment-detector type quasiparticle beam experiments in superfluid3He for some time. Recently we have developed a new source and detector based on the quasiparticle equivalent of the blackbody radiator. We have already used these devices for a few simple experiments but many more are possible, for example the direct observation of quasiparticle transmission across an A-B interface. The radiators are so sensitive that we can easily see the heat leak entering the liquid from the epoxy walls and also note that the leak from copper walls is more than 100 times smaller than from the epoxy.