V. Tsepelin

Generation, evolution, and decay of pure quantum turbulence : a full Biot-Savart simulation

A zero-temperature superfluid is arguably the simplest system in which to study complex fluid dynamics, such as turbulence. We describe computer simulations of such turbulence and compare the results directly with recent experiments in superfluid He-3-B. We are able to follow the entire process of the production, evolution, and decay of quantum turbulence. We find striking agreement between simulation and experiment and gain insights into the mechanisms involved.

Absence of low temperature anomaly on the melting curve of 4He

The melting pressure and pressure in the liquid at a constant density of ultrapure 4He (0.3 ppb of 3He impurities) have been measured with an accuracy of about 0.5 μbar in the temperature range from 10 to 320 mK. The measurements show that the anomaly on the melting curve below 80 mK, which was recently observed [I. A. Todoshchenko et al., Phys. Rev. Lett. 97, 165302 (2006)], is entirely due to an anomaly in the elastic modulus of Be-Cu from which our pressure gauge is made. Thus, the melting pressure of 4He follows the T4 law due to phonons in the whole temperature range from 10 to 320 mK without any attribute of a supersolid transition.

Andreev reflection, a tool to investigate vortex dynamics and quantum turbulence in 3He-B

Andreev reflection of quasiparticle excitations provides a sensitive and passive probe of flow in superfluid 3He-B. It is particularly useful for studying complex flows generated by vortex rings and vortex tangles (quantum turbulence). We describe the reflection process and discuss the results of numerical simulations of Andreev reflection from vortex rings and from quantum turbulence. We present measurements of vortices generated by a vibrating grid resonator at very low temperatures. The Andreev reflection is measured using an array of vibrating wire sensors. At low grid velocities, ballistic vortex rings are produced. At higher grid velocities, the rings collide and reconnect to produce quantum turbulence. We discuss spatial correlations of the fluctuating vortex signals measured by the different sensor wires. These reveal detailed information about the formation of quantum turbulence and about the underlying vortex dynamics.

Annihilation of an AB/BA interface pair in superfluid helium-3 as a simulation of cosmological brane interaction

This study presents measurements of the transport of quasiparticle excitations in the B phase of superfluid 3He at temperatures below 0.2Tc. We find that creating and then removing a layer of A-phase superfluid leads to a measurable increase in the thermal impedance of the background B phase. This increase must be due to the survival of defects created as the AB and BA interfaces on either side of the A-phase layer annihilate. We speculate that a new type of defect may have been formed. The highly ordered A–B interface may be a good analogy for branes discussed in current cosmology. If so, these experiments may provide insight into how the annihilation of branes can lead to the formation of topological defects such as cosmic strings.

The Thermal Boundary Resistance of the Superfluid 3He A‐B Phase Interface in the Low Temperature Limit

We have constructed a vertical cylindrical cell in which we cool superfluid 3He to the low temperature limit. At the top and bottom of this cylinder are pairs of vibrating wire resonators (VWRs), one to act as a heater and the other as a thermometer. Quasiparticle excitations are created by driving the heater VWRs. These excitations can only leave the cylinder via a small hole at the top. Using a shaped magnetic field, we can produce a layer of A phase across the tube, while maintaining low field B phase in the vicinity of the VWRs for reliable thermometry. Preliminary results show that the two A‐B interfaces lead to a measurable extra resistance for quasiparticles between the top and bottom of the cylinder.

Thermal Transport by Ballistic Quasiparticles in Superfluid 3He‐B in the Low Temperature Limit

In the temperature range below 0.2Tc, the gas of thermal excitations from the superfluid 3He‐B ground state is in the ultra‐dilute ballistic regime. Here we discuss preliminary measurements of the transport properties of this quasiparticle gas in a cell of cylindrical geometry with dimensions much smaller than any mean free path. The vertical cylinder, constructed from epoxy‐coated paper, has vibrating wire resonator (VWR) heaters and thermometers at the top and bottom, and a small aperture at the top which provides the only exit for quasiparticles. Using the thermometer VWRs, we measure the difference in quasiparticle density between the top and bottom of the tube when we excite the top or bottom VWR heater. This gives information about the transport of energy along the cylindrical 3He sample and hence about the scattering behaviour involved when a quasiparticle impinges on the cylinder wall.

A Levitated Droplet of Superfluid 3He‐B Entirely Surrounded by 3He‐A

From our long experience of using profiled magnetic fields to stabilize and manipulate the A-B phase boundary in superfluid He-3, we have constructed a cell in which we can create and move a droplet of B phase, levitated within A phase away from any walls at T similar to 0.15 T-c. Uniquely, the A and B condensates are coherent across the A-B interface and at such low temperatures the superfluid is essentially pure, providing the most ordered phase boundary to which we have laboratory access. We configure the field so that within a bulk volume of superfluid, a region of high field (stabilizing the A phase) completely surrounds a region of lower field (stabilizing the B phase). Our preliminary measurements are at zero pressure and temperatures below 0.3T(c) where the first-order transition from B to A phase is at 340 mT. We observe the formation of the droplet as we ramp the field, and we also study the transport of thermal excitations out of the droplet. Future plans include measurements at higher pressures where the A phase can be stabilized in low magnetic field at temperatures close to T-c. Upon cooling into the B phase we should then be able to make the first studies of nucleation uninfluenced by the presence of container walls.

Operating nanobeams in a quantum fluid

Microelectromechanical (MEMS) and nanoelectromechanical systems (NEMS) are ideal candidates for exploring quantum fluids, since they can be manufactured reproducibly, cover the frequency range from hundreds of kilohertz up to gigahertz and usually have very low power dissipation. Their small size offers the possibility of probing the superfluid on scales comparable to, and below, the coherence length. That said, there have been hitherto no successful measurements of NEMS resonators in the liquid phases of helium. Here we report the operation of doubly-clamped aluminium nanobeams in superfluid 4He at temperatures spanning the superfluid transition. The devices are shown to be very sensitive detectors of the superfluid density and the normal fluid damping. However, a further and very important outcome of this work is the knowledge that now we have demonstrated that these devices can be successfully operated in superfluid 4He, it is straightforward to apply them in superfluid 3He which can be routinely cooled to below 100 μK. This brings us into the regime where nanomechanical devices operating at a few MHz frequencies may enter their mechanical quantum ground state.

Visualization of quantum turbulence in superfluid 3He-B : combined numerical and experimental study of Andreev reflection

We present a combined numerical and experimental study of Andreev scattering from quantum turbulence in superfluid

Experimental setup for the observation of crystallization waves in 3He

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