Demagnetisation of solid 3He and supercritical superflow

This work describes the efforts with two ultra low temperature experiments with superfluid 3He as medium of interest. The experiments are mostly performed at temperatures below 200uK, in the regime where superfluid quasiparticle excitations are ballistic. Recently, a novel experimental tool has been built in Lancaster - a superconducting goalpost-shaped wire that can be moved trough the superfluid in oscillatory as well as in uniform linear motion. An object moving with high enough velocity that the excitation spectrum becomes gapless can create excitations at no energy cost and initiate the breakdown of the condensate - this limit is the well-known Landau velocity. In superfluid 3He, flow around an oscillating body displays a very clear onset of such dissipation. However, with this experiment it was found that for a uniform linear motion there is no discontinuity whatsoever in the dissipation as the Landau critical velocity is passed and exceeded, entering a supercritical flow regime. This regime allows for studying the dynamics of the Andreev bound states on the surface of the wire. This work presents a recent experimental estimation of the relaxation time of the bound states and a description of the relaxation mechanism. Next, the work describes the design and initial testing of a new experiment. Here a layer of solid 3He formed on the surface of a large aerogel sample submersed in superfluid 3He will be cooled down to below 100uK in a double nuclear demagnetisation process. NMR on the solid 3He will be used to search for a possible magnetic phase transition as well as to study superfluid 3He virtually free of quasiparticle excitations. The work reports progress to the present state of the experiment and discusses setbacks due to a large unexpected heating which appeared during the demagnetisation of the copper stage

Fundamental dissipation due to bound fermions in the zero-temperature limit

The ground state of a fermionic condensate is well protected against perturbations in the presence of an isotropic gap. Regions of gap suppression, surfaces and vortex cores which host Andreev-bound states, seemingly lift that strict protection. Here we show that in superfluid 3He the role of bound states is more subtle: when a macroscopic object moves in the superfluid at velocities exceeding the Landau critical velocity, little to no bulk pair breaking takes place, while the damping observed originates from the bound states covering the moving object. We identify two separate timescales that govern the bound state dynamics, one of them much longer than theoretically anticipated, and show that the bound states do not interact with bulk excitations

Transport of bound quasiparticle states in a two-dimensional boundary superfluid

The B phase of superfluid 3He can be cooled into the pure superfluid regime, where the thermal quasiparticle density is negligible. The bulk superfluid is surrounded by a quantum well at the boundaries of the container, confining a sea of quasiparticles with energies below that of those in the bulk. We can create a non-equilibrium distribution of these states within the quantum well and observe the dynamics of their motion indirectly. Here we show that the induced quasiparticle currents flow diffusively in the two-dimensional system. Combining this with a direct measurement of energy conservation, we conclude that the bulk superfluid 3He is effectively surrounded by an independent two-dimensional superfluid, which is isolated from the bulk superfluid but which readily interacts with mechanical probes. Our work shows that this two-dimensional quantum condensate and the dynamics of the surface bound states are experimentally accessible, opening the possibility of engineering two-dimensional quantum condensates of arbitrary topology

Effect of the boundary condition on the Kapitza resistance between superfluid 3He-B and sintered metal

Understanding the temperature dependence of thermal boundary resistance, or Kapitza resistance, between liquid helium and sintered metal has posed a problem in low temperature physics for decades. In the ballistic regime of superfluid 3He-B, we find the Kapitza resistance can be described via scattering of thermal excitations (quasiparticles) with a macroscopic geometric area, rather than the sintered metal's microscopic area. We estimate that a quasiparticle needs on the order of 1000 collisions to successfully thermalize with the sinter. Finally, we find that the Kapitza resistance is approximately doubled with the addition of two mono-layers of solid 4He on the sinter surface, which we attribute to an extra magnetic channel of heat transfer being closed as the non-magnetic solid 4He replaces the magnetic solid 3He

Magnetotransport of dirty-limit van Hove singularity quasiparticles

Tuning of electronic density-of-states singularities is a common route to unconventional metal physics. Conceptually, van Hove singularities are realized only in clean two-dimensional systems. Little attention has therefore been given to the disordered (dirty) limit. Here, we provide a magnetotransport study of the dirty metamagnetic system calcium-doped strontium ruthenate. Fermi liquid properties persist across the metamagnetic transition, but with an unusually strong variation of the Kadowaki-Woods ratio. This is revealed by a strong decoupling of inelastic electron scattering and electronic mass inferred from density-of-state probes. We discuss this Fermi liquid behavior in terms of a magnetic field tunable van Hove singularity in the presence of disorder. More generally, we show how dimensionality and disorder control the fate of transport properties across metamagnetic transitions

Acoustic Damping of Quartz Tuning Forks in Normal and Superfluid 3^3He

We investigate the damping experienced by quartz tuning fork resonators in normal and superfluid 3He as a function of their resonance frequency from 22 kHz to 250 kHz and contrast it with the behavior of the forks in 4He. For our set of tuning forks the low frequency damping in both fluids is well described by the existing hydrodynamic models. We find that the acoustic emission becomes the dominating dissipation mechanism at resonator frequencies exceeding approximately 100 kHz. Our results show that the acoustic emission model used in 4He fluid also describes acoustic damping in superfluid 3He and normal 3He at low temperatures using same geometrical prefactor. The high temperature acoustic damping in normal 3He does not exceed prediction of this model and thus the acoustic damping of moderate frequency devices measured in 4He should be similar or smaller in 3He liquid

Low conductive support for thermal insulation of a sample holder of a variable temperature scanning tunneling microscope

We have designed a supporting system to fix a sample holder of a scanning tunneling microscope in an UHV chamber at room temperature. The microscope will operate down to a temperature of 20 K. Low thermal conductance, high mechanical stiffness, and small dimensions are the main features of the supporting system. Three sets of four glass balls placed in vertices of a tetrahedron are used for thermal insulation based on small contact areas between the glass balls. We have analyzed the thermal conductivity of the contacts between the balls mutually and between a ball and a metallic plate while the results have been applied to the entire support. The calculation based on a simple model of the setup has been verified with some experimental measurements. In comparison with other feasible supporting structures, the designed support has the lowest thermal conductance

Probing liquid 4He with quartz tuning forks using a novel multifrequency lock-in technique

We report on a novel technique to measure quartz tuning forks, and possibly other vibrating objects, in a quantum fluid using a multifrequency lock-in amplifier. The multifrequency technique allows to measure the resonance curve of a vibrating object much faster than a conventional single frequency lock-in amplifier technique. Forks with resonance frequencies of 12 kHz and 16 kHz were excited and measured electro-mechanically either at a single frequency or at up to 40 different frequencies simultaneously around the same mechanical mode. The response of each fork was identical for both methods and validates the use of the multifrequency lock-in technique to probe properties of liquid helium at low fork velocities. Using both methods we measured the resonance frequency and drag of two 25-μm-wide quartz tuning forks immersed in liquid 4He in the temperature range from 4.2 K to 1.5 K at saturated vapour pressure. The damping and shift of resonance frequency experienced by both tuning forks at low velocities are well described by hydrodynamic contributions in the framework of the two-fluid model. The sensitivity of the 25-μm-wide tuning forks is larger compared to similar 75-μm-wide forks and in combination with the faster multifrequency lock-in technique could be used to improve thermometry in liquid 4He. The multifrequency technique could also be used for studies of the onset of non-linear phenomena such as quantum turbulence and cavitation in superfluids

Magnetotransport of dirty-limit van Hove singularity quasiparticles

Tuning of electronic density-of-states singularities is a common route to unconventional metal physics. Conceptually, van Hove singularities are realized only in clean two-dimensional systems. Little attention has therefore been given to the disordered (dirty) limit. Here, we provide a magnetotransport study of the dirty metamagnetic system calcium-doped strontium ruthenate. Fermi liquid properties persist across the metamagnetic transition, but with an unusually strong variation of the Kadowaki-Woods ratio. This is revealed by a strong decoupling of inelastic electron scattering and electronic mass inferred from density-of-state probes. We discuss this Fermi liquid behavior in terms of a magnetic field tunable van Hove singularity in the presence of disorder. More generally, we show how dimensionality and disorder control the fate of transport properties across metamagnetic transitions