Investigations of solid-liquid interfaces in helium at ultralow temperatures

This Thesis describes ultralow temperature studies of helium quantum crystals. Owing to the surrounding superfluid, small latent heat of crystallization and correspondingly short relaxation times, which are unreachable in ordinary crystals, helium crystals offer a unique and clean modeling system to study surface phenomena in a solid. The measurements of the crystal shape and growth rates are essential in providing the microscopic understanding of crystal growth. Optical observations are probably the most direct way to quantify the surface of crystals. The results presented in this Thesis were obtained with the help of two very powerful experimental techniques that were successfully adopted for ultralow temperature applications: optical interferometry and high-precision pressure measurements. The optical investigations on 3He crystals revealed altogether eleven types of facets at temperatures well below 1 mK, while previously only three facet types have been seen. The growth rates of rough and smooth surface states were explored and show significant anisotropy. The measured growth velocities of different facet types indicate that the main growth mechanism is spiral growth in the regime of suppressed mobility. Important thermodynamic parameters of an interface such as the width of an elementary step and the step free energy were directly deduced from the observed growth kinetics. Results suggest that coupling of the interface to the underlying crystal lattice is relatively "strong" in 3He crystals. Measurements of the spiral growth of the c-facet on 4He crystals in the presence of a small number of 3He atoms were also conducted. They show suppression of the crystal growth velocity with the increase of the 3He atom concentration and indicate "weak" coupling of the interface to the crystal lattice in 4He.reviewe

Scattering length of Andreev reflection from quantized vortices in 3^3He-BB

Andreev reflection of thermal quasiparticles from quantized vortices is an important technique to visualize quantum turbulence in low temperature $^3$He-$B$. We revisit a problem of Andreev reflection from the isolated, rectilinear vortex line. For quasiparticle excitations whose impact parameters, defined as distances of the closest approach to the vortex core, do not exceed some arbitrary value, $b$, we calculate exactly the reflected fraction of the total flux of excitations incident upon the vortex in the direction orthogonal to the vortex line. We then define and calculate exactly, as a function of $b$, the scattering length, that is the scattering cross-section per unit length of the vortex line. We also define and calculate the scattering lengths for the flux of energy carried by thermal excitations, and for the net energy flux resulting from a (small) temperature gradient, and analyze the dependence of these scattering lengths on temperature.Comment: 8 pages, 4 figure

Multiple critical velocities in oscillatory flow of superfluid 4He due to quartz tuning forks

We report recent investigations into the transition to turbulence in superfluid $^4$He, realized experimentally by measuring the drag forces acting on two custom-made quartz tuning forks with fundamental resonances at 6.5 kHz and 55.5 kHz, in the temperature range 10 mK to 2.17 K. In pure superfluid in the zero temperature limit, three distinct critical velocities were observed with both tuning forks. We discuss the signicance of all critical velocities and associate the third critical velocity reported here for the first time with the development of large vortical structures in the flow, which thus starts to mimic turbulence in classical fluids. The interpretation of our results is directly linked to previous experimental work with oscillators such as tuning forks, grids and vibrating wires, focusing on the behavior of purely superfluid $^4$He at very low temperature

Observation of quantum turbulence in superfluid 3He-B using reflection and transmission of ballistic thermal excitations

We report measurements of quantum turbulence generated by a vibrating grid in superfluid $^3$He-B at zero pressure in the zero temperature limit. Superfluid flow around individual vortex lines Andreev-reflects incoming thermal ballistic quasiparticle excitations, and allows non-invasive detection of quantum vortices in $^3$He-B. We have compared two Andreev reflection-based techniques traditionally used to detect quantum turbulence in the ballistic regime: quasiparticle transmission through and reflection from ballistic vortex rings and a turbulent tangle. We have shown that the two methods are in very good agreement and thus complement each other. Our measurements reveal that vortex rings and a tangle generated by a vibrating grid have a much larger spatial extent than previously realised. Furthermore, we find that a vortex tangle can either pass through an obstacle made from a mesh or diffuse around it. The measured dependence of vortex signal as a function of the distance from the vibrating grid is consistent with a power-law behaviour in contrast to turbulence generated by a vibrating wire which is described by an exponential function

On the origin of the controversial electrostatic field effect in superconductors

In semiconductor electronics, the field-effect refers to the control of electrical conductivity in nanoscale devices, which underpins the field-effect transistor, one of the cornerstones of present-day semiconductor technology. The effect is enabled by the penetration of the electric field far into a weakly doped semiconductor, whose charge density is not sufficient to screen the field. On the contrary, the charge density in metals and superconductors is so large that the field decays exponentially from the surface and can penetrate only a short distance into the material. Hence, the field-effect should not exist in such materials. Nonetheless, recent publications have reported observation of the field-effect in superconductors and proximised normal metal nanodevices. The effect was discovered in gated nanoscale superconducting constrictions as a suppression of the critical current under the application of intense electric field and interpreted in terms of an electric-field induced perturbation propagating inside the superconducting film. Here we show that ours, and previously reported observations, governed by the overheating of the constriction, without recourse to novel physics. The origin of the overheating is a leakage current between the gate and the constriction, which perfectly follows the Fowler-Nordheim model of electron field emission from a metal electrode.c

Frequency-dependent drag from quantum turbulence produced by quartz tuning forks in superfluid He4

We have measured the drag force from quantum turbulence on a series of quartz tuning forks in superfluid helium. The tuning forks were custom made from a 75-μm-thick wafer. They have identical prong widths and prong spacings, but different lengths to give different resonant frequencies. We have used both the fundamental and overtone flexure modes to probe the turbulent drag over a broad range of frequencies f=ω/2π from 6.5 to 300 kHz. Optical measurements show that the velocity profiles of the flexure modes are well described by a cantilever beam model. The critical velocity for the onset of quantum turbulence at low temperatures is measured to be vc≈0.7κω−−−−−√ where κ is the circulation quantum. The drag from quantum turbulence shows a small frequency dependence when plotted against the scaled velocity v/v

Nanoscale Real-Time Detection of Quantum Vortices at Millikelvin Temperatures

Since we still lack a theory of classical turbulence, attention has focused on the conceptually simpler turbulence in quantum fluids. Reaching a better understanding of the quantum case may provide additional insight into the classical counterpart. That said, we have hitherto lacked detectors capable of the real-time, non-invasive probing of the wide range of length scales involved in quantum turbulence. Here we demonstrate the real-time detection of quantum vortices by a nanoscale resonant beam in superfluid 4He at 10mK. Essentially, we trap a single vortex along the length of a nanobeam and observe the transitions as a vortex is either trapped or released, detected through the shift in the beam resonant frequency. By exciting a tuning fork, we control the ambient vortex density and follow its influence on the vortex capture and release rates demonstrating that these devices are capable of probing turbulence on the micron scale

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

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 $^3$He-B at ultralow temperatures. We simulate the evolution of moderately dense, three-dimensional, quasiclassical vortex tangles and the Andreev reflection of thermal quasiparticle excitations by these tangles. This numerical simulation enables us to generate the two-dimensional map of local Andreev reflections for excitations incident on one of the faces of a cubic computational domain, and to calculate the total coefficient of Andreev reflection as a function of the vortex line density. Our numerical simulation is then compared with the experimental measurements probing quantum turbulence generated by a vibrating grid. We also address the question of whether the quasiclassical and ultraquantum regimes of quantum turbulence can be distinguished by their respective total Andreev reflectivities. We discuss the screening mechanisms which may strongly affect the total Andreev reflectivity of dense vortex tangles. Finally, we present combined numerical-experimental results for fluctuations of the Andreev reflection from a quasiclassical turbulent tangle and demonstrate that the spectral properties of the Andreev reflection reveal the nature and properties of quantum turbulence