Thesis (Ph. D.)--Florida State University, 2003. / Advisor: Dr. James S. Brooks, Florida State University, College of Arts and Sciences, Dept. of Physics. Title and description from dissertation home page (viewed Sept. 29, 2003). Includes bibliographical references.

Direct measurements have been made of the velocity of second sound in liquid helium over the temperature range T[subscript ʎ]-T from 1.3 x 10⁻² K to 5 x 10⁻⁶K. Using previously determined relationships for the specific heat, superfluld density, and thermal conductivity near the lambda point, consistency has been demonstrated between the measurements, velocities predicted by superfluld hydrodynamics, and certain scaling law predictions. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Previous experimental studies have reported wave-like transport of heat in a small number of material systems, such as superfluids like helium II and crystal solids like bismuth. This phenomenon was henceforth referred to as 'second sound'. These rare observations of second sound are partly due to the challenge of obtaining accurate theoretical predictions. In this work, we use an ab-initio framework to study phonon hydrodynamics in 3D crystal fluorides and alkali hydrides, including sodium fluoride (NaF), lithium fluoride (LiF), lithium hydride (LiH), and sodium hydride (NaH). Moreover, we predict the existence of phonon hydrodynamics regime in bilayer graphene systems, including AA-bilayer graphene and AB-bilayer graphene. First, we obtain the second and third-order interatomic force constants using first-principles calculations, which are based on density functional theory (DFT). Secondly, we calculate the lattice thermal conductivity and phonon scattering rates by solving the Boltzmann transport equation (BTE). Thirdly, we apply Guyer's condition to show the phonon hydrodynamics regime based on the average Normal, Umklapp, and Boundary scattering rates. Finally, we examine the effect of different pseudopotentials on the thermal, electronic, and mechanical properties as well as the phonon hydrodynamics regime. In addition, we report the effect of isotopes on the lattice thermal conductivity and phonon hydrodynamics regime. Our calculations predict the existence of the second sound in NaF at 15 K and 8.3 mm characteristic length, consistent with previous experimental work. Based on Guyer's condition, the hydrodynamic window was determined in terms of characteristic lengths (~10² - ~10⁸ nm) and temperatures (up to ~80 K) for fluorides and alkali hydrides. On the other hand, second sound in 2D materials has been predicted to exist at much higher temperatures relative to 3D materials. We report the existence of a second sound for AA-bilayer graphene and AB-bilayer graphene above room temperature at a characteristic length of ~100 nm.

Experimental investigations of liquid helium flows Selected turbulent flows of He II, the superfluid phase of liquid 4 He, are inves- tigated experimentally. The second sound attenuation technique is employed to directly probe the tangle of quantized vortices, thin topological defects within the superfluid, while relatively small particles made of solid hydrogen are dispersed in He II to visualize the overall flow of the liquid via the particle tracking ve- locimetry. Considering the known particle-vortex interaction mechanisms, steady thermal counterflow in a square channel is investigated. Significant inhomogene- ity of the vortex tangle density along the channel height (near the flow-generating heater) is shown to develop. The means of energy transport in turbulent flows of He II are found strikingly different from those taking place in turbulent flows of viscous fluids. Moreover, individual particles in counterflow are observed to intermittently switch between two distinct motion regimes along their trajecto- ries. The regimes are identified and qualitatively described. Steady counterflow jets in He II are realized and the spatial arrangement of the underlying vortex tangle is explored. Finally, macroscopic vortex rings are thermally generated and observed in He II. A method for tracking their...

Steady state and decay of quantum turbulence generated in channel flows and detected by second sound attenuation Bc. Emil Varga Abstract Quantum turbulence is studied in superfluid 4 He under classical flow condi- tions. Turbulence is generated by a flow through a 7 × 7 mm square channel with a flow conditioner either with an additional grid or without it. The flow is generated mechanically by squeezing a stainless steel bellows. Vortex line den- sity is measured by attenuation of second sound in both steady state and decay for a range of temperatures 1.17 - 2.16 K. In the steady state, temperature- independent scaling of the vortex line density with flow velocity of the form L ∝ V 3/2 is observed. In the decay the expected late-time behaviour L ∝ t−3/2 is observed. Explanation for both of these observations is based on a quasi- classical model of quantum turbulence, that allows the extraction of the effective kinematic viscosity νeff, which approximately agree with the values available in the literature. Two models based on counterflow theory are also explored and the effect of inhomogeneous vortex line distribution on the measurement tech- nique is studied.

We have performed a study of quantum turbulence generated in oscillatory counterflow as a continuation of previous experiments on various channel flows of superfluid helium, in the form of coflow, thermal DC counterflow and pure superflow. We have investigated its development, steady state properties and temporal decay, as well as the effect of the resonant mode used to generate the turbulence at three different temperatures, 1.45 K, 1.65 K and 1.83 K. The attenuation of low amplitude second sound, orientated perpendicularly to the long axis of the resonator, was used to determine the amount of quantized vortices created. One of the main goals of this work was to characterize the critical parameters for the onset of instabilities in oscillatory counterflow and to determine their values. Decay measurements of the vortex line density allowed us to distinguish between Vinen-type and Kolmogorov- type decays of quantum turbulence.