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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Critical velocity and thermal effects in '3He-B at ultra-low temperatures and a search for superfluidity in dilute '3He-'4He mixtures

Coates, K. F. January 1988 (has links)
No description available.
2

Observation of Freedericksz transition in superfluid '3HeA

Gould, S. G. January 1986 (has links)
No description available.
3

Experimental study of two-dimensional helium mixture films absorbed on graphite

Patel, Hetal January 2002 (has links)
3He-4He mixture films adsorbed on graphite were studied down to temperatures below 1 mK using heat capacity and nuclear magnetic resonance (NMR) to probe the system. This work represents the first measurements in this temperature range of the various systems studied. The work focused on two types of mixtures, 3He °He mixtures within the second layer, and 'He floating on three layers of 4He. For mixtures within the layer, a homogeneous mixed phase was observed, followed by a phase interpreted as a Fermi liquid of 'He quasiparticle excitations tunneling within a solid 'He lattice. For the second type of mixture, the expected occupancy of the first excited surface normal state was observed. This is accompanied by anomalous behaviour at low temperatures (T < 10 mK) which arises due to a low concentration of 3He atoms being pushed into the third superfluid 4He layer. An NMR spectrometer utilising a cooled differential preamplifier operating at 4K was developed and used to measure the magnetic properties of helium mixtures. However, its signal to noise ratio was too low to provide the required accuracy in the measurements. An automated computer controlled data acquisition system was also developed to measure heat capacity and magnetic properties of the mixtures. The accompanying software required to process the data collected from both types of measurements was also developed and used successfully to obtain the data presented in this thesis
4

A microscopic theory for quantum evaporation from helium-II

Mulheran, Paul Alexander January 1991 (has links)
No description available.
5

Roton-roton scattering in liquid helium HE II

Forbes, Andrew Christopher January 1991 (has links)
No description available.
6

Flow properties of superfluid sup(3)He

Ling, R. January 1984 (has links)
No description available.
7

Experimentální a numerické studium kvantové turbulence v He II / Experimental and numerical investigation of quantum turbulence in He II

Varga, Emil January 2018 (has links)
Superfluid 4 He (He II) is a quantum liquid whose flow is strongly affected by quantum mechanical effects. This thesis presents experimental and numerical studies of turbulent flows in He II - quantum turbulence. Experimentally, quan- tum turbulence is investigated in thermal counterflow, pure superflow and coflow using second sound attenuation, precision local thermometry and by visualisa- tion of helium excimer molecules. The steady state and decay of the vortex line density in pure superflow and counterflow is studied and the universal quasi clas- sical decay is characterised by measurements of the effective kinematic viscosity. General dynamical behaviour is studied in detail in unsteady thermal counter- flow, with various theoretical models tested. A new model where the mean tangle curvature is dependent on the vortex line density is proposed. Temperature de- pendence and enhancement of intermittency in quasi-classical flow in the wake of a moving grid is found using visualisation. Numerically, the interaction of the tangle of quantized vortices with solid tracers is investigated, where a back reaction of the seeding particles on the tangle is identified and its relevance to visualisation experiments is discussed. Additionally, an interesting and as-yet overlooked spherical counterflow is...
8

Excitations in '3HE-B at ultra-low temperatures

Cousins, Derek John January 1997 (has links)
No description available.
9

Detecting continuous gravitational waves with superfluid 4He

Singh, S, Lorenzo, L A De, Pikovski, I, Schwab, K C 21 July 2017 (has links)
Direct detection of gravitational waves is opening a new window onto our universe. Here, we study the sensitivity to continuous-wave strain fields of a kg-scale optomechanical system formed by the acoustic motion of superfluid helium-4 parametrically coupled to a superconducting microwave cavity. This narrowband detection scheme can operate at very highQ-factors, while the resonant frequency is tunable through pressurization of the helium in the 0.1-1.5 kHz range. The detector can therefore be tuned to a variety of astrophysical sources and can remain sensitive to a particular source over a long period of time. For thermal noise limited sensitivity, we find that strain fields on the order of h similar to 10(-23)/root Hz are detectable. Measuring such strains is possible by implementing state of the art microwave transducer technology. Weshow that the proposed system can compete with interferometric detectors and potentially surpass the gravitational strain limits set by them for certain pulsar sources within a few months of integration time.
10

Structure and dynamics of vortices in superfluid helium-3

Karimäki, J. (Janne) 06 February 2012 (has links)
Abstract In this thesis, the structure and the dynamics of vortices are studied from the standpoint of the hydrodynamical theory of superfluids. In the hydrodynamical theory a superfluid is described by a continuous order parameter field. In the case of superfluid helium-4 this field is a complex-valued function of position and time. However, in superfluid helium-3 the order parameter is a complex-valued 3 × 3 matrix. The first part of this work consists of studies on structures that appear in the order parameter field, when a vessel filled with superfluid helium-3 in the A phase (3He-A) is rotated in an external magnetic field. Among the most common of these structures are the so-called continuous vortices. They exist in several different forms. In addition to vortices, other possible structures include the vortex sheet of 3He-A that was discovered at the Low Temperature Laboratory of Helsinki University of Technology (currently Aalto University) in late 1993. In this thesis, these structures were studied by finding stationary vortex configurations that minimize the free energy of the superfluid. An algorithm for minimizing the free energy was implemented by writing a computer program. This program was then used to study the structure of a few vortex types, inferred to be the most probable ones. In addition, regular lattices formed by these vortices, including the vortex sheet, were studied. A phase diagram for vortex lattices was constructed by comparing the free energy of various lattice structures as a function of rotational velocity and external magnetic field. The study of vortex structures also lead to a discovery of a new type of vortex in 3He-A, later named the LV3 vortex. In the second part of the work, the dynamics of vortices was studied using a filament model of vortex motion, which also has its theoretical justification in the hydrodynamical model of superfluids, but where the detailed structure of the vortex core is not relevant. The specific problem under consideration here was the motion of a quantized vortex in a rotating elongated cylinder filled with superfluid, and how the motion of the vortex depends on temperature and the rotational velocity of the vessel. The study of vortex motion was simplified using scaling laws. A new type of scaling law was discovered, which both simplified the specific problem under study, and made the results more general. In summary, the research in this thesis touched upon two somewhat complementary areas, i.e. the structure of continuous vortices in 3He-A and the dynamics of thin vortex lines, which is more applicable to superfluid 4He or to the B phase of superfluid helium-3 (3He-B). However, these areas complement each other in advancing the general scientific understanding about the properties of superfluids.

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