Acoustic waves in crystals: I. Ultrasonic flux imaging and internal diffraction. II. Imaging phonons in superconducting niobium
Hauser, Matt Russell
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https://hdl.handle.net/2142/19679
Description
Title
Acoustic waves in crystals: I. Ultrasonic flux imaging and internal diffraction. II. Imaging phonons in superconducting niobium
Author(s)
Hauser, Matt Russell
Issue Date
1995
Doctoral Committee Chair(s)
Wolfe, J.P.
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
Abstract
Acoustic wave propagation is studied via imaging techniques in two regimes not previously amenable to such experiments. (I) The low-temperature phonon-imaging technique is extended to ultrasonic frequencies at room temperatures where it is used to study the anisotropic propagation of acoustic waves in crystals. (II) The low-temperature heat-pulse technique is applied to a study of phonon transport in superconducting niobium, providing the first phonon images of a metal and revealing interesting interactions between phonons and quasiparticles.
"In the first part of this thesis the apparatus devised to perform ""ultrasonic flux imaging"" is described in detail. Measurements of acoustic flux in silicon show focusing patterns similar to the phonon focusing observed in heat-pulse experiments, but with additional structures. A theory is presented to explain these new structures in terms of interference between folded segments of the acoustic wave surfaces, an effect we call internal diffraction. Calculations based on this theory are shown to be in good agreement with experimental observations of silicon. Experimental results are also presented for several other materials."
The second part of the thesis presents the results of phonon imaging experiments on a crystal of superconducting niobium. Phonons are generated using a focused laser which either irradiates the niobium surface directly or heats a metal film evaporated onto the surface. For metal-film excitation, the dependence of ballistic-phonon flux on excitation energy indicates that the film emits phonons with a Planck frequency distribution characterized by a temperature that increases with excitation power. Phonons with energy less than the superconducting gap propagate ballistically while those with greater energy scatter frequently due to Cooper-Pair breaking. For direct optical excitation of the niobium and for high-energy excitation of the metal film, a broad peak is observed in the detected signal-versus-time. It is hypothesized that this peak is due to recombination of diffusing non-equilibrium quasiparticles, producing high frequency phonons which anharmonically decay into low-frequency phonons that are able to propagate ballistically. Scattering rates determined from the shapes of the heat pulses are higher than those expected from quasiparticle scattering. We postulated that at T $\leq$ 2K an additional scattering mechanism (possibly defect scattering) dominates the system.
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