Light Scattering Studies of Heavy Fermion and High T(c) Superconductors
Cooper, Stephen Lance
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https://hdl.handle.net/2142/77424
Description
Title
Light Scattering Studies of Heavy Fermion and High T(c) Superconductors
Author(s)
Cooper, Stephen Lance
Issue Date
1988
Doctoral Committee Chair(s)
Klein, Miles
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
Physics, Optics
Language
eng
Abstract
In this thesis, light scattering techniques are used to probe the diverse excitation spectra of heavy fermion and high T$\sb{\rm c}$ superconductors.
Our studies of heavy fermion compounds principally demonstrate the efficacy of using light scattering techniques to probe spin fluctuations and crystal field excitations in metals. For example, light scattering from spin fluctuations in UBe$\sb $, UPt$\sb3$, and URu$\sb2$Si$\sb2$ reveals a substantial q = 0 contribution to the magnetic susceptibility in these compounds. This result demonstrates that a spin-conserving Fermi liquid description of heavy fermion compounds is invalid. Furthermore, the q = 0 spin relaxation rates we observe in several heavy fermion superconductors differ markedly from those observed in higher-q neutron scattering data, suggesting that intersite magnetic correlations are present in these materials. Additionally, crystal field excitations are evident in CeCu$\sb2$Si$\sb2$, and are found to exhibit damping effects which are consistent with theoretical predictions. Finally, an investigation of the phonon spectra of several heavy fermion compounds presents evidence for anomalous magnetoelastic and deformation potential coupling in some heavy fermion compounds.
Polarized light scattering studies of YBa$\sb2$Cu$\sb3$O$\sb{7-\delta}$ reveal strong interband electronic scattering which interferes strongly with certain phonons. These interference effects indicate the presence of strong electron-phonon coupling in this compound. Below T$\sb{\rm c}$, the electronic continuum exhibits both a loss of spectral weight at low energies, and a broad peak at high energies due to the direct excitation of superconducting quasiparticle pairs above the gap. The observed anisotropy in the Raman matrix elements associated with these superconducting gap excitations presents strong evidence for substantial gap anisotropy in YBa$\sb2$Cu$\sb3$O$\sb{7-\delta}$. However, our lowest temperature data also demonstrates the presence of residual electronic scattering well below the expected 2$\Delta$ quasiparticle pair-breaking energy. This result suggests that a continuum of electronic states exists well inside the superconducting gap. The transition to the superconducting state is also reflected in a Raman active phonon which demonstrates interesting quasiparticle self-energy effects below T$\sb{\rm c}$.
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