Light scattering studies of heavy fermion and high Tc superconductors
Cooper, Stephen Lance
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https://hdl.handle.net/2142/23925
Description
Title
Light scattering studies of heavy fermion and high Tc superconductors
Author(s)
Cooper, Stephen Lance
Issue Date
1988
Doctoral Committee Chair(s)
Klein, Miles V.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
light scattering
heavy fermion
high Tc superconductors
excitation spectra
spin fluctuations
crystal field excitations
Language
en
Abstract
In this thesis, light scattering techniques are used to probe the diverse excitation
spectra of heavy fermion and high T 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 UBe13, UPt3, and
URu2Si2 reveals a substantial q=O 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=O spin relaxation rates we
observe in several heavy fermion superconductors differ markedly from those observed
in highu-q neutron scattering data, suggesting that intersite magnetic correlations are
present in these materials. Additionally, crystal field excitations are evident in
CeCu2Si2, 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 YBa2Cu307-5 reveal strong interband
electronic scattering which interferes strongly with certain phonons. These
interference effects indicate t_he presence of strong electron-phonon coupling in this
compound. Below Tc, 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
lV
presents strong evidence for substantial gap anisotropy in YBa2Cu301-B· However,
our lowest temperature data also demonstrates the presence of residual electronic
scattering well below the expected 2.!\ 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 c·
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