Phonon anomalies in high temperature superconductors
Author
Türeci, E Hakan
Advisor
Hakioğlu, Tuğrul
Date
1996Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
Anomalously large low temperature phonon anharmonicities can lead to static
as well as dynamical changes in the low temperature properties of the vibrational
system. In this work, we focus our attention on the low temperature lattice
anharmonicity and its effect on the electron-phonon ground state. We are,
in particular, motivated by certain high temperature superconductors. The
third and fourth order anharmonic coupling constants for YBCO, LBCO and
several other superconducting compounds are extracted from their measured
elastic constants using the anharmonic elastic continuum model. The coupling
constants are then used to extract the average anharmonic potential energy for the
transverse and longitudinal modes in the Cu-0 planes. We find that, anharmonic
contribution to the lattice potential relative to the harmonic one is unusually high
for all examined high Tc compounds.
The presence of anharmonic phonons elicits non-perturbative dynamical
effects in the ground state of the electron-phonon system. Phonon correlations
induced by anharmonic effects enhance the electron-phonon interaction which
then create a self-consistent mechanism to act back on the ground state of
the electron-phonón system. In result, strong momentum correlations are
created and the ground state comprises fluctuating polarons. The zero point
fluctuations and other ground state properties are obtained by self-consistent
numerical calculations. The influence of low temperature phonon anharmonicity
on the superconducting properties in the intermediate coupling range is also
investigated. It is shown that, the otherwise bare electron-phonon coupling is
strengthened in the presence of correlated polarons and the zero point fluctuations
are enhanced. Within this frame, it is plausible to achieve superconducting
transition temperatures as high as %20 of the characteristic vibrational energy
scale. The non-perturbative, self-consistent formalism thus introduced also offers
an account for the recently observed temperature anomalies near Tc in the
Debye-Waller factor and dynamical pair correlations of certain high temperature
superconductors.
Keywords
Phonon anharmonicityelectron-phonon interaction
Fröhlich Hamiltonian
squeezed states
polarons
polarons
high temperature superconductors
elastic continuum theory