Electronic instabilities and the Martensitic transition in A-15 compouns
Bhatt, Ravindra
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https://hdl.handle.net/2142/25646
Description
Title
Electronic instabilities and the Martensitic transition in A-15 compouns
Author(s)
Bhatt, Ravindra
Issue Date
1976
Doctoral Committee Chair(s)
McMillan, W.L.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
electronic instabilities
Martensitic transition
A-15 compounds
A-15 superconductors
Landau theory
charge density wave transition
Language
en
Abstract
The structural transition and anomalous properties of the high temperature A-15 superconductors have been investigated from two different standpoints. The first is a Landau theory based on Gorkov's physical model of a Peierls-like charge density wave (CDW) transition involving electronic CDW order parameters coupled to phonon coordinates. Pretransition elastic anomalies, softening of the [llOJ transverse ([110J polarized) phonon, sublattice distortions, variation of transition temperature with stress and alloying and other effects have been accurately predicted, and a detailed comparison is made with experimental results. Central peaks in neutron scattering are shown to be non-dynamic in nature and nopretransition forbidden (300) reflection is predicted. The f12 optic mode does not go soft at the transition, though its frequency is expected to be temperature dependent right up to room temperature.
Next, a tight binding, two (3-D) band model of the A-15 compounds,
using delta 1 (x2-y2) d-orbitals has been formulated, on the basis of Mattheiss' APW-LCAO band calculations, and with essentially one adjustable parameter, a calculation done of the magnetic susceptibility, electronic specific heat, low temperature distortion, and specific heat jump at the transition. The transition is pictured as a mixed Jahn-Teller and Peierls effect, and agreement with experiment is generally satisfactory. The results of the
calculation imply a reinterpretation of the CDW order parameters of the Landau theory; however, because of the phenomenological nature of the latter, the fits and results are still valid. Effects of enhancement due to strong electron-phonon coupling and lattice entropy, not included in the present model are discussed, and appear to be the cause of the few inadequacies of the present model.
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