Dynamic Properties of Anharmonic Phonons at Low Temperatures

Authors: E. H. TÜRECİ, T. HAKİOĞLU

Abstract: In this work we investigated the influence of low temperature lattice anharmonicity on the ground state properties of the lattice subsystem. The inspiration sprang from certain experiments revealing that strong low temperature anomalies of certain vibrational modes are common in most high temperature superconductors [1-5], and a recent study of the effect of phonon correlations on superconducting properties by one of the authors, Hakioglu et.al. [6]. We propose a model Hamiltonian comprising the harmonic and fourth order contributions and in conjunction put down an argument for the absence of the third order coupling. Relying on the fact that low temperature anharmonicity produces phase space correlations between otherwise independent phonon modes we assume lowest order pair correlations \phi^{ss'}(\vec{k}) = and proceed by writing down a squeezed variational ground state wavefunction depending on a single parameter, which is to be found via a self-consistent mean field theory within the LTDA model [7].

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