C. P. J. Adolphs, M. Berciu
Using the momentum average approximation we show that the linear Holstein Hamiltonian can be used to study single polarons only for weak electron-phonon couplings. For medium and strong linear coupling, quadratic and higher order electron-phonon coupling terms need to be included in the model. Doing so has drastic consequences for the properties of the polaron, which cannot be captured by a linear Holstein Hamiltonian with renormalized parameters. This shows that the linear Holstein Hamiltonian is inappropriate to model systems with strong electron-phonon coupling, at least for low carrier concentrations.
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http://arxiv.org/abs/1302.3843
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