Peierls phonons in organic molecular crystals and in charge transfer salts
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We review the Quasi-Harmonic Lattice Dynamic (QHLD) method, which we have recently implemented and adopted to carefully reproduce the crystal structure and lattice phonon dynamics of molecular crystals as a function of temperature and pressure. Association with mean field electronic structure calculations allows us to characterize the Peierls coupling, namely the coupling between electrons and lattice phonons. We apply this method to organic superconductors based on bis-ethylene-dithio-tetrathiafulvalene (BEDT-TTF), showing that many experimental findings related to superconducting properties are rationalized in terms of the Peierls coupling. Electron-intramolecular phonon coupling and electron -electron interactions, however, have to be taken into account for a full characterization. We also present results concerning another class of molecular crystals, the acenes. In this case, the focus is on the understanding of the temperature dependences of mobilities. First and foremost, however, we emphasize the possibility of accurately predicting both the crystal structure and lattice phonon spectral signatures. We analyse pentacene and tetracene, showing that both systems can crystallize into two different polymorphs. The two polymorphs have comparable stabilities, and can coexist in the same crystallite. Raman spectroscopy in the lattice phonon region is used as a convenient tool to identify the two phases. The Peierls coupling strength of pentacene is evaluated.
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