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Relying on the anharmonic special displacement method, we introduce an ab initio quasistatic polymorphous framework to describe local disorder, anharmonicity, and electron-phonon coupling in superionic conductors. Using cubic Cu2Se, we show that positional polymorphism yields the breakdown of the phonon quasiparticle picture, leading to extremely overdamped anharmonic vibrations while preserving transverse acoustic phonons, consistent with experiments. We also demonstrate highly broadened electronic spectral functions with band gap openings of 1.0 eV due to polymorphism, and that anharmonic electron-phonon coupling leads to a band gap narrowing with increasing temperature. Our approach, relying on generating a handful of configurations, opens the way for efficient calculations in superionic crystals to elucidate their compelling high figure of merit.
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Peculiar anharmonicity of Ruddlesden Popper metal halides: temperature-dependent phonon dephasing
The anharmonicity of the Ruddlesden Popper metal-halide lattice, and its consequences for their electronic and optical properties, are paramount in their basic semiconductor physics. It is thus critical to identify specific anharmonic optical phonons that govern their photophysics. Here, we address the nature of phonon–phonon scattering probabilities of the resonantly excited optical phonons that dress the electronic transitions in these materials. Based on the temperature dependence of the coherent phonon lifetimes, we isolate the dominant anharmonic phonon and quantify its phonon–phonon interaction strength. Intriguingly, we also observe that the anharmonicity is distinct for different phonons, with a few select modes exhibiting temperature-independent coherence lifetimes, indicating their predominantly harmonic nature. However, the population and dephasing dynamics of excitons are dominated by the anharmonic phonon.
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- Award ID(s):
- 1904293
- PAR ID:
- 10310188
- Date Published:
- Journal Name:
- Materials Horizons
- ISSN:
- 2051-6347
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1039/D1MH01010G
