Phonon-polaritons in Cu halides: Anomalies and their temperature dependence

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Abstract
The polariton Raman spectra of CuCl, CuBr, and CuI crystals are studied at various temperatures of their zinc-blende phase. Two polar modes are proven to exist in CuCl at room temperature and below; their oscillator-strength ratio changes with temperature and follows the temperature dependence of the lattice constant. In CuBr also two polar modes were observed at 300 and 80 K, but not at 2 K. In the latter case their oscillator-strength ratio shows an exponential temperature dependence. The two polar modes and their oscillator-strength temperature dependence are explained, assuming a temperature-dependent secondary mimimum in the potential energy of the copper ion. Polariton spectra of CuI at room temperature and below show just one polar mode and behave regularly.

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