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XKEYWORD: IP-phonon

# Interface phonons

FK-phonons (Fuchs and Kliewer FK-1965 PR140) are interface phonon-polaritons, the decaying solutions to the (TO-phonon) transverse part of the phonon equations (see Babiker-1992) (WARNING: THIS IS NOT CERTAIN). Because they are transverse, their interaction with electrons is by an A.p interaction term (Babiker-1992 p B58). There seem to be no s-polarised interface phonons (mentioned in Babiker-Z-1998).

There are other "interface" (evanescent) phonons, corresponding to LO-phonons with k_L imaginary.

The FK-phonon mode displacements are

w_x = -i k_x ( e^{k_L z} \pm \exp^{-k_L z} ) e^{ i (k_x x - \omega t) }
w_z = -  k_x ( e^{k_L z} \mp \exp^{-k_L z} ) e^{ i (k_x x - \omega t) }

\phi_{LO} = -\rho_{IP} ( e^{k_L z} \pm \exp^{-k_L z} ) e^{ i (k_x x - \omega t) }


OLD: These are phonons localised on interfaces between different layers. More specifically, this file deals with Fuchs and Kliewer interface phonons (FK-phonons), and not the decaying LO-phonon or TO-phonon modes that can be attached to interfaces when k_L or k_T is imaginary.

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