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6. A one-dimensional diatomic crystal contains 2 different atoms with masses ?1 and ?2, with ?1<...

6. A one-dimensional diatomic crystal contains 2 different atoms with masses ?1 and ?2, with ?1< ?2, in each of its unit cell. Assume that there is interaction between every nearest neighbor atoms that can represented by a spring with spring constant ?. Derive the phonon frequency dispersion relation ?(?) of the crystal. Indicate which one is the acoustic branch and which one is the optical branch.

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