Posted by Steve Simon on February 26, 2014, 7:37 am, in reply to "Re: Normal modes of a 1D Chain"
Yes, the cosine is the optical and the sin is the acoustic mode.
The confusion is from the fact that you are viewing the diatomic chain in reduced zone scheme so you have two modes per value of k. If you viewed it in exteneded zone scheme you would find only one mode per k and it would match perfectly with the monatomic dispersion in the limit where m1 =m2.
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