Posted by Ali on May 26, 2013, 10:18 am, in reply to "Re: Longitudinal modes of Germanium"
So this is the lecture where you applied degenerate perturbation theory to the divalent, 2dimensional metal? Q5, 2011? OK, so [100] can't scatter to [100] because the structure factor disappears for [200]. [111] can't scatter to [111] for the same reason, I found that the structure factor also disappears for [222] so it seems [111] can't scatter to [111] either. BUT, scattering occurs for [220] so [1,1,1] can scatter to [1,1,1], [1,1,1] and [1,1,1], so the states are split and it's NOT degenerate, is that the right idea? So the fact that the question specifies 'longitudinal modes' doesn't have any relevance to the calculation? 
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