<html><head><style type="text/css"><!-- DIV {margin:0px;} --></style></head><body><div style="font-family:arial, helvetica, sans-serif;font-size:10pt"><DIV>Hello everyone</DIV>
<DIV>I have a question, i wonder what is the physics behind the flatterning of the acoustic and optical phonon branches in some specific directions of the 1st Brillouin zone of some of the crystals? </DIV>
<DIV>Thanks for any comment</DIV></div><br>
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