
#1
Oct613, 06:03 PM

P: 992

My book gives a treatment of this problem for crystal vibrations, but I don't really understand it. It says: There is one allowed value of K per volume (2[itex]\pi[/itex]/L)^{3}. But at the same time it has just shown that Kx,Ky,Kz can take values ±2[itex]\pi[/itex]/L which would certainly lead to more combinations of Kx,Ky,Kz within the volume confined by (2[itex]\pi[/itex]/L)^{3}. What am I misunderstanding.
Also: applying periodic boundary conditions yields the condition that Kx,Ky,Kz=±n2[itex]\pi[/itex]/L, while fixed ends yielded K=n[itex]\pi[/itex]/L, but my book says the two approaches yield identical results. How is that?? 



#2
Oct613, 11:26 PM

P: 37

if you read the sentence without pausing, the meaning is slightly different.
"There is one allowed value of K per volume , for each polarization and for each branch." so there are multiple K values allowed per volume 



#3
Oct713, 07:11 AM

P: 992

Well in this context, what does the word branch refer to? Different combinations of Kx,Ky,Kz?




#4
Oct813, 08:02 AM

P: 1,909

density of states in 3d
No, same combination of kx,ky,kz may correspond to different states with different energies.
Look at the acoustic and optic branch in a 1D chain with two types of atoms. This is the simplest example of "branches". Here for each k there are two energies (or frequencies). 


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