Why is the valence band maxima at k = 0?

[nmbr28albert]

In general, materials are either direct or indirect materials. For direct materials, both the valence band maxima and the conduction band minima are at k = 0. For indirect materials, only the valence band maxima is at k = 0 and the conduction band minima is at some other non-zero value of k. In all the books I've read, it seems to always be assumed that the valence band maxima is at k = 0. Is this true, and if so why?

 

[Dr Transport]

by definition

 

[Anton Schneider]

I have always seen the same, I think it's arbitrary defined like this.

 

[nmbr28albert]

After some digging, I found that it actually isn't true that the valence band maxima is always at k = 0. In lead sulfide for example, the valence band maximum is at the L point.

 

[DrDu]

Graphene and gaphite also come to my mind.