What is the valley degree of freedom in Transition metal Di-Chalcogenides?
I'm studying the electronic properties of Transition metal Di-Chalcogenides (TMDs), and after reading a lot of papers, I became really confused about what is the valley degree of freedom. I tried to understand, but some places say that a valley is the conduction band minimum (that has a very nearly parabolic form), and others say that it is both the conduction band minimum and the valence band maximum.
I would like to understand how the valley degree of freedom is defined for the general case of an indirect band gap semiconductor, but also how it it defined for a monolayer of TMDs (specifically for MoS2).
Thank you , best regards!