(b). In this case, since the coupling in between layers are weaker than the coupling inside the layers, at extremely low temperatures, the degrees of freedom for the motion inside the layers will be frozen, and the motion of the atoms is restricted in the direction perpendicular to the layers. Therefore the system becomes 1- D.Follow a similar calculation as in part (a), we can find the heat capacity for 1- D system in low temperature will be proportional to T.Remark: A general statement is: for a d dimensional system with a dispersion relation in the form ω = AKα, the heat capacity of the system in low temperature limit will be proportional to Td/α. You can easily prove this statement in the way similar as in above.
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