  When two planar atomic membranes are placed within the van der Waals
distance, the charge and heat transport across the interface are coupled by the
rules of momentum conservation and structural commensurability, leading to
outstanding thermoelectric properties. Here we show that an effective
"interlayer phonon drag" determines the Seebeck coefficient (S) across the van
der Waals gap formed in twisted bilayer graphene (tBLG). The cross-plane
thermovoltage, which is non-monotonic in both temperature and density, is
generated through scattering of electrons by the out-of-plane layer breathing
(ZO'/ZA2) phonon modes and differs dramatically from the expected
Landauer-Buttiker formalism in conventional tunnel junctions. The tunability of
the cross-plane Seebeck effect in van der Waals junctions may be valuable in
creating a new genre of versatile thermoelectric systems with layered solids.
