  Heat conduction mechanisms in superlattices could be different across
different types of interfaces. Van der Waals superlattices are structures
physically assembled through weak van der Waals interactions by design, and may
host properties beyond the traditional limits of lattice matching and
processing compatibility, offering new types of interfaces. In this work,
natural van der Waals (SnS)1.17(NbS2)n superlattices are synthesized, and their
thermal conductivities are measured by time-domain thermoreflectance as a
function of interface density. Our results show that heat conduction of
(SnS)1.17(NbS2)n superlattices is dominated by interface scattering when the
coherent length of phonons is larger than the superlattice period, indicating
incoherent phonon transport dominates cross-plane heat conduction in van der
Waals superlattices even when the period is atomically thin and abrupt.
Moreover, our result suggests that the widely accepted heat conduction
mechanism for conventional superlattices that coherent phonons dominate when
the period is short, is not applicable due to symmetry breaking in most van der
Waals superlattices. Our findings provide new insight for understanding the
thermal behavior of van der Waals superlattices, and devise approaches for
effective thermal management of superlattices depending on the distinct types
of interfaces.
