  A continuum theory to describe periodic ripple formation in planar
graphene/boron nitride superlattices is formulated. Due to the lattice mismatch
between the two materials, it is shown that flat superlattices are unstable
with respect to ripple formation of appropriate wavelengths. A competition
between bending energy and transverse stretching energy gives rise to an
optimal ripple wavelength that depends on the superlattice pitch. The optimal
wavelengths predicted by the continuum theory are in good agreement with atomic
scale total energy calculations previously reported in Nandwana and Ertekin,
Nano Lett. 15 1468 (2015).
