  Two-dimensional atomic crystals can radically change their properties in
response to external influences such as substrate orientation or strain,
resulting in essentially new materials in terms of the electronic structure. A
striking example is the creation of flat-bands in bilayer-graphene for certain
'magic' twist-angles between the orientations of the two layers. The quenched
kinetic-energy in these flat-bands promotes electron-electron interactions and
facilitates the emergence of strongly-correlated phases such as
superconductivity and correlated-insulators. However, the exquisite fine-tuning
required for finding the magic-angle where flat-bands appear in twisted-bilayer
graphene, poses challenges to fabrication and scalability. Here we present an
alternative route to creating flat-bands that does not involve fine tuning.
Using scanning tunneling microscopy and spectroscopy, together with numerical
simulations, we demonstrate that graphene monolayers placed on an
atomically-flat substrate can be forced to undergo a buckling-transition,
resulting in a periodically modulated pseudo-magnetic field, which in turn
creates a post-graphene material with flat electronic bands. Bringing the
Fermi-level into these flat-bands by electrostatic doping, we observe a
pseudogap-like depletion in the density-of-states, which signals the emergence
of a correlated-state. The described approach of 2D crystal buckling offers a
strategy for creating other superlattice systems and, in particular, for
exploring interaction phenomena characteristic of flat-bands.

Mesoscale and Nanoscale Physics