  The ability to manipulate two-dimensional (2D) electrons with external
electric fields provides a route to synthetic band engineering. By imposing
artificially designed and spatially periodic superlattice (SL) potentials, 2D
electronic properties can be further engineered beyond the constraints of
naturally occurring atomic crystals. Here we report a new approach to fabricate
high mobility SL devices by integrating surface dielectric patterning with
atomically thin van der Waals materials. By separating the device assembly and
SL fabrication processes, we address the intractable tradeoff between device
processing and mobility degradation that constrains SL engineering in
conventional systems. The improved electrostatics of atomically thin materials
moreover allows smaller wavelength SL patterns than previously achieved.
Replica Dirac cones in ballistic graphene devices with sub 40nm wavelength SLs
are demonstrated, while under large magnetic fields we report the fractal
Hofstadter spectra from SLs with designed lattice symmetries vastly different
from that of the host crystal. Our results establish a robust and versatile
technique for band structure engineering of graphene and related van der Waals
materials with dynamic tunability.
