Inducing flat bands in two-dimensional materials by zone folding and hybridization*

A two-dimensional material hosting a flat electronic dispersion at the Fermi level is an appealing system¹ since the associated singularity in the density of states promotes a variety of electronic and structural instabilities and can drive phase transitions into a superconductor, a charge-density wave state or magnetic order. In graphene the presence of the Van Hove singularity is a possible way to induce a flat band, though its realization is hindered by the extreme doping needed. We present a method to induce a flat band at the Fermi level of graphene by sandwiching it in between two ordered metal layers. We realize this system by depositing cesium on graphene grown on Ir(111) substrate. By ARPES, we observe a flat dispersion in a large portion of the Brillouin zone. We propose a microscopic structural model, demonstrating that zone-folding due to cesium reconstruction and the consequent cesium/graphene hybridization are at the origin of the flat band formation² and predicting the experimental results with excellent accuracy.

¹Cao, Y. et al. Nature 556, 43–50 (2018)
² Ehlen, N. et al. ACS Nano 2020, 14, 1, 1055–1069 (2019)