Strain-induced Pseudo-Magnetic Fields and flat band in Graphene*

The discovery of free-standing 2D atomic layers ushered in a new era of materials whose properties can be shaped controlled and engineered by unconventional means such as strain, bend or twist. An externally imposed periodic potential breaks up the electronic band structure into a series of mini-bands which, under certain circumstances, become almost dispersionless. Such flat bands can occur naturally in the presence of a vector potential generated by a strain-induced periodic pseudo-magnetic field (PMF). Within these flat bands, the strongly suppressed kinetic energy favors the formation of interaction-driven exotic phases which can give rise to novel electronic properties. In this talk, I will present experimental results demonstrating two pathways to generate strain induced PMFs and flat bands. In the first method, a graphene membrane is supported and strained by a two-dimensional (2D) periodic array (~ 1mm) of Au nano-pillars grown on a hexagonal Boron Nitride (hBN) substrate¹. A direct measure of the local strain is achieved by scanning tunneling microscopy (STM) through the magnifying effect of the Moiré pattern formed against the hBN substrate. The strain-induced PMF (~10T) is obtained from the pseudo-Landau level spectra observed in scanning tunneling spectroscopy (STS). The second method utilizes the buckling transition of a compressed graphene membrane to generate a 2D periodic strain pattern. This produces spatially modulated PMFs with periods of order ~10nm and amplitudes exceeding 100T. Using STM, STS and numerical simulations we find that the pseudo-Landau levels associated with the strain-induced periodic PMF give rise to a flat-band at charge-neutrality, providing a pathway to engineering correlated phases by non-chemical means.
¹ Jiang, Y. et al. Visualizing Strain-Induced Pseudomagnetic Fields in Graphene through an hBN Magnifying Glass. Nano Lett 17, 2839-2843, (2017).