Strain Modulated Superlattices in Graphene
by Dr. Riju Banerjee (Department of Physics, Pennsylvania State University, USA)
Tuesday, August 6, 2019 from to (Asia/Kolkata)
Heterostructures created by combining two-dimensional (2D) materials provide a large platform for tunability where material choice, stacking order and rotational angle can all affect the resultant electronic properties. In this talk, I will present a different approach to generate an effective 2D lateral heterostructure – by periodic modulation of lattice strain. We engineer extreme (>10%) strain in graphene by draping it over large (up to 35nm) Cu step edges. Analogous to a draped tablecloth, nanoscale periodic ripples arise as the graphene is pinned and pulled by the contact forces of the substrate. Scanning tunneling microscopy (STM) of these ripples reveals that classical scaling laws fail to explain their shape. Instead, graphene behaves like a bizarre fabric that regardless of how it is pulled always buckles at the same angle. The ripples are characterized by large variations in carbon-carbon bond length. Such variations directly impact electronic coupling between atoms, which in one graphene ripple can be as different as in two different materials. The result is a single graphene sheet which effectively acts as an electronic superlattice in which novel electronic states arise at the interfaces. Combining scanning tunneling microscopy and atomistic calculations, we find that these electronic states are driven by interfacial pseudo-gauge fields on the order of 100 T and 107 V/m. Such intense, highly inhomogeneous fields create a new electronic quantization distinct from the usual Landau quantization observed in uniform fields, and their nanoscale periodicity creates a previously unrealizable electronic system which can aid in the realization of various theoretical proposals including valley filters, snake states and electron optics in graphene and other 2D materials.