Strain-Engineered Correlated Phases and Band Flattening in Graphene via Stressor Arrays (Part 2)

In this work, we investigate the spatially varying heterostrains, induced by square thin-film stressor arrays, on graphene using scanning tunneling microscopy/spectroscopy (STM/STS). Approaching the stressor, we measure an increasing strain gradient, generating a pseudo magnetic field on the order of 100 T. Near the corner of the stressor, the anisotropic strain gradient leads to band splitting and flattening, suggesting additional broken symmetry. Our STS map taken here shows spatially localized states with a series of flat fractal energy bands on strain-induced wrinkles. We will discuss the possible origins of these spectral features, including quantum confinement and Hofstadter butterfly, of the observed fractal energy bands. Furthermore, within a few 100 nm from the stressor edge, we observe a Kekulé-like extra charge order of about √(3) x √(3) R30° , tripling the unit cell of graphene, and our STS map and STM scans identify vortex-like textures of this charge order in the lattice. The STS maps in this region also reveal the opening of a large, unprecedented, semiconducting gap. To summarize, we observe various strain-driven phenomena in graphene-based devices via stressor arrays, and establish a robust platform for strain engineering in graphene-based devices.