Quantum straintronics: controlling the valley polarization, quantized flat bands and 2D electronic correlation by nanoscale strain engineering of single-layer graphene

We employed nanoscale strain engineering techniques to induce periodic strain on single-layer graphene (SLG) [1]. By placing strain-free SLG[2] and h-BN on top of e-beam lithography-fabricated periodic nano-pillar/cone arrays on a SiO₂/Si substrate, periodic graphene wrinkles with controlled strain were induced. Besides scanning tunneling spectroscopic evidence of strain-induced giant pseudo-magnetic fields and valley polarization [1], electrical transport studies of valley-Hall transistors built on strained graphene wrinkles were carried out. Nonlocal resistance and Hall resistance taken on multiple devices of different strain in zero external magnetic fields and at 1.8 K confirmed the existence of valley Hall effect (VHE), quantized Landau levels, and varying electron correlation proportional to the strain. Moreover, quantum VHE (QVHE), quantum anomalous Hall effect (QAHE) and spontaneous spin polarization were observed in the ballistic limit for strong correlation. Our approach thus establishes a new paradigm of quantum straintronics for tuning the 2D electron correlation. This work is jointly supported by ARO/MURI (Award #W911NF-16-1-0472), and NSF/IQIM at Caltech (Award #1733907).