Reconfigurable Internal Quantum Hall Boundaries in Graphene Visualized by Multimodal Nano-Imaging

Internal, gate-tunable quantum Hall (QH) boundaries offer a way to steer topological charge flow beyond static device edges. In this talk, we demonstrate the real-space formation and control of an incompressible quantum Hall strip inside monolayer graphene placed on a high-dielectric SrTiO₃ (STO) substrate. These stripes are formed spontaneously due to a large charge carrier gradient in graphene induced by the position-dependent doping from the STO substrate. Using terahertz (THz) space-time near-field nanoscopy, we map local plasmon phase velocities to quantify the built-in carrier-density gradient across the device. In addition, infrared photocurrent nanoscopy directly images the chiral current channel that follows the incompressible strip, which moves in real space under applied gate voltage and magnetic field. Our results show that the multimodal imaging tools we developed open a new path toward engineering and probing internal quantum Hall boundaries in two-dimensional systems.