Join us for the Physics Condensed Matter Seminar at 10:00 am
Title: Visualizing broken symmetry, topological excitations and Landau orbits in the graphene quantum Hall system.
abstract: The interaction between electrons in graphene under high magnetic fields drives the formation of a rich set of quantum Hall ferromagnetic phases (QHFM) and fractional quantum Hall (FQH) states. On ultra-clean graphene devices, we demonstrate observations of QHFM and FQH states in scanning tunneling spectroscopy (STS) for the first time. Visualizing atomic scale electronic wavefunctions with STS, we resolve microscopic signatures of valley ordering in QHFM and FQH phases of graphene. At charge neutrality, we observe a field-tuned continuous quantum phase transition from a valley polarized state to an intervalley coherent state, with a Kekule distortion of its electronic density. Mapping the valley texture extracted from STS measurements of the Kekule phase, we visualize valley skyrmion excitations localized near charged defects. The charged defects also induce energy splitting between different angular momentum states, which enables us to visualize Landau orbits at various filling factors. Our techniques can be applied to examine valley ordered phases and their topological excitations in a wide range of materials.