Visualizing broken symmetry and topological defects in a quantum Hall ferromagnet

The interaction between electrons in graphene under high magnetic fields drives the formation of a rich set of quantum Hall ferromagnetic (QHFM) phases with broken spin or valley symmetry. Visualizing atomic-scale electronic wave functions with scanning tunneling spectroscopy (STS), we resolved microscopic signatures of valley ordering in QHFM phases and spectral features of fractional quantum Hall phases of graphene. At charge neutrality, we observed 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 could visualize valley skyrmion excitations localized near charged defects. Our techniques can be applied to examine valley-ordered phases and their topological excitations in a wide range of materials.

Automated summary

The interaction between electrons in graphene under high magnetic fields leads to the formation of various quantum Hall ferromagnetic (QHFM) phases with broken spin or valley symmetry. Using scanning tunneling spectroscopy (STS), we were able to visualize atomic-scale electronic wave functions and observe valley ordering in QHFM phases, as well as spectral features of fractional quantum Hall phases in graphene. The results provide insights into the electronic properties of materials and their topological excitations.