Scanning tunneling microscopy of the π magnetism of a single carbon vacancy in graphene

Pristine graphene is strongly diamagnetic. However, graphene with single carbon atom defects could exhibit paramagnetism. Theoretically, the π magnetism induced by the monovacancy in graphene is characteristic of two spin-split density-of-states (DOS) peaks close to the Dirac point. Since its prediction, many experiments have attempted to study this π magnetism in graphene, whereas only a notable resonance peak has been observed around the atomic defects, leaving the π magnetism experimentally elusive. Here, we report direct experimental evidence of π magnetism by using a scanning tunneling microscope. We demonstrate that the localized state of the atomic defects is split into two DOS peaks with energy separations of several tens of meV. Strong magnetic fields further increase the energy separations of the two spin-polarized peaks and lead to a Zeeman-like splitting. Unexpectedly, the effective g factor around the atomic defect is measured to be about 40, which is about 20 times larger than the g factor for electron spins.