Tunable Kondo effect in graphene with defects

Graphene is a model system for the study of electrons confined to a strictly two-dimensional layer(1) and a large number of electronic phenomena have been demonstrated in graphene, from the fractional(2,3) quantum Hall effect to superconductivity(4). However, the coupling of conduction electrons to local magnetic moments(5,6), a central problem of condensed-matter physics, has not been realized in graphene, and, given carbon's lack of d or f electrons, magnetism in graphene would seem unlikely. Nonetheless, magnetism in graphitic carbon in the absence of transition-metal elements has been reported(7-9), with explanations ranging from lattice defects(10) to edge structures(11) to negative curvature regions of the graphene sheet(12). Recent experiments suggest that correlated defects in highly-ordered pyrolytic graphite (HOPG), induced by proton irradiation(8) or native to grain boundaries(7), can give rise to ferromagnetism. Here we show that point defects (vacancies) in graphene(13) are local moments which interact strongly with the conduction electrons through the Kondo effect(6,14-16), providing strong evidence that defects in graphene are indeed magnetic. The Kondo temperature T(K) is tunable with carrier density from 30 to 90 K; the high T(K) is a direct consequence of strong coupling of defects to conduction electrons in a Dirac material(16).