Kondo effect refers the phenemena where the local magnetic moment is screened by the antiferromagnetic spin alignment of free electrons from a metallic background, forming a resonance-type many-body ground state, so-called Kondo singlet state. The Kondo effect is one of the key elements to understand many-body effects in a solid system related to its magnetic properties.

However, the two-dimensional nature of the Kondo effect has been barely understood, especially when two-dimensional Kondo lattice has not been realized yet. We have explored the Kondo effect in a prototypical two-dimensional system, graphene. Kondo impurity and Kondo lattice has been realized in graphene by the presence of a magnetic impurity, Ce, and by the proximity of topological Kondo insulator, SmB₆, respectively. Most importantly, we report, for the first time, an experimental evidence of the Kondo effect, controlled by charge carrier density.