Kondo effect due to a hydrogen impurity in graphene: A multichannel Kondo problem with diverging hybridization*

We consider the Kondo effect arising from a hydrogen impurity in graphene. Approximating the C-H covalent bond as infinitely strong and the Hubbard interaction to be present only on the three nearest neighbors of hydrogen impurity, we obtain a Kondo model with three Z₃-symmetric impurity spins and three conduction channels, two of which support a diverging local density of states ∝1/[|ω| ln²(Λ/ω)] near the Dirac point ω→0. When the particle-hole (p-h) symmetry breaking at the impurity is not too strong, numerical renormalization group shows that the ground state is either a p-h symmetric spin-1/2 doublet with ferromagnetic impurity spin correlations, or a p-h asymmetric spin singlet with antiferromagnetic impurity spin correlations. This behavior is inherited by the Anderson model containing the hydrogen impurity and all four carbon atoms in its vicinity.