Theory of Defect-Induced Kondo Effect in Graphene: Effect of Zeeman Field

The effect of the Zeeman field on the defect-induced Kondo effect in graphene is investigated. The effective model of the Kondo effect is derived, and is analyzed on the basis of the numerical renormalization group method. It is found that, under the Zeeman field, at the border of the spin polarized state and the Kondo-Yosida singlet state, there is an unusual fixed point where the entropy of the defect is k_B ln2 and the expectation value of S^2_z is 1/8.