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Quantum phase transition in a gapped Anderson model: A numerical renormalization group study

We use the numerical renormalization group method to investigate the spectral properties of a single-impurity Anderson model with a gap δ across the Fermi level in the conduction-electron spectrum. For any finite δ > 0, at half filling the ground state of the system is always a doublet. Away from half filling a quantum phase transition (QPT) occurs as function of the gap value δ, and the system evolves from the strong-coupling (SC) Kondo-type state, corresponding to δ <δ_C toward a localized moment (LM) regime for δ > δ_C. The opening of the gap leads to the formation of one (two) bound states when the system is in the SC (LM) regime. The evolution across the QPT of their positions and the corresponding weights together with the dynamic properties of the model are investigated.