An Anderson Impurity Interacting with the Helical Edge States in a Quantum Spin Hall Insulator

Using the natural orbitals renormalization group (NORG) method, we investigate the screening of the local spin of an Anderson impurity interacting with the helical edge states in a quantum spin Hall insulator. It is found that there is a local spin formed at the impurity site and the local spin is completely screened by electrons in the quantum spin Hall insulator. Meanwhile, the local spin is screened dominantly by a single active natural orbital. We then show that the Kondo screening mechanism becomes transparent and simple in the framework of the natural orbitals formalism. We project the active natural orbital respectively into real space and momentum space to characterize its structure. We confirm the spin-momentum locking property of the edge states based on the occupancy of a Bloch state on the edge to which the impurity couples. Furthermore, we study the dynamical property of the active natural orbital represented by the local density of states, from which we observe the Kondo resonance peak.