Quantum impurity with 23 local moment in one-dimensional quantum wires: A numerical renormalization group study

We study a Kondo state that is strongly influenced by its proximity to an w-1/2 singularity in the metallic host density of states. This singularity occurs at the bottom of the band of a one-dimensional chain, for example. We first analyze the noninteracting system: A resonant state ed, located close to the band singularity, suffers a strong renormalization, such that a bound state (Dirac 8 function) is created below the bottom of the band in addition to a resonance in the continuum. When ed is positioned right at the singularity, the spectral weight of the bound state is 23, irrespective of its coupling to the conduction electrons. The interacting system is modeled using the single -impurity Anderson model, which is then solved using the numerical renormalization group method. We observe that the Hubbard interaction causes the bound state to suffer a series of transformations, including level splitting, transfer of spectral weight, appearance of a spectral discontinuity, changes in binding energy (the lowest state moves farther away from the bottom of the band), and development of a finite width. When ed is away from the singularity and in the intermediate valence regime, the impurity occupancy is lower. As ed moves closer to the singularity, the system partially recovers Kondo regime properties, i.e., higher occupancy and lower Kondo temperature TK. The impurity thermodynamic properties show that the local -moment (LM) fixed point is also strongly affected by the existence of the bound state. When ed is close to the singularity, the LM fixed point becomes impervious to charge fluctuations (caused by bringing ed close to the Fermi energy), in contrast to the LM suppression that occurs when ed is away from the singularity. We also discuss an experimental implementation that shows similar results to the quantum wire if the metallic host of the impurity is an armchair graphene nanoribbon.