Andrew K. Mitchell, Lars Fritz
Defects in the honeycomb lattice of graphene are known to induce local moments and strong correlation effects. Distortions due to structural reconstruction around vacancies in graphene were studied recently by Cazalilla et al [arXiv:1207.3135 (2012)], who formulated an effective model consisting of a localized \sigma-level hybridized with the \pi-band. We analyze the rich quantum impurity physics of this system using a combination of numerical renormalization group and analytical techniques, focusing on the special role played by the unusual local density of states, which is enhanced at low energies due to potential scattering. Depending on microscopic parameters, the model hosts both exactly-screened spin-1/2 (doublet) Kondo or underscreened spin-1 (triplet) Kondo phases, and we study the quantum phase transition separating them. Although the effective Kondo models also support new stable phases characterized by strong renormalized particle-hole asymmetry, such phases cannot in fact be accessed in the full Andersonian model describing the vacancy. We show that distinctive signatures of the modified powerlaw Kondo effect thus always appear at low energies in thermodynamic quantities and the scattering t matrix.
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http://arxiv.org/abs/1212.2631
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