Midgap states of a two-dimensional antiferromagnetic Mott-insulator: Electronic structure of meron vortices

– We demonstrate from first principles that ferromagnetic core, meron vortex configurations of the spin1 2 antiferromagnet in two-dimensions give rise to midgap electronic states in the Mott-Hubbard charge gap. Merons are collective mode excitations of the antiferromagnet and are induced by doping the system with charge carriers. They are the topological analogs of charged bosonic domain wall solitons in one-dimension. Doped Mott insulators [1] consisting of spin12 local moments exhibit a host of unconventional electronic, magnetic and optical properties [2]. These include non-Fermiliquid transport behaviour of the metallic state, quantum spin-liquid correlations in the local moment background, and anomalous optical absorption in the mid-infrared. It has been suggested [3] that these are intrinsic properties of an antiferromagnetic Mott-Hubbard gap in the presence of charge carriers and that they play a central role in the occurrence of high-temperature superconductivity. While some phenomenological pictures [4], [5] of this anomalous metal have been introduced, a microscopic theory has yet to be formulated. In this paper, we compute the electronic spectrum of antiferromagnetic (AFM) core—and ferromagnetic (FM) core—meron vortices of the spin12 antiferromagnet on a bipartite, square lattice using a simple continuum approximation. These are collective mode excitations which dominate the low-energy charge excitation spectrum of the doped Mott insulator, giving rise to non–Fermi-liquid behaviour. We demonstrate that the FM-core meron induces a degenerate pair of localized electronic midgap states within the Mott-Hubbard charge gap, which are independent of the meron core radius. Electronic excitations between the Mott-Hubbard bands and these midgap levels may contribute to sub-gap optical absorption [6], [7]. Lattice effects, meron-meron interactions and meron-spin-wave interactions lead to a broadening of the midgap levels into a mid-infrared band. Translational motion of charged merons may give rise to non-Drude behaviour in the a.c. conductivity. Merons are the two-dimensional analogues of magnetic domain wall solitons [8], [9] in the one-dimensional antiferromagnet and