Mid-infrared conductivity from mid-gap states associated with charge stripes

Abstract

The optical conductivity of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{NiO}}_{4}$ has been interpreted in various ways, but so far the proposed interpretations have neglected the fact that the holes doped into the ${\mathrm{NiO}}_{2}$ planes order in diagonal stripes, as established by neutron and x-ray scattering. Here we present a study of optical conductivity in ${\mathrm{La}}_{2}{\mathrm{NiO}}_{4+\ensuremath{\delta}}$ with $\ensuremath{\delta}=\frac{2}{15},$ a material in which the charge stripes order three dimensionally. We show that the conductivity can be decomposed into two components, a mid-infrared peak that we attribute to transitions from the filled valence band into empty mid-gap states associated with the stripes, and a Drude peak that appears at higher temperatures as carriers are thermally excited into the mid-gap states. The shift of the mid-IR peak to lower energy with increasing temperature is explained in terms of the Franck-Condon effect. The relevance of these results to understanding the optical conductivity in the cuprates is discussed.