Mid-Infrared Conductivity from Mid-Gap States Associated with Charge Stripes

TL;DR

This study investigates the optical conductivity in stripe-ordered La(2)NiO(4+d), revealing mid-gap states linked to charge stripes and their temperature-dependent behavior, providing insights relevant to cuprate superconductors.

Contribution

It demonstrates the decomposition of optical conductivity into components associated with stripe-related mid-gap states and thermal excitation effects, highlighting the role of charge stripes in optical properties.

Findings

Mid-infrared peak linked to stripe-related mid-gap states

Drude peak appears with thermal excitation of carriers

Mid-IR peak shifts to lower energy with temperature

Abstract

The optical conductivity of La(2-x)Sr(x)NiO(4) has been interpreted in various ways, but so far the proposed interpretations have neglected the fact that the holes doped into the NiO(2) planes order in diagonal stripes, as established by neutron and X-ray scattering. Here we present a study of optical conductivity in La(2)NiO(4+d) with d=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.