Mid-infrared absorption in YBa2Cu3O6: Failure of spin-wave theory in undoped cuprates?

TL;DR

This paper investigates the mid-infrared optical conductivity of undoped YBa2Cu3O6, revealing that spin-wave theory fails to explain the observed absorption features, especially at higher temperatures.

Contribution

It provides detailed experimental analysis of absorption processes and demonstrates the limitations of spin-wave theory in describing undoped cuprates.

Findings

Identification of bimagnon-plus-phonon absorption at 2800 and 3800 cm^-1

Unrealistically large coupling required for phonon-2-magnon scattering

High frequency spectral weight increases with temperature

Abstract

The optical conductivity sigma (omega) of undoped YBa2Cu3O6 is studied in detail in the mid-infrared range. Substitutions on all but the Ba site are used to identify the prominent absorption processes at 2800 and 3800 cm^-1. Experimental evidence for bimagnon-plus-phonon absorption is collected. A more critical analysis of the lineshape and the spectral weight reveals the limits of this approach. Although phonon-2-magnon multiple scattering seems to reproduce the lineshape, the necessary coupling is unrealistically large. The strong increase of high frequency spectral weight with increasing temperature makes the failure of spin-wave theory even more evident.