Anomalous oxygen isotope effect on the in-plane FIR conductivity of detwinned YBa2Cu3$^{16,18}$O$_{6.9}$

TL;DR

This study reveals an unusual oxygen isotope effect on the far-infrared conductivity of detwinned YBa2Cu3O6.9, showing a low-energy electronic mode that shifts with aging and isotopic composition, indicating collective electronic behavior influenced by isotopic defects.

Contribution

It demonstrates an anomalous isotope-dependent electronic mode in YBa2Cu3O6.9, highlighting the role of isotopic defects in collective electronic phenomena.

Findings

A pronounced low-energy mode appears at 240 cm$^{-1}$ for $^{18}$O.

The mode shifts to 190 cm$^{-1}$ after one year.

No comparable mode is observed for $^{16}$O above 120 cm$^{-1}$.

Abstract

We observe an anomalous oxygen isotope effect on the a-axis component of the far-infrared electronic response of detwinned YBa2Cu3$^{16,18}$O$_{6.9}$. For $^{18}$O a pronounced low-energy electronic mode (LEM) appears around 240 cm$^{-1}$. This a-axis LEM exhibits a clear aging effect, after one year it is shifted to 190 cm$^{-1}$. For $^{16}$O we cannot resolve a corresponding a-axis LEM above 120 cm$^{-1}$. We interpret the LEM in terms of a collective electronic mode that is pinned by `isotopic defects', i.e. by the residual $^{16}$O in the matrix of $^{18}$O.