Coexistence of superconductivity and density wave in $\mathrm{Ba_2Ti_2Fe_2As_4O}$: an optical spectroscopy study

TL;DR

This study uses optical spectroscopy to investigate the coexistence of superconductivity and density wave order in Ba2Ti2Fe2As4O, revealing a large density wave gap and multiple superconducting gaps, indicating complex electronic interactions.

Contribution

First optical spectroscopy analysis of Ba2Ti2Fe2As4O demonstrating coexistence of superconductivity and density wave order with detailed gap measurements.

Findings

Density wave gap forms below 125 K, removing half of the Fermi surface.

Density wave gap ratio exceeds weak-coupling BCS value.

Superconducting gaps of 3.4 meV and 7.9 meV observed at low temperature.

Abstract

We performed an optical spectroscopy measurement on single crystals of $\mathrm{Ba_2Ti_2Fe_2As_4O}$, which is a newly discovered superconductor showing a coexistence of superconductivity and density wave order. The study reveals a spectral change related to the formation density wave energy gap below $T_{DW}\approx$125 K, leading to the removal of about half of the multiple Fermi surface sheets. The ratio of 2$\Delta_{DW}$/$k_B T_{DW}\approx$ 11.9 is considerably larger than the mean-field value based on the weak-coupling BCS theory. At the lowest temperature in the superconducting state, we observed opening of superconducting energy gaps $\Delta_1(0) =3.4$ meV and $\Delta_2(0)=7.9$ meV. The properties of superconducting state in $\mathrm{Ba_2Ti_2Fe_2As_4O}$ are similar to that in $\mathrm{BaFe_{1.85}Co_{0.15}As_2}$.