Direct observation of a Fermi liquid-like normal state in an iron-pnictide superconductor

TL;DR

This study provides direct optical evidence that the normal state of electron-doped BaFe₂₋ₓCoₓAs₂ iron-pnictide superconductors behaves like a Fermi liquid at low temperatures, with residual scattering effects and a high Fermi liquid scale.

Contribution

It demonstrates that the normal state of electron-doped BaFe₂₋ₓCoₓAs₂ is a weakly correlated Fermi liquid, supported by optical spectroscopy and theoretical modeling, clarifying its electronic nature.

Findings

Normal state exhibits Fermi liquid properties at low temperatures

Residual elastic resonant scattering influences scattering rate scaling

Fermi liquid scale T₀ is approximately 1700 K

Abstract

There are two prerequisites for understanding high-temperature (high-T$_c$) superconductivity: identifying the pairing interaction and a correct description of the normal state from which superconductivity emerges. The nature of the normal state of iron-pnictide superconductors, and the role played by correlations arising from partially screened interactions, are still under debate. Here we show that the normal state of carefully annealed electron-doped BaFe$_{2-x}$Co$_{x}$As$_2$ at low temperatures has all the hallmark properties of a local Fermi liquid, with a more incoherent state emerging at elevated temperatures, an identification made possible using bulk-sensitive optical spectroscopy with high frequency and temperature resolution. The frequency dependent scattering rate extracted from the optical conductivity deviates from the expected scaling $M_{2}(\omega,T)\propto(\hbar\omega)^{2}+(p\pi k_{B}T)^{2}$ with $p\approx$ 1.47 rather than $p$ = 2, indicative of the presence of residual elastic resonant scattering. Excellent agreement between the experimental results and theoretical modeling allows us to extract the characteristic Fermi liquid scale $T_{0}\approx$ 1700 K. Our results show that the electron-doped iron-pnictides should be regarded as weakly correlated Fermi liquids with a weak mass enhancement resulting from residual electron-electron scattering from thermally excited quasi-particles.