Optical properties of superconducting EuFe2(As1-xPx)2

TL;DR

This study investigates the optical properties of EuFe2(As1-xPx)2 superconductors, revealing non-Fermi-liquid behavior, spectral weight transfer due to Hund's coupling, and evidence for clean-limit, two-gap superconductivity with anisotropic gaps.

Contribution

It provides the first broadband optical-conductivity analysis of EuFe2(As1-xPx)2, identifying non-Fermi-liquid behavior and characterizing the superconducting gaps as isotropic and anisotropic.

Findings

Linear temperature dependence of scattering rates indicating strong correlations

Spectral weight transfer attributed to Hund's-rule coupling

Evidence for clean-limit, two-gap superconductivity with anisotropic gaps

Abstract

We present a broadband optical-conductivity study of superconducting single-crystalline EuFe2(As1-xPx)2 with three different substitutional levels. We analyze the normal-state electrodynamics by decomposing the conductivity spectra using a Drude-Lorentz model with two Drude terms representing two groups of carriers with different scattering rates. The analysis reveals that the scattering rate of at least one of the Drude components develops linearly with temperature for each doping level. This points towards strong electron-electron correlations and a non-Fermi-liquid behavior in the P-substituted superconducting Eu-122 pnictides. We also detect a transfer of the spectral weight from mid-infrared to higher frequencies and assign it to the Hund's-rule coupling between itinerant and localized carriers. The conductivity spectra below the superconducting transition show no sharp features to be associated with the dirty-limit superconducting BCS gaps. We interpret these results in terms of clean-limit superconductivity in EuFe2(As1-xPx)2. The best parametrization fit can be achieved using a two-gap model. We find that the larger gap at the hole pockets of the Fermi surface is likely to be isotropic, while the smaller gap at the electron pockets is anisotropic or even nodal.