Optical conductivity of superconducting Rb2Fe4Se5

TL;DR

This study investigates the optical properties of Rb2Fe4Se5 single crystals, revealing a coexistence of superconductivity and magnetism, with detailed spectroscopic analysis across a broad energy range and temperature spectrum.

Contribution

It provides the first detailed spectroscopic analysis of Rb2Fe4Se5, showing coexistence of superconducting and semiconducting responses and identifying a superconducting gap and metallic behavior.

Findings

Identification of a 0.45 eV band-gap.

Observation of a superconducting gap below 8 meV.

Detection of metallic response below 100 K.

Abstract

We report the complex dielectric function of high-quality nearly-stoichiometric Rb2Fe4Se5 (RFS) single crystals with Tc=32 K determined by wide-band spectroscopic ellipsometry and time-domain transmission spectroscopy in the spectral range 1 meV<=$\hbar$\omega<=6.5 eV at temperatures 4 K<=T<=300 K. This compound simultaneously displays a superconducting and a semiconducting optical response. It reveals a direct band-gap of 0.45 eV determined by a set of spin-controlled interband transitions. Below 100 K we observe in the lowest THz spectral range a clear metallic response characterized by the negative dielectric permittivity \epsilon 1 and bare (unscreened) \omega pl=100 meV. At the superconducting transition this metallic response exhibits a signature of a superconducting gap below 8 meV. Our findings suggest a coexistence of superconductivity and magnetism in this compound as two separate phases.