Phase separation in iron chalcogenide superconductor Rb0.8+xFe1.6+ySe2 as seen by Raman light scattering and band structure calculations

TL;DR

This study investigates phase separation in Rb0.8+xFe1.6+ySe2 superconductors using Raman scattering and band structure calculations, revealing magnetic origins and conductive interfaces that influence superconductivity.

Contribution

It provides new insights into the magnetic and electronic structure of phase-separated Rb-iron selenides through combined experimental and theoretical approaches.

Findings

Detected phonons from both magnetic and conductive phases.

Identified superconducting gap with dx2-y2 symmetry.

Proposed a conductive spacer phase facilitating percolative superconductivity.

Abstract

We report Raman light scattering in the phase separated superconducting single crystal Rb0.77Fe1.61Se2 with Tc = 32 K. The spectra have been measured in a wide temperature range 3K -500K. The observed phonon lines from the majority vacancy ordered Rb2Fe4Se5 (245) antiferromagnetic phase with TN= 525 K demonstrate modest anomalies in frequency, intensity and halfwidth at the superconductive phase transition. We identify phonon lines from the minority compressed Rb{\delta}Fe2Se2 (122) conductive phase. The superconducting gap with dx2-y2 symmetry is also detected in our spectra. In the range 0-600 cm-1 we observed the low intensive but highly polarized B1g-type background which becomes well structured under cooling. The possible magnetic or multiorbital origin of this background has been discussed. We argue that phase separation in M0.8+xFe1.6+ySe2 has pure magnetic origin. It occurs below Neel temperature when iron magnetic moment achieves some critical magnitude. We state that there is a spacer between the majority 245 and minority 122 phases. Using ab-initio spin polarized band structure calculations we demonstrate that compressed vacancy ordered Rb2Fe4Se5 phase can be conductive and therefore may serve as a protective interface spacer between the pure metallic Rb{\delta}Fe2Se2 phase and the insulating Rb2Fe4Se5 phase providing the percolative Josephson-junction like superconductivity in the whole sample of Rb0.8+xFe1.6+ySe2 Our lattice dynamics calculations show significant difference in the phonon spectra of the conductive and insulating Rb2Fe4.Se5 phases.