Local structural disorder and superconductivity in KxFe2-ySe2

TL;DR

This study reveals that local structural disorder, especially the population of specific Fe sites, significantly influences superconductivity in KxFe2-ySe2, with post-annealing enhancing magnetic moments and superconducting properties.

Contribution

It demonstrates that the population of Fe1 sites and their structural disorder are key factors governing bulk superconductivity in KxFe2-ySe2, highlighting the importance of local structure control.

Findings

Quenching increases Fe-Se bond length and reduces static disorder.

Fe1 sites carry higher magnetic moments than Fe2 sites.

Nonzero Fe1 population is crucial for bulk superconductivity.

Abstract

We report significantly enhanced magnetic moment in K0.69(2)Fe1.45(1)Se2.00(1) single crystals with sharp Tc and bulk superconductivity obtained by postannealing and quenching process. There are two Fe sites in the K0.69(2)Fe1.45(1)Se2.00(1) unit cell: Fe1, which has higher symmetry with longer average Fe-Se bond length, and Fe2, which has lower symmetry with shorter average Fe-Se bond length. Temperature-dependent x-ray absorption fine-structure (XAFS) analysis results on quenched and as-grown K0.69(2)Fe1.45(1)Se2.00(1) crystals show that quenched K0.69(2)Fe1.45(1)Se2.00(1) have increased average Fe-Se bond length and decreased static disorder. Our results indicate that nonzero population of Fe1 sites is the key structural parameter that governs the bulk superconductivity. We also show clear evidence that Fe1 sites carry higher magnetic moment than Fe2 sites.