Antiferromagnetic order and superlattice structure in nonsuperconducting and superconducting RbyFe(1.6+x)Se2

TL;DR

This study uses neutron diffraction to analyze the magnetic and lattice structures of Rb$_{y}$Fe$_{1.6+x}$Se$_2$, revealing phase separation and distinct superlattice structures in superconducting and insulating forms.

Contribution

It provides detailed neutron diffraction evidence for phase separation and superlattice structures in Rb$_{y}$Fe$_{1.6+x}$Se$_2$, advancing understanding of its magnetic and structural properties.

Findings

Insulating Rb$_{y}$Fe$_{1.6+x}$Se$_2$ has a $ mf{ extit{ extbf{ extsqrt{5}} imes ext{ extsqrt{5}}}}$ block antiferromagnetic structure.

Superconducting samples exhibit both $ mf{ extit{ extbf{ extsqrt{5}} imes ext{ extsqrt{5}}}}$ and $ mf{ extit{ extbf{ extsqrt{2}} imes ext{ extsqrt{2}}}}$ superlattice structures.

Superconductivity coexists with phase-separated superlattice structures, indicating complex structural behavior.

Abstract

Neutron diffraction has been used to study the lattice and magnetic structures of the insulating and superconducting Rb$_y$Fe$_{1.6+x}$Se$_2$. For the insulating Rb$_{y}$Fe$_{1.6+x}$Se$_2$, neutron polarization analysis and single crystal neutron diffraction unambiguously confirm the earlier proposed $\sqrt{5}\times\sqrt{5}$ block antiferromagnetic structure. For superconducting samples ($T_c=30$ K), we find that in addition to the tetragonal $\sqrt{5}\times\sqrt{5}$ superlattice structure transition at 513 K, the material develops a separate $\sqrt{2}\times \sqrt{2}$ superlattice structure at a lower temperature of 480 K. These results suggest that superconducting Rb$_{y}$Fe$_{1.6+x}$Se$_2$ is phase separated with coexisting $\sqrt{2}\times \sqrt{2}$ and $\sqrt{5}\times\sqrt{5}$ superlattice structures.