Coexistence of Superconductivity and Magnetism in FeSe_1-x under Pressure

TL;DR

This study investigates how superconductivity and magnetism coexist in FeSe$_{1-x}$ under pressure, revealing that both phenomena are stabilized and increase with pressure up to 2.4 GPa, with detailed magnetic structure analysis.

Contribution

The paper provides the first detailed phase diagram of FeSe$_{1-x}$ under pressure, demonstrating coexistence of superconductivity and magnetism and proposing magnetic structures based on muon and symmetry analysis.

Findings

Magnetic order appears at pressures above 0.8 GPa.

Superconducting volume fraction remains near 100% up to 2.4 GPa.

Both $T_c$ and $T_N$ increase with pressure.

Abstract

An extended investigation of the electronic phase diagram of FeSe$_{1-x}$ up to pressures of $p\simeq2.4$\,GPa by means of ac and dc magnetization, zero field muon spin rotation (ZF $\mu$SR), and neutron diffraction is presented. ZF $\mu$SR indicates that at pressures $p\geq0.8$\,GPa static magnetic order occurs in FeSe$_{1-x}$ and occupies the full sample volume for $p\gtrsim 1.2$\,GPa. ac magnetization measurements reveal that the superconducting volume fraction stays close to 100% up to the highest pressure investigated. In addition, above $p\geq1.2$\,GPa both the superconducting transition temperature $T_{\rm c}$ and the magnetic ordering temperature $T_{\rm N}$ increase simultaneously, and both superconductivity and magnetism are stabilized with increasing pressure. Calculations indicate only one possible muon stopping site in FeSe$_{1-x}$, located on the line connecting the Se atoms along the $c$-direction. Different magnetic structures are proposed and checked by combining the muon stopping calculations with a symmetry analysis, leading to a similar structure as in the LaFeAsO family of Fe-based superconductors. Furthermore, it is shown that the magnetic moment is pressure dependent and with a rather small value of $\mu\approx 0.2\,\mu_B$ at $p\simeq2.4$\,GPa.