Disappearance of static magnetic order and evolution of spin fluctuations in Fe$_{1+\delta}$Se$_{x}$Te$_{1-x}$

TL;DR

This study investigates how static magnetic order disappears and spin fluctuations evolve in Fe$_{1+ ext{delta}}$Se$_x$Te$_{1-x}$, revealing a link between magnetic behavior and superconductivity in Fe-based chalcogenides.

Contribution

It provides detailed neutron scattering insights into the transition from static magnetic order to dynamic spin fluctuations associated with superconductivity in FeSeTe compounds.

Findings

Static magnetic order with bicollinear spin configuration exists in non-superconducting samples.

Spin excitations shift from (0.5,0) to (0.5,0.5) wave-vector as superconductivity emerges.

Collinear spin fluctuations are universal in Fe-based superconductors.

Abstract

We report neutron scattering studies on static magnetic orders and spin excitations in the Fe-based chalcogenide system Fe$_{1+\delta}$Se$_{x}$Te$_{1-x}$ with different Fe and Se compositions. Short-range static magnetic order with the "bicollinear" spin configuration is found in all non-superconducting samples, with strong low-energy magnetic excitations near the $(0.5,0)$ in-plane wave-vector (using the two-Fe unit cell) for Se doping up to 45%. When the static order disappears and bulk superconductivity emerges, the spectral weight of the magnetic excitations shifts to the region of reciprocal space near the in-plane wave-vector $(0.5,0.5)$, corresponding to the "collinear" spin configuration. Our results suggest that spin fluctuations associated with the collinear magnetic structure appear to be universal in all Fe-based superconductors, and there is a strong correlation between superconductivity and the character of the magnetic order/fluctuations in