Coexistence of superconductivity and short-range double-stripe spin correlations in Te-vapor annealed FeTe$_{1-x}$Se$_{x}$ with $x\le0.2$

TL;DR

This study demonstrates that Te-vapor annealing of FeTe$_{1-x}$Se$_{x}$ crystals with $x\,\leq0.2$ induces superconductivity while maintaining short-range double-stripe magnetic correlations, revealing a complex magnetic environment in these superconductors.

Contribution

It provides new insights into the coexistence of superconductivity and specific magnetic correlations in FeTe$_{1-x}$Se$_{x}$, highlighting the role of annealing in tuning magnetic and superconducting properties.

Findings

Superconductivity is induced by Te vapor annealing in FeTe$_{1-x}$Se$_{x}$ with $x\le0.2$.

Short-range double-stripe magnetic correlations persist in superconducting samples.

A spin gap and resonance are observed, with a mixed magnetic character in the resonance signal.

Abstract

In as-grown bulk crystals of Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ with $x\lesssim0.3$, excess Fe ($y>0$) is inevitable and correlates with a suppression of superconductivity. At the same time, there remains the question as to whether the character of the antiferromagnetic correlations associated with the enhanced anion height above the Fe planes in Te-rich samples is compatible with superconductivity. To test this, we have annealed as-grown crystals with $x=0.1$ and 0.2 in Te vapor, effectively reducing the excess Fe and inducing bulk superconductivity. Inelastic neutron scattering measurements reveal low-energy magnetic excitations consistent with short-range correlations of the double-stripe type; nevertheless, cooling into the superconducting state results in a spin gap and a spin resonance, with the extra signal in the resonance being short-range with a mixed single-stripe/double-stripe character, which is different than other iron-based superconductors. The mixed magnetic character of these superconducting samples does not appear to be trivially explainable by inhomogeneity.