Conformity of spin fluctuations in alkali-metal iron selenide superconductors inferred from the observation of a magnetic resonant mode in K(x)Fe(2-y)Se(2)

TL;DR

This study maps spin excitations in K(0.77)Fe(1.85)Se(2) superconductors, revealing a universal magnetic resonant mode that suggests a common electronic structure across alkali-metal iron selenides, differing from iron pnictides.

Contribution

It provides the first detailed mapping of spin excitations in K(0.77)Fe(1.85)Se(2), showing a universal magnetic resonant mode in alkali-metal iron selenide superconductors.

Findings

Magnetic resonant mode observed at Q_res=(1/2 1/4)

Resonance energy and shape similar to Rb-based compound

Resonance spectral weight larger than in optimally doped BaFe2-xCoxAs2

Abstract

Spin excitations stemming from the metallic phase of the ferrochalcogenide superconductor K(0.77)Fe(1.85)Se(2) (T_c=32 K) were mapped out in the ab plane by means of the time-of-flight neutron spectroscopy. We observed a magnetic resonant mode at Q_res=(1/2 1/4), whose energy and in-plane shape are almost identical to those in the related compound Rb(0.8)Fe(1.6)Se(2). This lets us infer that there is a unique underlying electronic structure of the bulk superconducting phase K(x)Fe(2)Se(2), which is universal for all alkali-metal iron selenide superconductors and stands in contrast to the doping-tunable phase diagrams of the related iron pnictides. Furthermore, the spectral weight of the resonance on the absolute scale, normalized to the volume fraction of the superconducting phase, is several times larger than in optimally doped BaFe(2-x)Co(x)As(2). We also found no evidence for any additional low-energy branches of spin excitations away from Q_res. Our results provide new input for theoretical models of the spin dynamics in iron based superconductors.