Persistent high-energy spin excitations in iron pnictide superconductors

TL;DR

This study reveals that optimally doped iron pnictide superconductors retain high-energy spin excitations similar to their undoped counterparts, supporting the idea that spin fluctuations may mediate superconductivity.

Contribution

It demonstrates the persistence of high-energy magnetic excitations in doped superconducting Fe-pnictides using resonant inelastic x-ray scattering, linking them to high-Tc cuprates.

Findings

High-energy spin excitations persist in doped superconductors.

Paramagnon modes are softer but as intense as undoped magnons.

Spin fluctuations may be involved in superconducting pairing.

Abstract

Motivated by the premise that superconductivity in iron-based superconductors is unconventional and mediated by spin fluctuations, an intense research effort has been focused on characterizing the spin excitation spectrum in the magnetically ordered parent phases of the Fe-pnictides2,3 and - chalcogenides4. For these undoped materials it is well-established that the spin excitation spectrum consists of a sharp, highly dispersive magnon spanning an energy range of up to 200 meV (ref. 3). The fate of these high-energy magnetic modes upon sizable doping is hitherto unresolved. Using resonant inelastic x-ray scattering we show that optimally doped superconducting Ba0.6K0.4Fe2As2 retains well defined, dispersive high-energy modes of magnetic origin. These paramagnon modes are softer than, though as intense as, the magnon of undoped antiferromagnetic BaFe2As2. The persistence of spin excitations well into the superconducting phase suggests that, if spin fluctuations are responsible for superconducting pairing, they originate from a distinctly correlated spin-state. This connects Fe-pnictide superconductors to the high-Tc cuprates, for which in spite of fundamental differences in the electronic structure, similar paramagnon modes are present5.