Reciprocity between local moments and collective magnetic excitations in the phase diagram of BaFe$_2$(As$_{1-x}$P$_x$)$_2$

TL;DR

This study investigates the interplay between local magnetic moments and collective spin excitations in BaFe$_2$(As$_{1-x}$P$_x$)$_2$, revealing how doping influences magnetic properties and their role in superconductivity through advanced spectroscopic techniques.

Contribution

It provides direct experimental evidence of the coexistence and evolution of local moments and collective excitations across the superconducting phase diagram, supported by an intermediate coupling theoretical model.

Findings

Collective magnetic excitations are gradually hardened with doping.

Local magnetic moments are strongly suppressed upon doping.

A spin fluctuations channel transfers excitations from local moments to the particle-hole continuum.

Abstract

Unconventional superconductivity arises at the border between the strong coupling regime with local magnetic moments and the weak coupling regime with itinerant electrons, and stems from the physics of criticality that dissects the two. Unveiling the nature of the quasiparticles close to quantum criticality is fundamental to understand the phase diagram of quantum materials. Here, using resonant inelastic x-ray scattering (RIXS) and Fe-K$_\beta$ emission spectroscopy (XES), we visualize the coexistence and evolution of local magnetic moments and collective spin excitations across the superconducting dome in isovalently-doped BaFe$_2$(As$_{1-x}$P$_x$)$_2$ (0.00$\leq$x$\leq0.$52). Collective magnetic excitations resolved by RIXS are gradually hardened, whereas XES reveals a strong suppression of the local magnetic moment upon doping. This relationship is captured by an intermediate coupling theory, explicitly accounting for the partially localized and itinerant nature of the electrons in Fe pnictides. Finally, our work identifies a local-itinerant spin fluctuations channel through which the local moments transfer spin excitations to the particle-hole (paramagnons) continuum across the superconducting dome.