Reciprocity between local moments and collective magnetic excitations in the phase diagram of BaFe2(As1\u2212xPx)2

Jonathan Pelliciari,Xiancheng Wang,Kenji Ishii,Shigeru Kasahara,Xingye Lu,Yuji Matsuda,Paul Olalde-Velasco,Yaobo Huang,Thorsten Schmitt,Changqing Jin,Marcus Dantz,Tanmoy Das,Takasada Shibauchi,Vladimir N Strocov,Lingyi Xing

doi:10.1038/s42005-019-0236-3

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 {rm{Fe}}-{{rm{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 le x le 0.52). 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.

Highlights

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
The non-interacting density of states (DOS) of electrons (Fig. 1c) is renormalised by the interaction in the two extreme limits as (i) the bands become sharper in the weak-coupling quasiparticle picture (Fig. 1d), and (ii) split into two Mott-like bands characterised by local moments in the strong-coupling limit (Fig. 1e). (i) In the weak-coupling limit, the low-energy spin excitations are very fragile and mix with the particle–hole continuum, failing to form a localised moment. (ii) In the strong-coupling limit, the spin excitations across the Mott bands feature a gapped behaviour on the order of the on-site energy U without the particle–hole continuum[13,14]
The spin excitation intensities detected in the portion of Brillouin zone (BZ) pertinent to RIXS possibly show gradual transfer of spectral weight upon doping

Summary

Introduction

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. On one side of the superconducting dome, the correlation strength is strongly enhanced, leading to diverse quantum manybody effects, such as the Mott insulating state, non-Fermi-liquid behaviour and magnetic orders (see Fig. 1a, b)[3,4,12,13,14]. SC is optimised in this intermediate region where the cooperation of a strongly enhanced non-Fermi-liquid behaviour, and the presence of a quantum criticality leads to unconventional and not understood physics[11] This generic phase diagram suggests that intertwined electronic and magnetic instabilities, arising from the intermediate correlation strength close to this critical region, induce strong Cooper pairing[3,4,15]. The detection of spin excitations is enabled in RIXS thanks to the spin–orbit coupling of the intermediate state mixing the quantum numbers L and S, thereby a b

Methods

Results

Conclusion