Persistent paramagnons in high-temperature infinite-layer nickelate superconductors

TL;DR

This study reveals persistent paramagnons in high-temperature superconducting nickelates, showing spin fluctuations that support a magnetic pairing mechanism, but with reduced exchange strength compared to similar cuprates, indicating different superconducting mechanisms.

Contribution

It provides the first direct RIXS evidence of paramagnons in Sm-based nickelates and compares magnetic interactions with cuprates, highlighting key differences in their superconducting behavior.

Findings

Dispersive paramagnonic excitations observed in nickelates.

Effective exchange coupling is reduced by ~20% despite higher T_c.

Magnetic interactions in nickelates differ from those in cuprates.

Abstract

The recent discovery of high-temperature superconductivity in hole-doped SmNiO$_2$, exhibiting the record-high transition temperature $T_c$ among infinite-layer (IL) nickelates, has opened a new avenue for exploring design principles of superconductivity. Experimentally determining the electronic structure and magnetic interactions in this new system is crucial to elucidating the mechanism behind the enhanced superconductivity. Here, we report a Ni $L$-edge resonant inelastic x-ray scattering (RIXS) study of superconducting Sm-based IL nickelate thin films Sm$_{1-x-y-z}$Eu$_x$Ca$_y$Sr$_z$NiO$_2$ (SECS). Dispersive paramagnonic excitations are observed in both optimally and overdoped SECS samples, supporting a spin-fluctuation-mediated pairing scenario. However, despite the two-fold enhancement of $T_c$ in the Sm-based nickelates compared to their Pr-based counterparts, the effective exchange coupling strength is reduced by approximately $20\%$. This behavior contrasts with hole-doped cuprates, where magnetic interactions correlate positively with $T_c$, highlighting essential differences in their superconducting mechanisms.