Local orthorhombic lattice distortions in the paramagnetic tetragonal phase of superconducting NaFe$_{1-x}$Ni$_x$As

TL;DR

This study reveals persistent local orthorhombic distortions and nematic fluctuations in NaFe$_{1-x}$Ni$_x$As across various doping levels, highlighting their potential role in the superconducting mechanism.

Contribution

It uncovers local orthorhombic distortions above structural transition temperatures and their suppression below $T_c$, providing new insights into nematicity in iron-based superconductors.

Findings

Local orthorhombic distortions extend into overdoped regime.

Distortions display Curie-Weiss temperature dependence.

Distortions are suppressed below $T_c$.

Abstract

Understanding the interplay between nematicity, magnetism and superconductivity is pivotal for elucidating the physics of iron-based superconductors. Here we use neutron scattering to probe magnetic and nematic orders throughout the phase diagram of NaFe$_{1-x}$Ni$_x$As, finding that while both static antiferromagnetic and nematic orders compete with superconductivity, the onset temperatures for these two orders remain well-separated approaching the putative quantum critical points. We uncover local orthorhombic distortions that persist well above the tetragonal-to-orthorhombic structural transition temperature $T_{\rm s}$ in underdoped samples and extend well into the overdoped regime that exhibits neither magnetic nor structural phase transitions. These unexpected local orthorhombic distortions display Curie-Weiss temperature dependence and become suppressed below the superconducting transition temperature $T_{\rm c}$, suggesting they result from a large nematic susceptibility near optimal superconductivity. Our results account for observations of rotational symmetry-breaking above $T_{\rm s}$, and attest to the presence of significant nematic fluctuations near optimal superconductivity.