Preferred magnetic excitations in Sr$_{1-x}$Na$_x$Fe$_2$As$_2$

TL;DR

This study uses inelastic neutron scattering to compare magnetic excitations in different magnetic phases of Sr$_{1-x}$Na$_x$Fe$_2$As$_2$, revealing the significance of c-axis polarized excitations for superconductivity.

Contribution

It provides new insights into how magnetic excitations, especially along the c-axis, influence superconductivity in Sr$_{1-x}$Na$_x$Fe$_2$As$_2$, highlighting differences between magnetic phases.

Findings

High-energy magnetic interactions are similar across phases.

c-axis polarized excitations are suppressed in the tetragonal phase.

The spin resonant mode appears only in the c-axis response in the superconducting state.

Abstract

We have used inelastic neutron scattering to determine magnetic excitations in a single-crystal sample of Sr$_{1-x}$Na$_x$Fe$_2$As$_2$. The material's two magnetic phases, which differ in their orthorhombic and tetragonal lattice symmetries, share very similar strengths of magnetic interactions as seen from the high-energy excitations. At low energies, excitations polarized along the $c$ axis are suppressed in the tetragonal magnetic phase, in accordance with the associated reorientation of the ordered moments. Although excitations perpendicular to the $c$ axis are still prominent, only the weak $c$-axis response exhibits a spin resonant mode in the superconducting state. Our result suggests that $c$-axis polarized magnetic excitations are important for the formation of the superconductivity, and naturally explains why the critical temperature is suppressed in the tetragonal magnetic phase.