Magnetic anisotropy in hole-doped superconducting Ba 0.67K 0.33Fe 2As2 probed by polarized inelastic neutron scattering

TL;DR

This study uses polarized inelastic neutron scattering to investigate spin excitations and anisotropy in the hole-doped superconductor Ba$_{0.67}$K$_{0.33}$Fe$_2$As$_2$, revealing energy-dependent spin gaps and anisotropic resonance behavior.

Contribution

First polarization analysis of spin excitations in hole-doped BaFe$_2$As$_2$, clarifying the origin of spin gaps and anisotropy in the superconducting state.

Findings

Spin anisotropy exists below 7 meV in the normal state.

Different spin gaps for c-axis and in-plane excitations in the superconducting state.

Resonance features have distinct energy widths for different spin polarizations.

Abstract

We use polarized inelastic neutron scattering (INS) to study spin excitations of optimally hole-doped superconductor Ba$_{0.67}$K$_{0.33}$Fe$_2$As$_{2}$ ($T_c=38$ K). In the normal state, the imaginary part of the dynamic susceptibility, $\chi^{\prime\prime}(Q,\omega)$, shows magnetic anisotropy for energies below $\sim$7 meV with c-axis polarized spin excitations larger than that of the in-plane component. Upon entering into the superconducting state, previous unpolarized INS experiments have shown that spin gaps at $\sim$5 and 0.75 meV open at wave vectors $Q=(0.5,0.5,0)$ and $(0.5,0.5,1)$, respectively, with a broad neutron spin resonance at $E_r=15$ meV. Our neutron polarization analysis reveals that the large difference in spin gaps is purely due to different spin gaps in the c-axis and in-plane polarized spin excitations, resulting resonance with different energy widths for the c-axis and in-plane spin excitations. The observation of spin anisotropy in both opitmally electron and hole-doped BaFe$_2$As$_2$ is due to their proximity to the AF ordered BaFe$_2$As$_2$ where spin anisotropy exists below $T_N$.