Spin excitation anisotropy in optimal-isovalent-doped superconductor BaFe2(As0.7P0.3)2

TL;DR

This study investigates spin excitation anisotropy in BaFe2(As0.7P0.3)2, revealing its development below Tc and linking it to spin-orbit coupling, AF order, and superconductivity, contrasting with other doped BaFe2As2 compounds.

Contribution

First neutron polarization analysis of spin excitation anisotropy in BaFe2(As0.7P0.3)2, showing its emergence in the superconducting state and its relation to spin-orbit coupling.

Findings

No spin excitation anisotropy above 2 meV in the normal state.

Anisotropy develops at the AF zone center below Tc.

Magnetic spectrum remains isotropic at the zone boundary.

Abstract

We use neutron polarization analysis to study spin excitation anisotropy in the optimal-isovalent-doped superconductor BaFe2(As0.7P0.3)2 (Tc = 30 K). Different from optimally hole and electron-doped BaFe2As2, where there is a clear spin excitation anisotropy in the paramagnetic tetragonal state well above Tc, we find no spin excitation anisotropy for energies above 2 meV in the normal state of BaFe2(As0.7P0.3)2. Upon entering the superconducting state, significant spin excitation anisotropy develops at the antiferromagnetic (AF) zone center QAF = (1, 0, L = odd), while magnetic spectrum is isotropy at the zone boundary Q = (1, 0, L = even). By comparing temperature, wave vector, and polarization dependence of the spin excitation anisotropy in BaFe2(As0.7P0.3)2 and hole-doped Ba0.67K0.33Fe2As2 (Tc = 38 K), we conclude that such anisotropy arises from spin-orbit coupling and is associated with the nearby AF order and superconductivity.