Preferred magnetic excitations in Sr1-xNaxFe2As2
Abstract
We have used inelastic neutron scattering to determine magnetic excitations in a single-crystal sample of Sr1-xNaxFe2As2. 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.