Block magnetic excitations in the orbitally selective Mott insulator BaFe2Se3

TL;DR

This study uses neutron scattering to explore the magnetic excitations in BaFe2Se3, revealing how orbital degrees of freedom influence the stabilization of an exotic magnetic state with localized spins.

Contribution

It provides a detailed characterization of the spin correlations and excitations in BaFe2Se3, demonstrating the partial spectral weight and the role of orbital effects in stabilizing the magnetic state.

Findings

All predicted magnetic excitations observed below 300 meV.

Spectral weight accounts for about 2/3 of the expected localized spin moments.

Orbital degrees of freedom contribute to stabilizing the magnetic state.

Abstract

Iron pnictides and selenides display a variety of unusual magnetic phases originating from the interplay between electronic, orbital, and lattice degrees of freedom. Using powder inelastic neutron scattering on the two-leg ladder BaFe2Se3, we fully characterize the static and dynamic spin correlations associated with the Fe4 block state, an exotic magnetic ground state observed in this low-dimensional magnet and in Rb0.89Fe1.58Se2. All the magnetic excitations of the Fe4 block state predicted by an effective Heisenberg model with localized spins are observed below 300 meV and quantitatively reproduced. However, the data only account for 16 mub^2 per Fe2+, approximatively 2/3 of the total spectral weight expected for localized S=2 moments. Our results highlight how orbital degrees of freedom in iron-based magnets can conspire to stabilize an exotic magnetic state.