Quantum magnetism of iron-based ladders: Blocks, spirals, and spin flux

TL;DR

This paper provides a comprehensive theoretical analysis of magnetic states in low-dimensional iron-based ladder materials, revealing exotic magnetic phases and phase diagrams consistent with experimental observations.

Contribution

It introduces a low-energy generalized Kondo-Heisenberg model to study the magnetic phase diagram of iron-based ladders in the orbital-selective Mott phase, uncovering new magnetic phases and explaining experimental phenomena.

Findings

Reproduces experimental block magnetism in BaFe₂Se₃.

Discovers interaction-induced frustrated block-spiral states.

Predicts new phases such as phase separation and quantum spin-flux phase.

Abstract

Motivated by increasing experimental evidence of exotic magnetism in low-dimensional iron-based materials, we present a comprehensive theoretical analysis of magnetic states of the multiorbital Hubbard ladder in the orbital-selective Mott phase (OSMP). The model we used is relevant for iron-based compounds of the AFe$_2$X$_3$ family (where A${}={}$Cs, Rb, Ba, K are alkali metals and X${}={}$S, Se are chalcogenides). To reduce computational effort, and obtain almost exact numerical results in the ladder geometry, we utilize a low-energy description of the Hubbard model in the OSMP - the generalized Kondo-Heisenberg Hamiltonian. Our main result is the doping vs interaction magnetic phase diagram. We reproduce the experimental findings on the AFe$_2$X$_3$ materials, especially the exotic block magnetism of BaFe$_2$Se$_3$ (antiferromagnetically coupled $2\times 2$ ferromagnetic islands of the $\uparrow\uparrow\downarrow\downarrow$ form). As in recent studies of the chain geometry, we also unveil block magnetism beyond the $2 \times 2$ pattern (with block sizes varying as a function of the electron doping) and also an interaction-induced frustrated block-spiral state (a spiral order of rigidly rotating ferromagnetic islands). Moreover, we predict new phases beyond the one-dimensional system: a robust regime of phase separation close to half-filling, incommensurate antiferromagnetism for weak interaction, and a quantum spin-flux phase of staggered plaquette spin currents at intermediate doping. Finally, exploiting the bonding/antibonding band occupations, we provide an intuitive physical picture giving insight into the structure of the phase diagram.