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

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 ladders 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 × 2 ferromagnetic islands of the ↑↑↓↓ form). As in recent studies of the chain geometry, we also unveil block magnetism beyond the 2 × 2 pattern (with block sizes varying as a function of the electron doping) and also an interaction-induced frustrated block-spiral exotic 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.

Magnetic ground states of honeycomb lattice Wigner crystals

“…Our discovery of a variety of exotic spin arrangements close to the dominant AFM and FM states, including the n ¼ 1=2 and n ¼ 2=3 exotic phases, is common occurrence in other families of materials where there is strong phase competition, such as in manganites [49][50][51][52][53] , ruthenates 54 , and ladder iron superconductors 55,56 . These unusual states all arise because near a possible first-order AFM-FM transition, there are spin arrangements mixing both characteristics of the dominant states that can reduce even further the energy.…”

Magnetization dynamics fingerprints of an excitonic condensate $t_{2g}^{4}$ magnet

“…We solved the above models in one-dimensional geometries using the accurate numerical technique density matrix renormalization group (DMRG) [54,55]. Note that a wide variety of real materials do have dominant one-dimensional geometries in their crystal structure, either chains or ladders, and recent theoretical investigations have unveiled a wide range of exotic phenomena including block, chiral, superconducting, dimerized, alternating, and Majorana states in one dimensional systems [57][58][59][60][61][62][63][64][65][66][67][68][69][70]. Thus, one dimensionality is not restrictive physically, and allows our study to be sufficiently accurate to provide reliable many-body information.…”

Formation of spinful dark excitons in Hubbard systems with magnetic superstructures

“…in materials where magnetic super structures are realized. Examples are CE-structures in manganites 82 , or orbital-selective Mott phases in iron-based ladder compounds such as BaFe 2 Se 3 [42][43][44][45] . Alternatively, this can also be studied in ultracold gases on optical lattices [83][84][85] , on which it is possible to realize superlattices 86,87 and to investigate for spectral functions.…”

“…In this paper, we investigate the formation of excitons in Hubbard systems with only on-site electron-electron interactions and an additional magnetic superstructure, but without longer-range Coulomb-interactions. This is motivated by a one-dimensional toy model of manganites 41 and the observation of orbital-selective Mott phases (OSMP) [42][43][44][45] . We treat the effect of a single, direct electron-hole excitation, in which an electron is assumed to be instantly excited over the gap with-out changing its momentum.…”