Magnon Crystallization in the Kagome Lattice Antiferromagnet

Chiral and flat-band magnetic quasiparticles in ferromagnetic and metallic kagome layers

Magnetic kagome metals are a promising platform to develop unique quantum transport and optical phenomena caused by the interplay between topological electronic bands, strong correlations, and magnetic order. This interplay may result in exotic quasiparticles that describe the coupled electronic and spin excitations on the frustrated kagome lattice. Here, we observe novel elementary magnetic excitations within the ferromagnetic Mn kagome layers in TbMn6Sn6 using inelastic neutron scattering. We observe sharp, collective acoustic magnons and identify flat-band magnons that are localized to a hexagonal plaquette due to the special geometry of the kagome layer. Surprisingly, we observe another type of elementary magnetic excitation; a chiral magnetic quasiparticle that is also localized on a hexagonal plaquette. The short lifetime of localized flat-band and chiral quasiparticles suggest that they are hybrid excitations that decay into electronic states.

Towards lattice-gas description of low-temperature properties above the Haldane and cluster-based Haldane ground states of a mixed spin-(1,1/2) Heisenberg octahedral chain

The rich ground-state phase diagram of the mixed spin-(1,1/2) Heisenberg octahedral chain was previously elaborated from effective mixed-spin Heisenberg chains, which were derived by employing a local conservation of a total spin on square plaquettes of an octahedral chain. Here we present a comprehensive analysis of the thermodynamic properties of this model. In the highly frustrated parameter region the lowest-energy eigenstates of the mixed-spin Heisenberg octahedral chain belong to flat bands, which allow a precise description of low-temperature magnetic properties within the localized-magnon approach exploiting a classical lattice-gas model of hard-core monomers. The present article provides a more comprehensive version of the localized-magnon approach, which extends the range of its validity down to a less frustrated parameter region involving the Haldane and cluster-based Haldane ground states. A comparison between results of the developed localized-magnon theory and accurate numerical methods such as full exact diagonalization and finite-temperature Lanczos technique convincingly evidence that the low-temperature magnetic properties above the Haldane and the cluster-based Haldane ground states can be extracted from a classical lattice-gas model of hard-core monomers and dimers, which is additionally supplemented by a hard-core particle spanned over the whole lattice representing the gapped Haldane phase.

Analytical solutions for the magnon frequencies at high-symmetry points of the Brillouin zone in anisotropic kagome antiferromagnets

The knowledge of the magnon dispersion relations in antiferromagnetic materials with nontrivial spin textures has considerable interest to the understanding of magnonic and spintronic phenomena involving these materials. One particularly interesting nontrivial spin texture existing in several antiferromagnets has spins at an angle of 120° with the in-plane neighbors and arranged in kagome lattices. Here we present a spin-wave calculation for antiferromagnets with kagome spin lattices considering exchange and single-ion anisotropy interactions between the spins. The theory yields exact analytical expressions for the frequencies of magnons at high-symmetry points of the Brillouin zone, that can be readily use to obtain the interaction parameters from experimental data with one-and two-magnon inelastic light scattering. The analytical expressions are used to obtain the field parameters for the kagome lattice antiferromagnet L1₂-IrMn₃from four experimentally measured frequencies. Both exchange field parameters are in reasonable agreement with the values obtained withab initiocalculations, while the anisotropy field is in very good agreement with the one calculated with atomistic spin models and Monte Carlo simulations.