Chiral enhancement of two-magnon bound states in an $S=1/2$ triangular-lattice magnet

Abstract

We study one- and two-magnon excitations above the fully polarized state of the spin-$1/2$ triangular-lattice $J_1$-$J_2$-$J_3$ Heisenberg model with an additional uniform scalar-chirality interaction. In the one-magnon sector of the Heisenberg model, we identify two special minimum manifolds by rewriting the dispersion in complete-square form. The scalar-chirality term cancels exactly in this sector, leaving the one-magnon dispersion and the single-magnon instability unchanged. In contrast, it survives in the two-magnon sector as an oriented interaction between neighboring flipped spins. Using symmetry-adapted triangular-lattice harmonics, we derive finite-dimensional gap equations at the $Γ$ point in the symmetry-resolved $\mathsf{A_1}$ and $\mathsf{E_2}$-type partial-wave channels. The chirality coupling splits the two opposite relative-motion chiralities in the $\mathsf{E_2}$-type sector, thereby selectively enhancing one two-magnon bound-state channel. Exact diagonalization confirms this mechanism and reveals enhanced binding, as well as additional bound states at $M$ and at incommensurate total momenta. Our results identify scalar chirality as an efficient microscopic mechanism for strengthening two-magnon binding without shifting the one-magnon spectrum, and provide a route toward high-field spin-nematic and multipolar instabilities.