Unusual dynamics of spin-1/2 antiferromagnets on the triangular lattice in magnetic field

TL;DR

This paper uses a bond-operator theory to analyze the dynamical properties of spin-1/2 Heisenberg antiferromagnets on a triangular lattice in a magnetic field, revealing complex quasiparticle behavior and phase transitions.

Contribution

It introduces a quantum bond-operator approach to describe elementary excitations and phase evolution in the triangular lattice antiferromagnet under magnetic field, capturing phenomena beyond semiclassical theories.

Findings

Identification of four magnetic phases with specific spin arrangements.

Discovery of spin-0 excitations with long lifetime in the UUD phase.

Observation of complex evolution of quasiparticle spectra across phases.

Abstract

We theoretically discuss dynamical properties of spin-1/2 Heisenberg antiferromagnet on the triangular lattice in magnetic field $\bf H$. We use the recently proposed bond-operator theory which operates with quantum states of the whole magnetic unit cell containing three spins. This technique describes accurately short-range spin correlations and provides a quantitative description of elementary excitations which appear in other approaches as bound states of conventional low-energy quasiparticles (e.g., magnons). In quantitative agreement with previous numerical and analytical findings, we observe four phases with coplanar spin arrangements upon the field increasing: the three-sublattice Y-phase, the collinear "up-up-down" (UUD) state, the non-collinear V-phase, and the collinear fully polarized (FP) state. We demonstrate that apart from magnons there are spin-0 elementary excitations in the UUD state one of which is long lived and its spectrum lies below magnon branches. This mode originates from a high-energy quasiparticle at $H=0$ and it produces anomalies only in the longitudinal spin correlator. In the V-phase, we obtain multiple short-wavelength spin excitations which have no counterparts in the semiclassical spin-wave theory. We demonstrate a highly nontrivial field evolution of quasiparticles spectra on the way from one collinear state (UUD) to another one (FP) via the non-collinear V-phase (in which the longitudinal and the transverse channels are mixed). In particular, some parts of the spin-0 branch in the UUD state become parts of the spin-1 (magnon) branch in the FP phase whereas some parts of one magnon branch turn into parts of spin-2 branch. Such evolution would be very difficult to find by any conventional analytical approach. Our results are in good agreement with neutron experimental data obtained recently in $\rm Ba_3CoSb_2O_9$, $\rm KYbSe_2$, and $\rm CsYbSe_2$.