Unusual dynamics of spin-\(\frac{1}{2}\) antiferromagnets on the triangular lattice in magnetic field

DOI10.1016/J.AOP.2023.169342arXiv2202.02120OpenAlexW4372235634MaRDI QIDQ6097736

Publication date: 7 June 2023

Published in: Annals of Physics (Search for Journal in Brave)

Abstract: We theoretically discuss dynamical properties of spin-1/2 Heisenberg antiferromagnet on the triangular lattice in magnetic field

. 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

m B a_{3} C o S b_{2} O_{9}

,

m K Y b S e_{2}

, and

m C s Y b S e_{2}

.

Full work available at URL: https://arxiv.org/abs/2202.02120

zbMATH Keywords

elementary excitations spin dynamics quantum spin models

Mathematics Subject Classification ID

Quantum theory (81-XX) Statistical mechanics, structure of matter (82-XX)