Effect of Magnetic Field on Dynamics of Antiferromagnets

Motivated by synthesis of new antiferromagnetic compounds with weak exchange coupling, a study of high magnetic field properties of the Heisenberg model on a square-lattice is undertaken here. Ordered spins at zero temperature cant toward the field direction inducing coupling of transverse and longitudinal magnon modes. Resulting interactions renormalize the ground-state energy and the dispersion relation below a threshold field H∗ = 0.76Hsat, where one-magnon excitations starts to become unstable and acquire finite lifetimes. Such decays originates from Van-Hove singularities in the two-magnons density of states lying below the one-particle energy. Decay rates are computed using a Self-Consistent Born Approximation, revealing strong magnon damping in the middle of the Brillouin’s zone quarters whereas sound and precession modes remain well defined. Far from these modes, transverse part of the dynamical structure factor display important broadening of the excitations peaks that might be accesible to neutron scattering experiments

Hierarchy of exchange interactions in the triangular-lattice spin-liquid YbMgGaO4_{4}

The spin-1/2 triangular lattice antiferromagnet YbMgGaO$_{4}$ has attracted recent attention as a quantum spin-liquid candidate with the possible presence of off-diagonal anisotropic exchange interactions induced by spin-orbit coupling. Whether a quantum spin-liquid is stabilized or not depends on the interplay of various exchange interactions with chemical disorder that is inherent to the layered structure of the compound. We combine time-domain terahertz spectroscopy and inelastic neutron scattering measurements in the field polarized state of YbMgGaO$_{4}$ to obtain better microscopic insights on its exchange interactions. Terahertz spectroscopy in this fashion functions as high-field electron spin resonance and probes the spin-wave excitations at the Brillouin zone center, ideally complementing neutron scattering. A global spin-wave fit to all our spectroscopic data at fields over 4T, informed by the analysis of the terahertz spectroscopy linewidths, yields stringent constraints on $g$-factors and exchange interactions. Our results paint YbMgGaO$_{4}$ as an easy-plane XXZ antiferromagnet with the combined and necessary presence of sub-leading next-nearest neighbor and weak anisotropic off-diagonal nearest-neighbor interactions. Moreover, the obtained $g$-factors are substantially different from previous reports. This works establishes the hierarchy of exchange interactions in YbMgGaO$_{4}$ from high-field data alone and thus strongly constrains possible mechanisms responsible for the observed spin-liquid phenomenology

Instabilities of heavy magnons in an anisotropic magnet

The search for new elementary particles is one of the most basic pursuits in physics, spanning from subatomic physics to quantum materials. Magnons are the ubiquitous elementary quasiparticle to describe the excitations of fully-ordered magnetic systems. But other possibilities exist, including fractional and multipolar excitations. Here, we demonstrate that strong quantum interactions exist between three flavors of elementary quasiparticles in the uniaxial spin-one magnet FeI2. Using neutron scattering in an applied magnetic field, we observe spontaneous decay between conventional and heavy magnons and the recombination of these quasiparticles into a super-heavy bound-state. Akin to other contemporary problems in quantum materials, the microscopic origin for new physics in FeI2 is the quasi-flat nature of excitation bands and the presence of Kitaev anisotropic magnetic exchange interactions.Comment: 29 pages, 3 main figure