Direct determination of exchange parameters in spin-1/2 Heisenberg triangular-lattice antiferromagnets: high-field ESR studies

Spin-1/2 Heisenberg antiferromagnets Cs₂CuCl₄ and Cs₂CuBr₄ with distorted triangular-lattice structures are studied by means of electron spin resonance spectroscopy in magnetic fields up to the saturation field and above. In the magnetically saturated spin polarized phase, quantum fluctuations are fully suppressed, and the spin dynamics is defined by ordinary magnons. This allows us to accurately describe the magnetic excitation spectra in both materials and, using the harmonic spin-wave theory, to determine their exchange parameters. The proposed approach has a broader impact and can be potentially used for any quantum magnet with reduced (e.g., by the staggered DM interaction) translational symmetry, resulting, as predicted, in emergence of a new exchange mode in the magnetically saturated phase.