Spin dynamics in triangular-lattice antiferromagnets Cs₂CuBr₄ and Cs₂CuCl₄: high-field ESR studies

A spin-1/2 Heisenberg antiferromagnet (AF) on a triangular lattice is the paradigmatic model in quantum magnetism, which was intensively studied. In spite of numerous theoretical studies (which predict a rich variety of grounds states, ranging from a gapless spin liquid to Néel order), many important details of the phase diagram of triangular-lattice AFs remain controversial or even missing. In order to test the theory experimentally, a precise information on the spin-Hamiltonian parameters for the materials of interest is highly demanded. Here, we present results of high-field electron spin resonance (ESR) studies of spin-1/2 Heisenberg AFs Cs₂CuCl₄ and Cs₂CuBr₄ with distorted triangular-lattice structures in magnetic fields up to 50 T. In the magnetically saturated phase (H > H_sat), 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 viability of the proposed method was first proven by applying it to Cs₂CuCl₄, revealing good agreement with inelastic neutron-scattering results. For the isostructural Cs₂CuBr₄ we obtain J/k_B = 14.9(7) K, J'/k_B = 6.1(3) K, [J'/J ~ 0.41], providing exact and conclusive information on the exchange coupling parameters in this frustrated spin system. The approach has a broader impact and can be potentially used for any quantum magnet with reduced (e.g., by the staggered Dzyaloshinskii-Moriya interaction) translational symmetry, resulting, as predicted, in emergence of a new exchange mode above H_sat