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 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 viability of the proposed method was proven by applying it to Cs2CuCl4, yielding J/kB=4.7(2) K, J′/kB=1.42(7) K [J′/J≃0.30] and revealing good agreement with inelastic neutron-scattering results. For the isostructural Cs2CuBr4, we obtain J/kB=14.9(7) K, J′/kB=6.1(3) K, [J′/J≃0.41], providing exact and conclusive information on the exchange couplings in this frustrated spin system.
@article{arxiv.1401.6793,
title = {Direct determination of exchange parameters in Cs2CuBr4 and Cs2CuCl4: high-field ESR studies},
author = {S. A. Zvyagin and D. Kamenskyi and M. Ozerov and J. Wosnitza and M. Ikeda and T. Fujita and M. Hagiwara and A. I. Smirnov and T. A. Soldatov and A. Ya. Shapiro and J. Krzystek and R. Hu and H. Ryu and C. Petrovic and M. E. Zhitomirsky},
journal= {arXiv preprint arXiv:1401.6793},
year = {2015}
}