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## Direct determination of exchange parameters in Cs2CuBr4 and Cs2CuCl4: 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\; 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$