High-field electron spin resonance (ESR) measurements of single crystals of (CH 3) 2NH 2CuCl 3 (DMACuCl 3) have been performed in the frequency range of 50 to 550 GHz in the paramagnetic region. In addition to the electron paramagnetic resonance, we have observed four ESR modes ω 1, ω 2, ω 3, and ω 4. The modes ω 2 and ω 3 have a frequency gap of 325 GHz (15.6 K), while the mode ω 1 has a gap of 420 GHz (20 K) at 0 T. The modes ω 2 and ω 3 are attributed to the transitions between the singlet and the triplet of the antiferromagnetic (AF) dimer. We found that the large spin-gap for the AF dimer still remains at the critical field B c2 = 3.5 T determined by magnetization and specific heat measurements.
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