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The putative quantum spin liquid compound 𝜅-(BEDT-TTF)2Ag2(CN)3 is investigated here by multifrequency electron spin resonance spectroscopy. At high temperatures, the spin susceptibility exhibits a weak temperature dependence, which can be described by the Heisenberg model with an antiferromagnetic exchange interaction. At low temperatures, however, the rapid drop of the static spin susceptibility, together with monotonic decrease of the ESR linewidth indicates that strong singlet correlations develop below a pairing energy scale accompanied by a spin gap.
[Picture: PI1]