The notion of quantum spin-liquids (QSLs), antiferromagnets with quantum fluctuation-driven disordered ground states, is now firmly established in one-dimensional (1D) spin systems as well as in their ladder cousins. The spin-1/2 organic insulator -(bis(ethylenedithio)-tetrathiafulvalene) 2 Cu 2 (CN) 3 (-(BEDT-TTF) 2 Cu 2 (CN) 3; ref.1) with a 2D triangular lattice structure is very likely to be the first experimental realization of this exotic state in D2. Of crucial importance is to unveil the nature of the low-lying elementary spin excitations, particularly the presence/absence of a spin gap, which will provide vital information on the universality class of this putative QSL. Here, we report on our thermal-transport measurements carried out down to 80 mK. We find, rather unexpectedly, unambiguous evidence for the absence of a gapless excitation, which sharply contradicts recent reports of heat capacity measurements. The low-energy physics of this intriguing system needs be reinterpreted in light of the present results indicating a spin-gapped QSL phase.
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