Thermal-transport measurements in a quantum spin-liquid state of the frustrated triangular magnet -(BEDT-TTF)2Cu2(CN)3

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 kappa-(bis(ethylenedithio)-tetrathiafulvalene)2Cu2(CN)3 (kappa-(BEDT-TTF)2Cu2(CN)3; ref. 1) with a 2D triangular lattice structure is very likely to be the first experimental realization of this exotic state in Dgreater than or equal to2. Of crucial importance is to unveil the nature of the low-lying elementary spin excitations2, 3, 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 measurements4. The low-energy physics of this intriguing system needs be reinterpreted in light of the present results indicating a spin-gapped QSL phase. [PUBLICATION ABSTRACT]