Gapped quantum spin-liquid state in a frustrated triangular magnet $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$

Unveiling the nature of quantum-spin-liquids (QSL) states, quantum fluctuation-driven disordered ground states, has been a central challenge in condensed matter physics. Especially the nature of the low-lying spin excitations and the presence/absence of the ``spin gap'' have been of great interest. Recently, NMR measurements have shown that a QSL state is realized in $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ with a nearly isotropic 2D triangular lattice structure. Here we report on our thermal-transport measurements in this compound down to 80~mK. We find that the QSL state has a full gap of $\sim 0.5$~K ($\sim J/500$) and the gap is hardly affected by magnetic fields up to 10~T [1], which sharply contradict recent reports of heat capacity measurements reporting a finite $\gamma$-term. We will discuss some possibilities to explain the tiny spin gap in this triangular system. \\[0pt] [1] M. Yamashita et al., Nature Physics (in press).