The nature of quantum spin liquids, a novel state of matter where strong quantum fluctuations destroy the long-range magnetic order even at zero temperature, is a long-standing issue in physics. We measured the low-temperature thermal conductivity of the recently discovered quantum spin liquid candidate, the organic insulator EtMe3Sb[Pd(dmit)2]2. A sizable linear temperature dependence term is clearly resolved in the zero-temperature limit, indicating the presence of gapless excitations with an extremely long mean free path, analogous to excitations near the Fermi surface in pure metals. Its magnetic field dependence suggests a concomitant appearance of spin-gap-like excitations at low temperatures. These findings expose a highly unusual dichotomy that characterizes the low-energy physics of this quantum system.

Electron correlation and spin frustration are among the central issues in condensed matter physics, and their interplay is expected to bring about exotic phases with both charge and spin fluctuations. Molecular materials are playgrounds suitable for this study. Fundamentals in physics of Mott transition and spin frustration on triangular lattices are seen in the organic materials ET and Pd(dmit)2 compounds. We review the experimental studies on the criticality of Mott transition with a continuously controllable pressure technique and on the ground state of the quasi-triangular-lattice Mott insulator. Mott criticality is well characterized in both charge and spin channels with unconventional critical exponents of possibly quantum nature. The ground state of the triangular-lattice Mott insulator is changed from antiferromagnet to spin liquid as the triangular lattice becomes more isotropic. The various experiments probing the nature of spin liquid are described in the light of proposed mechanisms.

The family of layered organic salts X͓Pd͑dmit͒ 2 ͔ 2 are Mott insulators and form scalene-triangular spin-1 / 2 systems. Among them, EtMe 3 Sb͓Pd͑dmit͒ 2 ͔ 2 has a nearly regular-triangular lattice. We have investigated the spin state of this salt by 13 C-NMR and static susceptibility measurements. The temperature dependence of the susceptibility is described as that of a regular-triangular antiferromagnetic spin-1 / 2 system with an exchange interaction J = 220− 250 K. The 13 C-NMR measurements reveal that there is no indication of either spin ordering/freezing or an appreciable spin gap down to 1.37 K, which is lower than 1% of J. This result strongly suggests that this system is in the quantum spin-liquid state with no appreciable spin gap, which has been long sought after.

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