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.

One of the most striking features deriving from quantum mechanics is the destabilization effect of classical ordering. In classical Newtonian dynamics, all systems lose their internal degrees of freedom and crystallize as the temperature tends towards zero. Quantum fluctuations work against this tendency and sometimes cause a quantum-liquid state without classical crystallization even at absolute zero. The best-known examples are 3 He, 4 He and electron systems in metals. Over the past one hundred years, these states have been intensively studied and shown to have several instabilities that are not identified with classical crystallization but described by quantum mechanics. One example is the Peierls instability in quantum fermion liquids, which breaks translational symmetry. Another profound example are the superfluid and superconducting instabilities, which have been one of the central topics in modern physics. Such instabilities arise because there is always an energy gain if a gapless quantum-liquid system creates a gap by undergoing symmetry breaking.Recently, a new quantum liquid has been discovered 1,2 -the quantum spin liquid. A classical spin system with antiferromagnetic interactions inevitably undergoes classical crystallization, that is, antiferromagnetic ordering. In spin-1/2 systems, quantum fluctuations take effect and destabilize the classical crystallization. If they completely destroy the classical crystallization, they may realize a quantum state with no trivial order, which is referred to as a quantum spin liquid. (Note that, in this definition, the quantum spin liquids may yet exhibit non-trivial symmetry breaking and/or topological ordering.) Apart from in one-dimensional (1D) systems, however, quantum fluctuations are so weak on 1 Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan, 2 Condensed Molecular Materials Laboratory, RIKEN, Wako-shi, Saitama 351-0198, Japan. *e-mail: tetsuaki@staff.mbox.media.kyoto-u.ac.jp.typical non-frustrated lattices that they cannot destroy classical crystallization [4][5][6] . Therefore, strong geometrical frustration, which works against the classical crystallization, is needed to realize the spin-liquid state for D ≥ 2. This spin liquid, although theoretically proposed early on 7 , has not been experimentally realized for many years, because real model materials of ideal frustrated 1/2-spins are limited. Recently, however, experimental studies of spin-liquid physics have taken an important step; some organics are now recognized as ideal frustrated spin-1/2 systems, and the spin-liquid state has been found 1,2 in κ-(BEDT-TTF) 2 Cu 2 (CN) 3 (BEDT-TTF = bis(ethylenedithio)-tetrathiafulvalene) and EtMe 3 Sb[Pd(dmit) 2 ] 2 (dmit = 1,3-dithiole-2-thione-4,5-dithiolate, Me = CH 3 , Et = C 2 H 5 ), which have sparked considerable interest in the condensedmatter community [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] . Thus, the first experimental goal of realizing the spin liquid in real materials h...

The series of [Pd(dmit) 2 ] salts (dmit = 1,3-dithiole-2-thione-4,5-dithiolate) is a Mott insulator on an anisotropic triangular lattice and has an antiferromagnetic exchange constant J ∼ 200 K. While it has been revealed that most of them undergo antiferromagnetic transitions, the nature of the spin state of EtMe 3 Sb[Pd(dmit) 2 ] 2 which has a nearly isotropic triangular lattice is still unknown. We investigated the spin state by means of 13 C NMR measurements down to 1.37 K for polycrystalline samples in which the carbon atoms in both ends of the Pd(dmit) 2 molecule were selectively enriched with the 13 C isotope. Both behaviours of the spin-lattice relaxation rate and the spectra indicate no spin ordering/freezing at least down to 1.37 K, which is lower than onehundredth of the exchange interaction. Furthermore, the spin-lattice relaxation rate does not show an exponential temperature dependence and seems to retain a finite value at low temperatures. Thus, the ground state of this system is possibly the almost-gapless spin-liquid state.

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