J. Phys. IV France 114 (2004) 377-378  EDP Sciences, Les Ulis DOI: 10.1051/jp4:2004114089 377 1 H-NMR study of Mott insulator N-(ET) 2 Cu 2 (CN) 3 with isotropic triangular lattice Y. Shimizu 1,2, * , K. Miyagawa 2,3 , K. Oda 2 , K. Kanoda 2,3 , M. Maesato 1 and G. Saito 1 1 Division of Chemistry, Graduate School of Science, Kyoto University, Oiwaketyo, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan 2 Department of Applied Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 3 CREST, Japan Science and Technology Corporation (JST), Kawaguchi, Saitama 332- 0012, Japan Abstract. 1H NMR measurement was performed for a single crystal sample of the organic Mott insulator κ-(ET)2Cu2(CN)3 with a nearly isotropic triangular lattice. The 1H NMR spectra and the spin-lattice relaxation rate, 1/T1, show no indication of magnetic transition down to 30 mK. However, the line shows a slight but gradual broadening accompanied with only a moderate increase in the spin-lattice relaxation rate, 1/T1, below 4 K. It suggests a possible change of spin dynamics of the frustrated quantum spins. Key words. Triangular lattice – Mott insulator – spin liquid. 1. INTRODUCTION Spin frustration is of interest in solid state physics, since it may break long-range spin ordering and bring out rich phenomena such as spin liquid state with large quantum fluctuations [1]. The quasi-two dimensional organic conductors inherently having triangular lattice structure can be models of the spin liquid. For instance, the electronic structure of κ-(ET) 2 X which has afforded many high T C superconductors is described as a frustrated triangular lattice consisting of ET dimer units. In fact, no indication of long-range magnetic order was observed in the 1 H NMR spectra of the isotropic triangular lattice compound, κ-(ET) 2 Cu 2 (CN) 3 , which suggests realisation of the spin liquid state [2]. In this paper we briefly discuss the spin dynamics of κ-(ET) 2 Cu 2 (CN) 3 at low temperatures based on the 1 H NMR results. 2. EXPERIMENTS The single crystals of κ-(ET) 2 Cu 2 (CN) 3 were obtained by electrooxidation of ET in a solution of KCN, CuCN and 18-crown-6 ether, which affords the Mott insulator exclusively [3]. The 1 H NMR experiments were carried out on the Cu 2+ free single crystal, since the presence of a small amount of Cu 2+ may give extrinsic behaviour of 1/T 1 . The 1 H NMR spectra were obtained from the solid echo signals. The magnetic field of 2.2 T was applied normal to the conducting plane. 3. RESULTS AND DISCUSSION Figure 1 (a) shows the temperature dependence of 1/T 1 . It decreases gradually with lowering temperature and rather steeply below 10 K, implying the depression of spin excitation. However, the * Corresponding author: Y. Shimizu, e-mail: yasuhiro@kuchem.kyoto-u.ac.jp