Magnetic properties of an organic Mott insulator with an approximately isotropic triangular lattice structure, κ-(ET)2Cu2(CN)3, are investigated by the muon spin relaxation (μSR) method. No magnetic ordering is observed down to 20 mK in the zero-field (ZF) μSR measurement, suggesting the realization of the spin liquid ground state. An enhancement of the distribution of internal fields, corresponding to broadening of the linewidth reported in the 13C NMR study under magnetic fields, is not observed under ZF. A slight change in the spin dynamics is detected below 4 K. This change is quite small and different from the critical slow-down effect attributable to a magnetic transition.
74.25.Ha 75.50.−y 76.75.+i
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