Magnetic properties of κ-(ET)2Cu2(CN)3

The 1 H-NMR measurements on a Mott insulator κ-(ET) 2 Cu 2 (CN) 3 , which shows a superconducting transition under pressure, have been performed for the first time. We have found no long-range magnetic ordering down to 1.4 K, suggesting the possible spin liquid state at low temperatures. This behavior is in marked contrast with that of another Mott insulator κ-(ET) 2 Cu[N(CN) 2 ]Cl showing an antiferromagnetic transition at 26-27 K. The difference of the low-temperature magnetic state between these Mott insulators is explained by the difference of the degree of geometrical frustration between the localized spins on the triangular lattice.