Ground state of the organic spin-liquid material EtMe3Sb[Pd(dmit)2]2

The organic Mott insulator EtMe3Sb[Pd(dmit)2]2 is a 1/2-spin system on a nearly-regular triangular lattice. It does not show classical antiferromagnetic ordering in spite of strong antiferromagnetic exchange interactions J = 220?250 K, and thus it is regarded as a spin liquid material. We have found that (T1T)?1 of this spin liquid state shows a peak at around 1 K and after that (T1T)?1 decreases on cooling. This is not attributed to slowing down of the spin dynamics because T2?1 does not show any enhancement down to low temperature limit. Therefore, the decrease of (T1T)?1 is attributed to disappearance of spin excitations, suggesting that magnetic excitations acquire an energy gap at least in some portion of q-space. This indicates that the ground state of the present spin-liquid material is not a state with spinon Fermi surface, where magnetic excitations are fully gapless and thus (T1T)?1 does not decrease down to low temperature limit.