NMR study of quantum spin liquid and its phase transition in the organic spin-1/2 triangular lattice antiferromagnet EtMe3Sb[Pd(dmit)2]2

We found by 13C-NMR experiment that a quasi-two-dimensional organic antiferromagnet with spin 1/2 on the triangular lattice, EtMe3Sb[Pd(dmit)2]2, has no classical magnetic ordering down to 19.4mK, although the magnetic exchange interaction J is 220-250K. The quantum spin liquid state is realized in this material due to the significant spin frustration and quantum fluctuation. The 13C nuclear spin-lattice relaxation rate T1−1 under an external magnetic field of 7.65T indicates that the excitations have no spin gap in this spin liquid phase above 1.0K. Then the relaxation rate exhibits a clear kink at 1.0K, and shows a steep decrease below this temperature proportional to T2. This clearly shows a continuous phase transition at 1.0K and the steep decrease of the spin fluctuation could show the appearance of the excitation gap in the lower spin liquid phase. The power law but not exponential law of the temperature dependence may imply a nodal gap rather than a full gap.