Coherent Behavior and Nonmagnetic Impurity Effects of the Spin Disordered State in NiGa${}_2$S${}_4$

Nonmagnetic impurity effects of the spin disordered state in the triangular antiferromagnet NiGa${}_2$S${}_4$ was studied through magnetic and thermal measurements for Ni${}_{1-x}$Zn${}_x$Ga${}_2$S${}_4$ (0.0\le x\le 0.3). Only 1 % substitution is enough to strongly suppress the coherence observed in the spin disordered state. However, the suppression is not complete and the robust feature of the T^2 dependent specific heat and its scaling behavior with the Weiss temperature indicate the existence of a coherent Nambu-Goldstone mode. Absence of either conventional magnetic order or bulk spin freezing suggests a novel symmetry breaking of the ground state.