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$_{4}$[1] was studied through magnetic and thermal measurements for Zn substituted insulating materials Ni$_{1-x}$Zn$_{x}$Ga$_{2}$S$_{4}$ (0.0 $\le \quad x \quad \le $ 0.3)[2]. Only 1 {\%} Zn substitution is enough to strongly suppress the coherence observed in the spin disordered state. However, suppression is not complete and the robust feature of the quadratic temperature 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 long-range order or bulk spin freezing suggests a novel symmetry breaking of the ground state. [1] Satoru Nakatsuji, Yusuke Nambu, Hiroshi Tonomura, Osamu Sakai, Seth Jonas, Collin Broholm, Hirokazu Tsunetsugu, Yiming Qiu and Yoshiteru Maeno, Science \textbf{309}, 1697 (2005). [2] Yusuke Nambu, Satoru Nakatsuji and Yoshiteru Maeno, preprint.