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

TL;DR

This study investigates how nonmagnetic impurities affect the spin disordered state in NiGa$_2$S$_4$, revealing persistent coherent excitations and suggesting a novel symmetry-breaking ground state.

Contribution

It demonstrates that even minimal impurity substitution disrupts coherence but does not eliminate the coherent excitations, indicating a unique ground state in NiGa$_2$S$_4$.

Findings

1% Zn substitution suppresses coherence but preserves T^2 specific heat.

Presence of a coherent Nambu-Goldstone mode despite disorder.

No conventional magnetic order or spin freezing observed.

Abstract

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.