### Article

## Microwave dynamics of the stoichiometric and bond-disordered anisotropic S=1 chain antiferromagnet NiCl2−4SC(NH2)2

We studied electron spin resonance in a quantum magnet NiCl2−4SC(NH2)2, demonstrating a field-induced quantum phase transition from a quantum-disordered phase to an antiferromagnet. We observe two branches of the antiferromagnetic resonance of the ordered phase, one of them has a gap, and the other is a Goldstone mode with zero frequency at a magnetic field along the fourfold axis. This zero-frequency mode acquires a gap at a small tilting of the magnetic field with respect to this direction. The upper gap was found to be reduced in the Br-substituted compound Ni(Cl1−xBrx)2−4SC(NH2)2 with x=0.21. This reduction is unexpected because of the previously reported rise in the main exchange constant in a substituted compound. Furthermore, a nonresonant diamagnetic susceptibility χ′ was found for the ordered phase in a wide frequency range above the quasi-Goldstone mode. This dynamic diamagnetism is as large as the dynamic susceptibility of the paramagnetic resonance. We speculate that it originates from a two-magnon absorption band of the low-frequency dispersive magnon branch.