Microwave dynamics of pure and doped anisotropic S=1 chain antiferromagnet NiCl2-4SC(NH2)2

TL;DR

This study investigates the microwave spin dynamics in a quantum antiferromagnet NiCl2-4SC(NH2)2, revealing field-induced phase transitions, mode behaviors, effects of doping, and unexpected diamagnetic responses in the ordered phase.

Contribution

It provides new insights into the field-dependent spin resonance modes and the impact of doping on the magnetic excitation spectrum of an anisotropic S=1 chain antiferromagnet.

Findings

Observation of two antiferromagnetic resonance branches with a gap and a Goldstone mode.

Doping reduces the upper gap despite increased exchange constants.

Detection of a large dynamic diamagnetic susceptibility above the quasi-Goldstone mode.

Abstract

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 four-fold 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 doped compound Ni(Cl(1-x)Br(x))2-4SC(NH2)2 with $x=0.21$. This reduction is unexpected because of the previously reported rise of the main exchange constant in a doped compound. Further, a nonresonant diamagnetic susceptibility $\chi^{\prime}$ 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 low-frequency dispersive magnon branch.