Magnetic excitations in the spin-1 anisotropic antiferromagnet $NiCl_2-4SC(NH_2)_2$

TL;DR

This paper investigates magnetic excitations in the spin-1 anisotropic antiferromagnet NiCl_2-4SC(NH_2)_2, combining experimental ESR data and theoretical analysis to understand bound states and spectral features across a field-induced quantum phase transition.

Contribution

It provides a comprehensive reinterpretation of ESR spectra and confirms the existence of single-ion two-magnon bound states consistent with theoretical predictions.

Findings

Reanalysis of frequency-field dependence aligns experiment with theory.

Identification of ESR signatures of two-magnon bound states.

Clarification of spectral structure in the intermediate phase.

Abstract

The spin-1 anisotropic antiferromagnet NiCl_2-4SC(NH2)_2 exhibits a field-induced quantum phase transition that is formally analogous to Bose-Einstein condensation. Here we present results of systematic high-field electron spin resonance (ESR) experimental and theoretical studies of this compound with a special emphasis on single-ion two-magnon bound states. In order to clarify some remaining discrepancies between theory and experiment, the frequency-field dependence of magnetic excitations in this material is reanalyzed. In particular, a more comprehensive interpretation of the experimental signature of single-ion two-magnon bound states is shown to be fully consistent with theoretical results. We also clarify the structure of the ESR spectrum in the so-called intermediate phase.