Spin Dynamics of $Ni Cl_2-4SC(NH_2)_2$ in the Field-Induced Ordered Phase

TL;DR

This study investigates the low-energy spin excitations in the field-induced ordered phase of the spin-1 material DTN, providing insights into its Bose-Einstein condensation behavior through high-field ESR measurements.

Contribution

The paper offers a systematic analysis of DTN's excitation spectrum using ESR and models the observed modes with a four-sublattice antiferromagnet framework, emphasizing the role of unbroken axial symmetry.

Findings

Identification of two gapped modes in the excitation spectrum.

Interpretation of modes within a four-sublattice antiferromagnet model.

Confirmation of unbroken axial symmetry's role in BEC-like ordering.

Abstract

$Ni Cl_2$-$4SC(NH_2)_2$ (known as DTN) is a spin-1 material with a strong single-ion anisotropy that is regarded as a new candidate for Bose-Einstein condensation (BEC) of spin degrees of freedom. We present a systematic study of the low-energy excitation spectrum of DTN in the field-induced magnetically ordered phase by means of high-field electron spin resonance measurements at temperatures down to 0.45 K. We argue that two gapped modes observed in the experiment can be consistently interpreted within a four-sublattice antiferromagnet model with a finite interaction between two tetragonal subsystems and unbroken axial symmetry. The latter is crucial for the interpretation of the field-induced ordering in DTN in terms of BEC.