ESR excitations in the BEC candidate DTN: what can we learn from that?


ESR excitations in the BEC candidate DTN: what can we learn from that?

Zvyagin, S.

NiCl2-4SC(NH2)2 (known as DTN) is a quantum S = 1 chain system with strong easy-plane anisotropy that is regarded as a new candidate for the Bose-Einstein condensation (BEC) of magnons, with critical fields Bc1 = 2.1 T, Bc2 = 12.6 T (defined at T = 0). Tuneable-frequency Electron Spin Resonance (ESR) studies of magnetic excitations in DTN in fields up to 25 T are presented. Based on analysis of the single-magnon excitation mode observed in the high-field spin-polarized phase at temperatures above Tc (Tc ≤ 1.2 K) and previous experimental results [Phys. Rev. Lett. 96, 077204 (2006)], a revised set of spin-Hamiltonian parameters is obtained. Our results yield D = 8.9 K, Jc = 2.2 K, and Ja,b = 0.18 K for the anisotropy, intrachain, and interchain exchange interactions, respectively. These values are used to calculate the antiferromagnetic phase boundary, magnetization and the frequency-field dependence of two-magnon bound-state excitations predicted by theory and observed in DTN for the first time. Furthermore, we present a systematic study of the low-energy excitation spectrum of DTN in the field-induced magnetically ordered phase (which has been regarded as the magnon BEC phase) 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 physically simplest scenario would correspond to the isotropic “corner-center” exchange. Since the exchange interaction within each sublattice is antiferromagnetic, the system in the BEC phase is supposed to be highly frustrated, and then its mean-field ground state at B > Bc1 should be infinitely degenerate. Our observations are of crucial importance for the interpretation of the field-induced ordering in DTN in terms of the BEC of magnons.

  • Lecture (Conference)
    Workshop Highly Frustrated Magnets, 02.-05.03.2008, Grenoble, France

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Publ.-Id: 11614