Abstract Motivated by recent experiments on quantum magnet NiCl2-4SC(NH2)2 (DTN) and its Br-doped counterpart DTNX we propose a theoretical description of optical magnon branch in the antiferromagnet with large single-ion anisotropy in the magnetically ordered phase. In the framework of the 1 / S expansion we derive analytical expressions for optical magnon with k = 0 energy magnetic field dependence Δ ( h ) . It is shown that in the linear spin wave approximation Δ ( h ) is monotonic without extrema whereas first order in 1 / S corrections makes it drastically different function with a minimum near the center of magnetically ordered phase. The latter behaviour was observed in ESR experiments. Moreover, we show that Δ ( h ) has nontrivial dependence on the system parameters. It solves the discrepancy between inelastic neutron scattering data where the growth of interaction constants in DTNX with small Br concentration was observed and ESR experiments showing almost unchanged Δ ( h ) in comparison with pure DTN.

中文翻译：

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具有大单离子易平面各向异性的反铁磁体中的磁子谱和电子自旋共振

摘要 受量子磁体 NiCl2-4SC(NH2)2 (DTN) 及其掺杂对应物 DTNX 的近期实验的启发，我们提出了在磁有序相中具有大单离子各向异性的反铁磁体中光学磁振子分支的理论描述。在 1/S 展开的框架内，我们推导出具有 k = 0 能量磁场依赖性 Δ ( h ) 的光学磁振子的解析表达式。结果表明，在线性自旋波近似中 Δ ( h ) 是单调的，没有极值，而 1 / S 校正中的一阶使其函数完全不同，在磁有序相位的中心附近具有最小值。在 ESR 实验中观察到后一种行为。此外，我们表明 Δ ( h ) 对系统参数具有非平凡的依赖性。