Abstract

In some magnets, despite the presence of a strong exchange interaction between magnetic ions, conventional magnetic ordering does not occur, but a collective paramagnetic state is formed. If, due to the particular architecture of the exchange bonds, the ground state turns out to be a singlet state and is separated from the triplet excited states by a gap in the energy spectrum, then this state persists down to T = 0. The spin dynamics of collective paramagnets with gap excitation spectrum (spin-gap magnets) at low temperatures can be described as the behavior of a dilute gas of triplet excitations. The application of a sufficiently strong magnetic field can close the gap in the spectrum, which leads to a gapless spin liquid state, or even to the unusual phenomenon of the formation of field-induced antiferromagnetism. The introduction of impurities into a spin-gap magnet leads to the formation of a paramagnetic center in the vicinity of a defect or exchange bonds randomly distributed in the lattice. This review presents the results of the study of several characteristic representatives of the class of quantum paramagnets with gapped excitation spectrum by the EPR spectroscopy method: a quasi-two-dimensional antiferromagnet (C₄H₁₂N₂)Cu₂Cl₆ and quasi-one-dimensional magnets of the spin tube, Cu₂C_l4 ⋅ H₈C₄SO₂, and the spin ladder, (C₇H₁₀N)₂CuBr₄, types. It has been shown that the electron spin resonance spectra make it possible to find common features in the behavior of these systems: to detect and characterize the fine structure of the energy levels of triplet excitations, to detect multiparticle relaxation processes in a gas of triplet excitations, and to observe the excitation of spin waves in the field-induced antiferromagnetically ordered state. Individual features of different systems are revealed as well.

中文翻译：

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带隙激励谱的集体顺磁体的磁共振

摘要

在某些磁体中，尽管在磁性离子之间存在强烈的交换相互作用，但常规的磁性排序并不会发生，而是形成了集体顺磁态。如果由于交换键的特定体系结构，基态变成单重态，并通过能谱隙与三重态激发态分开，则该态持续到T=0。具有间隙激发谱（自旋隙磁体）的集体顺磁体在低温下的自旋动力学可描述为三重激发的稀气体的行为。施加足够强的磁场可以缩小光谱中的间隙，从而导致无间隙自旋液体状态，甚至导致形成磁场感应反铁磁性的异常现象。将杂质引入自旋间隙磁体导致在缺陷附近形成顺磁中心或随机分布在晶格中的交换键。这篇综述介绍了通过EPR光谱法研究具有带隙激发谱的量子顺磁体类别的几个特征代表的研究结果：准二维反铁磁体（C ₄ħ ₁₂ Ñ ₂）的Cu ₂氯₆的旋管，将Cu和准一维磁铁₂ ç _L4 ⋅ħ ₈ c ^ ₄ SO ₂，和自旋梯子，（C ₇ H ^ ₁₀ N）₂的CuBr ₄，类型。已经证明，电子自旋共振谱使得可以找到这些系统行为的共同特征：检测和表征三重态激发能级的精细结构，检测三重态激发气体中的多粒子弛豫过程。 ，并观察场感应的反铁磁有序状态下自旋波的激发。还揭示了不同系统的个别功能。