Dynamical fractal and anomalous noise in a clean magnetic crystal
Dynamical fractal and anomalous noise in a clean magnetic crystal
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DOI：
10.1126/science.add1644
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发表时间：
2022-10
期刊：
影响因子：
56.9
通讯作者：
Jonathan N. Hallén;S. Grigera;D. Tennant;C. Castelnovo;R. Moessner
中科院分区：
文献类型：
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作者：
Jonathan N. Hallén;S. Grigera;D. Tennant;C. Castelnovo;R. Moessner
分形-具有非整数维的物体-在自然界中存在于多种设置和长度尺度中。在这项工作中，我们确定了一个新兴的动力学分形无序，化学计量，三维磁性晶体的热力学平衡。这一现象是由于自旋冰中的磁共振激发动力学受到限制而产生的，这种限制限制了它们在分形上的运动。这一观察结果解释了自旋冰化合物Dy 2 Ti 2 O 7的磁噪声实验中发现的异常指数，并解决了一个长期存在的关于其快速发散弛豫时间的难题。自旋冰能够表现出如此显著的现象表明，即使是简单的拓扑多体系统的合作动力学也会有更多意想不到的发现。描述分形跳跃单极自旋冰具有由磁性离子四面体组成的晶格。在基态，每个四面体上的四个自旋中有两个指向内，两个指向外。当一个被称为磁共振的激励被产生时，磁共振在晶体中移动时，这一规则就被违反了。单极动力学反映在磁噪声等数量上，其测量结果显示出与最简单模型预测的频率依赖性不同。Hallén等人解决了这个难题，他们意识到，平行运动比以前认为的更受限制，并且仅限于具有分形结构的簇（参见闪烁的透视图）。-JS一种将它们的运动限制在分形簇中的磁动力学模型解释了异常磁化噪声数据。
Fractals—objects with noninteger dimensions—occur in manifold settings and length scales in nature. In this work, we identify an emergent dynamical fractal in a disorder-free, stoichiometric, and three-dimensional magnetic crystal in thermodynamic equilibrium. The phenomenon is born from constraints on the dynamics of the magnetic monopole excitations in spin ice, which restrict them to move on the fractal. This observation explains the anomalous exponent found in magnetic noise experiments in the spin ice compound Dy2Ti2O7, and it resolves a long-standing puzzle about its rapidly diverging relaxation time. The capacity of spin ice to exhibit such notable phenomena suggests that there will be further unexpected discoveries in the cooperative dynamics of even simple topological many-body systems. Description Fractal-hopping monopoles Spin ices have crystal lattices that consist of tetrahedra of magnetic ions. In a ground state, two of the four spins on each tetrahedron point in and two point out. When an excitation called the magnetic monopole is created, this rule is violated as the monopole moves through the crystal. Monopole dynamics are reflected in quantities such as magnetic noise, the measurements of which have shown a different frequency dependence from the one that the simplest model predicts. Hallén et al. solved this puzzle by realizing that the monopole motion is more restricted than previously thought and is limited to a cluster with a fractal structure (see the Perspective by Flicker). —JS A model of monopole dynamics that restricts their motion to a fractal cluster explains anomalous magnetization noise data.