Heat transport in Spin-Ice Systems

该项目的目的是对自旋-冰材料中的热输运有一个详细的了解。这涉及到磁激发热输运的研究以及磁激发对声子热输运的影响。自旋-冰材料是由具有大磁矩的稀土离子组成的角共用四面体，例如Dy3。由于晶场效应，每个力矩都有一个简单的轴，将角点与四面体的中心连接起来，并指向四面体的内部或外部。自旋冰是结晶冰的磁性类似物，其中磁矩的方向对应于质子相对于氧离子的移动。基态具有宏观简并性，这是由所谓的2in-2out冰规则决定的。对自旋冰特别感兴趣的原因是有证据表明，存在像磁单极子激发一样的分数元激发。因此，这与通常发生在低维和(或)磁受阻系统中的孤子激发有密切的相似之处。本项目要回答的主要问题是，自旋-冰中的磁激发对总的热输运有多大的贡献，以及是传统的磁偶极子模型还是考虑其衰变为磁单极子是更合适的描述，或者在哪个温度和/或磁场范围内其中一个模型更合适。此外，我们还将阐明磁激发是如何相互作用的，我们的目标是确定磁激发的有效平均自由程和特征速度，并研究这些量是否依赖于它们相对于不同晶体方向的取向。最重要的是外加磁场的影响，它提高了不同2in-2out自旋冰态的简并度，并根据场的方向稳定了不同的自旋组态作为基态。对于每个不同的基态，我们想要弄清楚是否存在磁场对热输运的贡献，以及这种贡献是否取决于热流相对于外加磁场的方向和晶轴的方向。在替代实验的基础上，我们想要探索磁激发与声子系统以及与杂质的耦合。各种不同的自旋冰材料将允许在一个宽且可控的实验参数范围内广泛地研究自旋冰基态及其激发。因此，我们不仅希望显著提高我们对自旋冰具体物理的了解，而且希望对受挫基态以及一般分数激发的动力学有更深入的了解。

The aim of this project is to gain a detailed understanding of the heat transport in spin-ice materials. This concerns the study of heat transport by magnetic excitations as well as the influence of magnetic excitations on the phonon heat transport.The spin-ice materials consist of corner-sharing tetrahedra of rare earth ions with large magnetic moments, e.g. Dy3+. Due to crystal field effects, each moment has an easy axis connecting the corner with the center of the tetrahedron and point either into or out of the tetrahedron. Spin ice is the magnetic analog of crystalline ice wherein the directions of the magnetic moments correspond to the proton shift relative to the oxygen ions. The ground state has a macroscopic degeneracy, which is determined by the so-called 2in-2out ice-rule. The particular interest in spin-ice results from the evidence that there are fractionalized elementary excitations, which behave like magnetic monopole excitations. Thus, there is a close analogy to soliton excitations, which often occur in low-dimensional and/or (magnetically) frustrated systems.The main questions to be answered by this project is to clarify, to what extent the magnetic excitations in spin-ice contribute to the total heat transport, and whether a conventional model of magnetic dipoles or the consideration of their decay into magnetic monopoles is a more adequate description, or in which temperature and/or magnetic-field range one or the other model is more adequate. Further, it shall be clarified how the magnetic excitations interact with each other and we aim to determine the effective mean free paths and characteristic velocities of the magnetic excitations and to study whether these quantities dependence on their orientation with respect to the different crystallographic directions. Of central importance is the influence of an external magnetic field which lifts the degeneracy of the different 2in-2out spin ice states and stabilizes different spin configurations as the ground state depending on the field direction. For each of the different ground states we want to clarify whether there is a magnetic contribution to the heat transport and whether this contribution depends on the orientation of the heat flow relative to the direction of the applied magnetic field and the crystal axes. On the basis of substitution experiments, we want to explore the coupling of the magnetic excitations to the phononic system as well as to impurities. The various spin ice materials will allow for extensive investigations of the spin-ice ground state and its excitations over a wide and well controllable experimental parameter range. Thus, we do not only hope to considerably improve our knowledge of the specific physics of the spin ice, but also expect to gain a deeper understanding of frustrated ground states as well as the dynamics of fractional excitations in general.