Heat transport in Spin-Ice Systems

本项目的目的是获得对自旋冰材料中的热输运的详细了解。本文主要研究磁激励下的热输运以及磁激励对声子热输运的影响。自旋冰材料由具有大磁矩的稀土离子（如Dy3+）共角四面体组成。由于晶体场效应，每个矩都有一个简单的轴连接四面体的中心和角，指向四面体的内部或外部。自旋冰是晶体冰的磁性类似物，其中磁矩的方向对应于质子相对于氧离子的位移。基态具有宏观简并，这是由所谓的2进2出冰律决定的。对自旋冰的特别兴趣来自于有证据表明存在分数化的基本激发，其行为类似于磁单极子激发。因此，这与孤子激励有着密切的相似之处，孤子激励经常发生在低维和/或（磁）受挫系统中。本项目要回答的主要问题是澄清，自旋冰中的磁激发对总热输运的贡献程度，以及传统的磁偶极子模型或考虑其衰变为磁单极子的模型是否更适当的描述，或者在温度和/或磁场范围内哪个模型更适当。此外，还应阐明磁激励如何相互作用，我们的目标是确定磁激励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学方向的取向。最重要的是外部磁场的影响，它提升了不同的2 - in-2 - out自旋冰态的简并，并根据场的方向稳定了不同的自旋构型作为基态。对于每一个不同的基态，我们想要弄清楚，是否有磁场对热传递的贡献，以及这种贡献是否取决于热流的方向，相对于外加磁场的方向和晶体轴的方向。在取代实验的基础上，我们想探索磁激励与声子系统以及杂质的耦合。各种自旋冰材料将允许在广泛和可控的实验参数范围内广泛研究自旋冰基态及其激发。因此，我们不仅希望大大提高我们对自旋冰的具体物理知识，而且希望对受挫基态以及一般的分数激励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.