Heat transport in Spin-Ice Systems

该项目的目的是获得一个详细的了解自旋冰材料的热传输。自旋冰材料是由具有大磁矩的稀土离子（如Dy ~（3+））组成的共角四面体组成。由于晶体场效应，每个力矩都有一个易磁化轴，连接四面体的角和中心，并指向四面体内部或外部。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位移。基态具有宏观简并性，这由所谓的2 in-2 out冰规则决定。对自旋冰的特别兴趣来自于存在分数元激发的证据，它的行为类似于磁共振激发。因此，存在与孤子激发的密切类似，孤子激发经常发生在低维和/或高维的情况下。（磁）挫系统。这个项目要回答的主要问题是澄清，自旋冰中的磁激发在多大程度上有助于总热传输，以及磁偶极子的传统模型或考虑其衰变为磁单极子是否是更充分的描述，或者在哪个温度和/或磁场范围内一个或另一个模型更合适。此外，它应澄清如何相互作用的磁激励，我们的目标是确定有效的平均自由程和特征速度的磁激励，并研究这些量是否依赖于他们的取向相对于不同的结晶方向。最重要的是外部磁场的影响，它解除了不同的2 in-2 out自旋冰状态的简并性，并根据磁场方向稳定不同的自旋配置作为基态。对于每一个不同的基态，我们想弄清楚是否有一个磁的热传输的贡献，以及这种贡献是否取决于相对于所施加的磁场和晶轴的方向的热流的方向。在替代实验的基础上，我们想探索磁激发与声子系统以及杂质的耦合。各种自旋冰材料将允许广泛的调查自旋冰基态和它的激发在一个广泛的和可控的实验参数范围。因此，我们不仅希望大大提高我们的自旋冰的具体物理知识，但也期望获得更深入的了解挫折基态以及一般的分数激发的动力学。

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.