Heat transport in Spin-Ice Systems

• Applicant: Professor Dr. Thomas Lorenz
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2013 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 229725134

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.

• DFG Programme: Research Grants