MSE Seminar: Thermal and thermoelectric properties of topologically non-trivial crystals
Speaker: Joseph Heremans, Professor, Materials Science & Engineering, Ohio State University
Title: Thermal and thermoelectric properties of topologically non-trivial crystals
First, an overview will be given of the Hereman group activities. They're focused on the fundamental research on thermal transport properties of electrons, phonons and magnons, and on the use of these properties in all-solid-state thermal energy conversion. Recent focus is on properties that are affected by topological properties of the Fermi surface in goniopolar materials (Nature Materials, https://doi.org/10.1038/s41563-019-0309-4) and in Weyl semimetals (Nature Materials, https://doi.org/10.1038/s41563-021-00983-8).
Second, a more in-depth view will be given of a new mechanism for heat transport in Weyl semimetals (WSMs). These are solids with a bulk band structure consisting of pairs of chirally distinct linear Dirac bands that intersect at the Weyl points. Thermal transport in WSMs in the extreme quantum limit of magnetic fields show the thermal version of the chiral anomaly. This is an additional thermal conductivity that results from the generation of energy by carriers of one chirality and the annihilation of the energy by carriers of the other. The effect is shown experimentally on single-crystal Bi1-xSbx alloys (x=11 and 15 at.%). It dominates the thermal conductivity at 9 T, where it increases the electronic thermal conductivity by 300%. The thermal chiral anomaly is related to the electrical one, which is due to the creation and generation of electron numbers, by the Wiedemann-Franz law with a Lorenz ratio of p2/3 (kB/e)2 where p2/3 is now a topological invariant, unlike in the classical free electron case. This very large effect extends to over 200 K and could be useful in designing heat switches useful, for example, in adiabatic demagnetization refrigeration.
Heremans is an Ohio Eminent Scholar and Professor in the Mechanical and Aerospace Engineering Department at the Ohio State University, with appointments in the Materials Science and Engineering Department and the Department of Physics. He is a member of the National Academy of Engineering, and a fellow of AAAS and the American Physical Society. He joined OSU after a 21-year career at the General Motors and later Delphi Research Laboratories. His research interests focus on experimental measurements of transport properties, energy conservation and recovery. In the last decade, he worked on the transport of heat, charge, and spin in solids.