An overview will be given of our studies of thermal properties of solids in which the topology plays a dominant role in the band structure. Two examples will be described in- more detail.
Goniopolar materials are conductors in which the dominant charge carriers behave like electrons along some crystallographic axes and holes along others. This arises in solids in which the Fermi surface is not simply connected (https://doi.org/10.1038/s41563-019-0309-4) . The effect opens a new path in thermoelectric generator (TEG) technology: it enables the generation of electrical power in a direction normal to that of the heat flux. This greatly simplifies the design of TEGs, by eliminating the need for electrical contacts at the hot side, where they are subject the thermal degradation. Transverse TEG’s also can be made from a single slab of material, unlike conventional TEG’s which are thermopiles with hundreds of n and p-type elements connected by contacts whose resistance decreases the thermal efficiency by close to 30%. We demonstrate experimentally, using Re4Si7, that the actual thermal efficiency of a transverse goniopolar TEG is very close to the theoretical limit (https://doi.org/10.1039/D1EE00923K).
Weyl semimetals (WSMs) 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 (https://doi.org/10.1038/s41563-021-00983-8) 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.
About the Speaker
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 magnetization in solids.