The Waterloo Institute for Nanotechnology (WIN) is pleased to present a Distinguished Lecture Series talk by Professor Joseph Heremans, an Ohio Eminent Scholar and distinguished professor in the Mechanical and Aerospace Engineering Department at the Ohio State University.
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. In ideal Weyl semimetals, where the electrochemical potential is located at the energy of the Weyl points, the Fermi surface in the bulk amounts to a pair of Weyl points of opposite chirality. Transport in ultra-quantum magnetic fields in an ideal WSM is subject to the chiral anomaly, an additional electrical conductivity that results from the generation of carriers of one chirality and the annihilation of carriers of the other in a magnetic field oriented parallel to the direction of the charge flux. In contrast the chiral anomaly in the electrical conductivity, there is no creation/annihilation of charge in the thermal conductivity, but there is an equivalent effect in the energy at both points, giving an excess thermal conductivity that we put in evidence experimentally.
This talk starts with a general introduction to the effects of topology on the Fermi surface. It then concentrates on the theory and the experimental observations of thermal magneto-transport on the field-induced WSM Bi1-xSbx (x = 11.5±0.5 at.%) in a high longitudinal magnetic field Bz along the trigonal direction z. The alloys are topological insulators at zero field. In a magnetic field, the very large Zeeman energy inverts the lowest spin-polarized conduction and valence band Landau levels (at Bz > 1-to-4 T), transforming the semiconductor into a WSM. The samples display freeze-out upon cooling and show no Shubnikov–de Haas oscillations despite their high mobility (1.9 × 106 cm2V-1s-1 at 10 K), ensuring that the electrochemical potential is at the Weyl points. By construction, no trivial pockets exist in the Fermi surface, making this an ideal WSM. We observe an increase in thermal conductivity kzz in longitudinal field Bzthat is consistent with the theory and obeys the Wiedemann-Franz law. The effects are reproducible in samples whose mobility vary by a factor of 100, and persist to temperatures double the Debye temperature, which we submit as evidence for topological protection of the thermal transport from defect and phonon scattering.
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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 in the automotive industry at the General Motors Research Laboratories, where he was the section manager for Semiconductor Physics, and at the Delphi Research Laboratories. His research interests focus on energy conservation and recovery, and lie at the intersection between experimental condensed matter physics and thermodynamics. In the last decade, he worked on the transport of heat, charge, and magnetization in solids.