MSE Seminar Series: Thermal Effects of Topologically Non-trivial Fermi Surfaces
Speaker: Dr. Joseph Heremans
Professor, Mechanical & Aerospace Engineering and MSE & Physics
The Ohio State University
Title: Thermal Effects of Topologically Non-trivial Fermi Surfaces: the Thermal Chiral Anomaly and Goniopolar Thermopower
Thermal conductivity and thermoelectric measurements provide some of the most conclusive experimental evidence for the existence of topologically non-trivial electronic band structures. In this talk, this statement will be illustrated using two examples.
The first example uses Bi89Sb11 alloys that are narrow-gap semiconductors identified in 2008 as the first topological insulators (TI). Thermal conductivity data from these alloys show a 300% increase in conductivity with magnetic field when it is increased from 3 to 9 Tesla. In that field regime, the TI phase is converted to an ideal Weyl phase. In the ultra-quantum limit, this gives rise to the chiral anomaly, an anomalous generation and annihilation of charge carriers due to the Berry curvature of the last Landau level. Prior work claimed that the onset of a negative longitudinal magnetoresistance is evidence for this chiral anomaly. However, the measurements suffer from the fact that the magnetic field distorts the current lines (current jetting), making the conclusions ambiguous. Thermal measurements avoid this ambiguity, as no electrical current is flowing; thus, there is no current jetting. The observed increase in electronic thermal conductivity is robust vis-à-vis defect and phonon scattering, illustrating topological protection, and fits the theoretical predictions quantitatively. We argue that these data provide clear evidence for the existence of the thermal chiral anomaly in the solid state.
The second example is the discovery of a new class of materials we label goniopolar materials. These are anisotropic metals with Fermi surfaces that fulfill specific conditions: they have concave sections and must be non-simply connected along one direction. Under a specific set of conditions, such materials can have a thermoelectric power that is negative along some directions, and positive along others; interestingly, the sign of the Hall effect is the exact opposite. An example of such materials is NaSn2As2. Such materials have potential for device applications, because conversion from p-type to n-type conduction can occur in a homogeneous material simply by manipulating the current line directions.
Joseph Heremans is an Ohio Eminent Scholar and Professor in the Department of Mechanical and Aerospace Engineering at Ohio State University, where he is also Professor of Physics and Materials Science and Engineering. He is a member of the National Academy of Engineering and a fellow of the American Association for the Advancement of Science and the American Physical Society. His current research interests focus on the thermal transport properties of magnons and of electrons in non-trivial topological phases and in thermal energy conversion. He received his Ph. D. in Applied Physics (1978) from the Catholic University of Louvain, Belgium. With a fellowship from the Belgian National Science Foundation, he was a visiting scientist at theUniversity of Copenhagen, the Massachusetts Institute of Technology, and the University of Tokyo. He joined General Motors Research Laboratories in 1984, became group leader of the Electro-optical Physics Group (1985), and manager of the Semiconductor Physics Section (1987). He moved to Delphi Corporation’s Research Labs in 1999, where he was a Research Fellow.