学术报告201534-美国俄亥俄州立大学Joseph P. Heremans院士报告会

Joseph P. Heremans is a member of the U.S. National Academy of Engineering. He joined the faculty of the Ohio State University in 2005 as Ohio Eminent Scholar and Professor in the Departments of Mechanical and Aerospace Engineering, Materials Science and Engineering, and Physics. He is a guest professor at Zhejiang University, and fellow of the American Associations for the Advancement of Science and the American Physical Society. He graduated from the Catholic University of Louvain (Belgium) with Ph.D. in Applied Physics (1978). After postdoctoral appointments, he had a 21-year career at the General Motors Research Labs and later at Delphi, as researcher and research manager. His research interests focus on the experimental investigation of electrical and thermal transport properties and on the physics of narrow-gap semiconductors, semimetals and nanostructures. In the last decade his group focuses on fundamental aspects of thermoelectric and thermal spin transport.

 

【报告摘要】

 This talk explores the potential to improve the thermoelectric figure of merit by adding a new variable to the optimization of materials: magnetism. The recent decade has seen a doubling of the efficiency of thermoelectric converters through the use of various band structure engineering techniques and nanotechnology. Here the spin degree of freedom is added to research on thermal solid-state energy converters, based on the recently discovered spin-Seebeck effect [SSE, 1,2].

The talk first explains the physics [3] behind the SSE, an effect that occurs at the interface between a ferromagnetic metal, insulator or semiconductor, and a normal metal with strong spin-orbit interactions. The temperature gradient creates flux of spin polarized particles, such as magnons [1,2] or spin-polarized conduction electrons [4]. This flux crosses the interface into a material with strong spin-orbit interactions, which then gives rise to an electric field via the spin-Hall effect [3]. This energy conversion technique has a very poor efficiency, and the loss mechanisms will be discussed.

Next, the talk will show how the efficiency can be increased by avoiding the interface altogether, and folding the magnon flux and the voltage generation step into the same material. This actually is the same physics as magnon drag, which is suspected to be the mechanism behind the very high thermopower of Fe [5]. New data is presented on the magnon drag thermopower and the Nernst coefficient of Fe. A model is developed and used to suggest how magnon drag could be used to design metals with a good thermoelectric figure of merit ZT. Metal thermoelectrics would have many advantages over semiconductors, in that they can be formed in any shape, can be welded, and, most importantly, are much less sensitive to impurities and defects because they contain far more electrons.

Conversely, the talk will also show how non-magnetic quasi-particles, like phonons, can also have a magnetic response in a heat flux. Phonon anharmonicity can be affected by magnetic fields, even in diamagnetic systems [6]. The local atomic displacements corresponding to the phonons locally modulate the valence band, which in turn creates a very small local modulation of the local diamagnetic susceptibility. In the presence of an external magnetic field, this exerts a local magnetic force on the atoms, which affects the probability of phonon-phonon interactions. The effect on the lattice thermal conductivity of InSb is measurable, and modeled by the theory without any adjustable parameter.

 

  [1] K. Uchida & al., Nature 455, 778 (2008); [2] C. M. Jaworski & al., Nature Materials, 9 898-903 (2010); [3] S. R. Boona & al., Energy Environ.Sci. 7 885-910 (2014) [4] C. M. Jaworski & al., Nature, 487, 210-213 (2012); [5] F. Blatt & al., Phys. Rev. Lett. 18 395 (1967); [6] Hyungyu Jin & al., Nature Materials 14, 601–606 (2015)