Mechanical and Civil Engineering Seminar

In the last decade, thermoelectric converter efficiency has doubled using various band structure engineering techniques and nanotechnology.  The first part of this talk shows how specific doping impurities in semiconductors, called resonant levels, catalyze an increase in the electronic density of states of the delocalized electrons near conduction or valence band edges.  The carriers in these resonant states can produce an enhancement in thermoelectric power and thermoelectric figure of merit [1].

The second part of the talk adds the spin degree of freedom to research on thermal solid-state energy converters, based on the recently discovered spin-Seebeck effect [2]. A temperature gradient applied to a spin-polarized material creates a spin flux that is driven into an adjacent material (Pt). There, it gives rise to a voltage by the inverse spin-Hall effect via spin/orbit interactions.  The thermal spin flux can be carried by various spin-polarized quasi-particles.   The magnon thermal conductivity in ferromagnets gives, under a temperature gradient, a magnon heat flux that is directly proportional to a spin flux [3].  Spin-polarized electrons can also sustain a spin flux: the effect can then be as large as the highest thermoelectric voltages in semiconductors [4].  In fact, even in diamagnetic solids under a magnetic field, the atomic motion due to phonons modulates the local diamagnetic moment (phonon-induced diamagnetism, [5]) enough to generate measurable changes in the anharmonicity and lattice thermal conductivity.

Both part of the talk are closely related: the magnon-drag thermopower, first identified on Fe [6], can be seen as a self-spin-Seebeck effect, eliminating the need for an interface between a ferromagnet and a normal metal.  Magnetism can now be considered as a new design tool that adds to thermoelectrics research. The engineering advantages of spin-Seebeck based devices over conventional thermoelectric generators will be described.

[1] Heremans & al., Science 321 554 -558 (2008); [2] K. Uchida & al., Nature 455, 778 (2008); [3] C. M. Jaworski & al.,  Nature Materials, 9 898-903 (2010); [4] C. M. Jaworski & al.,  Nature, 487, 210-213 (2012); [5] Hyungyu Jin & al.,  Nature Materials (2015; doi:10.1038/nmat4247); [6] F. Blatt & al., Phys. Rev. Lett. 18 395 (1967)