Thermoelectric devices can cool materials by passing currents, or convert temperature differences into electric power. However, metallic structures have a very poor thermoelectric performance, and therefore most thermoelectrics are made of semiconductors. Now a group of researchers from the University of Jyväskylä, Aalto University (Finland), San Sebastian (Spain) and Oldenburg University (Germany) have shown how a proper combination of magnetic metals and superconductors could allow reaching very strong thermoelectric conversion efficiency.
The electronic structure of semiconductors and superconductors looks superficially similar, because both contain an “energy gap”, a region of energies forbidden for the electrons. The difference between the two is that doping semiconductors allows moving this energy gap with respect to the average electron energy. This is in contrast to superconductors, where the energy gap is symmetric with respect to positive and negative energies, and therefore the thermoelectric effect from positive energy electrons cancels the effect from the negative energy electrons. Heikkilä and the international research group showed how this symmetry can be broken by the presence of an extra magnetic field, and driving the electric current through a magnetic contact. As a result, the system exhibits a very large thermoelectric effect.
Because conventional superconductors require temperatures of the order of a few Kelvin, this mechanism cannot be used directly in consumer devices such as portable coolers or waste heat converters. However, it could be used in accurate signal detection, or a similar mechanism could be applied in semiconductors to improve their thermoelectric performance.
A huge thermoelectric effect can be observed by contacting a superconductor whose density of states is spin split by a Zeeman field with a ferromagnet with a nonzero polarization. The resulting thermopower exceeds kB/e by a large factor, and the thermoelectric figure of merit ZT can far exceed unity, leading to heat engine efficiencies close to the Carnot limit. We also show that spin-polarized currents can be generated in the superconductor by applying a temperature bias.
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