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Thermoelectric devices can cool materials by passing currents, or convert temperature differences into electric power. However, especially 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. In the work published yesterday 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.

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