Heat one end of a conductor and keep the other cold, and electrons start flowing from hot to cold. Usually, this creates a voltage along the conductor, like water running down a tilted trough. But in some materials, electrons veer sideways, and the voltage appears across the conductor. It's like a magician blowing on a ball, and instead of flying forward, it shoots off to the side. This sideways response is called the transverse thermoelectric effect.
In the new study, researchers took an iron-silicon alloy — Fe5Si3—and made it into a thin film. At room temperature, the transverse response was remarkably strong: about 1.5 microvolts per degree of temperature difference. For comparison, in ordinary metals this figure is hundreds of times smaller. What's more, part of the signal persisted even when heated above room temperature, which is rare.
Calculations showed that the reason for this efficiency lies in the peculiar arrangement of electronic levels. In this material, several unusual states that usually occur separately coexist simultaneously. As a result, electrons moving in different directions respond differently to a magnetic field, giving rise to a strong sideways response. Importantly, the material consists of just two common elements, making it cheap and accessible.
Such an alloy could find use in devices that capture waste heat (for example, from exhaust pipes or microchips) and convert it into electricity. For now, this is just a laboratory result, but it shows that simple materials can behave in surprisingly complex and useful ways.
The transverse thermoelectric effect is named after the German physicist Walther Nernst, who discovered it in 1886. Since then, it has been sought in a wide variety of materials, but only recently did it become clear that some magnetic alloys give a particularly strong response.
In Frank Herbert's novel "Dune," desert dwellers wear special suits that recycle body heat into useful energy. Similar principles, turning heat into electricity, underlie thermoelectric materials, though they are still far from that fictional efficiency.