Researchers at Osaka University were able to improve the power factor of a promising thermoelectric material by more than 100% as a result of varying the pressure. (See the graph above showing the pressure dependence of thermoelectric power factor at 150K and 300K for SnSe. The inset shows the schematic illustration of the corresponding change in valley topology in SnSe.)
This drastic improvement in power factor could lead to new materials with improved thermoelectric properties. Thermoelectric materials have the unusual ability to generate electricity from temperature differences.
Such materials could potentially be used to convert otherwise wasted heat (such as heat from hot servers or laptops) into usable electricity.
In addition to enhancing the material’s thermoelectric properties, the researchers showed that its thermoelectric properties originate from a transition in the topology of the electronic band structure, which is known as the Lifshitz transition.
This transition differs from the traditional Landau-type phase transition because it occurs without any breaking of symmetry.
Experts have long had reason to believe that the Lifshitz transition plays a critical role in many quantum phenomena, such as complex magnetism, superconductivity, and thermoelectric properties, but they needed direct proof.
In this new study, Osaka University researchers were able to demonstrate a direct link between the Lifshitz transition and the physical properties of a thermoelectric material. “We were able to keep track of the Lifshitz transition by applying pressure and measuring the quantum oscillations as the pressure was increased,” the corresponding author Hideaki Sakai said.
The researchers examined tin selenide (SnSe), a thermoelectric material, which is also a semiconductor with a small amount of conducting carriers.
In semiconductors, the lower energy valence band is filled with electrons, but the higher energy conduction band has none of them. When some impurities and/or chemical defects are introduced, conducting carriers are also introduced as electrons and holes in the conduction and valence bands, respectively.
Then, the semiconductor acts like a conductor.
The band structure, in addition to having an effect on the material’s electrical conduction, also has an effect on quantum phenomena, such as their thermoelectric properties. The valence bands of tin selenide are not entirely flat, but usually have two valleys in them.
“When we increased the pressure on the material, we observed a change from two to four valleys in the material when the Lifshitz transition occurred,” Hideaki Sakai said.
As a result of applying pressure up to 1.6 GPa, they observed a large enhancement of the thermoelectric power factor by greater than 100% over a wide temperature range (10K to 300K).
The researchers were able to demonstrate experimentally and show theoretically that this change in the number of valleys was directly responsible for the significant improvement in tin selenide’s thermoelectric properties.
They published their findings in Physical Review Letters.
The researchers speculate that in the future, the results of the study may help in the development of improved thermoelectric materials and could also help clarify the effect of the Lifshitz transition on various transport properties. The findings could also lead to possible applications including novel electronics using valley degrees of freedom in the band structure.
Nishimura, T., Sakai, H., Mori, H., Akiba, K., et al. “Large Enhancement of Thermoelectric Efficiency Due to a Pressure-Induced Lifshitz Transition in SnSe,” Physical Review Letters 122, 226601 -June 7, 2019. DOI: https://doi.org/10.1103/PhysRevLett.122.226601