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Thermoelectric (TE) materials have recently garnered significant attention toward realizing a low-carbon society. The maximum power of a TE material can be characterized by the power factor (PF), which is determined as PF = σS², where σ and S denote the conductivity and Seebeck coefficient, respectively. Thus, both a large |S| and high σ are needed for the development of high-PFmaterials. However, it is well known that there is a tradeoff between |S|and σ; |S| is large but σ is small in semiconductors, and vice versa in metals.

Recently, Shimizu et al. reported that a high-quality ultrathin FeSe under a perpendicular electric field exhibits a high PF of 500 mW/(m·K²) at 100 K¹⁾. The film exhibits metallic character, with σ~4 × 10⁶ S/m at 100 K and becomes a superconductor below 50 K. In contrast, |S| exhibits a large value of 350 μV/K, similar to that of a semiconductor. Such a high PF with both a large |S| and high σ cannot be explained in terms of a conventional one-band model. Thus, we proposed a two-band model with a chemical potential between upper and lower band bottoms as a simple theoretical model and elucidated the high PF. Here, we assumed that the two-band structure with finite splitting is realized by field in the ultrathin FeSe.

The present two-band model provides a guideline for designing high TE materials. For instance, nanowires and nanotubes with nanosized diameters are also promising materials with large |S| and high σ values because of sub-band structures. In contrast to FeSe thin films, tunes the chemical potential without affecting the sub-band gap suited for optimization of TE properties.

(Written by M. Matsubara on behalf of all the authors.)