Possible Origin of High Thermoelectric Power Factor in Ultrathin FeSe: A Two-band Model
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Two-band Model with High Thermoelectric Power Factor and Its Application to FeSe Thin Film
J. Phys. Soc. Jpn. 92, 104704 (2023).
The high thermoelectric power factor observed in ultrathin FeSe can be theoretically explained by a two-band model with chemical potential between upper and lower band bottoms.
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-PF materials. 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.)
Two-band Model with High Thermoelectric Power Factor and Its Application to FeSe Thin Film
J. Phys. Soc. Jpn. 92, 104704 (2023).
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