Fig. 2

Thinning-induced enhancement of thermoelectric effect. a Thermoelectric voltage ΔV under temperature difference ΔT in an FeSe thin film, Sample A. The thermoelectric measurements were performed at 200 K with V_G = 5 V. The value of ΔV changed its sign and dramatically increased with decreasing thickness d from ~18 nm to ~1 nm. b Thickness d dependence of the Seebeck coefficient α. The values of α were estimated from the slope of the ΔV−ΔT plot in a as α = −ΔV/ΔT. The inset shows the temperature T dependence of the normalized sheet resistance ρ_2D with respect to 200 K. Superconductivity appeared when V_G = 5 V was applied. c Variation of sheet resistance ρ_2D^200 K at 200 K as a function of d. The value of ρ_2D^200 K for V_G = 5 V showed a weak d dependence (blue circles), whereas that for V_G = 0 V increased with decreasing d (gray circles). d Thickness d dependence of thermoelectric power factor α²/ρ exhibiting anomalous enhancement in the ultrathin limit. Here, ρ is the electrical resistivity, which is estimated as ρ = ρ_2D × d. The value of α²/ρ at 200 K in the thick region is comparable to that in bulk FeSe¹⁹, whereas α²/ρ increased with decreasing d because of the double-digit increase of α in ultrathin regions in b. The error bar of d corresponds to ~ ± 0.5 nm, which was estimated from the surface roughness of the initial thin film¹⁵