Enhanced Seebeck effect in ion-gated FeSe

The reduction of the dimensionality leads to the manifestation of quantum phenomena and the development of electronic correlation. Such low dimensional effects often become even more pronounced in nano-scale materials including exfoliated atomic layers and their hetero structures, triggering the emergence of the novel electronic, optical, and magnetic properties. Here we report the thermoelectric effect of FeSe ultrathin films with an electric double layer configuration, which allows us to control not only the carrier density but also the film thickness down to monolayer [J. Shiogai et al., Nature Phys. 12, 42 (2016)]. By utilizing a chemical etching induced by applying a large gate bias, the thickness of thin films is reduced in a layer-by-layer manner. Accompanying the emergence of the high-T_c superconductivity, the ionic liquid gating on FeSe thin films induces the anomalous enhancement of the Seebeck effect. We will discuss the thickness and the temperature dependence of the thermoelectric properties in FeSe thin films in detail.