Thermoelectric Effect in Ultrathin FeSe*

The enhancement of the superconducting transition temperature T_c in monolayer FeSe/SrTiO₃ has been a central issue in 2D materials research in the last few years. However, the experimental investigation of monolayer FeSe has been limited to STM, ARPES, and electrical resistivity measurements due to the sensitivity to the air. Here we report the thermoelectric measurements in ultrathin films of FeSe through the in-situ electrochemical etching technique. The thickness of the FeSe films is controllable in ionic liquid media and can be reduced to monolayer [J. Shiogai et al., Nature Phys. 12, 42 (2016)]. With thinning the thin film with 18 nm thickness, T_c was enhanced up to ~40 K, and the anomalous enhancement of the Seebeck coefficient was observed. The thickness and the temperature dependence of the thermoelectric properties in FeSe will be discussed in detail.