Ultrathin FeSe films on ${\mathrm{SrTiO}}_3$ substrate show interface-enhanced superconductivity. However, how the superconductivity is established including superconducting (SC) fluctuations remains unclear. This study investigates the Nernst effect, which is sensitive to superconducting fluctuations in ultrathin FeSe films on ${\mathrm{SrTiO}}_3$. Temperature dependence of Nernst signals in the normal state is similar to bulk FeSe, suggesting that the electrons of ${\mathrm{SrTiO}}_3$ are transferred only to a few layers near the $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ interface. The Nernst effect caused by SC fluctuations was observed only below $T\sim 1.2T_{\mathrm{c}}^{\mathrm{onset}}$ within our measurement resolution, which is similar to other Fe chalcogenide systems. Our results suggest that the pseudogap in monolayer FeSe/STO possibly originates in other electronic states rather than superconductivity.

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• Nernst effect
• Superconducting fluctuations
• Superconductivity
• Iron-based superconductors
• Ultrathin films