High temperature superconductivity in single unit-cell FeSe films on SrTiO$_{3}$

High transition temperature ($T_{C})$ superconductivity was discovered in single unit-cell thick FeSe films grown on a SrTiO$_{3}$(001) substrate by molecular beam epitaxy. \textit{In situ} scanning tunneling microscopy revealed a superconducting gap as large as 20 meV in single unit-cell thick FeSe films [1]. By \textit{ex situ} transport measurements on single unit-cell thick FeSe films protected with FeTe layer, we demonstrated an onset $T_{C}$ above 40 K and a critical current density $J_{C}$ $\sim$ 1.7 $\times$ 10$^{6}$ A/cm$^{2}$ at 2 K, which are much higher than $T_{C}$ $\sim$ 8 K and $J_{C}$ $\sim$ 10$^{4}$ A/cm$^{2}$ for bulk FeSe [2,3], and that the characteristics of the transition are consistent with a two-dimensional superconductor undergoing a Berezinskii-Kosterlitz-Thouless transition. The superconductivity is further confirmed by measuring Meissner effect. The simple structure of the current system provides an ideal platform for understanding the underlying physics of high-$T_{C}$ superconductivity.\\[4pt] [1] Wang, Q. Y. \textit{et al.}, Interface-induced high-temperature superconductivity in single unit-cell FeSe films on SrTiO$_{3}$. \textit{Chinese Physics Letters}, \textbf{29}, 037402 (2012).\\[0pt] [2] Hsu, F. C. \textit{et al.}, Superconductivity in the PbO-type structure $\alpha $-FeSe. \textit{Proc. Natl. Acad. Sci. USA}\textbf{ 105}, 14262 (2008).\\[0pt] [3] Lei, H. C.\textit{ et al.}, Critical fields, thermally activated transport, and critical current density of $\beta $-FeSe single crystals. \textit{Phys}.\textit{Rev. B} \textbf{84}, 014520 (2011).