Competing superconductivity and charge-density wave in Kagome metal CsV3Sb5: evidence from their evolutions with sample thickness

Recently superconductivity and topological charge-density wave (CDW) were discovered in the Kagome metals $A$V${}_3$Sb${}_5$ ($A$ = Cs, Rb, and K), which have an ideal Kagome lattice of vanadium. Here we report resistance measurements on thin flakes of CsV${}_3$Sb${}_5$ to investigate the evolution of superconductivity and CDW with sample thickness. The CDW transition temperature $T_{\mathrm{C}\mathrm{D}\mathrm{W}}$ decreases from 94 K in bulk to a minimum of 82 K at thickness of 60 nm, then increases to 120 K as the thickness is reduced further to 4.8 nm (about five monolayers). Since the CDW order in CsV${}_3$Sb${}_5$ is quite three-dimensional (3D) in the bulk sample, the non-monotonic evolution of $T_{\mathrm{C}\mathrm{D}\mathrm{W}}$ with reducing sample thickness can be explained by a 3D to 2D crossover around 60 nm. Strikingly, the superconducting transition temperature $T_{\mathrm{c}}$ shows an exactly opposite evolution, increasing from 3.64 K in the bulk to a maximum of 4.28 K at thickness of 60 nm, then decreasing to 0.76 K at 4.8 nm. Such exactly opposite evolutions provide strong evidence for competing superconductivity and CDW, which helps us to understand these exotic phases in $A$V${}_3$Sb${}_5$ Kagome metals.