Anomalous enhancement of charge density wave in kagome superconductor CsV₃Sb₅ approaching the 2D limit

The recently discovered kagome metals AV₃Sb₅ (A = Cs, Rb, K) exhibit a variety of intriguing phenomena, such as a charge density wave (CDW) with time-reversal symmetry breaking and possible unconventional superconductivity. Here, we report a rare non-monotonic evolution of the CDW temperature (T_CDW) with the reduction of flake thickness approaching the atomic limit, and the superconducting transition temperature (T_c) features an inverse variation with T_CDW. T_CDW initially decreases to a minimum value of 72 K at 27 layers and then increases abruptly, reaching a record-high value of 120 K at 5 layers. Raman scattering measurements reveal a weakened electron-phonon coupling with the reduction of sample thickness, suggesting that a crossover from electron-phonon coupling to dominantly electronic interactions could account for the non-monotonic thickness dependence of T_CDW. Our work demonstrates the novel effects of dimension reduction and carrier doping on quantum states in thin flakes and provides crucial insights into the complex mechanism of the CDW order in the family of AV₃Sb₅ kagome metals.