Optimized superconductivity in the vicinity of a nematic quantum critical point in the kagome superconductor Cs(V(1-x)Ti(x))(3)Sb(5)

CsV(3)Sb(5) exhibits superconductivity at T(c) = 3.2 K after undergoing intriguing two high-temperature transitions: charge density wave order at ~98 K and electronic nematic order at T(nem) ~ 35 K. Here, we investigate nematic susceptibility in single crystals of Cs(V(1-x)Ti(x))(3)Sb(5) (x = 0.00-0.06) where double-dome-shaped superconducting phase diagram is realized. The nematic susceptibility typically exhibits the Curie‒Weiss behaviour above T(nem), which is monotonically decreased with x. Moreover, the Curie‒Weiss temperature is systematically suppressed from ~30 K for x = 0 to ~4 K for x = 0.0075, resulting in a sign change at x = ~0.009. Furthermore, the Curie constant reaches a maximum at x = 0.01, suggesting drastically enhanced nematic susceptibility near a putative nematic quantum critical point (NQCP) at x = ~0.009. Strikingly, T(c) is enhanced up to ~4.1 K with full Meissner shielding realized at x = ~0.0075-0.01, forming the first superconducting dome near the NQCP. Our findings directly point to a vital role of nematic fluctuations in enhancing the superconducting properties of Cs(V(1-x)Ti(x))(3)Sb(5).