Wetting transitions of He-4 on alkali-metal surfaces from density-functional calculations

We have studied the wetting properties of He-4 adsorbed on the surface of h eavy alkali metals by using a nonlocal free-energy density functional which describes accurately the surface properties of liquid He-4 in the temperat ure range 0<T<3 K. Our results for He-4 on the Cs surface give both the tem perature dependence of the contact angle and the wetting temperature in goo d agreement with the experimental findings. For the He-4/Rb system we find that a wetting transition on the Rb surface occurs at T similar to1.4 K, wh ereas;the experiments show either wetting down to T=0 or a wetting transiti on at T similar to0.3,K; We suggest that this disagreement is due either to an inaccuracy of the fluid-substrate potential used in our calculations or the consequence of substrate roughness, which is known to affect the Rb su rface, and whose effect would be to lower the wetting temperature. The sens itivity of the wetting transition to the He-surface potential is stressed f or the He/Rb surface, which may justify the controversial experimental find ings for this system.