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

We have studied the wetting properties of He4 adsorbed on the surface of heavy alkali metals by using a non-local free-energy density-functional which describes accurately the surface properties of liquid He4 in the temperature range 0<T<3 K.Our results for He4 on the Cs surface give both the temperature dependence of the contact angle and the wetting temperature in good agreement with the experimental findings. For the He/Rb system we find that a wetting transition on the Rb surface occurs at T=1.4 K, whereas the experiments show either wetting down to T=0 or a wetting transition at T=0.3 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 surface, and whose effect would be to lower the wetting temperature. The sensitivity of the wetting transition to the He-surface potential are stressed for the He/Rb surface, which may justify the controversial experimental findings for this system.