Novel wetting behavior in quantum films

Theory and experiment have revealed that novel behavior occurs when a Cs surface is exposed to ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ vapor. For temperature $T$ below the wetting temperature ($T_w\simeq 2$ K), ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ forms only a microscopically thin film as the vapor pressure $P$ increases from zero to its saturated value. For intermediate values of $T \left(T_w\le T\le T_c\simeq 2.5 \mathrm{K}\right)$, the film thickness jumps discontinuously at a prewetting transition pressure $P_{\mathrm{pw}}\mathrm{}\left(T\right)\mathrm{}$. For $P>\mathrm{}{P}_{\mathrm{pw}}$, and for all $P$ if $T>\mathrm{}{T}_c$, the film grows continuously to infinite thickness as saturation is approached. Microscopic theories of these phenomena are discussed in terms of the basic physics of electrons and atomic interactions. Initial experimental observations are summarized. Related phenomena of interest are described, including observation of prewetting with adsorbates other than He and substrates other than Cs.