STM OBSERVATIONS OF HELIUM-ATOMS ADSORBED ON GRAPHITE SURFACES

We present preliminary results of real-space imaging of two-dimensiona l (2D) submonolayer He-4 films adsorbed on graphite basal planes using a cryogenic STM. Below 1 K, we observed an additional root 3 x root 3 atomic corrugation superimposed to the triangular lattice of the B-si te carbon atoms of the graphite surface. This superlattice has the sam e periodicity as the commensurate solid He-4. It disappears on warming from 1 to 2 K presumably due to melting of the 2D solid. Furthermore, an energy gap of the order of +/- (10 to 30) mV was observed in tunne ling spectroscopy measurements associated with the adsorption of He-4.