IMAGING STANDING WAVES IN A 2-DIMENSIONAL ELECTRON-GAS

ELECTRONS occupying surface states on the close-packed surfaces of nob le metals form a two-dimensional nearly free electron gas1-3. These st ates can be probed using the scanning tunnelling microscope (STM), pro viding a unique opportunity to study the local properties of electrons in low-dimensional systems4. Here we report the direct observation of standing-wave patterns in the local density of states of the Cu(111) surface using the STM at low temperature. These spatial oscillations a re quantum-mechanical interference patterns caused by scattering of th e two-dimensional electron gas off step edges and point defects. Analy sis of the spatial oscillations gives an independent measure of the su rface state dispersion, as well as insight into the interaction betwee n surface-state electrons and scattering sites on the surface.