QUANTUM CORRALS

Electrons occupying surface states on the close-packed faces of the no ble metals form a two-dimensional (2D) nearly-free electron gas that c an be imaged with a scanning tunneling microscope (STM). We find that Fe adatoms strongly scatter metallic surface state electrons, acid so are good building blocks for constructing atomic-scale barriers to con fine these electrons. The barriers (''quantum corrals'') are construct ed by individually positioning Fe adatoms using the tip of a cold (4K) STM. Tunneling spectroscopy performed inside of the corrals reveals d iscrete resonances, consistent with size quantization. A more quantita tive understanding is obtained by accounting for the multiple-scatteri ng of the surface state electrons with the corrals' constituent adatom s. This scattering is characterized by a complex phase shift which can be extracted from the electronic density pattern inside a quantum cor ral.