ELECTRON RESONANCES IN SHARP TIPS AND THEIR ROLE IN TUNNELING SPECTROSCOPY

The influence of the shape and electronic structure of the tip in scan ning tunneling spectroscopy is studied by a combined theoretical and e xperimental approach. Tunneling conductance spectra of a Cu(111) surfa ce are used to detect and characterize the electronic states of W tips . The characteristic tunneling conductance of the Cu(111) surface stat e is the only feature present in spectra recorded with a blunt tip, wh ile spectra obtained with sharp tips display also tip resonances; By m eans of a self-consistent tight-binding model of realistic tip structu res we show that a sharp W tip exhibits, in contrast with blunt tips, narrow resonant states around the Fermi level.