Theory of tunneling from transition metal tips

A transition metal tip consisting of a single tungsten atom adsorbed on a W(110) surface is modelled. Chemisorpion theory is applied to treat the interaction of the tip atom with the flat W(110)-surface. The basis set on the tip atom includes 6s-, 6p- and 5d-orbitals. The importance of the d-electrons for the tunneling to an Al(111)-surface is investigated. Inclusion of the d- orbitals affects strongly the local electronic structure and modifies the current. For short distances a significant part of the total tunnel current flows directly via the tip d-orbitals. The theory is used to investigate the ‘barrier height’ of the clean surface. A corrugation of the Al(111)-surface is obtained in STM-theory, if a elastic deformation of the tip is taken into account.