ONE-ATOM POINT CONTACTS

A mechanically controllable break junction is used to study the transition between vacuum tunneling and contact for three different metals. In the tunnel regime a faster-than-exponential behavior is seen at close distances, followed by a jump to contact, both interpreted as being due to metal bonding forces. We show that stable contacts of a single atom can be formed. For Cu, the conductance value for a one-atom contact is very close to 2e2/h. For Al this value is less well defined but of similar magnitude, while for Pt it is noticeably higher, implying that the electronic structure of the atoms is relevant to the one-atom conduction process.