Metallic adhesion and tunnelling at the atomic scale

We simultaneously measured the distance dependence of the force and the tun nelling current between a W(111) tip and a Au(111) sample in an ultrahigh v acuum at T = 150 K. The tip was characterized by field ion microscopy. Even at atomically close contact no evidence of structural instabilities was fo und. The scaling of the force curves show an unexpectedly long distance sca ling parameter of lambda = 0.2 nm. We conclude that not only the apex atoms contribute to the adhesion forces, but the first three layers play an almo st equal role. Using a model that correlates the force and the tunnelling c urrent, we are able to extract the tip density of states. Possible reasons for the long scaling length are discussed.