M ay 2 00 6 Resonant photon tunneling enhancement of the van der Waals friction

We study the van der Waals friction between two flat metal surfaces in relative motion. For good conductors we find that normal relative motion gives a much larger friction than for parallel relative motion. The friction may increase by many order of magnitude when the surfaces are covered by adsorbates, or can support low-frequency surface plasmons. In this case the friction is determined by resonant photon tunneling between adsorbate vibrational modes, or surface plasmon modes. A great deal of attention has been devoted to non-contact friction between nanostructures, including, for example, the frictional drag force between two-dimensional quantum wells [1, 2, 3] , and the friction force between an atomic force microscope tip and a substrate [4, 5, 6, 7, 8]. In non-contact friction the bodies are separated by a potential barrier thick enough to prevent electrons or other particles with a finite rest mass from tunneling across it, but allowing interaction via the long-range electromagnetic field, which is always present in the gap between bodies. The presence of inhomogeneous tip-sample electric fields is difficult to avoid, even 1 under the best experimental conditions [6]. For example, even if both the tip and the sample were metallic single crystals, the tip would still have corners present and more than one crystallographic plane exposed. The presence of atomic steps, adsorbates, and other defects will also contribute to the inho-mogeneous electric field. The electric field can be easily changed by applying a voltage between the tip and the sample. The electromagnetic field can also be created by the fluctuating current density, due to thermal and quantum fluctuations inside the solids. This fluctuating electromagnetic field is always present close to the surface of any body, and consist partly of traveling waves and partly of evanescent waves which decay exponentially with the distance away from the surface of the body. The fluctuating electromagnetic field originating from the fluctuating current density inside the bodies gives rise to the well-known long-range attractive van der Waals interaction between two bodies [9]. If the bodies are in relative motion, the same fluctuating electromagnetic field will give rise to a friction which is frequently named as the van der Waals friction. Although the dissipation of energy connected with the non-contact friction always is of electromagnetic origin, the detailed mechanism is not totally clear, since there are several different mechanisms of energy dissipation connected with the electromagnetic interaction between bodies. …