Friction - sometimes electrons matter!

While I don't do any research on the subject myself, over the last few years I've become more interested in the origins of friction, a subject about which almost no physics progress was made between from around 1650 to 1950. Since the development of the tools of surface science (ultrahigh vacuum, for example) and scanned probe microscopy, however, people have learned much about where friction comes from.

We all have an intuitive grasp of what friction is, and in freshman physics (or even high school), we learn that we can model friction as a (shear) force between two surfaces as they slide (or attempt to slide) relative to one another. That force is modeled as proportional to the normal force between the surfaces, with the surface-dependent friction coefficient as the proportionality constant. The force is further traditionally modeled as being independent of the contact area between the two surfaces, and independent of the relative speeds of the two surfaces (except for the distinction between static friction - with no relative motion - and kinetic or sliding friction). That approach does a very good job at describing many many experiments on friction between macroscopic objects.

The problem is, as many famous scientists (e.g., Coulomb) discovered, it's very difficult to come up with a microscopic model of the interaction between surfaces that has these properties. One of the essential difficulties is rather deep: friction has to result in real dissipation. Energy has to be transferred from macroscopic degrees of freedom (the motion of a hockey puck relative to the ice) into microscopic degrees of freedom (the relative vibrational motions of the atoms in the hockey puck, and similar motions of the atoms in the ice - heat, in short.). That transfer of energy from macroscopic coordinates to microscopic motions or coordinates is irreversible in the same sense that the motion of water in a pond is irreversible after a stone is tossed in. (Yes, it's physically conceivable from the point of view of Newton's laws that all the little bits of water at the edge of the pond could jiggle just right so as to send coordinated ripples inward toward the center of the pond, spitting the stone back out. However, that's incredibly unlikely, given all of the possible microscopic states of the water, so from the standpoint of macroscopic thermodynamics, the water rippling process is irreversible.)

There has been some beautiful work on friction at the nanoscale, and much of it has focused on chemical interactions between surfaces, as well as vibrations (phonons) as the relevant microscopic degrees of freedom. However, in the case of metals, there are other excitations where the energy could end up: electrons! That's one defining characteristic of a metal, the existence of possible electronic excitations of (almost) arbitrarily low energy. How can you tell if the energy is ending up in the electrons? Well, you'd really like to do an experiment where none of the vibrational properties are changed, but that allows you to compare between with-electrons and without-electrons. Amazingly, it is possible to do something close to that by working with a metal that is superconducting! Above the superconducting transition temperature, T_c, the metal has plenty of low energy electronic excitations. Below T_c, however, in the superconducting state, electronic excitations are forbidden below some threshold energy (this "gap" in the excitation spectrum is one key reason why superconductors have no electrical resistance). In this new paper (sorry about not having an arxiv version to link), the investigators have demonstrated that the (noncontact) friction between a metal tip and a niobium film drops dramatically once the niobium becomes superconducting. This argues that electronic dissipation is responsible for much of the friction in this case (in the normal state). I should point out that previous work with lead films had hinted at similar physics.  The new experiment is very clear and benefits from technique developments in the meantime.