Friction and pinning of nitrogen films on lead substrates near the superconducting transition

We have used a quartz crystal microbalance to study the dynamical properties of thin films of solid nitrogen adsorbed on a lead substrate in the temperature range 4 K,T,14 K. Simple mechanical models relate the frequency shift and the Q of the microbalance to the friction at the N2 /Pb interface. By varying either the magnetic field or the temperature, the Pb substrate could be prepared in the superconducting or normal state. Our experiments indicate that superconductivity has no effect on the behavior of the microbalance. Furthermore, experiments in which the N2 layer thickness was varied by in situ evaporation showed that solid nitrogen films are rigidly pinned on our low-temperature substrate for velocities below approximately 0.6 m/sec.