Superconductivity-dependent sliding friction

Ne have employed a quartz crystal microbalance technique to measure the friction associated with sliding of solid nitrogen along a lead surface, above and below the lead's superconducting transition temperature. The friction is observed to drop abruptly at the transition as the substrate enters the superconducting slate. The observation signifies a new phenomenon, most likely attributable to electronic contributions to friction. While such contributions have been discussed and debated theoretically for a number of years, they have heretofore not been observed in any direct experimental fashion. [S0031-9007(98)05391-5].