A superconductor that works at standard room temperature and around one atmosphere of pressure... or works at whatever pressure and temperature machines need it to be.
The Ringworld used to have superconductor, but some mold or fungus has broken it all down.
It is revealed that the Pierson's Puppeteers (an alien race of the series) released a fungus that eats and destroys the type of superconductor used by the Ringworld civilization to eliminate what they viewed as a powerful threat. Additionally, the ships have a supply of superconductor wire and cloth. Superconductor wire is used to instantly transfer heat from a weapon into a very large body of water, which then boils.