Universal nodal Fermi velocity

The mechanism that causes high-temperature superconductivity in copper oxide materials (cuprates) is still unknown, more than 15 years after it was discovered1. As the charge carriers (electrons or holes) are introduced into the parent antiferromagnetic insulator, a process called doping, the material evolves from an insulator to a superconductor, and eventually to a normal metal. This marked change of physical properties with doping2,3,4,5,6,7 indicates that doping dependence (non-universality) might be a general feature of these materials, but we find that, on the contrary, the low-energy Fermi velocity of electrons is in fact universal, even among different superconductor families.