Superconductor is supreme at 77K

A new method boosts the power capacity of superconducting wires at liquid nitrogen temperatures.

Over the past five years, thin films of yttrium barium copper oxide (YBCO) have become the favoured material for superconducting wires. However, the supercurrent passing through the material has always been limited due to boundaries in the films where the crystals are not perfectly aligned. These grain boundaries act like insulating "Josephson junctions" - the current flow is restricted as charge-carrying holes become depleted by interface charging and bending of the electronic band structure at the boundary. Previous experiments have shown that doping the YBCO film with calcium, introducing more holes, does help - but only at temperatures much lower than 77K.

However, a collaboration of Dutch and German physicists has found a way to "overdope" specifically at the grain boundaries. They grew a 25 nm calcium-doped YBCO film over an undoped YBCO film and found that some of the calcium migrated into the grain boundaries, effectively bridging the weak links. Supercurrent flow across the boundary increased by up to a factor of six - and the critical temperature T_c, at which the material becomes superconducting, was also unchanged.

Paul Grant of the US Electric Power Research Institute welcomed these findings, saying that they "could have profound consequences for future developments of high T_c technology". Nature