Superconductors lose their unique properties above a critical current density of about 105 A cm-2. The best candidates for high-current applications - silver sheathed tapes of the bismuth-lead-strontium-calcium-copper-oxygen alloy known as BSCCO - reach about 25% of this value. To explore the factors limiting current performance, the researchers analyzed some of the best multi-filament tapes available. They found that individual filaments had critical current densities much higher than the average value of the tape.

Electron micrographs and magneto-optic images of the tapes show a series of cracks which reduce the critical current density. The researchers also found that currents along the crystalline planes of the material were sometimes blocked by a series of nanometre and micrometre scale barriers.

The team point out that although cracks are very difficult to engineer out of the production process, it should now be possible to design new processes to eliminate them.