Researchers at the University of Durham in the UK have set a new record in the strength of magnetic field that a superconductor can support before turning normal. H J Niu and Damian Hampshire found that a superconductor made from lead, molybdenum and sulphur, and fabricated out of nanocrystals, can reach a field of almost 100 Tesla. This is 10 times higher than can be achieved in the niobium-titanium alloys that are currently used in superconducting magnets for particle accelerators, and about five times higher than in the niobium-tin used in high-frequency NMR applications and high-field laboratories.
Theory shows that high critical fields, where the material turns normally conducting, can be achieved through increasing disorder in a material, either by increasing the resistivity in the normal state or by reducing the electron mean-free path. Chemical doping or alloying can produce such effects, but in this case the researchers used mechanical milling followed by hot isostatic pressing to make a disordered nanocrystalline form of the superconductor PbMo6S8. The milling produces a disordered nanocrystalline powder, which is then formed into bulk material in the pressing stage.
The superconductor made in this way has higher normal resistivity, but unlike materials produced by doping techniques, does not have a reduced superconducting transition temperature. The disorder reduces the electron mean- free path to 1 nm, which is less than a tenth the size of the grains, and together with the higher resistivity, ensures the high critical field. While conventionally produced PbMo6S8 has a critical field of around 50 T, the disordered nanocrystalline form reaches around 100 T.
Further reading
H J Niu and D Hampshire 2003 Phys. Rev. Lett. 91 027002.