When a superconductor in a magnetic field is cooled below its transition temperature, it expels the field. The details of how this Meissner effect occurs in real materials depend on the shape and physical properties of the sample, the type of superconductivity it exhibits, and the experimental conditions, but all superconductors are expected to behave as follows: As long as the external magnetic field is below a certain critical value, currents on the surface of the superconductor will form to cancel the field inside, but above this critical field magnetic flux will start to penetrate. The experimental signature of this field dependence is that the magnetization of the superconductor, which opposes the applied field, reaches a maximum at the critical field and then starts to decrease.
However, in a Rapid Communication published in Physical Review B, Ruslan Prozorov and collaborators from Ames Laboratory at Iowa State University, US, and the Institute of Physics of the Chinese Academy of Science in Beijing, show that two iron arsenide (pnictide) superconductors seem to defy this expected behavior. The researchers find that the magnetizations of and continually increase in an approximately linear fashion, without reaching a maximum, even when the applied field far exceeds the estimated critical field for either material. Based on their results, Prozorov et al. suggest that the magnetic field suppresses magnetic scattering, yielding more resilient Cooper pairing in iron arsenide compounds than in other superconductors, but more experiments are needed to support or refute this proposal. – Matthew Eager