Oriol T. Valls and students, Klaus Halterman and Paul Barsic, have been studying some unusual properties of nanostructures consisting of intercalated superconductor and ferromagnetic materials. Such structures are very interesting for a variety of technological (application to "spintronics") and scientific reasons. They exhibit what is called "proximity effects:" the superconductivity leaks into the magnetic layers and the magnetism into the superconducting ones.

In the process of addressing some intriguing and puzzling recent experimental results, in which superconductivity was found to leak into a magnet over a distance very much longer than expected, investigators were led to speculate that this could happen if the Cooper pairs in the magnet had parallel spins, not anti-parallel. But this would violate the Pauli principle, one of the most fundamental laws in Physics: two fermions cannot be in the same state. Superconductors have zero electrical resistivity hence their name. This unusual property is known to be due to the electrons pairing up, forming so-called Cooper pairs. In nearly all of the known superconductors, the wave function of the electron pair is symmetric in space. The overall antisymmetry means then that the electrons must have their spins (proportional to their magnetic moments) pointing in opposite directions. If that were not the case, the Pauli principle would be violated. How can this be?

Professor Valls and his group found the answer. They managed to prove that there is after all a loophole which allows triplet (parallel spins) spatially symmetric Cooper pairs to exist in these nanostructures. The crucial trick is to look at the time dependence. It would not violate the Pauli principle if it turned out that the spin-symmetric state occurs with a time delay: one member of the Cooper pair has its spin parallel to its mate, but not at the same time, just a little later. Valls and his group showed through calculations that this state does exist in nanostructures and that it has the right properties to explain the experiments.