In most known superconductors, electrons pair up to form spin singlets, combinations of spin up and spin down with zero spin angular momentum. But strontium ruthenate (Sr2RuO4, or SRO) shows signs of superconductivity based on spin triplets. (See the article by Yoshiteru Maeno, Maurice Rice, and Manfred Sigrist, PHYSICS TODAY, January 2001, page 42.) There’s also evidence that the triplets in SRO take a particular form, called an equal-spin-pairing (ESP) state, that can be thought of as two weakly interacting condensates, one of spin-up pairs and one of spin-down pairs. Helium-3, another substance that forms a superfluid of pairs of fermions, has a superfluid phase with similar structure.
Magnetic vortices in superconductors are quantized. Conventional superconductors are characterized by a scalar order parameter, whose phase must change by an integer multiple of 2π over a closed loop around the vortex core. In unconventional superconductors,...
