A common feature of all superconductors - both the low-temperature and high-temperature varieties - is that electrons in the material somehow overcome their mutual electrostatic repulsion to form Cooper pairs below a certain transition temperature. These pairs can then condense into a single quantum state and move without electrical resistance.

In a low-temperature superconductor the total orbital angular momentum of the two electrons in a Cooper pair is zero - a so-called *s*-wave state. By contrast the Cooper pairs in a high-temperature superconductor exist in a *d*-wave state with *L*=2, where *L* is the total orbital angular momentum. The latest results confirm that the Cooper pairs in strontium ruthenate form a *p*-wave state with *L*=1. States with *p*-wave symmetry are also formed when liquid helium-3 becomes a superfluid.

According to the laws of quantum mechanics, the wave function of a pair of electrons must change sign when the electrons are exchanged. This means that only certain combinations of orbital and spin angular momentum are possible. If the *L* is an even number, then the two spins must point in opposite directions to form a "spin-singlet" or "even-parity" state. However, if *L* is an odd number, the spins must point in the same direction in a "spin-triplet" or "odd-parity" state. Although evidence for odd-parity superconductivity has been seen in strontium ruthenate before, other explanations are also possible.

Liu and colleagues connected a sample of superconducting strontium ruthenate to a conventional superconductor through two Josephson junctions. Cooper pairs are able to tunnel through the junctions in both directions and the entire ensemble is known as a superconducting quantum-interference device (SQUID). By measuring the current through the SQUID as a function of an applied magnetic field, the Penn State-Kyoto team was able to confirm that the Cooper-pair currents passing through each junction had interfered destructively. This is only possible if the Cooper pairs in the strontium ruthenate are in a spin-triplet state.

"Our work completes a 40 year quest to find an odd-parity superconductor," Liu told *PhysicsWeb*. "We now have a new playground to study such superconductors, which may lead to further discoveries."