Superconductivity occurs when electrons throughout a material form pairs, which then merge into a single quantum-mechanical state that carries electrical current with no resistance. In different materials, the pairs have different properties. In most superconductors, the electron pairs have ¿even parity,¿ meaning that they look the same in a mirror. In SRO, in contrast, experiments suggest that the pairs form with odd parity at temperatures below one degree Celsius above absolute zero--that is, when the material becomes superconducting. A related state occurs in superfluid 3He, and experts have theorized on the basis of indirect evidence that a handful of other superconductors exhibit this state as well.
To make a direct measurement, Liu and his colleagues first had to figure out how to make a junction to an ordinary superconductor that could inject electron pairs into the SRO. They then made two of these junctions on opposite sides of a tiny polished crystal of SRO, forming a Superconducting Quantum Interference Device, or SQUID (see image). The superconducting current that can flow through the two junctions in parallel depends on whether they are in phase. For an odd-parity superconductor, the pairs should have opposite phases, and destructive interference between them should reduce the total current. However, magnetic fields less than 1 percent of the earth's field can completely scramble the phase, so the researchers had to carefully exclude all stray fields. They also had to correct for the field arising from the measurement current itself.
Because of these corrections, says Dale Van Harlingen of the University of Illinois at Urbana-Champaign, the Penn State work is ¿very suggestive, but not definitive.¿ Van Harlingen's group pioneered the interference technique on high-temperature superconductors, and has also been trying to extend it to SRO.