DISCOVERY OF A METASTABLE PI-STATE IN A SUPERFLUID HE-3 WEAK-LINK

Under certain circumstances(1,2), a superconducting Josephson junction can maintain a quantum phase difference of rr between the two samples that are weakly connected to form the junction. Such systems are call ed 'pi-junctions' and have formed the basis of several experiments des igned to investigate the much-debated symmetry of the order parameter of high-temperature superconductors(3-8) More recently, the possibilit y that similar phenomena might occur in another macroscopic quantum sy stem - a pair of weakly coupled Bose-Einstein condensates - has also b een suggested(9). Here we report the discovery of a metastable superfl uid state, in which a quantum phase difference of pi is maintained acr oss a weak link separating two reservoirs of superfluid He-3. The exis tence of this state, which is the superfluid analogue of a superconduc ting pi-junction, is likely to reflect the underlying 'p-wave' symmetr y of the order parameter of superfluid He-3, but a precise microscopic explanation is at present unknown.