Discovery of a metastable pi-state in a superfluid sup 3 He weak link

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Under certain circumstances [1,2], a superconducting Josephson junction can maintain a quantum phase difference of pi between the two samples that are weakly connected to form the junction.Such systems are called 'pi-junctions' and have formed the basis of several experiments designed to investigate the much-debated symmetry of the order parameter of high-temperature superconductors [3-8]. More recently, the possibility that similar phenomena might occur in another macroscopic quantum system-a pair of weakly coupled Bose-Einstein condensates-has also been suggested [9]. Here we report the discovery of a metastable superfluid state, in which a quantum phase difference of pi is maintained across a weak link separating two reservoirs of superfluid3 He. The existence of this state, which is the superfluid analogue of a superconducting pi-junction, is likely to reflect the underlying 'p-wave' symmetry of the order parameter of superfluid3 He, but a precise microscopic explanation is at present unknown.

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