Browsing by Author "Takamara, K"

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    Anomalous time-decay in neutron depolarization by high-Tc superconductors
    (Australian and New Zealand Institutes of Physics, 1994-11-09) Miles, PA; Kennedy, SJ; Taylor, KNR; Russell, GJ; Wang, J; Gu, GD; Takamara, K; Koshizuka, N
    In studies of the- field and time dependence of the polarisation state of a polarised neutron beam transmitted through single crystals of YBa2Cu- (YBCO) and BI2Sr2CaCu202-6 (BSCCO), we have observed some unusual behaviour for temperatures > 60*K for YBCO and 30>T>40K. The observed results suggest that at these temperatures, after removing an external magnetic field, the trapped flux density seen by the neutrons is essentially zero for all time less than = 200msecs. With increasing tune, the sampled flux density increases in magnitude, reaching the values observed in long time (20secs) measurements only after a delay of several seconds. At this time, the trapped flux density reaches a maximum and subsequently decays logarithmically at a rate determined by the effective pinning energy U(H,T). Possible mechanisms responsible for the inverse flux decay are considered. Large anisotropy in this behaviour is observed with the effect not being observed at all with Hlab for BSCCO and no marked temperature onset with YBCO with Hlab. For both YBCO and BSCCO the Hjfc observed flux decay show this behaviour. We conclude that the observed results follow, not from a change in the magnitude of the trapped flux, but rather from a change in the flux distribution within the sample. We interpret this in terms of a domain structure in the flux lattice system which is associated with the fringing closure flux which remains once the external applied field is removed.