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Using a sensitive differential pressure gauge, we have measured the limits to dissipationless superflow in bulk superfluid 3He-A far from stabilizing walls. We report the first quantitative measurements of the breakdown of bulk superflow in the A phase. In addition to confirming the existence of a finite criticai current in bulk uniform textures, we have discovered an unexpected tortuoua texture in which there is an even larger critical current. These results indicate that the current theoretical models for determining textures are incomplete.

with natural modes of oscillation of the fiber. The enhancement mechanism is identical to proposed local field enhancement mechanisms for SERS from metallic microstructures, where the analogous resonant modes are surface plasmon modes and the n 9 l values correspond to various multipolar plasmon resonances.

A system has been developed to directly observe NMR free induction signals at very low frequencies. We use a nonresonant pickup arrangement with a commercial dc SQUID to monitor the precessing magnetization in a 0.5-50-kHz range of Larmor frequencies. A relocking interval for the SQUID's flux locked loop presently sets our pulse recovery around 20-30 J.ls throughout this frequency range. The noise for this system is less than 1.5 times that recorded for the SQUID with shorted input tenninals.

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