A flexible combination of superconducting integrated circuits was used to construct a low-temperature magneto-optic microsusceptometer utilizing a dc superconducting quantum inteference device (SQUID) detector operating near the quantum limit (coupled energy sensitivity of 1.7ℏ). Miniature pick-up loop assemblies on transparent substrates were joined by superconducting interchip connections to a thin-film dc SQUID, which is in turn read out by a second dc SQUID connected to room-temperature electronics. Measurements on an 8.5-μm-diam titanium dot evaporated directly into the pick-up loop demonstrate a spin sensitivity of ∼103 spins/(Hz)1/2 at T=290 mK.
The free decay of the quantized-vortex tangle in a channel is observed to occur in two distinct stages, an initial rapid decay in agreement with current theory, followed by a regime of anomalously slow decay. It is argued that the second stage represents a state of coupled turbulence involving not only the quantized vortex tangle, but also macroscopic random motion of the normal and superfluid fields.PACS numbers: 67.40.Vs, 47.25.-c Superfluid 4 He at very low temperatures moves as a frictionless, irrotational fluid describable by a superfluid velocity field v s . At finite temperatures, the thermalexcitation gas can also flow, with a normal-fluid velocity field v". It has long been known 1 that when a sufficient relative velocity exists between the normal and superfluid components of the motion, a transition occurs to a new dynamical state characterized by the appearance of a dense tangle of quantized vortex lines in the v s field. The fully developed vortex-tangle state is well understood theoretically 2 and its properties can be calculated quantitatively from the equation of motion for quantized vortices. An important exception, however, has been the observation that when the driving fields are suddenly reduced to zero, the decay of the vortex tangle occurs much more slowly than predicted by the theory.
We have invented a three superconducting quantum interference device (SQUID) gradiometer (TSG) that uses three SQUID magnetometers and a novel feedback method to measure magnetic field gradients. One SQUID, designated the reference SQUID, operates normally except that its feedback loop output is directed to all three SQUIDs through identical nonsuperconducting coils around each SQUID. The feedback loops for the remaining two SQUIDs, the sensor SQUIDs, measure the differences between the magnetic field at the reference SQUID location and those at the sensor SQUID locations. The voltage difference between the two sensor SQUID outputs divided by the gradiometer base line, the distance between the sensor SQUIDs, represents the average magnetic field gradient. We have measured gradient sensitivities of 10−12 and 10−10 T/m√Hz for TSGs made from bare low-Tc and high-Tc SQUIDs. An advantage of a TSG is that a sensitive gradiometer, free of hysteresis error, can be made using relatively small substrates.
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