Superfluid helium-4 weak links.

摘要

Josephson effects in superconductors, since their discovery in 1962, have not only provided a fascinating example of the counter-intuitive behavior of macroscopic quantum systems, but have also given rise to important technologies. The search for Josephson effects in superfluid 4He began not long after their discovery in superconductors, in 1962. We report, more that four decades later, the first observation of Josephson frequency quantum oscillation in superfluid 4He.{09}We observe these oscillations in a 65 x 65 array of sub-micron sized apertures drilled in a 50 nm thick silicon nitrite membrane. We find that these oscillations can be driven not only by a pressure difference applied across the array, but also by a temperature difference. The frequency of the oscillations obeys the Josephson frequency relation, fj = Deltamu/h, where h is Plank's constant and Deltamu = m4(Delta P/rho - sDeltaT) is the chemical potential difference across the array. Furthermore, we find that for temperatures a few mK below the superfluid transition temperature T lambda, the amplitude of the oscillations indicates that they are occurring synchronously in all apertures of the array.; We have developed a method of extracting the current-phase relation of the super-fluid 4He array from the response of the cell to a step in the chemical potential difference across the array. When the current-phase relation is plotted as a function of temperature near T lambda, we observe a cross-over from a low-temperature strong coupling regime in which the Josephson frequency oscillation is a result of periodically generated phase slips associated with singly quantized vortices, to a weak coupling regime exhibiting the sinusoidal current-phase signature of the Josephson effect.; We have investigated the synchronicity of the oscillations in the array in the strong coupling regime as a function of temperature. We find that as the temperature drops, the apertures become less and less synchronous. We suggest several possible explanations for this behavior, including the idea that as the temperature rises toward the cross-over to the weak coupling regime, the vortex phase slip mechanism gives way to a wave function collapse mechanism.; Finally, we present a "Chemical potential battery" for superfluid 4He weak link cells, whereby a constant heater power is used to generate a constant chemical potential difference, giving rise to steady Josephson frequency oscillations. This may be an ideal method of operating a superfluid 4He dc-SQUID, a device constructed from two weak link arrays in a torus, which will be highly sensitive to rotation.; The experiments reported in this dissertation represent a breakthrough in superfluid 4He weak link research, and provide a big step in the direction of a practical superfluid dc-SQUID operating at 2 K, a regime accessible to mechanical cryo-coolers. Such a device may find application in geodesy, detection of rotational seismic waves, and basic physics.