Research on Superfluid 3He Weak Links

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

This project is focused on understanding the static and dynamic properties of weak link arrays (WLA) in superfluid 3He. The WLA consists of a quantum coherent array of apertures with diameter on the scale of the superfluid coherence length: typically a few tens of nanometers. Past research has already exposed several unanticipated features including: bi-stability, p-states, and new sources of superfluid dissipation. The past research was performed only at zero ambient pressure. The present project will explore the entire phase diagram of 3He, up to the melting pressure. The specific questions which will be answered include: What is the form of the current-phase relation, I(f), for all relevant temperatures and pressures? In the Josephson region, how does the critical current density scale? What is the limit, if any, to the number of apertures that will remain coherent in a weak link array? How do the dissipation processes change at elevated pressure? Are there always two stable states of the weak link array? Can a Josephson oscillation be driven by a temperature gradient as well as a pressure gradient? How does the I(f) function change in the transition from A to B phase? What is the nature of I(f)when there is an A-B bias across the array? Graduate students trained in the project will learn state-of-the-art techniques in microfabrication, ultra-cryogenics, SQUID magnetometry, electronic instrumentation and computer control and analysis. "Macroscopic quantum" systems span the range from the interior of neutron stars to ultra-cold Bose-Einstein condensed gases. Relevant systems include superconductors, both conventional and high Tc, superfluid 4He and superfluid 3He, both A and B phases. It is a major goal of physics to understand why such diverse systems display phenomena which are generic to the entire class. This project will be directed toward understanding the physics of weak links in superfluid 3He. Weak links consist of devices which join two macroscopic quantum systems into a single, weakly connected whole. In the case of 3He the weak link is an array of nanometer-sized apertures, micro-machined in a 50nm thick "window". These devices have been shown to display a variety of unexpected phenomena, which have already led to a better understanding of superfluidity, high Tc superconductivity and quantum computing. Since past research has only covered experiments at zero ambient pressure, the present project will extend the range of exploration over a wide range of pressure and will include two different superfluid phases of 3He. To prepare them for future scientific careers, students participating in the project will receive state-of-the-art training in a variety of physics techniques including ultra-cryogenics, micro-machining, ultrasensitive magnetometry, electronics and computer control.