Superfluid 3He Far from Equilibrium

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This fellowship proposal answers the EPSRC's call to tackle outstanding grand challenges in the areas of physics of systems far from equilibrium and physics of emergent phenomena. My vision is to approach these challenges by studying non-equilibrium phenomena in a well-known system with an established theoretical framework - superfluid 3He. Coherent condensates (or superfluids) are the simplest "complex" systems that we have, in that the condensate is governed by just one global wave function describing the whole macroscopic ensemble. This simplicity makes them ideal playgrounds for studying and testing ideas that cover a vast range of apparently unrelated physical systems from the subnuclear to the cosmological. Of interest here, superfluids are absolutely ideal for studying systems far from equilibrium and also host an abundance of emergent phenomena. Coherent condensates (or at least those to which we have experimental access) are fragile objects only existing at very low temperatures. Almost uniquely that means that we can study them over the whole range of conditions from the virtually zero-entropy zero-temperature quiescent state all the way through to the regime where we have the complete destruction of the coherence. Producing and studying the simplest forms of states far from equilibrium is essential for creating falsifiable theories and reliable numerical models. By understanding the simplest states we can progress to the understanding of more complex phenomena, adjusting the theoretical models with the firm knowledge that they do work in simpler situations. That said, in this application I propose several experiments which take our model condensate, superfluid 3He, to the limit. The main interest here is that although perceived wisdom suggests that the destruction of coherence is pretty well understood, in reality that is very far from the truth. It is "common knowledge" that when we move a scatterer through a superfluid, then at some critical velocity the superfluidity should catastrophically break down and return the system to the normal state. Recently, we have shown at Lancaster that this does not happen in superfluid 3He up to velocities well in excess of the accepted Landau value. This was quite unexpected. In the proposed experimental programme I will aim to find the reason for the existence of supercritical supercurrents which flow around the scatterer and also find the domain of their stability. To this end I will utilise a combination of nuclear magnetic resonance to probe the condensate's wave function with a device capable of uniform motion through the condensate - a recently pioneered addition to the arsenal of the superfluid research techniques.Using for the first time the powerful combination of nuclear magnetic resonance with steady superflow in 3He at ultralow temperatures will also enable us to investigate several emergent phenomena. For example, the superfluid 3He system is an ideal medium for the study of quantum critical phase transitions between different superfluid phases which can be accessed by changing the pressure of the superfluid near the absolute zero of temperature.