Superfluid 3He Far from Equilibrium

这一奖学金建议响应了EPSRC的号召，即在远离平衡的系统物理和新兴现象物理领域解决突出的重大挑战。我的愿景是通过研究一个著名系统中的非平衡现象来应对这些挑战，该系统具有既定的理论框架--超流体3He。凝聚态(或超流体)是我们所拥有的最简单的“复杂”系统，因为凝聚态只受一个描述整个宏观系综的全局波函数支配。这种简单性使它们成为研究和测试各种想法的理想场所，这些想法涵盖了从亚核到宇宙学的大量看似无关的物理系统。令人感兴趣的是，超流体绝对是研究远离平衡的系统的理想选择，也是大量新出现的现象的宿主。凝聚态(或者至少是我们可以通过实验获得的凝聚态)是只存在于极低温度下的易碎物体。这几乎是独一无二的，这意味着我们可以在整个条件范围内研究它们，从几乎零熵的零温度静止状态一直到我们完全摧毁相干性的制度。产生和研究远离平衡态的最简单形式对于创建可证伪的理论和可靠的数值模型至关重要。通过理解最简单的状态，我们可以进步到理解更复杂的现象，用坚定的知识调整理论模型，这些模型在更简单的情况下确实有效。也就是说，在这个应用中，我提出了几个实验，将我们的模型凝聚态，超流体3He，发挥到了极限。这里的主要兴趣在于，尽管感知到的智慧表明，人们很好地理解了一致性的破坏，但实际上，这与事实相去甚远。众所周知，当我们移动散射体穿过超流体时，在某一临界速度下，超流性会灾难性地破裂，使系统恢复到正常状态。最近，我们在兰开斯特会议上表明，在超流3He中，这种情况不会发生，速度远远超过可接受的朗道值。这是相当出乎意料的。在拟议的实验计划中，我将致力于找出绕着散射体流动的超临界超流存在的原因，并找到它们的稳定区域。为此，我将利用核磁共振来探测凝聚体的波函数，并使用一种能够在凝聚体中匀速运动的装置--这是最近开创的超流体研究技术武器库的补充。首次使用核磁共振与超低温下3He中稳定的超流的强大组合，也将使我们能够研究几个紧急现象。例如，超流3He系统是研究不同超流相之间量子临界相变的理想介质，可以通过在绝对零度附近改变超流的压力来实现。

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.