Superfluid 3He at UltraLow Temperatures

超流3He具有许多奇特的性质，为研究基本过程提供了一个模型系统。它是唯一可以接触到的具有绝对纯度的材料。它支持质量、自旋和轨道角动量的无耗散流动，这导致了各种宏观相干现象。相干轨道和自旋性质由矢量序参数描述，这些参数在很大的长度尺度上变化，仅受实验晶胞大小的限制。我们的团队专门从事超低温下的新型冷却和测量技术。我们通常冷却超流3He以记录低温，在那里少数剩余的热激发是高度弹道的，而超流基本上处于基态。我们计划进行一系列实验来研究相变和界面的基本性质。低温下的A-B相变发生在0.4特斯拉量级的较大磁场下。使用定制的超导螺线管，我们可以精确地控制磁场分布，以稳定和操纵A-B相界面。我们将结合技术来测量当界面移动时非常小的、毫微瓦的能量消耗。我们将振荡界面，以研究不同的耗散机制如何依赖于速度、振幅和频率。在低速/低频率时，界面通过散射热激励来耗散能量。在更高的速度/频率下，可以通过类似于高能物理中粒子产生的过程产生新的激发来发生耗散。我们还期望这一运动将引起界面两侧轨道织构的有趣的动力学变化。冷却时从A相到B相的转变是一个长期悬而未决的难题。根据标准理论，即使是在天文时间尺度上，这种转变也不应该发生。实验表明，这种转变很容易发生。基于亚稳态之间的“共振隧穿”，提出了一种可能的解释。我们将进行对照实验来验证这一点。通过形成一个尖锐的最小磁场，我们将B相成核区塑造成远离细胞壁的气泡，以消除表面机制。根据共振隧穿模型，这种转变应该只在特定的温度和磁场下发生，这给出了一个明确的实验信号。我们将构建一个基于气凝胶的3He冷却阶段来获得更低的温度。我们将通过退磁直接冷却覆盖在气凝胶上的固体3He层的纳米网络。由于快速交换，这些层与周围液体具有极好的热接触。该装置将有一个封闭的空腔，以便能够对块状超流体3He进行实验。空腔将被冷消磁气凝胶完全包围，以消除热泄漏。我们相信有可能达到一个新的温度区域，在整个体积中只有几个剩余的热激发。我们将利用核磁共振同时探测块体超流体、气凝胶受限流体和纳米固体3He层。这将使我们能够探索极端低温极限下的新现象。我们将研究固体层中的超低温磁性相变。在块状B相中，我们将研究持久进动域(PPD)，这是一种介子的玻色-爱因斯坦凝聚。在最低温度下，PPD的自由衰变可能持续数小时。在这种极端条件下，可能会出现额外的耗散机制，例如宇宙射线留下的电离轨迹。这项研究将有助于更好地理解量子系统中的基本过程、相变和相界，并扩展冷却技术的能力。

Superfluid 3He has many exotic properties and provides a model system for investigating fundamental processes. It is the only accessible material which has absolute purity. It supports dissipationless flow of mass, spin and orbital angular momentum which give rise to a variety of macroscopic coherence phenomena. The coherent orbital and spin properties are described by vector order parameters which vary on very large length scales, limited only by the size of the experimental cell. Our group specialises in novel cooling and measurement techniques at ultralow temperatures. We routinely cool superfluid 3He to record low temperatures where the few remaining thermal excitations are highly ballistic and the superfluid is essentially in its groundstate.We plan a series of experiments to study fundamental properties of phase transitions and interfaces. The A-B phase transition at low temperatures occurs at a fairly large magnetic field of order 0.4Tesla. Using custom made superconducting solenoids we can accurately control the magnetic field profile to stabilise and manipulate the A-B phase interface. We will incorporate techniques to measure very small, femtowatt, energy dissipations when the interface moves. We will oscillate the interface to investigate how the different dissipation mechanisms depend on the velocity amplitude and the frequency. At low velocity/frequency the interface dissipates energy by scattering thermal excitations. At higher velocities/frequencies dissipation may occur by generating new excitations via processes analogous to particle production in high energy physics. We also expect the motion to induce interesting dynamics of the orbital textures on both sides of the interface.The transition from A-phase to B-phase on cooling presents a long outstanding puzzle. According to standard theories the transition should not occur, even on astronomical timescales. Experiments show that the transition occurs quite readily. A possible explanation has been proposed based on `resonant tunneling' between metastable states. We will perform controlled experiments to test this. By forming a sharp magnetic field minimum we will shape the B-phase nucleating region into a bubble remote from the cell walls to eliminate surface mechanisms. According to the resonant tunneling model, the transition should only occur at particular temperatures and magnetic fields, giving a clear experimental signature.We will construct an aerogel-based 3He cooling stage to access lower temperatures. We will directly cool, by demagnetization, the nano-network of solid 3He layers which coat the aerogel strands. These layers are in extremely good thermal contact with the surrounding liquid owing to rapid exchange. The stage will have an enclosed cavity to enable experiments on bulk superfluid 3He. The cavity will be completely surrounded by cold demagnetised aerogel to eliminate heat leaks. We believe it is possible to reach a new temperature regime where there are only a few remaining thermal excitations in the entire volume. We will use Nuclear Magnetic Resonance to simultaneously probe the bulk superfluid, the aerogel-confined fluid and the nanometer solid 3He layers. This will enable us to explore new phenomena in the extreme low temperature limit. We will study ultralow temperature magnetic phase transitions in the solid layers. In the bulk B-phase we will investigate the Persistent Precessing Domain (PPD), a Bose-Einstein condensate of magnons. The free decay of the PPD may last several hours at the lowest temperatures. Under such extreme conditions, additional dissipation mechanisms may emerge, such as from ionisation tracks left by cosmic rays.The research will lead to a better understanding of fundamental processes in quantum systems, phase transitions and phase interfaces, as well extending the capabilities of cooling technology.