Novel Experiments in Multiphase Superfluid 3He at Ultralow Temperatures

超流体、超导体和玻色凝聚稀气体是非常有趣的，因为它们的组成原子形成相干基态，在这种基态中，整个系统的行为是相关的，导致宏观量子现象。然而，超流体3He由于其多组分序参数而具有独特性。3He原子形成对，同时具有自旋和轨道角动量。质量、自旋和轨道运动都表现出相干的量子行为，从而产生了一系列奇异的性质。这种增加的复杂性使得超流体3He可以存在于两个非常不同的相干相A和B中，这取决于温度和磁场。我们能够将超流体3He冷却到几乎所有原子都成对的温度，所以我们有一个几乎纯的量子态。通过应用适当形状的磁场剖面，我们可以稳定不同区域的不同相，包括被a相包围的b相孤立气泡。气泡的几何形状将使我们能够研究基本过程，否则这些过程可能会受到壁的影响，如相成核、热输运和湍流。a - b界面是一个分离两个高度相干相的相干结构。序参量必须在从一个相到另一个相的交叉过程中做一个复杂的旋转，使质量、自旋和轨道自由度平滑地匹配。这个独特的系统为我们提供了一个进入广泛的新物理学的入口。显然，它本身就是一个有趣的系统。然而，它也为不易访问的系统提供了一个模型。例如，它是我们在实验室中最接近宇宙膜（时空中的等效结构）的东西。通过碰撞两个A-B边界，我们可以在实验室中模拟膜湮灭。这些都是宇宙学的膜世界场景的基本兴趣。通过将气凝胶浸泡在3He中，我们可以研究纳米级二氧化硅链在气凝胶中引起的无序所产生的脏限内的超流相。此外，当我们将气凝胶浸入超流体中时，一些3He原子层被吸附到二氧化硅链上，从而在氦气凝胶体系中形成大量的固体3He成分。固体氦具有很强的磁性，与液体紧密接触。这为在固体上使用磁冷却技术和近乎完美的热接触来冷却超流体提供了一个独特的机会。二氧化硅链周围的固体层形成了一个3he纳米管系统，它也有可能揭示新的令人兴奋的物理学。我们打算开发一个3he气凝胶冷却阶段来研究固体3he纳米管，并将超流体冷却到一个新的低温状态，在这个低温状态下，液体的宏观体积上基本上没有热激发。最近，我们还发现超流体3He为研究低温下的量子湍流提供了一个特别有用的工具。量子湍流本质上是量子涡旋线（超流体循环被量子化的线缺陷）的纠缠。量子湍流与经典湍流有密切的相似之处，但由于量子化的涡流和缺乏粘性，量子湍流要简单得多，因此可能为理解一般的湍流提供更好的见解。我们计划使用我们之前开发的高灵敏度量热技术来测量零温度极限下量子湍流的能量衰减，为基本衰变机制提供更好的信息。最后，我们正在利用这些实验来试点开发一种新型高灵敏度石英谐振器，这种谐振器是定制设计的，可以在最低温度下最大限度地发挥其与超流体3He的相互作用。我们希望这将取代目前普遍存在的振动线谐振器（也是在兰开斯特大学开发的）作为标准的低温量子流体传感器/温度计。

Superfluids, superconductors and Bose condensed dilute gases are extremely interesting since their constituent atoms form coherent ground states, in which the behaviour of the whole system is correlated, leading to macroscopic quantum phenomena.However, superfluid 3He is unique due to its multi-component order parameter. The 3He atoms form pairs which have both spin and orbital angular momentum. The mass, spin and orbital motions each exhibit coherent quantum behaviour giving rise to a whole range of exotic properties. This added complexity allows superfluid 3He to exist in two very different coherent phases, A and B, depending on the temperature and magnetic field.We are able to cool superfluid 3He to temperatures where virtually all atoms are paired, so we have an almost pure quantum state. By applying a suitably shaped magnetic field profile we can stabilise different phases in different regions, with various geometries including an isolated bubble of B-phase surrounded by A-phase. The bubble geometry will allow us to study fundamental processes, which might otherwise be influenced by walls, such as phase nucleation, thermal transport and turbulence.The A-B interface is a coherent structure separating two highly coherent phases. The order parameter must make a complex pirouette in crossing from one phase to the other, matching-up smoothly the mass, spin and orbital degrees of freedom. This unique system gives us an entre into a wide range of new physics. It is clearly an interesting system in its own right. However, it also provides a model for less accessible systems. For example, it is the nearest thing we have in the laboratory to a cosmological brane (equivalent structures in space-time). By colliding two A-B boundaries, we can simulate brane-annihilations in the laboratory. These are of fundamental interest to the braneworld scenarios of cosmology.By immersing aerogel in 3He we can study the superfluid phases in the dirty limit generated by the disorder induced by the nanometre sized silica strands in the aerogel. Furthermore, when we immerse the aerogel in superfluid, a few atomic layers of 3He atoms are adsorbed onto the silica strands to make a substantial solid 3He component in the helium-aerogel system. The solid 3He is highly magnetic and is in intimate contact with the fluid. This provides a unique opportunity for using magnetic cooling techniques on the solid and the near perfect thermal contact to cool the superfluid. The solid layers around the silica strands form a system of 3He-nanotubes which also have potential for revealing new exciting physics. We intend to develop a 3He-aerogel cooling stage to study both the solid 3He-nanotubes and to cool the superfluid to new a low temperature regime where there are essentially no thermal excitations over macroscopic volumes of the liquid.Recently, we have also found that superfluid 3He provides a particularly useful tool for studying quantum turbulence at low temperatures. Quantum turbulence is essentially a tangle of quantum vortex lines (line defects around which the superfluid circulation is quantised). Quantum turbulence has close analogues with classical turbulence but is much simpler, due to the quantised vortices and lack of viscosity, and might therefore provide better insights to understanding turbulence in general. We plan to use highly sensitive calorimetric techniques, which we have developed earlier, to measure the energy decay of quantum turbulence in the zero temperature limit, providing better information on fundamental decay mechanisms.Finally we are using these experiments to pilot the development of a new form of the highly sensitive quartz resonator, custom-designed to maximise its interaction with superfluid 3He at the lowest temperatures. We hope that this will replace the currently ubiquitous vibrating wire resonator (also developed at Lancaster) as the standard low temperature quantum fluids sensor/thermometer.