Exotic Phenomena in Superfluid 3He at Ultralow Temperatures

超流体3 He是现存最奇特的液体。它只存在于绝对零度以上千分之几度以下。它的存在完全归功于量子力学，因此研究它以促进对量子系统的更好理解是很有趣的。虽然它是超流体，可以在没有摩擦的情况下流动，但它也具有相关的自旋“超流体”和轨道角动量“超流体”的事实赋予了它许多独特的性质，其中许多仍有待发现。我们的团队率先研究了超流3 He中的弹道热传输。超流体中的热量是由准粒子激发携带的。在非常低的温度下，这些是如此之少，几乎从来没有分散。因此，我们可以产生弹道准粒子束，并将它们射向由超流体本身形成的各种障碍物。到目前为止，我们已经掌握了观察这种光束的Andreev反射（超流体和超导体特有的一种反射形式）的技术。我们现在希望发展出测量准粒子束透射率的方法，并将用这种技术来研究量子湍流的衰减。在超流中，~ 3 He湍流的形式是量子化涡线的缠结。这比具有可变尺寸的涡流/涡旋的经典湍流简单得多。对超流湍流的研究将使我们从总体上更好地理解湍流。准粒子传输技术，我们希望获得更多的定量信息的湍流衰减机制在零温度的限制，耗散机制应该由量子效应，而不是由传统的粘度。我们还将研究脏3 He的超流相图。杂质可以通过将其限制在气凝胶中有效地添加到液体3 He中，气凝胶是二氧化硅线的纳米级网络。由于纯3 He被很好地理解，它是研究这种效应的理想物质。我们将研究杂质如何影响各种超流体相以及它们之间的转变。无带隙超流性是一种奇特的现象，常见于脏超导体中，其中提供超流行为的组分对的结合能为零。我们最近在超流体的热导率中看到了这种行为。用类似的技术，我们计划研究从无隙超流到（通常的）有隙超流的交叉。此外，在气凝胶中的超流体转变发生在零温度下的情况下，这构成了由量子效应主导的量子相变。这似乎是已知的最干净的这种转变，我们很好地调查相关的量子涨落。超流体轨道性质的3 He是不明显的，在温度下提供给大多数研究小组。我们的新技术使我们能够达到最低的温度，在那里轨道超流性变得明显。我们有间接的证据，轨道超流性的外来NMR信号从持续进动域（PPD）。这些是具有激光性质的相干自旋进动的超长寿命域。通过观察两个PPD的相互作用，我们希望获得轨道超流性的更直接的证据。最后，我们计划展示一种独特的超流机制，即3 He影响液体中物体的运动。在绝对零度下，没有激发，物体应该像通过真空一样自由地通过超流体。然而，如果物体被加热，准粒子的热辐射应该阻尼它的运动。我们将通过移动加热的气凝胶样品通过超流体来研究这一点。我们强调，大多数拟议的实验只有在我们独特的冷却技术所能实现的最低温度下才是可行的。

Superfluid 3He is the most exotic liquid in existence. It only exists below a few thousandths of a degree above absolute zero. It owes its existence entirely to quantum mechanics and is therefore interesting to study to promote the better understanding of quantum systems in general. While it is superfluid and may flow without friction, the fact that it also has an associated spin 'superfluid' and an orbital angular momentum 'superfluid' gives it many unique properties, many of which remain to be discovered. Our group has pioneered the study of ballistic heat transport in superfluid 3He. Heat in the superfluid is carried by quasiparticle excitations. At very low temperatures these are so few as to hardly ever scatter. We can thus generate beams of ballistic quasiparticles and fire them at various obstacles formed by the superfluid itself. We have so far mastered the techniques for observing the Andreev reflection of such beams (a form of reflection unique to superfluids and superconductors). We now wish to develop the methods needed to measure the transmission of quasiparticle beams.We will use this technique to investigate the decay of quantum turbulence. In superfluid 3He turbulence takes the form of a tangle of identical quantised vortex lines. This is much simpler than classical turbulence which has eddies/vortices of variable sizes. The study of superfluid turbulence will give us a better understanding of turbulence in general. With quasiparticle transmission techniques we hope to obtain more quantitative information on the turbulence decay mechanisms in the zero-temperature limit where the dissipation mechanism should be determined by quantum effects rather than by conventional viscosity. We will also investigate the superfluid phase diagram of dirty 3He. Impurities may be effectively added to liquid 3He by confining it in aerogel, a nanoscale network of silica strands. Since pure 3He is so well understood, it is the ideal substance for investigating such effects. We will study how impurities influence the various superfluid phases and the transitions between them. Gapless superfluidity is an exotic phenomenon common in dirty superconductors where the binding energy of the constituent pairs providing the superfluid behaviour vanishes. We have recently seen this behaviour in the thermal conductivity of superfluid. With similar techniques we plan to study the cross-over from gapless to (the usual) gapped superfluidity. Further, where the superfluid transition in aerogel occurs at zero temperature, this constitutes a quantum phase transition dominated by quantum effects. This seems to be the cleanest such transition known and we are well placed to investigate the associated quantum fluctuations.The superfluid orbital properties of 3He are not evident at the temperatures available to most research groups. Our novel techniques allow us the lowest achievable temperatures where orbital superfluidity becomes apparent. We have indirect evidence of orbital superfluidity from exotic NMR signals from persistent precessing domains (PPDs). These are ultra long lived domains of coherent spin precession which have laser-like properties. By looking at the interaction of two PPDs we hope to gain more direct evidence of orbital superfluidity.Finally we plan to demonstrate an exotic mechanism unique to superfluid 3He affecting the motion of an object in the liquid. At absolute zero there are no excitations and an object should move freely through the superfluid as through a vacuum. However, if the object is heated, the thermal emission of quasiparticles should damp its motion. We will investigate this by moving a heated aerogel sample through the superfluid.We emphasise that most of the proposed experiments are only feasible at the very lowest temperatures made possible by our unique cooling techniques.