Exotic Phenomena in Superfluid 3He at Ultralow Temperatures

超流体3是现存的最奇特的液体。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，因此研究它是有趣的，可以促进对量子系统的更好理解。虽然它是超流体，可以在没有摩擦的情况下流动，但它还具有相关的自旋“超流体”和轨道角动量“超流体”，这一事实赋予了它许多独特的性质，其中许多性质仍有待发现。我们小组率先研究了超流体3He中的弹道热输运。超流体中的热量由准粒子激发传递。在极低的温度下，它们很少，几乎不会分散。因此，我们可以产生弹道准粒子束，并将它们发射到由超流体本身形成的各种障碍物上。到目前为止，我们已经掌握了观察这种光束的安德烈夫反射（一种超流体和超导体特有的反射形式）的技术。我们现在希望发展测量准粒子束传输所需的方法。我们将使用这种技术来研究量子湍流的衰减。在超流体3He中，湍流以一团相同的量子化涡线的形式出现。这比具有可变大小的涡流/漩涡的经典湍流要简单得多。对超流体湍流的研究将使我们更好地理解一般的湍流。利用准粒子传输技术，我们希望获得更多关于零温度极限下湍流衰减机制的定量信息，在零温度极限下，耗散机制应该由量子效应而不是传统的粘度来决定。我们还将研究脏3He的超流体相图。杂质可以通过气凝胶（一种纳米级的二氧化硅链网络）将其限制在气凝胶中而有效地添加到液体3He中。由于对纯氦的了解如此透彻，所以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过渡。无间隙超流体是脏超导体中常见的奇异现象，在脏超导体中，提供超流体行为的组成对的结合能消失。我们最近在超流体的导热性中看到了这种行为。我们计划用类似的技术研究从无间隙到（通常的）间隙超流体的交叉。此外，当气凝胶中的超流体转变发生在零度时，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跃迁，我们可以很好地研究相关的量子涨落。3He的超流体轨道性质在大多数研究小组可用的温度下并不明显。我们的新技术使我们能够达到轨道超流动性变得明显的最低温度。我们从持续处理域（PPDs）的外来核磁共振信号中获得了轨道超流动性的间接证据。这些是超长寿命的相干自旋进动域，具有类似激光的性质。通过观察两个ppd的相互作用，我们希望获得轨道超流动性更直接的证据。最后，我们计划展示一种独特的超流体3He影响液体中物体运动的奇特机制。在绝对零度下，没有激发态，物体在超流体中应该像在真空中一样自由运动。然而，如果物体被加热，准粒子的热辐射应该会抑制它的运动。我们将通过在超流体中移动加热的气凝胶样品来研究这一点。我们强调，大多数提出的实验只能在我们独特的冷却技术所能达到的最低温度下进行。

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.