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Exotic Phenomena in Superfluid 3He at Ultralow Temperatures

超低温超流体 3He 中的奇异现象

基本信息

批准号：
EP/D078407/1
负责人：
George Pickett
金额：
$ 85.87万
依托单位：
Lancaster University
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2006
资助国家：
英国
起止时间：
2006 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FD078407%2F1
关键词：
Exotic Phenomena Superfluid 3He Ultralow

项目摘要

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.

超流体3He是现存的最奇异的液体。它只存在于绝对零度之上的千分之几度以下。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，但它也有一个相关的自旋“超流体”和轨道角动量“超流体”，这一事实给了它许多独特的性质，其中许多还有待发现。我们小组开创了超流3He弹道热输运研究的先河。超流体中的热量由准粒子激发携带。在非常低的温度下，它们非常稀少，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列夫反射(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光束传输所需的方法。我们将使用这项技术来研究量子湍流的衰变。在超流中，3He湍流以相同的量化涡线缠绕的形式存在。这比具有不同大小的涡旋/涡旋的经典湍流要简单得多。对超流湍流的研究将使我们对湍流有更好的认识。利用准粒子传输技术，我们希望在零温极限下获得更多关于湍流衰变机制的定量信息，在这种情况下，耗散机制应该由量子效应而不是传统的粘性来决定。我们还将研究脏3He的超流相图。通过将3He限制在气凝胶中，可以有效地将杂质添加到液体3He中，气凝胶是由二氧化硅链组成的纳米网络。由于人们对纯3He的了解如此之深，因此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相以及它们之间的转变。无间隙超流是肮脏超导体中常见的一种奇特现象，提供超流行为的组成对的结合能消失。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，我们计划研究从无间隙到(通常)有间隙的超流的交叉。此外，当气凝胶中的超流转变发生在零温度时，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这种转变，我们很好地研究了相关的量子涨落。在大多数研究小组可用的温度下，3He的超流轨道性质并不明显。我们的新技术使我们能够在轨道超流变得明显的情况下达到最低的温度。我们从持久进动结构域(PPD)的奇异核磁共振信号中获得了轨道超流的间接证据。这些是具有激光性质的相干自旋进动的超长寿命区域。通过观察两个PPD的相互作用，我们希望获得更多轨道超流的直接证据。最后，我们计划展示超流3He影响液体中物体运动的一种奇特机制。在绝对零度时，没有任何激发，物体应该像在真空中一样在超流体中自由运动。然而，如果物体被加热，准粒子的热发射应该会抑制它的运动。我们将通过移动加热的气凝胶样本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可能的最低温度下才是可行的。