Critical and surface phenomena of quantum fluids

中子散射为研究凝聚态物质中的激发态提供了一个强有力的工具。作为一种技术，它在量子流体(液体HE4、液体HE3和液体同位素混合物)的情况下具有特殊的优势。目前的研究计划包括两个主要部分：1.研究HE3、HE4和HE3-HE4混合物中的临界现象；探索利用中子反射研究超低温下量子流体表面的量子激发的可能性。与传统的光学技术相比，利用中子研究临界乳浊度等现象具有特殊的优势。特别是，它不要求已经很复杂的低温设备必须有光学窗口。例如，建议的方法允许研究流体3He和3He-4He混合物的临界行为。对临界点附近的中子乳光的测量预示着对重整化群理论的准确预测的严格检验，包括几种不同类别的临界现象。我们还希望确定如果我们将流体放入纳米多孔材料中，临界现象如何变化：临界涨落的大小将如何受宿主纳米基质的尺寸的影响。中子为这类研究提供了独特的探测器，特别是在低温下。我们现在提出利用中子反射来研究液氦表面，使我们能够检验Fomin的长期理论，即在高频范围内，费米液体不仅应该支持普通的毛细波，而且应该支持瑞利波。似乎也会观察到不可预测的、不可预见的物理影响。慢中子表现出一系列与经典光学中观察到的现象非常相似的现象，包括反射、折射和干涉。慢中子的全反射最先是由费米和他的同事报道的，此后被广泛应用于各种问题。例如，用中子衍射法实验研究了声表面波的传播和衰减特性。然而，据我们所知，这种方法还没有被应用于量子流体表面的研究，可能是因为将中子反射仪(例如ISIS ral的CRISP)与必要的超低温样品环境相结合是困难的。研究3He原子在液体4He自由表面上的Andreev量子态的形成是本研究的另一个挑战。我们还将使用中子散射来研究量子流体表面的波动湍流。类似的光学实验已经在液氢自由表面上进行过，但还没有关于HE4或HE3的结果。

Neutron scattering provides a powerful tool for investigation of excitations in condensed matter. As a technique, it offers special advantages in the case of quantum fluids (liquid He4, liquid He3, and liquid isotopic mixtures). It can be applied both to excitations in the bulk and to those on the free surface of the fluid.The current research plan consists of two main parts:1. investigation of critical phenomena in He3, He4, and He3-He4 mixtures; and2. exploration of the possibility of using neutron reflection for studying quantum excitations on the surfaces of quantum fluids in the ultra-low temperature limit. The use of neutrons to study phenomena such as critical opalescence has special advatantages, compared to the traditional optical technique. In particular, it does not require the already-complicated cryogenic equipment to have optical windows. For example proposed method allows studies of the critical behaviour of fluid 3He and 3He-4He mixtures. Measurements of neutron opalescence in the vicinity of the critical point promise a rigorous test of the exact predictions of Renormalization Group theory, including several different classes of critical phenomena. We also wish to establish how the critical phenomena change if we place the fluid inside a nanoporous material: how the size of the critical fluctuations will be affected by the dimension of the host nano-matrix. Neutrons provide unique probes for this kind of research, especially at low temperatures.We now propose the use of neutron reflection to study the liquid helium surface, enabling us to test the long-standing theory of Fomin that, in the high-frequency limit, the Fermi liquid should support, not only ordinary capillary waves, but Rayleigh-type waves as well. It seems likely that unpredicted, unforeseen, physical effects will be observed as well. Slow neutrons exhibit a range of phenomena closely analogous to those observed in classical optics, including reflection, refraction and interference. Total reflection of slow neutrons was first reported by Fermi and co-workers and has since been extensively applied to a diversity of problems. For example the propagation and attenuation characteristics of surface acoustic waves have been experimentally investigated by neutron diffraction . To our knowledge, however, this method has not yet been applied to studies of the surfaces of quantum fluids, probably on account of the difficulty of combining a neutron reflectometer (e.g. CRISP at ISIS RAL) with the necessary ultra-low temperature sample environment. The investigation of the formation of Andreev's quantum states of 3He atoms on free surface of liquid 4He is another challenge of this research. We will also use neutron scattering to investigate wave turbulence on the surfaces of quantum fluids. The analogous optical experiment has already been carried out for a liquid hydrogen free surface, but no results have yet been reported for either He4 or He3.