"Non-equilibrium and emergent phenomena in superfluid 3He"

在这个项目中，我们将研究一个新发现的现象——流体速度远远超过临界值时超流动性的存在。我们将在低温物理领域的技术极限下工作，在微开尔文范围内。在这些温度下，超流体3He处于深度量子状态，只有很少的正常激发，这使我们能够研究其远离热平衡的性质。其中之一是将超流体区域扩展到远远超过朗道临界值的速度。我们将探索一个巨大的参数空间，以确定超流体在朗道速度以上存在的区域。其中一个参数将是在超流体中移动的散射体的表面反射率：这可以通过在表面添加4He来调整。我们将研究超流是否可以存在于所有三种可用于实验的超流体状态：A相，B相和最近发现的极性相。后者可以在由薄（10纳米）平行链组成的强各向异性纳米结构介质中稳定。我们将与世界各地的理论家合作，发展一种合适的理论来解释所观察到的现象。该项目将从可行性研究开始，即确定在实验电池表面覆盖几层单层4He将如何影响制冷剂（铜和银粉）与超流体之间的热卡皮察阻力。这些实验将确定我们设计实验的策略，即让一块纳芬以匀速通过超流体。我们将需要设计和建造一个3He-4He同位素分离仪器，以净化实验后从污染物4He中回收的3He气体。nafen实验将需要核磁共振测量，以确定超流体状态的结构。我们将需要开发和建造一个合适的核磁共振光谱仪，包括超导矢量场磁体。测量在超流体中运动的非均匀样品的阻力需要设计一个合适的准粒子辐射热计。开发的实验装置将允许测量临界速度以及观察其他现象。其中之一是可能的量子临界——在绝对零度下的相变。新的实验还将允许测量所有超流体状态下的超流体密度（使用小振幅机械振荡）和自旋轨道相互作用的强度（使用核磁共振频移数据）。这将使我们能够描述杂质（nafen）对超流动性的影响，并将具有更广泛的含义，包括更好地理解纵波超导性。

In this project we will study a newly discovered phenomenon - existence of superfluidity at fluid velocities by far exceeding the critical value. We will work at the limit of what is technically possible in the field of low temperature physics, in the microkelvin region. At these temperatures superfluid 3He is in deep quantum regime with only few normal excitations, which enables us to study its properties emerging far from its thermal equilibrium. One of them is an ability to extend the superfluidity region to velocities far beyond the Landau critical value. We will explore a vast parameter space to determine the region where superfluidity survives above the Landau velocity. One of the parameters will be the surface specularity of the scatterer moving in the superfluid: this can be tuned by adding 4He on the surface. We will study whether the superflow can exist in all three superfluid states available for experiments: the A phase, the B phase and the recently discovered polar phase. The latter can be stabilised in the strongly anisotropic nanostructured medium consisting of thin (10 nm) parallel strands - nafen. We will collaborate with theoreticians world-wide to develop a suitable theory that would explain the observed phenomena.The project will start with the feasibility studies, namely determining how covering the surfaces of experimental cell with a few monolayers of 4He will affect the thermal Kapitza resistance between the refrigerant (copper and silver powder) and the superfluid. These experiments will define our strategy of designing the experiment with a piece of nafen moving through the superfluid at a uniform velocity. We will need to design and build an instrument for the 3He-4He isotope separation in order to purify the 3He gas recovered after the experiments from the contaminant 4He.The experiment with nafen will require NMR measurements in order to determine the structure of the superfluid state. We will need to develop and build a suitable NMR spectrometer including superconducting vector-field magnets. The measurements of the drag force on the nafen sample moving in the superfluid will require designing a suitable quasiparticle bolometer. The developed experimental setup will allow the measurement of the critical velocity as well as observation of other phenomena. One of them is the possible quantum criticality - a phase transition at absolute zero of temperatures. The new experiment will also allow measuring the superfluid density (using small-amplitude mechanical oscillations) and the strength of the spin-orbital interaction (using NMR frequency shift data) in all superfluid states. This will allow us to characterise the influence of impurities (nafen) on superfluidity and will have wider implications, including better understanding of p-wave superconductivity.