Acoustic Faraday effect in superfluid He-B

We have observed that the direction of linearly polarized transverse sound in superfluid ³He-B rotates in a magnetic field. This acoustic Faraday effect proves the existence of a propagating transverse sound mode in ³He and provides clear evidence for spontaneously broken relative spin–orbit symmetry.     

He superfluidity in the presence of aerogel

Aerogels introduce disorder into the p-wave-paired superfluid ³He and suppress T_c. Quantifiable (by small angle X-ray scattering) differences in the long-range structure of two identical density aerogels are primarily responsible for their different transition temperatures. We also demonstrate that alteration of the short-range correlations by the addition of ⁴He does not strongly affect T_c. Acoustic measurements of the fast and slow modes of ³He in aerogel are described. These can be used to explore the superfluid component. We also outline future prospects.     

Low-temperature helium in bulk and in restricted geometries

Experiments with helium in thin films and small pores show that the bulk properties are greatly modified. With ⁴He, the onset of superfluidity is depressed and the specific heat anomaly is rounded and also depressed to lower temperatures. With ³He new features are found such as surface magnetism and free path effects. Bulk superfluid ⁴He is shown to have a sharp transition to turbulence, and bulk ³He the characteristics of a Fermi fluid.     

Electrons on superfluid He

Low-frequency transport properties of the Wigner solid on normal and superfluid ³He have been studied. It is known that the Wigner solid deforms the free surface of liquid He to form a dimple lattice. The low-frequency resistivity is successfully interpreted by the ballistic and specular scattering of ³He quasiparticles from the dimple lattice. The resistivity decreases abruptly below the superfluid transition temperature. For superfluid ³He-A, the l-texture, presumably perpendicularly oriented to the free surface, has an essential influence on the resistivity.     

Current–phase relationship measurements in the flow of superfluid He through a single orifice

We report accurate measurements of the current-phase relationship in the flow of superfluid ³He through a single orifice. While ideal sinusoidal 2π-periodic Josephson behavior prevails close to T_c, increasingly strong π-periodic admixture is usually observed at lower temperatures.     

Decay of quantized vorticity in superfluid He at mK temperatures

An experimental investigation of the free decay of quantized turbulence in isotopically pure superfluid ⁴He at mK temperatures is discussed. Vortices are created by a vibrating grid, and detected by their trapping of negative ions. Preliminary results suggest the existence of a temperature-independent vortex decay mechanism below T70 mK.     

Fluctuation effects in spin diffusion measurements in liquid He

We have measured spin diffusion coefficients of liquid ³He at a frequency of 920 kHz by pulsed NMR. By analyzing our data in the framework of the Leggett–Rice theory we got a spin diffusion coefficient and a Leggett–Rice parameter =λωτ simultaneously at each temperature. On approaching the superfluid transition the spin diffusion coefficients showed a deviation from predictions of the Fermi liquid theory. The deviation at low pressure was larger than that at high pressure. This anomaly may be due to the effects of fluctuations of superfluidity which were recently observed in the viscosity measurement of liquid ³He.     

Adsorption and superfluid onset of He films on intermediate strength substrates

On strong binding substrates, such as graphite or mylar that are wetted by ⁴He at all temperatures, an adsorbed ⁴He film consists of 2 atomic layers of “inert” helium covered by a liquid layer that becomes superfluid via a Kosterlitz–Thouless (KT) transition. On weak substrates, for example cesium, superfluid onset above the wetting temperature also conforms to the KT picture. In contrast, superfluid onset on intermediate strength substrates, specifically heavier alkali metals and monolyer films of cesium on gold, deviates strongly from KT behavior. Here we describe superfluid onset of ⁴He on intermediate strength substrates and discuss the contributions of weak bindings and disorder to the non-KT behavior.     

The thermodynamics of the A–B transition in superfluid He as a probe of the A-phase gap node geometry

We have used a magnetic field gradient to stabilise the phase boundary between the A- and B-phases of superfluid ³He inside a quasiparticle black body radiator down to 146 μK. The radiator technique enables us to measure very accurately the density of quasiparticle excitations and therefore the temperature of the B-phase inside the radiator. As we ramp the field through the transition we can observe the cooling (warming) effect when the volume of A-phase is increased (decreased). From these measurements we deduce the temperature dependence of the latent heat, which provides the lowest temperature verification yet of the existence of nodes in the A-phase order parameter.     

Critical Casimir effect in dilute He–He mixture films

Recently, we have found that ⁴He films thin near the lambda point. Based on our measurements, the thinning appears to be caused by the confinement of superfluid order parameter fluctuations in the films. The phenomenon of the thinning is believed to be analogous to the Casimir effect in electromagnetism where the confinement of electromagnetic fluctuations causes the vacuum layer that separates two metal plates to thin. In this paper, we report the result of an experiment extending our previous study to ³He–⁴He mixture films. Thinning is found near the new lambda points and the characteristics of the thinning are similar to that found in pure ⁴He films.     

Simulation of the scattering in an atomic beam evaporated from a He film

We have simulated the scattering in a pulsed atomic beam evaporated from a superfluid ⁴He film. Assuming that the atoms leaving the surface of the film have an equilibrium Maxwellian distribution, we find that the experimentally observed deviations from Lambert's law are explained by scattering of the atoms after evaporation.     

Symmetry breaking patterns in superfluid He

By means of the time reversal operator a general form of the Landau free energy functional of superfluid ³He is investigated. Various possible phase diagrams of superfluid ³He in a magnetic field are discussed in the context of group theory. Both the cases of neglible and strong spin-orbit coupling are treated.     

极化3He系统核磁共振FID信号的激光探测

极化率测量装置是极化³He系统的重要组成部分之一.本文介绍了一套在极化³He系统上搭建的激光探测自由感应衰减核磁共振（FID NMR）信号的实验装置,该装置有望为³He极化率的高精度实时测量提供新的途径.激光探测法的原理是法拉第旋光效应,该方法使用一束线偏振光探测极化³He磁矩绕主磁场进动的FID信号,实验结果表明相较于拾波线圈探测方法,激光探测方法的信噪比提高了106%,对³He的极化率测量更精确.该探测方法有望替代传统的拾波线圈,广泛地应用于³He极化系统的极化率测量或精密测量实验中.     

Experimental evidence against zero temperature spin-wave damping in He

We have discovered unexpected features in the NMR signal of the ³He– ⁴He mixture inside the mixing chamber of a ⁴He circulating dilution refrigerator. The dilution process not only cools the liquid in the mixing chamber to 12–15 mK but also enhances the nuclear spin polarization of the concentrated as well as the dilute phase to nearly four times the equilibrium value in a magnetic field of 9.36 T. The NMR features are ascribed to standing spin waves trapped by falling dilute phase droplets. The temperature and polarization dependence of the quality factor of the spin-wave modes contradicts evidence from previous work that favored zero temperature spin-wave damping in moderately polarized degenerate Fermi liquids.     

A Phenomenological Model on the Superfluid Transition of 4He in Porous Media

The superff uid transition of ⁴He in porous media is discussed. The eigenstates of the Hamiltonian may be localized whether the disorder is spatially correlated or not. Based on this argument we proposed a phenomenological model to describe the effects of localization on phase transitions. The system may be divided into many isolated blocks because of the localization. We calculated the superfluid density and the flee energy density for the superfluid transition according to this model. These thermodynamical quantities are described by power Jaws in a certain temperature region but the hyperscaling relation is violated as one can expect. The comparison of the result of this model with the experiment on the superfluid transition of ⁴He in gels is given.     

光泵抽运3He极化程度监控系统的设计与实现

极化³He的一项重要应用是中子的极化.中国绵阳研究堆（CMRR）已建立国内首个自旋交换光学泵浦（SEOP）极化³He中子极化系统.为了监测³He的极化率随时间的相对变化情况，本文首先设计了基于核磁共振（NMR）技术的³He相对极化率测量系统，通过Matlab控制程序实现了对³He相对极化率的定时检测.然后对拾波线圈的构形和信噪比（SNR）进行了优化.结果表明当绕线长度一样时，Brooks构形的线圈有利于提高SNR；当线圈的平均半径为（a₀+d）/√2（a₀为³He气室的半径，d为拾波线圈与气室之间的距离）时，其SNR最高.最后对该系统的本底噪声进行了测量，发现其主要来源于环境噪声（0.27 μV/√Hz）和数据采集（DAQ）卡的噪声（0.40 μV/√Hz），系统的总噪声功率谱密度约为√0.16+0.073G² μV/√Hz（G为放大器的增益倍数）.     

Sound velocity measurement of nuclear-ordered U2D2 solid He along the melting curve

The sound velocity of a single-domain ³He crystal was measured in the nuclear-ordered low-field phase and the paramagnetic phase along the melting curve, using 10.98 MHz longitudinal sound. The temperature dependence of the sound velocity along the melting curve was explained by a nuclear spin contribution and the molar volume change along the melting curve. By comparing the measured velocity with thermodynamic quantities, we extracted the Grüneizen constant for the exchange energy. The anisotropy of the velocity in the ordered phase was investigated for three samples and was found to be smaller than 2×10⁻⁵ in Δv/v. The attenuation coefficient of the sound was much smaller than 0.2 cm⁻¹.     

一种可用于极化~3He实验的新型磁场系统

原子核自旋极化的~3He气体已被深入研究并广泛用于各种科学实验.在过去的极化~3He实验中,为了减小磁场梯度对纵向弛豫时间的影响,通常会建造大尺寸的亥姆霍兹线圈来提供所需均匀度的主磁场环境.本文通过计算得到了新的六正方形线圈系统,可以为极化~3He实验提供小型高均匀性的磁场装置.其中线圈系√统内部超过30%的区域磁场梯度满足(|▽B_x|~2+|▽B_y|~2)/B_0 10~(-4)cm~(-1),这一均匀区域比例超过了现在所有用于极化~3He实验的线圈装置.对于其他需要大均匀区域磁场环境的研究实验,新的六线圈系统也具有很好的应用价值.     

Adsorption of He on He crystal surfaces

We have compared the surface properties of crystals respectively grown from normal ⁴He containing 130 ppb of ³He and from ultrapure ⁴He (0.4 ppb). Below 0.4 K, ³He impurities are found to decrease both the surface tension and the step energy. Our results are consistent with the existence of two-dimensional bound states for ³He atoms at the solid-liquid interface. Quantitative agreement with the data is found with a binding energy _s ≈ 4.3 K and a ³He density saturating around 0.4 monolayer. The presence of steps is found to increase the binding energy by about 10 mK.     

The origin of nuclear magnetism in solid He films: determination of multi-spin exchange frequencies

Solid ³He films adsorbed on graphite display remarkable magnetic properties due to quantum exchange, reduced dimensionality and frustration. We discuss the relevance of the multiple spin exchange Hamiltonian to describe these systems. Our high precision NMR data and the results of heat capacity measurements performed in other laboratories allow us to unambiguously determine the most significant multi-spin exchange frequencies. This result gives a quantitative understanding of the thermodynamical properties of solid ³He films, explaining the evolution from a pure Heisenberg behavior at high areal densities to a novel spin liquid state at low coverages.