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.     

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.     

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.     

Effect of He pre-plating on third sound in superfluid He

We recently reported observation of third sound resonances in superfluid films of pure ³He on a disk-shaped polished copper substrate. From the observed spectra of resonant modes, the third sound speed, the average superfluid density, and information on dissipation were obtained. The film thicknesses studied were larger than or comparable to the superfluid coherence length, which is near 100 nm. We describe preliminary results from efforts aimed at using third sound in thinner films on a substrate which has been coated with a few atomic layers of ⁴He.     

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.     

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.     

X-ray scattering studies of liquid surfaces

X-ray reflectivity experiments on the structure of the liquid-vapor interface that were recently carried out by our group will be described. Systems that were studied include thin wetting layers adsorbed onto a solid substrate, the interfacial electron density profile of superfluid ⁴He, and preliminary measurements of the roughness of the surface of liquid gallium. The fundamental theoretical background to these experiments will be reviewed.     

Nonwetting of He and dilute He–He mixtures on Cs

For pure ⁴He on Cs a nonwet phase exists below ≈2 K but for dilute ³He–⁴He mixtures the wetting is reentrant. Measurements of the contact angle with dilute mixtures of liquid helium on Cs can be explained in detail in terms of ³He states together with ripplons at the Cs–He mixture interface, but it is impossible to account for the lower wetting temperatures or contact angles with only ³He states or ripplons. We discuss the influence of surface roughness of the Cs on the contribution of interface excitations to the free energy and suggest that the variety of contact angles found with different samples of Cs is due mainly to their degree of roughness.     

Introduction to the vortex sheet of superfluid He-A

It is well known that superfluids respond to rotation by forming vortex lines. It has been recently discovered that a different type of state consisting of a vortex sheet, instead of lines, can be created in the A phase of superfluid ³He. This paper presents an introduction to the vortex sheet. We first discuss ⁴He, where a vortex sheet is unstable. The way to realize a stable sheet in ³He-A is called a vortex soliton. It consists of a topologically stable domain wall to which nonsingular vorticity is bound. The vortex soliton has been observed by nuclear magnetic resonance, and its most prominent experimental properties are explained. The macroscopic shape of the sheet and the superfluid flow in a rotating container are discussed.     

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.     

Optical interferometry in phase separated He---He mixture films

We report on interferometric studies of ³He---⁴He liquid mixtures. Anomalous wetting phenomena were observed in phase separated mixture films: the upper ³He-rich phase did not wet the lower ⁴He-rich phase under a small continuous feed of ⁴He atoms to the vapor phase. Instead, under this dynamic situation, floating pools of the ³He-rich phase were stabilized with a contact angle on the order of 10 mrad. In equilibrium, the concentrated ³He-rich phase was found to nucleate uniformly on the free surface, consistent with Antonow's rule.     

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.     

Relaxation processes in phonon–impuriton system and effective thermal conductivity of superfluid He–He solutions

By the method of stationary thermal flow, the measurements of effective thermal conductivity in the superfluid solution ³He–⁴He with initial molar concentration 9.8% ³He in the temperature range 0.1–0.5 K have been carried out. At temperatures below 0.23 K, where the phase separation of the mixture occurs, the measurements were carried out in the lower superfluid phase with changing concentration following the phase diagram with decreasing the temperature. Experimental data were compared with the prediction of the kinetic theory of the phonon–impuriton system of a solution. The analysis allows us to establish the relaxation time hierarchy in such a system and to provide the quantitative estimation of the times of impuriton–impuriton and phonon–impuriton relaxation. It is shown that a rather good agreement between experimental data and theory can be obtained by the use of one fit parameter.     

Diaphragm-driven flow in rotating superfluid He-B

We present the theory used to analyse experiments at Manchester University in which we observe the normal modes of transverse vibration of a Kapton diaphragm separating two nominally identical disc-shaped regions of superfluid ³He, each of height 100 μm and diameter 40 mm. From the mode frequencies we deduce information on the superfluid density and hence on strong coupling corrections to the energy gap. From the dissipation the first and second viscosities, η and ξ₃, of the fluid can be obtained. Rotation of the experiment about an axis perpendicular to the diaphragm creates a lattice of quantised vortex lines. We show how the mutual friction parameters B and B′ can be determined from the effect of the vortices on the normal modes of the diaphragm.     

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.     

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.     

Spectroscopy of molecules in helium droplets

The work reviewed in the talk explores the possibilities of using molecules immersed in liquid helium as probes to study superfluidity on the microscopic level. For this purpose liquid ⁴He, ³He and mixed ⁴He/³He droplets consisting of 10³–10⁵ atoms have been doped with single molecules or a small defined number of molecules which form clusters in the droplet interior. The dopants were investigated with high-resolution optical spectroscopy. The electronic and rovibrational spectra show unusually sharp spectral lines and unexpected features such as a distinct gap between the zero phonon line and the phonon wing as well as the free rotations of single molecules. Both are not observed in colder nonsuperfluid ³He droplets and therefore are considered to be new microscopic manifestations of superfluidity. Spectroscopic studies of small p-H₂ and o-D₂ clusters surrounding a single chromophore molecule formed in the interior of the He droplets are also reported.     

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.     

Texture of superfluid 3He probed by a Wigner solid

The resistivity of the Wigner solid floating on the free surface of superfluid 3He has been measured in both the A and B phases down to 200 microK in the magnetic field. The resistivity in the A phase shows the asymptotic behavior to the temperature-square dependence at low temperatures. The temperature dependence is successfully explained by the uniform l[over] texture oriented normal to the surface and by the specular scattering of quasiparticles excited along l[over] vector. In the B phase, the resistivity exhibits the exponential decrease at low temperatures. The steep increase of the resistivity observed at high magnetic field is attributable to the nonuniform texture of the field-distorted order parameter induced near the surface.     

Pionic K X-rays in liquid He

Energies and lorentzian widths of pionic K X-ray transitions have been measured in liquid ³He with improved accuracy. The strong interaction on the π³He 1s level is found to be attractive and to produce an increase in the K transition energies of 34 ± 4 eV; the measured lorentzian width is 36 ±7 eV. Measured values are also presented for K X-ray energies in liquid π⁴He and μ⁴He, the lorentzian width of the 1s level in π⁴He, and relative intensities of K X-ray transitions in π³He, π⁴He and μ⁴He. The measurements are compared with those of others and with recent theoretical calculations.