4He Films near Monolayer Completion

We study the role played by the substrate–helium interaction in determining whether a ⁴He film is fluid or solid near monolayer completion. In order to glean information concerning the possible fluid-solid transition we calculate the equation of state as well as the behavior of phonons and rotons for a high density monolayer fluid film. By analyzing the behavior of the long-wavelength excitations and the roton gap, we argue that we can infer instability in the fluid phase. We study a model Lennard-Jones two parameter potential for the substrate–helium interaction. We map out in the two-parameter space the separatrix between those weak substrates that can only support a fluid monolayer from those strong substrates that support a solid. Our approach utilizes a combination of information from both variational calculations and also correlated basis function theory to examine in detail the excitation structure in the monolayer liquid as a function of film coverage and substrate potential.     

On the Existence of Supersolid 4He Monolayer Films

Extensive Monte Carlo simulations of ⁴He monolayer films adsorbed on weak substrates have been carried out, aimed at ascertaining the possible occurrence of a quasi-two-dimensional supersolid phase. Only crystalline films not registered with underlying substrates are considered. Numerical results yield strong evidence that ⁴He will not form a supersolid film on any substrate strong enough to stabilize a crystalline layer. On weaker substrates, continuous growth of a liquid film takes place.     

Quartz Oscillator Study of Monolayer 4He and 3He - 4He Mixture Films Adsorbed on Solid H2

The superfluid transition in submonolayer and monolayer ⁴ He films and ³ He - ⁴ He mixture films on solid H ₂ has been studied using a quartz crystal microbalance technique. Kosterlitz-Thouless (KT) transitions were observed in submonolayer ⁴ He films with density greater than 0.062 ± 0.002 Å ^–2 . We determine a binding energy of ⁴ He to 0.241 Å ^–2 H ₂ of –15.7 K in the. presence of 1 monolayer of ⁴ He. At several ⁴ He coverages, a range of submonolayer ³ He coverages was studied (n ₃ 0.0567 Å ^–2 ). With each increase in the ³ He coverage, the KT transition temperature decreased. For the higher coverage mixture films studied (n ₄ 0.0726 Å ^–2 ) we observed an apparent second decoupling of the film from the quartz oscillator frequency in addition to the KT transition. We have studied the. coverage dependence of this new feature.     

The Superfluid Transition and Vortex Dynamics in Submonolayer Superfluid 4He Films in Porous Glass Under Rotation

A combination of a rotating dilution refrigerator and high-Q torsional oscillator technique has been used to study dynamics of vortices in thin ⁴ He films adsorbed on the porous glass (d=1m pore size). Under rotation an additional dissipation peak with the amplitude proportional to the angular velocity is seen at the middle of the superfluid transition, on the low temperature side of the stationary peak which is present even at =0. We attribute this peak to the 3D Type vortices created in multiply connected ⁴ He film by the rotation. Peak shape of the rotation-induced dissipation could be interpreted as a freezing of the 3D vortices well below T _c     

Nuclear Spin Relaxation Characteristic of Submonolayer ^3He Films in Nanochannels

In order to obtain information on dynamics of helium films in the nondegenerate fluid region, we have performed a pulsed-NMR experiment at 3.29 MHz on $^3$ He films adsorbed in straight 2.4 nm channels of FSM silicates down to 0.54 K. In general, the spin-lattice and spin-spin relaxation times $T_1$ and $T_2$ were explained in terms of the two-dimensional Bloembergen–Purcell–Pound model for dipolar relaxation. Temperature dependences of $T_1$ in submonolayer $^3$ He films show a minimum, indicating that the dipolar-field correlation time $\tau _\mathrm {c}$ is about $\omega ^{-1}=4.8\times 10^{-8}$ s. The temperature $T_\mathrm {min}$ of the $T_1$ minimum monotonically lowers with increasing coverage, suggesting that $^3$ He adatoms become more mobile at higher coverages. The low-dimensional property of $^3$ He adatoms is observed as the separation of $T_1$ and $T_2$ above $T_\mathrm {min}$ where $\omega \tau _\mathrm {c}<1$ . On the other hand, several features specific to films in the nanochannel geometry were also found. Especially, the temperature dependence of $T_2$ becomes very small just below $T_\mathrm {min}$ and shows a shoulder at lower temperatures. This anomaly has not been observed in $^3$ He adsorbed in wider pores or on flat surfaces, so that it is considered to be characteristic of $^3$ He films confined in narrow channels with a diameter of a few nm.     

The bcc-hcp Phase Transition in 4He: Comparison with the Theory of Homogeneous Nucleation

Precise pressure experiments in supercooled bcc ⁴He are carried out under cooling along an isochore near an equilibrium line of the bcc-hcp structural phase transition. It is established that the transition from bcc to hcp occurs after significant delay which can be of a few seconds to a few hours depending on the extent of supercooling. The data obtained allow to establish of a line of maximum supercooling in the P-T phase diagram and to describe it within the framework of homogeneous nucleation theory. It is shown that the reverse hcp-bcc transition occurs without any delay, i.e., this structural phase transition is strongly asymmetric. The additional peculiarities are observed in the pressure and temperature time dependences. The possible reasons appearing of them are discussed      

Self Diffusion in Solid 4He and 3He-4He Mixtures Near the bcc-hcp Phase Transition

We report experiments on the plastic flow of solid ⁴ He and ³ He- ⁴ He mixtures of 1.4% and 2.8% near the bcc-hcp transition. Plastic flow was generated by moving a wire through a macroscopic single crystal. We found that the plastic flow rate both in pure ⁴ He and in mixture helium crystals is enhanced in vicinity of the bcc-hcp phase transition. The results are interpreted in terms of self diffusion in the solid. Values of the self diffusion coefficient D_s at the respective transition temperatures of pure ⁴ He and of the mixtures are very close, and reach that found in normal liquids. The activation energy for self diffusion in the mixtures is lower by up to 3 K than in pure ⁴ He. We suggest that similar to what is observed in bcc metals, self-diffusion in solid He takes place through phonon assisted atom-vacancy exchange. The enhancement of the diffusion near the bcc-hcp transition is a result of the softening of a short wavelength transverse phonon. The temperature dependence of the energy of the phonon calculated using our data is in accord with the Landau theory of a phase transition driven by a soft mode. Work hardening was observed in mixture crystals, but not in pure ⁴ He. This implies that ³ He impurities pin dislocation lines.     

Fast Mode in Dense Aerogel Filled with Superfluid He II and Dilute Mixture of 3He in 4He

Longitudinal sound wave propagation in 90% porous silica aerogel filled with superfluid He II and dilute mixture of ³He in ⁴He has been studied using a low frequency resonance method. The observed fast mode was identified as a mode intermediate between the sound in the aerogel matrix and first sound. It was shown that the behavior of the fast mode in dense aerogel differs from both high porosity aerogel and rigid porous medium. We discuss the obtained results within the framework of theoretical models available.      

Can He3 - He4 dilution refrigerators operate aboard spacecraft?

Cooling to temperatures in the millikelvin range could be beneficial as a means of reducing noise and increasing sensitivity of instruments in certain experiments and measurements that are being considered for future space missions. The possibility of using a He³ - He⁴ dilution refrigerator for that purpose is explored in this paper. Calculations reported here indicate that electrostriction can be substituted for gravity as a practical means of achieving phase separation with well-defined interfaces in mixing chambers of dilution refrigerators operating aboard spacecraft.     

One- and Two-Electron Bubbles in Superfluid 4He

Properties of one- and two-electron bubbles in superfluid ⁴He at 0 K were studied by density functional theory. The model allows for accurate treatment of both the electronic and liquid degrees of freedom and as such, enables accurate calculation of bubble energetics for the ground and excited electronic states. The obtained results were compared against the earlier “bubble model” calculations and the limits and accuracy of the bubble model were established. The calculations were carried out in 3-D space and the non-spherical solvation structures for the 1P and 1D excited states were calculated. The 1P state was found to be stable within the radiative lifetime and no plausible non-radiative relaxation channels were found. Finally, a coupled boson and fermion density functional theory was used to show that two-electron bubbles are unstable in both the singlet and triplet electronic states.      

Heat Capacities of Submonolayer Solid 3He Films on Graphite at Highly Frustrated Density Regime

No Heading The heat capacities (C) of submonolayer solid films adsorbed on a graphite surface are measured down to 100 K at areal densities between 6.6 and 7.5 nm^–2, where there has been no previous measurement below 2 mK. The temperature (T) variation of C shows almost single power low behavior, C T, in a wide temperature range. And the exponent is much smaller than –2, an expected exponent for localized spins in the high temperature limit. These behaviors should imply that the competition of multiple spin exchange interactions is strong at this density range.PACS numbers: 67.70.+n, 67.80.Gb     

Behavior of 4He Near T λ in Films of Infinite and Finite Lateral Extent

We report studies of a ⁴He film confined between two silicon wafers separated by 3189 Å. The film is connected to a bulk helium reservoir via small channels 100 Å high, 8 µm wide by 2000 µm long. This cell design has allowed us to study the heat capacity in a planar confinement (a film of ∞ lateral size), and the superfluid density in the connecting channels (a film of finite lateral size). This work is relevant to finite-size scaling of the specific heat for 2D confinement and it is compared with earlier data. It is also relevant to finite-size 2D behavior for the superfluid density which is related to the recent theory of Sobnack and Kusmartsev. Analysis of the data is presented as well as a discussion of future cell designs to address, in particular, the behavior of laterally confined films.     

A Faraday Magnetometer for Studies of Monolayer 3He Films on Graphite

No Heading A Faraday-type magnetometer has been developed to measure the nuclear magnetization of monolayer ³He films adsorbed on Grafoil at temperatures down to 0.1 mK under high magnetic fields up to 10 T. The magnetic force is measured capacitively by monitoring the displacement of a wire-suspended copper plate. A double gradient coil system is employed to produce opposite field gradients at two regions the same distance apart from the center. At each region, exfoliated and non-exfoliated graphite foils are put one by one to eliminate a large background signal from Grafoil and copper.PACS numbers: 67.70.+n, 75.10.Jm     

Flow of Solid 4He Near Melting

Torsional oscillator measurements on solid ⁴He have demonstrated non-classical rotational inertia (NCRI), indicative of a supersolid phase transition. Recent experiments indicate that the NCRI fraction depends on isotopic purity and perhaps on details of crystal growth and annealing, suggesting that defects may be involved. Our recent experiments have shown that solid helium does not flow in response to pressure gradients at low temperatures. Close to the melting temperature we do observe mass flow, but it decreases rapidly with temperature. For solid helium in the pores of Vycor the flow appears to be thermally activated and disappears below about half the melting temperature. Flow in bulk helium is restricted to a much narrower temperature range. Very close to melting (within 20 mK) the flow completely eliminates pressure differences in less than a minute. At slightly lower temperatures we saw flow, but significant pressure differences remained even after annealing.     

The Two-Dimensional World of 3He in 3He-4He Mixture Films

For thin films of ⁴ He at low temperatures the addition of sub-monolayer numbers of ³ He atoms results in the ³ He residing in a ground state at the free surface of the ⁴ He film. For low ³ He coverage the ³ He are a nearly ideal two-dimensional Fermi system. With increasing numbers of ³ He atoms the interactions among the ³ He increase. The results from a number of experiments that reveal the remarkable properties of this rich, interacting, two-dimensional ³ He system are described. These results include the magnetization, the energetics of the ³ He atoms in quantum states, the diffusion of the ³ He along the ⁴ He film, and the ³ He effective mass. The techniques employed include NMR, specific heat, quartz crystal microbalance and third sound. A clearer picture of the properties of the ³ He is now emerging, and preliminary numbers for the ³ He coverage dependence of the Fermi Liquid parameters are presented. Prospects for the observation of superfluidity of the ³ He in this system are discussed.     

Superfluid Phase Transition of 3He-4He Mixture Films Adsorbed on Alumina Powder

The superfluid phase transition of ³ He- ⁴ He mixture films adsorbed on 500 Å alumina powder has been studied for mixture films whose superfluid thickness is less than a monolayer. The transitions are found to be controlled by the Kosterlitz-Thouless critical line, but a strong broadening of the transition is observed as the ³ He concentration is increased. Analyzing the broadening in terms of a KT vortex-pair theory modified for the the finite powder size yields a vortex core parameter which increases nearly linearly with added ³ He. Also observed in these measurements is a temperature-dependent and ³ He-dependent depletion of the superfluid density at low temperatures, which is thought to arise from the high-frequency ripplon/third sound excitations of the film.     

Electron Conductivity on Helium Films

Electrons on liquid helium films form a two-dimensional (2D) array with a wide range of electron density. This system is also very interesting for applications in restricted geometry. The conductivity σ of the electron arrays, however, strongly depends on the thickness d of the helium films adsorbed above solid substrates. This behaviour of σ is discussed in detail for a randomly rough substrate. It turns out that for the dependence of the conductivity σ(d) there exist three regions of helium thicknesses: d>d _min , d∼d _min , and d<d _min . Here d _min is the helium film thickness which corresponds to a relatively deep minimum of the 2D conductivity. In the first interval, d>d _min , a two-fraction scenario determines the behaviour of σ(d). In the vicinity of d _min percolation phenomena develop and the conductivity exhibits different types of the so-called dip effect. For even thinner helium films, i.e., when d<d _min , an activation type of mobility is stimulated. The presented model fits quite well to existing data of ac and dc electron mobility.      

Superfluid-Normal Interface of 4He Near a Wall

We observed a profile of nonequilibrium superfluid-normal (SN) interface of ⁴He near a vertical wall. A glass, brass or copper wall was used. The SN interface was produced by cooling liquid ⁴He in a bath from the bottom, where liquid ⁴He was pumped through a flow impedance in order to cool down the liquid. Superfluid (Normal fluid) occupied the lower (upper) part of the bath. The SN interface was visualized by three methods: simple visualization, shadowgraphy and schlieren method. The interface touched a vertical glass wall at almost 90°. A large hollow was observed near a brass wall which had intermediate thermal conductivity. Downward flow was observed on a copper wall due to the very good thermal conductivity of the wall. Various types of interface profile were observed depending on the thermal conductivity of the wall used.      

密封器件压氦和预充氦细检漏过程中环境氦分压的影响

分别推导和分析了环境大气氦分压对压氦法的影响,地球干洁大气氦分压对预充氦法和预充氦密封器件压氦法复检的影响.证明对于压氦法,不需要考虑地球干洁大气氦分压的影响.但是如果候检室环境大气氦分压显著升高,对于内腔有效容积大,且等效标准漏率小的密封器件,会加大测量漏率值,所以压氦后,被检器件应尽快离开压氦设备所在的房间.对于预充氦法,地球干洁大气氦分压会使测量漏率通过极大值后出现极小值,且当候检时间与内腔有效容积之比大于100 h/cm3时,极小值点的气流仍处于分子流状态,不能靠粗检鉴别,所以需压氦法复检加粗检,才能防止漏检.另外,地球干洁大气氦分压会使预充氦法候检时间的第一特征点变大,从而扩大了需压氦法复检加粗检的范围,但第二特征点不变,也不影响压氦法复检加粗检的结果.