Continuous adsorption refrigerator for producing temperatures below 1 K

This paper describes an evaporation refrigerator in which the pumping of the vapour above liquid He³ or He⁴ was accomplished with two alternately operating adsorption pumps. In this way temperature can be maintained constant with an accuracy to ～0.003 K for a long time; ～ 10 l helium vapour being used. The refrigeration capacity of the apparatus has been determined.The characteristics of the dilution refrigerator (namely, the rate of He³ circulation, the starting and operating temperatures) have been calculated for He³ circulation using the adsorption pumps and pumping line.This system is shown to be successful for producing very low temperatures at an He³ circulation rate of ～ 10^?5?10^?4 mole s^?1.     

Low-temperature heat capacity of helium films adsorbed on copper

Heat capacity measurements are described for films of pure⁴ He and a 12%He ³ mixture ofHe ³ andHe ⁴He adsorbed on copper, for 0.1<T<1.2 K.He ⁴ heat capacity isotherms show a step-type behavior as a function of coverage for the first two layers, while the mixture isotherms only show the first step. A comparison is made with recent multilayer data for helium films on Vycor published by other authors.Work supported by the National Science Foundation.     

Diffusion Monte Carlo Study of the Small Mixed 3He-4He Clusters

The ground state properties of helium mixed clusters ³He_2,3,4 ⁴He₂, ³He_2,3,4 ⁴He₃, ³He_2,3,4 ⁴He₄, ³He_2,3 ⁴He₅, consisting of up to eight helium atoms are studied using variational and diffusion Monte Carlo calculations. For clusters with three and four ³He atoms released-node diffusion Monte Carlo method is used. Our calculations show that, within errorbars, clusters ³He_3,4 ⁴He₂ have the same binding energy as the cluster ³He₂ ⁴He₂, and that ³He₃ ⁴He₃ has the same binding energy as ³He₂ ⁴He₃. The clusters ³He_3,4 ⁴He₂ and ³He₃ ⁴He₃ are in states in which one or two ³He atoms are far away from the rest of the system. Other considered clusters are bound. In particular, we have shown the stability of the cluster ³He₄ ⁴He₃, which was previously considered unstable. The calculations are performed using several different interatomic potentials and the conclusions concerning stability are insensitive to the particular form of the interaction potential. We compare our results with the recent experiment and other theoretical calculations.     

Structure and Energetics of Helium Adsorption on Nanosurfaces

The ground and excited state properties of small helium clusters, ⁴He _N , containing nanoscale (～3–10 Å) planar aromatic molecules have been studied with quantum Monte Carlo methods. Ground state structures and energies are obtained from importance-sampled, rigid-body diffusion Monte Carlo. Excited state energies due to helium vibrational motion are evaluated using the projection operator, imaginary time spectral evolution technique. We examine the adsorption of N helium atoms (N≤24) on a series of planar aromatic molecules (benzene, naphthalene, anthracene, tetracene, phthalocyanine). The first layer of helium atoms is well-localized on the molecule surface, and we find well-defined localized excitations due to in-plane vibrational motion of helium on the molecule surface. We discuss the implications of these confined excitations for the molecule spectroscopy.     

A new superleak to remove He for UCN experiments

The production of superleaks to remove He³ in helium for UCN experiments is described. Using one of these superleaks, He³/He⁴ ratio was found to be less than 3 × 10⁻⁹ as indicated by the UCN storage lifetime.     

Quasithermodynamic theory of He3 and He4 solutions

The conventional theory of nonideal solutions is modified on the basis of Gorter's hypothesis, and various properties of He³ and He⁴ solutions are investigated. The variation of -temperature with He³ concentration is explained adequately right up to the phase separation region. A comparison between the theoretical and experimental results for excess chemical potentials, excess entropy, and excess enthalpy is then carried out. In view of the recognized complex behavior of the solution in the region 1 to 2° K, the observed satisfactory agreement between theory and experiment may be regarded as a strong support for the assumptions made in our work.     

Anomalies in adsorbed films of He4 above the lambda temperature

Previous measurements of adsorption isotherms of He⁴ on an argon-coated copper substrate have suggested that a phase transition may take place in adsorbed He⁴ films at temperatures above the normal bulk liquid lambda point. We have made high-precision adsorption measurements on a substrate of porous Vycor glass (mean pore diameter 62.4 Å, surface area 580 m²) and failed to find any evidence of anomalous adsorption. Reasons for the conflict in results are discussed.     

Adsorption of 4He N and 4He N 3He Clusters on Cesium

The ground-state properties of helium clusters ⁴He _N and ⁴He _N ³He, for N≤ 40, adsorbed on the surface of cesium are studied using variational and diffusion Monte Carlo calculations. Binding properties are determined using two different Cs–He interaction potentials. For the smallest clusters, self-binding on a Cs surface is stronger than in two or three dimensions. For N > 10 self-binding in three dimensions is stronger than on Cs for both types of Cs–He interaction potential considered. The obtained binding energies and structure are compared to results of recent density functional calculations. The emergence of edge states of ³He atom, localized along the contact line of ⁴He cluster with a cesium surface, is studied. First indication that ³He atom prefers to be close to the contact line appears already for the ⁴He₃ ³He cluster.     

A cryostat for investigating inelastic scattering of neutrons on liquid He4 between 4.2 and 0.5 K

A cryostat is described which is designed for the research of inelastic scattering of neutrons on a large volume (3.5 l) of He⁴ between 4.2 and 0.5 K. Temperatures lower than 1.3 K are produced by means of pumping out vapour above liquid He³ (～ 330 cm³) with an adsorption pump (1 kg of coal CKT-2) located in the device itself. A minimum temperature is held during 140 h. The essential time for the cryostat start is about 10 h. The cryostat was used for the measurement of temperature dependence of the Bose-condensate density in He⁴.     

Rough model of a quantum liquid

As applied to condensed helium and hydrogen, a theory of quantum effects is suggested, one that is insensitive to the type of statistics of particles—the theory of quantum nondegenerate liquids. Evidently such a theory assumes that the mass of the atom and its polarization are small. For a zero-approximation, a system of hard spheres is chosen, and the attractive forces and the softness of the atom are considered to be small effects. The correlations between thermodynamic and kinetic characteristics of liquid He³ and He⁴ are obtained. On the basis of the experimental data for He⁴, and an isotropic law of the corresponding states for quantum systems with a large value of the de Boer parameter, all thermodynamic functions of liquid He³ are defined and tabulated in the regions of temperatures and molar volumes where they have not yet been measured. A rough model of a quantum liquid is suggested, based on the assumption of a dominating contribution by the diffusion excitations to the free energy of a dense condensed medium. The equation of state of the quantum liquid is derived. On the basis of the experimental data for He⁴, the equation of state of a quantum liquid consisting of hard spheres is compared with the similar equations obtained by a numeric computer simulation. The energy of the ground state of spin-oriented condensed hydrogen isotopes is defined.     

Active roles of helium in the growth of hydrogenated microcrystalline silicon germanium thin films

A combination of hydrogen and helium dilutions was introduced when the microcrystalline silicon germanium (μc-SiGe:H) thin films were prepared by very high frequency plasma enhanced chemical vapor deposition on a low-temperature substrate. An optimum helium flow rate was found to achieve the structural uniformity in the growth direction, while Ge content was found to nearly keep constant with varying flow rates of helium. An abundance of atomic H was detected in plasma due to the attendance of helium and no obvious photosensitivity deterioration was observed on the thin film with a high crystalline volume fraction. The active roles of helium were identified by analyzing the mechanism in the plasma, where both metastable He_m^? and He⁺ can accelerate the diffusion of Ge related radicals and passivation of the dangling bonds on the growth surface, respectively. These phenomena have been revealed by experimental results. Therefore, a combination of hydrogen and helium dilutions can improve the structure of the μc-SiGe:H thin films with little degradation of photo-electronic properties.     

Adsorption of3He and4He on copper and on argon-coated copper below 20 K

Measurements have been made of adsorption isotherms of ³ He and of ⁴ He on copper and on a monolayer of argon deposited on copper in the temperature range 6.18–18.55 K and in the pressure range 0.25 to 75 Torr. From these many isotherms, calculations have been made of the isosteric heat of adsorptionQ _st/R. In the limit of zero coverage on the argon monolayerQ _st/R=76±2 K for ³ He and 76±2 K for ⁴ He. For adsorption on the bare copper,Q _st/R is difficult to extrapolate to zero coverage, but it probably lies (for both ³ He and ⁴ He) between 135 and 165 K. At theoretical monolayer helium coverage,Q _st/R=44±2 K for ³ He on the argon monolayer and 47±2 K for ⁴ He. At theoretical monolayer helium coverage on the bare copper,Q _st/R=61±4 K for ³ He and 77±5 K for ⁴ He. The results are compared with theoretical evaluations for helium adsorbed on an argon monolayer and with some previous experimental data, and the agreement is found to be fair. All the data are summarized in tables. Finally, a review is given of evaluations, including those from this work, of the monolayer capacity of ³ He and ⁴ He on the substrates studied.Work supported by a contract with the Department of Defense (Themis Program) and with the Office of Naval Research and by a Grant from the National Science Foundation.     

Hybrid Density Functional for Liquid 4He

No Heading An alternative density functional for studying adsorption of ⁴He onto solid surfaces is suggested, where the helium-helium interaction is screened at small distances by the two-body distribution function g(r) and the gradient-gradient term of the Orsay-Trento proposal is replaced by a contribution written in terms of the gradient of g(r). Results for helium adsorption onto planar Na substrates are reported.PACS numbers: 67.40.Db, 67.70.+n, 68.10.–m, 68.15.+e     

A modified blister test to study the adhesion of thin coatings based on local helium ion implantation

A modified blister test has been developed based on helium ion implantation into selected areas of the metal substrate prior to the coating deposition. After a post-deposition thermal annealing, blisters are formed by agglomeration of the implanted gas at the ceramic–metal interface. This method can be used to control the pressure in the blister which eventually may lead to delamination at the periphery of the blister. A microsieve with a regular array of circular holes is used during the implantation to assure the initial blister size. Two different microsieves were employed in this work, with pore diameters of 1.5 and 4.5 μm, respectively. The distance between the centres of neighbour pores is twice the pore diameter. Scanning Electron Microscopy (SEM) and Confocal Scanning Optical Microscopy (CSOM) observations allowed the determination of the blistering parameters such as the radius, the height and the blister volume. From the gas content and these parameters, the work of adhesion or energy release rate can be obtained.In this work, we present the first results of this blister test applied to W–C:H films and multilayers of Ti and Al deposited by Physical Vapour Deposition on polycrystalline copper substrates. The copper substrates were implanted with 34 keV He⁺ ions up to fluences of 3 and 5×10¹⁶ cm⁻² before the deposition of the coatings and annealed afterwards in vacuum at temperatures from 773 to 1073 K for 30 min. Delamination of the Ti/Al multilayer coatings was already detected after annealing at 873 K with an energy release rate estimated to be 0.5 J m⁻² at a typical helium pressure of 10⁷ Pa. No delamination but only helium swelling was observed for W–C:H coatings annealed at 1073 K. Results of experiments on uncoated copper samples are also shown in order to explain the mechanism of helium bubble growth and helium release that causes the creation of the blisters.     

Compact dilution refrigerator with a cryogenic circulation cycle of He3

A He³-He⁴ dilution refrigerator has been constructed which circulates He³ by using two alternately operating adsorption pumps which are in the same cryostat as the dilution refrigerator. The scheme described here organised the low temperature circulation cycle such that the construction and cryostat communications were essentially simplified, the dimensions of the arrangement were decreased which resulted in a circulation velocity of 10^?4 mole s^?1, which was sufficient to obtain a temperature in the order of 10 mK.     

Theory ofT 2 heat capacities of monolayer helium films

Heat capacities of adsorbed monolayers of He³and He⁴indicate that the dominant thermal excitations at low temperatures are phonons rather than single-particle modes when the films are near maximum density. A recent theoretical study of noninteracting particles adsorbed on a crystalline substrate yielded a two-dimensional band structure, and explored the thermodynamic properties of the single-particle modes. The theory is now extended to include the influence of hard-core interactions between the particles moving in the lowest band. Resembling the high-density approximations for liquid and solid helium, the hard-core interactions are shown to give rise to a collective zero-point kinetic energy from which a two-dimensional pressure and a compressibility are derived. The corresponding velocity of sound and phonon heat capacity are calculated in the weak-binding and tight-binding approximations. Comparison with experiment indicates that the weak-binding approximation is more appropriate and that it yields effective mass ratiosm*/m1.7 for the two isotopes at the experimental film densities. A novel feature of the tight-binding calculation that may be applicable to other experimental systems is the prediction of phase condensation for adsorbed gases of relatively small molecular diameter.Research supported by the National Science Foundation.     

In Situ Mitigation of Subsurface and Peripheral Focused Ion Beam Damage via Simultaneous Pulsed Laser Heating

Focused helium and neon ion (He⁺/Ne⁺) beam processing has recently been used to push resolution limits of direct‐write nanoscale synthesis. The ubiquitous insertion of focused He⁺/Ne⁺ beams as the next‐generation nanofabrication tool‐of‐choice is currently limited by deleterious subsurface and peripheral damage induced by the energetic ions in the underlying substrate. The in situ mitigation of subsurface damage induced by He⁺/Ne⁺ ion exposures in silicon via a synchronized infrared pulsed laser‐assisted process is demonstrated. The pulsed laser assist provides highly localized in situ photothermal energy which reduces the implantation and defect concentration by greater than 90%. The laser‐assisted exposure process is also shown to reduce peripheral defects in He⁺ patterned graphene, which makes this process an attractive candidate for direct‐write patterning of 2D materials. These results offer a necessary solution for the applicability of high‐resolution direct‐write nanoscale material processing via focused ion beams.     

Bulk fluid helium: A review for users

This review of helium is a collection of graphs together with some discussion of topics on which, it seemed to the author, something useful could be said. The resulting article, intended primarily for users rather than pure researchers, gives pictorial representations and/or references to recent tabulations of all macroscopic bulk properties. There is stress on some areas (flow and heat transfer, behaviour near the lambda-line and critical point) and total exclusion of others (He³ impurity, films, adsorption). Naturally there has been bias in the selection of topics but the intention has been to draw attention to recent additions to our technical knowledge and to avoid repeating undue quantities of familiar material.     

Anomalous vapour pressure over liquid D-T

The presence of a few tenths mol % helium in hydrogen causes anomalous vapour pressures as high as 10 kPa. This effect is caused by the combination of a small cold cell, a single, long fill tube, and the insolubility of helium in liquid hydrogen. This effect is important in the handling of deuterium-tritium, as radioactive decay produces He³.     

An apparatus for preparing isotopically pure He4

A cryostat has been constructed for extracting pure He⁴ from He II of natural isotopic composition by means of a heat flush technique. It is shown that the method is in principle capable of yielding He⁴ which is entirely devoid of He³ isotopic impurities. A secondary heat flush, operated in conjunction with a conical heat exchanger of novel design, was used to place an experimental lower bound of 2 × 10¹⁵ on the $\text{He}{}^4\text{He}{}^3$ ratio of the product, a standard sufficient for all present or projected applications requiring isotopically pure He⁴. The apparatus produces 2 000 1 (at standard temperature and pressure) of He⁴ per thermal cycle. The design of future purifiers is discussed.