Adsorption of3He on cesium surfaces

Vapor pressure isotherms of³He on Cs are reported which show that, in contrast to⁴He,³He weis Cs for 1.26K<T<2K. Although the isotherms are distinctly different from those taken on strong binding substrates, we have observed no evidence for a first order prewetting transition. The data suggest that the prewetting critical point is substantially below T=1.26K.     

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.     

Prediction of reentrant wetting of3He-4He mixtures on cesium

We examine the effect of ³ He impurities on the wetting behavior of⁴He on cesium, predicting a phase diagram which includes reentrant wetting transitions. This phase diagram is shown to be very sensitive to effects such as a theoretically predicted bound state of³He at the liquid-cesium interface, and the contact angle may be sensitive to interesting temperature dependences of the helium-cesium surface tension resulting from surface rotons or Rayleigh waves.     

Investigation of adsorption and wetting of3He on cesium and cesiated glass

Experiments have been carried out to investigate the binding of ³ He on cesium substrates, using optical pumping to spin-polarize the atoms. The behaviour of ³ He on the walls at low temperature can be analyzed through the evolution of the nuclear magnetization of the sample. Preliminary results are presented, including: (i) adsorption studies of gaseous ³ He on cesiated glass; (ii) magnetic relaxation time of polarized liquid ³ He on cesium and cesiated glass; (iii) evidence for wetting of liquid ³ He on cesium.     

Separation of 3He from 3He-4He Mixture by means of Adsorption

No Heading A simple technique for purifying ³He from a ⁴He admixture is presented. The technique is based on different adsorption energies of ³He and ⁴He.PACS numbers: 81.20.Ym, 67.60.–g.     

Adsorption Structure of 3He Monolayer Solid Film on Grafoil

The adsorption structure of ³ He submonolayer solid film on a, Grafoil Surface is considered. The adsorption energies of some supposed structures are calculated by the path integral Monte Carlo simulations. According to these calculations the evolution of the adsorption structure is discussed. Two structural, phase transitions are predicted there. The phase boundaries coincide with the specific features in the results of heat-capacity measurements. The commensurate - incommensurate phase transition seems responsible for the sudden ferromagnetic peak of the exchange frequency.     

Competitive Adsorption of 3He and 4He to Carbon Nanotube Bundles

No Heading We have studied the competitive adsorption of the two helium isotopes to single-walled carbon nanotube bundles. It is observed that the presence of ⁴He suppresses subsequent ³He adsorption while the presence of ³He has little influence on the subsequent adsorption of ⁴He. These observations are generally consistent with expectations based on the difference in the masses of the isotopes.PACS numbers: 68.35.Md, 68.43.Fg, 81.07.De     

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.     

Adsorption pumping for obtaining ULT in 3He Cryostats and 3He-4He Dilution Refrigerators

Novel designs of ³ He Cryostats and Dilution Refrigerators (DR), based on adsorption pumping of ³ He gas, are described for obtaining temperatures well below 0.3K. Using a new technique, extremely low temperature ³ He refrigerators have been tested to T<235mK. The same technique enables the manufacture of a compact travelling ³ He system, that can move up to a distance of about 0.5m. The ability to travel from warm to cold zones can be important for sample preparation in ultra high vacuum and to carry out STM measurements. Also described is a continuously operating sorption pumped DR, that can be installed inside a transport dewar with a neck diameter of only 51mm. The cooling time of the mixing chamber from 4.2K down to 50mK is 2 hours. Removal of the insert from the cryostat requires approximately 30 minutes. The cooling power of the system is 40 microwatts at 100mK. With two miniature silver heat exchangers the mixing chamber temperature cools to a continuous temperature of 15mK. A key distinguishing feature of this DR is the absence of a traditional gas handling system and sealed helium-3 rotary pump. This makes it easy for transportation and quick commissioning at a new site within a few hours.     

Novel behavior of3He-4He mixture films on a cesium coated substrate at low temperatures

We report quartz crystal measurements of the adsorption of³He —⁴He mixture films to gold and cesium substrates as a function of temperature and concentration. Measurements for higher concentrations than reported earlier show steps in the adsorption at low temperatures. Speculations as to the origin of these steps are presented.     

Influence of Adsorption Potential on 3He Fluid States Formed on 4He-Preplated Substrates

No Heading From heat capacity measurements for ³He adsorbed on ⁴He-preplated mesopore in substrates, FSM-16, HMM-2, and Hectorite, we found fluid states of the ³He adatoms. At very dilute ³He densities, however, it strongly depends on the substrate whether the adatoms are in a localized or a fluid state. The molar heat capacity of a two-dimensional gas or a degenerate Fermi fluid appears above densities n_c = 0, 0, and 2 mol/m² for FSM-16, HMM-2, and Hectorite, respectively. The critical coverages n_c are extremely small or zero for the former two SiO₂-based substrates. On the other hand, n_c for Hectorite is large. Since Hectorite has strong adsorption sites due to the electric-dipole field gradients around ions located on the substrate surface, these potentials are likely to affect the localization of the dilute ³He adsorbed even on the ⁴He film.PACS numbers: 67.55.Cx, 67.70+n, 71.30+h     

Adsorption of 4He on neon-coated exfoliated graphite

Adsorption isotherms of ⁴He on Grafoil coated with a monolayer of neon were made for the temperature range of 2–10 K and in the pressure range of 0.10–15.00 Torr, using a standard volumetric method. From these isotherms the isosteric heats of adsorption were calculated as a function of coverage. The binding energy of ⁴He on Ne was obtained from the isosteric heat for V 0, yielding a value of 39.4 K. The differential molar entropy and the internal molar energy were calculated as a function of coverage. A comparison of monolayer coverage obtained by analyzing different procedures is presented. Whenever possible the results are compared with theoretical evaluations and previous experimental data.Work supported by FINEP, CNPq, and CEPG. Part of the Cooperative Program UFRJ/Stevens Institute of Technology, sponsored by CNPq/NSF International Program.     

Adsorption of sodium and cesium on graphite: Work function calculations

Changes in the charge of adatoms and the substrate work function have been calculated for the adsorption layers of sodium and cesium on graphite using the Anderson-Newns model. The results of calculations are in good agreement with the available experimental data. It is established that there is electron transfer from Na and Cs adatoms to the graphite substrate, which leads to a decrease in the substrate work function.     

Evidence for the absence of 4He adsorption on cesium coated graphite surface

In agreement with recent theoretical predictions, volumetric vapor pressure isotherm studies indicate a substantial reduction in the adsorption of ⁴He at temperature near and below 2K when the graphite surface is coated with Cs.     

Adsorption of 4He on a Single C60

The adsorption of ⁴He inside and outside a single fullerene C₆₀ is studied. A physisorption potential is proposed. The energetics and structural features of C₆₀-⁴He _N clusters are investigated within the framework of nonlocal density functionals. Particular attention is paid to the growth of the highly pronounced layered density profile. The evolution towards bulk liquid and surface thickness at the free interface are discussed.     

Imaging the edge of a 4He prewetting film on a cesium metal surface

The edge of a metastable ⁴ He prewetting film on a cesium metal surface is investigated using polarization interference microscopy. This technique images the local gradient of the coverage through its optical thickness. The cesium metal is a film evaporated on a polished copper substrate. A liquid helium film is deposited on the surface through raising and lowering the bulk liquid surface. Its edge is clearly observed for temperatures below 1.6 K. The apparent optical thicknesses of the films, larger than what is expected for a normal saturated film, remain to be explained.     

4He Adsorption and Superfluid Transition on C60

We present adsorption isotherm data of⁴He on C₆₀ for 1.50 K<T<1.68 K determined by measurements of the frequency of quartz crystal microbalances (QCM’s) coated with C₆₀ films. We find a Kosterlitz-Thouless transition with a jump in the areal superfluid density close to predictions. By comparing the adsorbed⁴He on two QCM’s we derive an upper limit for the amount of⁴He intercalated into C₆₀ at low temperature of 0.05⁴He atoms per C₆₀ molecule. The low coverage portion of the adsorption data shows an apparent excess adsorption of⁴He onto the C₆₀.     

Effects of 4He impurities on 3He nuclear spin-lattice relaxation in hcp solid 3He

Nuclear spin-lattice relaxation properties of hcp solid ³He with ⁴He impurities have been studied. At temperatures below the exchange plateau region, three kinds of relaxation time were observed. To analyze the data, a phenomenological four-bath model was developed, the four baths being the Zeeman bath, the phonon bath, an X-bath, and a Y-bath. The X-bath consists of the exchange bath and a part of the ⁴He-⁴He elastic interaction bath. The Y-bath is the main part of the elastic interaction bath. We measured the concentration dependence of the energy constants of all the baths, as well as the temperature and concentration dependences of the three kinds of relaxation time. The relaxation behavior corresponding to the process between the X-bath and the Y-bath could not be expressed by a single exponential function of time and the relaxation rate was strongly dependent on the concentration. This process may be related to the internal thermal equilibrium process within the elastic interaction bath. The impurity-dependent relaxation time between the X-bath and the phonon bath had a temperature dependence of T ^–n with n = 7.4±0.3.     

3He Spin Diffusion Measurements for 3He - 4He Mixture Films on Nuclepore

Preliminary ³ He spin diffusion results for ³ He - ⁴ He mixture films on Nuclepore are presented as a function of ³ He coverage for 0.1 T 0.3 K. We report measurements for a constant ⁴ He coverage of 4.41 bulk density atomic layers with ³ He coverages between 0.3 and 2.1 layers. The diffusion is observed to decrease with increasing amounts of ³ He, and some structure as a function of temperature is present.     

NMR Studies on 3He-3He and 3He-4He tunneling motions in solid 3He-4He mixtures

Helium-3 nuclear spin relaxation times T ₁, T ₂, and T ₁have been measured for ³He-⁴He solid mixtures at the exchange plateau region (~0.5K). The ³He concentrations X ₃of the samples were 7.2, 2.9, 1.8, 1.4, 0.67, 0.65, and 0.22%, and their molar volumes varied between 19.9 and 20.9cm³/mole in hcp phase. The spectral density function J() for dipolar field fluctuations was determined in the low-frequency branch from T ₁measurements and in the high-frequency branch from conventional T ₁measurements. It was found that J() is given by J() = cJ()|_3–4 + (1–c)J()|_3–3, where J()|_3–4 is the spectral density function due to the ³He-⁴He tunneling motions, and J()|_3–3 is that due to the ³He-³He tunneling motions. Using the Torrey theory, the correlation frequency of the ³He-⁴He tunneling motions was evaluated from T ₁data, and was found to be in good agreement with Landesman 's theory.Supported in part by the Japan Society for the Promotion of Science through a grant to Y.H.