Superfluidity of solid4He

Using a procedure suggested by Leggett, an upper bound to the superfluid fraction in ground state solid ⁴ He slightly above the melting density is obtained numerically. The value obtained is 0.3±0.1. To judge the usefulness of this upper bound, we examine the conditions under which a symmetrized product of single-particle functions times a Jastrow function exhibits ODLRO, a necessary and sufficient condition for superfluid flow. It is found that ifU _ij (U _ij=? φ _i (x)φ _j (x) dx, and φ _i (x) is a single-particle wave function centered on the pointi) satisfies σ′_i U _ij>x, wherex varies from unity for long rangeU _ij (i.e.,U _ij decreases slowly enough asR _i?R_j increases) to a value of 12/7 for nearest-neighbor overlap only in the hcp lattice, then there is ODLRO, but not otherwise. Therefore, if the accepted single-particle functions are the true ones, then there is no ODLRO in solid ⁴ He, since the overlap is too small. We have explored the possibility of adding a flat tail, of magnitude λ′(VN)^?1/2 to the accepted single-particle functions. It is shown that if λ → 1 [λ ² =(λ′) ² +2(vNV ^?1)^1/2, andv=(? φ_i(x)dx)²], the system wave function becomes a pure Jastrow function, whereas if λ²?1??2×10 ^?1 , we have in effect the case where λ′=0; furthermore, there is ODLRO if λ ² ?1～?2×10 ^?1 . It is also concluded that the superfluid fraction upper bound of 0.3±0.1 obtained here as well as one suggested by Leggett are not very useful. We have not attempted to establish if there is some value of λ satisfying the above inequality such that the ground-state energy is lower than the value it takes for λ′=0.     

Towards Superfluidity of 3He Diluted by 4He

Search for the superfluid state of dilute ³He dissolved to ⁴He is one of the major remaining problems of low temperature physics. We describe our two experiments designed to pursue the lowest achieved temperature in such mixtures essentially below the values reported before.     

Observation of 4He Superfluidity in 1.8 nm-Pores

No Heading Superfluid properties of ⁴He adsorbed in uniform straight pore 1.8 nm in diameter were studied using a torsional oscillator. In the pore, the first one or two layers of adsorbed. ⁴He are solid, therefore the pore diameter is effectively reduced to about 1.1 or 0.4 nm. In order to investigate whether ⁴He becomes superfluid in such a narrow pore, we performed the oscillator experiments for two cases: ⁴He is adsorbed (1) on the bare substrate and (2) on the pore completely filled with N₂ atoms. In the latter case, only superfluid film coating the surface of the substrate grain can be detected. Compared with this case, an additional superfluid signal originating from ⁴He in the pore is observed for the bare substrate. This strongly suggests that ⁴He in the pore is superfluid.PACS numbers: 67.40.–w, 67.70.+n     

Superfluidity of4He films adsorbed on hydrogen plated graphite

The superfluidity of⁴He on graphite with various preplatings of HD is under investigation using a torsional oscillator. Results for a bilayer of HD follow a similar pattern to those obtained earlier on pure⁴He films and are consistent with third sound measurements on graphite plated with a bilayer of hydrogen. In our case the first⁴He layer solidifies, consistent with behaviour observed in NMR studies of³He on a similarly prepared substrate. At a coverage of 11nm^–2 there is the onset of a region in which the transition temperature increases rapidly with coverage and the transition itself becomes sharper. At certain coverages pronounced signatures have been observed in the period shift and dissipation, which may indicate a further phase transition in the film at lower temperatures.     

Slippage of 4He Films Adsorbed on Grafoil

We applied the quartz-crystal microbalance (QCM) technique to ⁴He films adsorbed on grafoil. It was found that both of nonsuperfluid ⁴He films and the inert layers underneath the superfluid film slip under a certain condition at low temperatures, and that the temperature at which these films start to slip depends on the ⁴He areal density. In addition, this slippage also depended strongly on the amplitude of the quartz crystal.     

3He Impurities in 4He Systems Adsorbed on Curved Substrates

It is shown that ³He impurities in sufficiently large ⁴He systems adsorbed onto substrates with curved geometries form surface bound states, analogous to the Andreev state on a planar liquid--vapor interface. We report the analysis performed for superfluid ⁴He adsorbed on the external surface of the nano-fullerene C₆₀ and on cylindrical nano-wires of Au. It is found that a single ³He impurity diluted into such adsorbed structures behaves as on films on planar substrates and as on pure ⁴He clusters.     

Local Superfluidity in 4He and para-H2 Clusters

No Heading We present a new definition of local superfluidity around impurities in quantum fluids that provides a consistent analysis of the response of the inhomogeneous fluid density to the impurity rotations. This definition is based on the local decomposition of the moments of inertia of the fluid, which can be estimated from the projected area of exchange-coupled Feynman paths in path integral Monte Carlo calculations. Application to helium droplets doped with a planar phthalocyanine molecule shows that the first solvation layers parallel to the molecular plane consist of localized helium atoms that are totally inert to superfluid response, while the second solvation layers and capping regions at the end of the molecule exhibit partial and anisotropic superfluidity. Application to weakly bound complexes of the linear OCS molecule with para-hydrogen molecules shows evidence for the existence of molecular supersolids. We find that five H₂ molecules constitute a single ring around OCS which possesses both a solid-like pair correlation function and a complete superfluid response to rotation around the molecular axis below T ~ 1 K.PACS numbers: 36.40.–c, 36.40.Mr, 67.40.–w. 67.40. Yv     

Superfluidity of Pure 3He and Mixtures of 3He and 4He in Aerogel

We describe new torsional oscillator experiments on ³ He confined in 98.2% open aerogel. In one, we monitored the superfluid fraction of pure ³ He at T << T _c while we gradually changed the sample pressure. The resulting change in density alters ₀ of the superfluid ³ He relative to the distribution of the length scales (correlations) of silica in the aerogel. We observed a T = 0 normal-to-super fluid transition at a pressure of about 6.5 bar, in marked contrast to the bulk where liquid ³ He is superfluid at all pressures. In the second experiment, we measured the temperature dependence of the ³ He _s at a pressure of 21.6 bar with different amounts of ⁴ He present in the cell. Adding 2-3% ⁴ He slightly increases both T _c and _s . We found that for ⁴ He concentrations between 2% and 34%, the ³ He T _c increases by a very small amount. However, _s , which for pure ³ He in aerogel at 0.5T _c is no more than 11%, falls by another factor of 7. This behavior (constant T _c , reduced _s ) is similar to that observed in granular superconducting films where the long-range order is controlled by phase coherence between adjacent grains.     

活性炭纤维吸附氙气机理的研究

研究了不同基体的活性炭纤维对氙气的吸附性能,同时对活性炭纤维结构修饰后的性能进行研究.结果表明活性炭纤维对氙气的吸附量不是随表面积增大而增加,修饰后的活性炭纤维的微孔尺寸变窄,对氙气的吸附量有所增加,吸附前后活性炭纤维的结构未发生变化.     

He Atoms Adsorbed in Carbon Nanotubes and Nanotube Bundles

We calculate the energetics of ⁴He and ³He atoms in the presence of one, two, and three carbon single-walled nanotubes (SWNTs). The spectrum is obtained for three characteristic sites: inside a tube in the presence of other tubes, within an interstitial channel, and in a groove. Calculations have been performed using both an axially symmetric averaged helium-carbon potential and for a potential constructed as a sum of individual carbon-helium interactions. Thermodynamic properties of helium atoms adsorbed in carbon nanonotube bundles are then studied using the single-particle spectrum. A range of one-dimensional (1D) to two-dimensional (2D) behavior is observed.     

Slippage of 4He Films and Precursor Phenomenon of Superfluidity

We have carried out quartz crystal microbalance (QCM) experiments for ⁴He films on an exfoliated single-crystalline graphite using a 32 kHz tuning fork, and have measured the temperature dependence of the resonance frequency and the Q value for various areal densities and oscillation amplitudes. Comparing with the previous experiments for Grafoil, the decoupling of the films due to the slippage or the superfluidity was larger than that of Grafoil, and the competition between the slippage and the superfluidity was observed in three-atom thick films. Furthermore, it was found that the slippage is suppressed gradually at higher temperature than the superfluid onset T _c , and that the relaxation time decreases at low temperatures while it obeys the Arrhenius law at high temperatures. These results suggest a precursor to the superfluidity of ⁴He films.     

Variational Monte Carlo Calculations of 4He Adsorbed on Graphite

We present the results of variational Monte Carlo calculations of the solid ⁴ He monolayer adsorbed on a corrugated graphite substrate with the shadow wave function technique. We find that the solid monolayer at the commensurate density is tightly bound to the substrate and that the presence of corrugation localizes the Helium atoms in the direction parallel to the substrate around the sites of a triangular lattice where 1/3 of the adsorption sites of graphite are occupied. We have calculated the energy per particle, the density profile of the monolayer and the static structure factor. We have also studied the system in the presence of a defect (vacancy or interstitial) and we report the results for the one-body density matrix.     

Interplay of Non-linear Elasticity and Dislocation-Induced Superfluidity in Solid 4He

The mechanism of the roughening induced partial depinning of gliding dislocations from ³He impurities is proposed as an alternative to the standard “boiling off”. We give a strong argument that ³He remains bound to dislocations even at large temperatures due to very long equilibration times. A scenario leading to the similarity between elastic and superfluid responses of solid ⁴He is also discussed. Its main ingredient is a strong suppression of the superfluidity along dislocation cores by dislocation kinks (D. Aleinikava, et. al., arXiv:0812.0983, 2008). These kinks, on one hand, determine the temperature and ³He dependencies of the generalized shear modulus and, on the other, control the superfluid response. Several proposals for theoretical and experimental studies of solid ⁴He are suggested.     

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.     

Observation of Superfluid 4He Adsorbed in One-Dimensional Mesopores

To study one-dimensional (1D) quantum liquid of ⁴ He, we measured the heat capacity and performed a torsional oscillator experiment for ⁴ He adsorbed on a new mesoporous substrate whose adsorption area consists of walls of straight one-dimensional 18Å diameter tunnels. The presence of adsorbed quantum liquid was examined by the isotope effect on the heat capacity for ³ He and ⁴ He adatoms. Above a coverage n _o , the heat capacity isotherms are completely different because of the Fermi and the Bose fluids, respectively. In the torsional oscillator experiment we observed superfluid ⁴ He above n _o . The fraction of the superfluid decoupled from the motion of the substrate is 0.13, which is the same order as 0.18 for packed Pt fine powder and 0.24 for 80 Å-porous glasses. The result indicates that the superfluid state exists in the one-dimensional ⁴ He adatoms formed in the 18 Å diameter pores.     

A New Approach to Bose Condensation and Superfluidity in 4He

An investigation is made of the implications that the presence of a Bose condensate (BC) has for the form of the many particle Schröedinger wavefunction. It is shown that many particle wavefunctions of states which contribute to the BC, contain long range structure in the position space of each particle. It follows from the requirement that the wavefunction is single valued that, in the presence of a BC, the angular momentum of each particle must be quantised over macroscopic length scales. The paper thus provides a new and simple proof from first principles, that Bose condensation implies macroscopic quantum behaviour. It is shown that this behaviour can be described in terms of the occupation by each particle of the same single particle-like macroscopic wavefunction. The structure in position space of this wavefunction is investigated, using a well known model of the many particle wavefunction for the ground state of⁴He. The model predicts that the probability density of each particle is delocalised in the presence of a BC, occupying all spaces in the sample volume, from which the particle is not excluded by the hard core interaction with other particles.     

Localization-Fermi Liquid Transition of 3He Adsorbed on 4He-Coated Hectorite

To create a phase diagram of two-dimensional (2D) ³ He fluids, we measured heat capacity of the ³ He adatoms down to 20mK as a function of the density. The ³ He adatoms are deposited on hectorite precoated with ⁴ He in the amount of 24.70mol/m ², just above the onset density for the ⁴ He super-fluid. Isotherms of the heat capacity steeply increase with the ³ He density up to n _c =2.4mol/m ². Above n _c , the heat capacity shows properties of the 2D Fermi liquids. We pointed out that the density dependence of the heat capacity is similar to those of the insulator-metal (I-M) transitions of some electron systems, e.g. Si:P. Assuming the transition from a localized state to the Fermi liquid at the density n _c , the diameter of a localization area a _H ^* was obtained as about 3.8Å using the Mott relation that the mean distance equals 2.2 a _H ^* at the transition. Effective mass and 2D spin fluctuations of the Fermi liquids are enhanced with the density by increasing correlations in the 2D liquids.     

4He in Nano-porous Media: Superfluidity and Quantum Phase Transition

This paper reviews recent findings of novel phenomena in ⁴He confined to a nano-porous glass. We examined pressure–temperature (P-T) phase diagram of ⁴He confined in a porous Gelsil glass that had nanopores 2.5 nm in diameter, by torsional oscillator and pressure studies. The obtained phase diagram is fairly unprecedented the superfluid transition temperature approaches zero at 3.4 MPa, and a novel nonsuperfluid phase exists between the superfluid and solid phase. These observations indicate that the confined ⁴He undergoes a superfluid-nonsuperfluid-solid quantum phase transition at zero temperature. We propose that the nonsuperfluid phase may be a localized Bose-condensed state in which global phase coherence is destroyed by a strong correlation between the ⁴He atoms or by a random potential. ⁴He in nanospace is an excellent model system for studying a strongly correlated Bose liquid and solid in a confinement potential.     

The Heat Capacity of 4He Monolayers Adsorbed on Evaporated Gold

The specific heat of ⁴He condensed on an evaporated gold surface has been measured for coverages between 0.007 and 0.096 Å^?2 and temperatures between 0.4 and 3.0 K. There are at least two types of physisorption site on the gold surface with binding energies ?83 ± 7 and ?94 ± 7 K. For coverages below 0.06 Å^?2 (～0.5 monolayers), the ⁴He is a two-dimensional classical gas at sufficiently high temperatures. At lower temperatures and higher coverages, the ⁴He forms condensed phases, including liquids, solids commensurate with the gold lattice, and incommensurate solids. The phase diagram of ⁴He/evaporated gold is similar to that for ⁴He/graphite, but the phase boundaries are shifted and not well defined, nor is the diagram as rich in structure.     

载贵金属活性碳纤维对氙吸附性能的研究

利用活性炭纤维的氧化还原特性,在活性炭纤维上负载了一定量的贵金属银或金。表征了负载贵金属后活性炭纤维的孔结构变化,以及活性炭纤维表面贵金属颗粒的分布和表面化学性质。研究并比较了负载贵金属后活性炭纤维对氙的吸附性能。研究结果表明,在活性炭纤维上负载适量的贵金属银或金,可以显著地提高活性炭纤维对氙的吸附容量,其可能的原因是由于这些贵金属对活性炭纤维孔宽和表面化学性质的修饰,以及提高了活性炭纤维对氙的吸附势。