Helium Film Formed in 1.2 nm Pore in Zeolite Templated Carbon

We have measured the vapor pressure of ⁴He adsorbed in zeolite templated carbon (ZTC) at 4.2 K, in order to examine ⁴He film growth in the nanopore. ZTC has a framework made of non-stacked graphene, and has a relatively uniform 1.2 nm pore with three-dimensional (3D) periodicity of 1.4 nm. The nanoporous substrate is a candidate to realize superfluid ⁴He film with closer 3D connectivity than those studied so far. From the vapor pressure, coverage dependences of the thickness and the two-dimensional (2D) isothermal compressibility of ⁴He film were derived using thermodynamic equations. The 2D compressibility indicates the onset coverage n ₁ of the second adsorption layer around 20 µmol/m². It is confirmed that the ⁴He film in the 1.2 nm pores grows uniformly at least up to 1.3–1.5 n ₁.     

One-Dimensional Phonon State of 4He Films Adsorbed in Straight Nanopores

We have studied heat capacities of ⁴He adsorbed in straight nanopores 1.8, 2.2, and 2.8 nm in diameter. Heat capacities of the ⁴He fluid film on the solid layer at 0.08–0.4 K show power laws close to T in 1.8 nm pores, close to T ² in 2.8 nm pores, and a crossover from T to T ² with increasing temperature in 2.2 nm pores. These heat capacities are explained by a model assuming a phonon dispersion with continuous one-dimensional (1D) states in the axial direction and discrete energy levels in the azimuthal direction. By fitting experimental data to the model, the phonon velocity along the pore axis and the energy gap for propagation in the cross section are derived. At temperatures sufficiently lower than the energy gap, where the thermal wave length of phonons is much longer than the effective pore diameter, the ⁴He fluid films show a T-linear heat capacity of 1D phonons propagating only along the pore axis. At higher temperatures, a 1D-2D crossover of phonons occurs.      

4He Fluid in Extremely Narrow 1D Channels 1.5?nm in?Diameter

To examine whether one-dimensional (1D) helium quantum fluid is realized in narrower channels than those studied previously, we have measured heat capacities of ⁴He adsorbed in nanoporous material FSM with straight 1D channels 1.5 nm in diameter. From the heat of desorption for adsorbed ⁴He, the coverage n _f, up to which ⁴He film grows in the channels, is determined to be 15.4 ??mol/m² using the Frenkel-Halsey-Hill model. At coverages sufficiently below n _f, the temperature dependence of the ⁴He heat capacity has a shoulder, above which adsorbed ⁴He is delocalized from the substrate. On the other hand, the depression of the heat capacity indicating quantum effects has not been observed up to n _f, which suggests that ⁴He film in the channels remains amorphous-like normal fluid. Just above n _f, the quantum effect is observed in ⁴He adatoms on the grain surface of FSM powder, which indicates that 1.5 nm channels are slightly below the limit required to realize quantum effect in the inside ⁴He fluid.     

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     

Possible Phase Transition at Low mK Temperatures in Liquid Helium Mixture Films Adsorbed on Graphite

We report heat capacity and magnetisation measurements of ³He adsorbed on the surface of graphite plated with three atomic layers of ⁴He. For ³He coverages n ₃>4 nm^?2 the heat capacity corresponds to a 2D Fermi fluid. The inferred hydrodynamic mass of the ³He quasiparticles is 1.38±0.05 m₃. The ³He effective mass ratio increases with coverage to 2.4 at n ₃=4 nm^?2, due to Fermi liquid interactions. The heat capacity isotherm exhibits a steplike increase centred on n ₃=4.5 nm^?2, similar to that previously observed on four layers of ⁴He. This is associated with the population of the first excited Andreev surface bound state. However, in the present case, as n ₃ is increased through the step a pronounced anomalous feature develops in the temperature dependence of the heat capacity, around 10 mK. Below 5 mK the heat capacity is approximately linear in temperature. With n ₃=7 nm^?2, we find that this behaviour is very sensitive to small changes in the ⁴He third layer coverage, around the completed third layer. Measurements of the ³He magnetisation,, by continuous wave NMR methods, find a significant increase with decreasing temperature below around 20 mK. Together the data suggest that a phase transition takes place in the film at low mK temperatures.     

Atomic Motion in Low-Coverage Helium Films Adsorbed in FSM Nanochannels

⁴He and ³He films adsorbed in nanoporous silicates have shown similar heat capacities until the quantum-fluid layer appears at coverages over the first-layer completion n ₁. To obtain information on dynamics of adatoms at low coverages below the quantum fluid region, we have done pulsed-NMR experiment at 3.3 MHz for ³He films adsorbed in straight 2.4 nm nanochannels of FSM silicates. The spin-lattice and spin-spin relaxation times T ₁ and T ₂ observed at 0.54–7 K were well described by the two-dimensional version of the Bloembergen-Purcell-Pound model. At coverages 0.4–1.7n ₁, minima of T ₁, indicating the spin correlation time τ _c of 4.8×10^?8 sec, were observed at temperatures between 6 and 3 K. With decreasing temperature, changes in T ₁ and T ₂ become small below about 1.5 K, suggesting crossover from thermally-activated motion to quantum tunneling. In contrast to large variations below n ₁, both relaxation times above n ₁ are almost independent of coverage, which is likely to indicate that τ _c is determined by interlayer exchange of adatoms. Below n ₁, onsets for localization of adatoms were suggested by a decrease in T ₂.     

Vapor Pressure and Heat Capacity Measurement of 3He Adsorbed on 18Å-Pores

We have measured the vapor pressure (0.84 K 3 He adsorbed on pores 18 Å in diameter. The results of the vapor pressure measurement indicate that ³ He film grows up to the second layer. In the first layer, the heat capacity of ³ He shows the same temperature and coverage dependence as ⁴ He, indicating a solid phase. Above a little higher coverage than second layer promotion, heat capacity isotherms for ³ He at several hundred mK increase with coverage while those for ⁴ He decrease. This large heat capacity of ³ He is the nuclear spin heat capacity of the second layer fluid ³ He.     

Observation of the Fermi Fluid in 3He-4He Mixture Films Formed in One-Dimensional 28 Å Pores

A ³He film formed in quite narrow pores is one of the possible model systems for a one-dimensional Fermi fluid. Here we report our new heat capacity resugts of ³He and ⁴He film adsorbed in a straight pore 28 Å in diameter down to 5 mK. The coverage and temperature dependence of the heat capacity indicates that a fluid phase appears between the second layer promotion and the complete filling of the pores. Since the heat capacity of ³He adsorbed on the bare substrate shows a large upturn due to the nuclear heat capacity of the localized ³He, it is difficugt to observe the true heat capacity of the fluid phase. By replacing the localized ³He, we can successfully suppress the upturn and observe the true heat capacity of the fluid phase.     

Quantum States of Helium Atoms Confined in Nanocage in Na-Y Zeolite

We have studied the heat capacities of heliums adsorbed in Na-Y zeolite down to 70 mK. The Y-type zeolite has void cages 1.3 nm in diameter connected through narrow apertures 0.8 nm in diameter. In the nanopores at low temperatures, the second-layer He adatoms can be confined in each cage by the first solid layer. In the quantum fluid region above the second-layer promotion, where the heat capacities show qualitative differences between ⁴He and ³He, the heat capacities of ³He adatoms and the thermal relaxation processes show large and non-monotonic changes when one ³He atom is added in each cage. These results strongly suggest that the quantum fluid formed in Na-Y zeolite is not fluid film spread over the entire substrate but quantum clusters confined in each cage.     

Heat Capacity Measurements of 3He-4He Mixture Films

We report heat capacity measurements from our on-going experiments on two-dimensional liquid ³He on superfluid ⁴He thin films absorbed on Nuclepore. The ³He coverage dependence of the heat capacity for 0.02–0.50 bulk-density layers of ³He with 3.10 bulk-density layers of ⁴He over the temperature range 40≤T≤200 mK is presented. We find the effective mass of ³He on 3.10 layers of ⁴He is greater than that on 4.33 layers of ⁴He, consistent with our previous magnetic susceptibility measurements.     

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.     

Pulsed Nuclear Magnetic Resonance on 3He Adsorbed on Bare and 4He Preplated MCM-41 Using DC SQUID Detection

We report low field DC SQUID NMR measurements down to 1.5 K of ³He adsorbed in the pores of the mesoporous molecular sieve MCM-41. In the first experiment measurements were made on ³He adsorbed onto the bare pore walls of MCM-41 with coverages ranging from $n_{^{3}\mathrm{He}}=0.86n_{1}$ to full pores at $n_{^{3}\mathrm{He}}=1.79n_{1}$ , where n ₁ is the coverage for monolayer completion. A second experiment was performed with low ³He coverages ( $n_{^{3}\mathrm{He}}\sim0.01n_{1}$ ) on ⁴He preplated pores, where a crossover to a quasi-1D state is expected to occur at temperatures sufficiently below 700 mK. In both experiments relaxation times T ₁ and T ₂ ^* were measured as a function of temperature and coverage at frequencies from 80 to 240 kHz. The frequency dependence of the linewidth in the pure ³He experiment is extremely weak therefore T ₂ ^* ≈T ₂. The 1.5 K isotherm shows a small minimum in T ₂ ^* at a coverage corresponding to monolayer completion. In the experiment with ⁴He preplating there was no significant change in T ₁ or T ₂ ^* when the ³He coverage was doubled from $n_{^{3}\mathrm{He}}=0.01n_{1}$ to 0.02n ₁ at a ⁴He preplating of $n_{^{4}\mathrm{He}}=1.05n_{1}$ . This suggests that the relaxation times are dominated by single particle effects in the low density regime.     

Possible 3He Boltzmann Gas Formed on Three-Dimensional Nanopores Preplated with 4He

We have measured the heat capacity of ³He adsorbed on three-dimensionally connected nanopores, 2.7 nm in diameter, preplated with about 1.3 atomic layers of ⁴He. At low coverages of ³He, the ³He heat capacity is roughly constant at the measured temperatures between 0.1 and 1 K. Its molar heat capacity is on the order of the gas constant R, between 1.1R and 1.8R. This suggests a Boltzmann gas state of the adsorbed ³He. At high coverages, the heat capacity is likely approaching linear in T at low temperatures, which suggests a degenerate state at further lower temperatures.     

Nuclear Spin-Kinetics of 3He in Carbonizates with Various Porosity

In the present work the NMR relaxation of the gaseous ³He inside carbonizate pores was investigated at temperature 1.5 K. The carbonizates synthesized from fructose and wood of the tropical tree astronium were used. The dependences of the ³He relaxation rates T ₁ and T ₂ on the gas pressure and the amount of the ³He atoms adsorbed on the surface of pore walls were measured. The analysis of obtained results reveals the existence of the ³He phases inside carbonizate—adsorbed solid layers, liquid and gas.     

Superfluid Transition of a 4He Thin Film Pressurized by Bulk Liquid 3He

We measured transverse acoustic impedance Z of normal fluid ³He at 46.6 MHz on a surface coated with a thin ⁴He film. The real component of the impedance, Z′, in the coated samples deviates from Z′ in the pure ³He in the low temperature region. Z′ on the coated samples is almost identical with Z′ in the pure sample at high temperature and gradually deviates below a particular temperature T _onset . T _onset  is possibly the superfluid onset temperature of the ⁴He film pressurized by the bulk liquid ³He. The gradual decrease in Z′ means that the superfluid component in ⁴He film increases gradually, which is expected from the dynamic KT transition at high frequency. The thicker is the film, the higher is the T _onset . The range of T _onset we observed was between 40 and 160 mK. This is much lower than that at the saturated vapor pressure. Suppression of T _onset achieved by the applied pressure from bulk liquid ³He was presumably caused by the dissolved ³He in the film, thickening of the inert layers and/or by the strong correlation effect. The result shows that the specularity of ³He quasiparticle scattering is strongly affected by superfluidity of the ⁴He film.     

Heat capacities of Ar,Ne,4He-Ar,and4He-Ne films adsorbed on Cu at low temperatures

Heat capacities of Ar and Ne monolayers adsorbed on Cu are measurable at 1–4 K. Temperature dependences are similar to the low-temperature law for harmonic oscillators, but the empirical characteristic temperatures of 11–15 K are much lower than can correspond to independent adatoms oscillating in substrate sites. The magnitudes and temperature dependences do not match those predicted for surface modes of an absorbent coated with inert impurities or for the collective modes of epitaxial films. A model involving the excitations in non-epitaxial films appears to offer an explanation of the observed magnitudes and temperature dependences. The heat capacities of⁴He adsorbed on preplated Ar or Ne show little variation with the number of Ar layers, or between Ar and Ne coatings.Research supported by The National Science Foundation.     

Ellipsometric Study of Superfluid Onset in Thin Liquid 4Helium Films

We have developed a modulated null ellipsometer capable of measuring single layers of adsorbed ⁴He films at 1.4 K. The small optical index of liquid helium, the extreme sensitivity to temperature gradients, and the requirement of sub-monolayer stability over many hours presents significant experimental challenges, which will be briefly discussed. The main goal of our experiments is to independently measure the superfluid and normal coverage in thin adsorbed ⁴He films. This is a particularly important issue for helium films on intermediate strength substrates such as rubidium and thin cesium, where previous measurements indicate that prewetting and the Kosterlitz-Thouless transition interact strongly, and the K-T transition appears to have nonuniversal features. Independent determination of the superfluid and normal fraction can be accomplished by using the ellipsometer in conjunction with a quartz crystal micro balance (QCM). QCM measurements rely on viscous coupling of the fluid layers, and therefore respond only to the normal component of a ⁴He film. In contrast, the ellipsometer is sensitive to the total thickness, independent of the state (superfluid or normal) of the film. By combining the QCM and ellipsometric measurements we can determine the total coverage, the normal fluid component and thus the superfluid fraction.     

Magnetic Susceptibility of Heavy Fermion 3He-Bilayers

A ³He bilayer film adsorbed on graphite preplated with a bilayer of solid ⁴He has recently been shown to be a new heavy fermion system that can be tuned towards a quantum critical point (QCP) by varying the ³He coverage. We report new measurements of the temperature dependence of the magnetic susceptibility of this system over the temperature range 0.3 to 300 mK. The magnetisation of the ³He film is measured using pulsed NMR at a frequency of 60 kHz, on a SQUID detector setup with an untuned input circuit. The signal sensitivity attainable with this setup enables us to collect high-precision data with reasonable acquisition times, despite the low surface area of the sample (2 m²) and the small liquid susceptibility. The detailed NMR response provides an essential diagnostic tool to precisely adjust the ⁴He preplating. On cooling, the magnetisation evolves from Curie-law, through a distinct Kondo-like maximum at a temperature T ^*, to a weakly rising magnetisation far below the Curie law. This behaviour signifies the formation of the heavy fermion state of the coupled ³He layers. As the ³He coverage is increased towards the putative QCP, T ^* is tuned to lower temperature. These preliminary observations demonstrate the power of the technique, confirm previous results obtained in a different sample cell by a different method, and pave the way for a detailed study close to the QCP.     

Specific heat of amorphous3He films and confined liquid3He

We have measured the heat capacities of³He films and liquid³He in porous Vycor glass at 10 to 600 mK. With increasing the film thickness from 1 to 3 atomic layers, the specific heat evolves gradually from that typical to solid to that of liquid³He. At about 2 atomic layers, however, its low-temperature part is nearly temperature-independent; we interpret this as a result of gradual freezing of spins in an amorphous solid³He film with decreasing the temperature. The contribution of liquid³He in the center of the Vycor pores can be described as the specific heat of bulk liquid³He at corresponding pressures in the range 0 to 28 bar. The thickness of amorphous solid on the pore walls increases with external pressure roughly linearly. Preplating the walls with⁴He allows to determine the positions of³He atoms contributing to the surface specific heat at 10 to 50 mK. In addition, the contribution from the specific heat of³ He -⁴He mixing at 100 to 600 mK is discussed as a function of pressure and amount of⁴He.0n leave from ISSP Acad. Sci. of Russia, Chernogolovka, Russia     

Adsorption Potentials and Film Growths of 4He in Nanometer Pores of FSM-16 (2.8 nm) and HMM-2 (2.7 nm)

No Heading To find the one- and the three-dimensionalities for the superfluid onset of the ⁴He films formed in nanopores, we have been studying ⁴He adsorbed on FSM-16 and HMM-2 that have pores of almost the same diameter, 2.8 and 2.7 nm, but ID and 3D connections of the pores, respectively. For these two substrates, the adsorption potential profile and layer-by-layer growth of ⁴He films were observed by the precise measurement of the vapor pressure for adsorption. The isothermal compressibility _T, the film thickness d, and the isosteric heat q_st of adsorption are derived from the vapor pressure. _T and d indicate that ⁴He adatoms form uniform layers up the second layer on both substrates. Almost the same q_st for both substrates suggest that the adsorption potential is the same between these SiO₂-based substrates. These results suggest that FSM-16 and HMM-2 are ideal substrates to use in investigating the dimensionality of the superfluid onset.PACS numbers: 67.40.–w, 67.70.+n