NMR Measurements of 2D 3He Adsorbed on ZYX Exfoliated Graphite

We describe first successful usage of ZYX exfoliated graphite to cool adsorbed two dimensional (2D) ³He to ugtra low temperatures (ULT). ZYX has much better characteristics as an adsorption substrate such as larger crystallite (platelet) size and smaller mosaic angle than Grafoil, a widely used exfoliated graphite in ULT experiments. In spite of the relatively small surface area of ZYX, we could perform a nuclear magnetization measurement of paramagnetic 2D solid ³He with continuous wave NMR below 10 mK. The data follow nicely the Curie law down to 200 µK. This indicates that ZYX can be used in experiments to study detailed density dependences of low temperature properties of 2D ³He including possible BCS transitions. We also show a preliminary resugt of pulsed NMR measurements of high density 2D fluid ³He adsorbed on ZYX preplated with a ⁴He monolayer.     

Preliminary Heat-Capacity Measurements of 2D Solid 3He Adsorbed on Graphite Preplated with 4He

We describe preliminary resugts of heat capacity measurements of the 4/7 phase in two dimensional ³He adsorbed on graphite preplated with a ⁴He monolayer in a temperature range between 0.17 and 20 mK. In zero magnetic field, we observed a double-peak structure similar to that reported previously for the same phase in a different system (³He/³He/gr) in which the gapless quantum spin-liquid ground state is proposed. The exchange interaction deduced from the high temperature data is by a factor of 30% smaller than the previous one, presumably because of the smaller lattice constant for the ³He/⁴He/gr system. The present resugt provides strong evidence that such a peculiar temperature dependence of heat capacity is characteristic of the 4/7 phase. Application of magnetic fields up to 0.65 T on this phase does not seem to change appreciably the higher temperature broad peak at around 1.4 mK.     

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.     

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.     

‘Heat from Above’ Heat Capacity Measurements in Liquid 4He

We have made heat capacity measurements of superfluid ⁴He at temperatures very close to the lambda point, T _λ, in a constant heat flux, Q, when the helium sample is heated from above. In this configuration the helium enters a self-organized (SOC) heat transport state at a temperature T _soc(Q), which for Q≥100 nW/cm² lies below T _λ. At low Q we observe little or no deviation from the Q=0 heat capacity up to T _SOC(Q); beyond this temperature the heat capacity appears to be sharply depressed, deviating dramatically from its bulk behaviour. This marks the formation and propagation of a SOC/superfluid two phase state, which we confirm with a simple model. The excellent agreement between data and model serves as an independent confirmation, of the existence of the SOC state. As Q is increased (up to 6 µW/cm²) we observe a Q dependent depression in the heat capacity that occurs just below T _SOC(Q), when the entire sample is still superfluid, This is due to the emergence of a large thermal resistance in the sample, which we have measured and used to model the observed heat capacity depression. Our measurements of the superfluid thermal resistivity are a factor of ten larger than previous measurements by Baddar et al.     

Heat Capacity of the Second Layer of 3He Adsorbed on Graphite Pre-plated with a 4He Monolayer

The second solid layer of ³He on graphite is a highly frustrated magnetic system, as a resugt of its triangular symmetry and the importance of ring exchange (mugtiple spin exchange). This is predicted to lead to a quantum spin liquid ground state. Experiments in which the first paramagnetic solid layer is replaced by ⁴He have been undertaken by several groups, allowing more precise measurements of the second layer magnetisation in an attempt to infer the spin gap of the putative spin liquid phase. We report the first measurements of the influence of such pre-plating on the second layer heat capacity, in the vicinity of the commensurate solid phase. We find the differences in heat capacity of the two systems to be significant. The effective exchange constant is reduced by pre-plating by a factor of order 2/3. In addition the value of the heat capacity at its maximum is significantly reduced by approximately 20%. Can these differences be accounted for within a model of intra-layer mugtiple spin exchange on a triangular lattice?     

Heat Capacity of 3He Solid Film on Graphite in Magnetic Fields

Heat capacities of a submonolayer ³He solid film adsorbed on a graphite surface are measured in magnetic fields up to 400 Oe. The measured heat capacity shifts to higher temperatures with an increasing magnetic field. The amplitudes of the shifts are twenty times larger than the magnitude of the Zeeman energy, which is anomalously large. With regard to the origin of these large shifts, the reduction of frustrations and the weakening of the competition between multiple spin exchange interactions are discussed.     

PVT and Vapor Pressure Measurements on Ethane

New measurements of the vapor pressures and PVT properties of ethane are reported. PVT determinations have been made from near the triple point to 320 K at pressures to 33 MPa. The density range investigated extends to more than three times the critical density. The new measurements of the vapor pressures of ethane extend from 160 K to near the critical point.     

Preliminary Measurements on Grid Turbulence in Liquid 4He

A grid has been pulled through a column of liquid helium at speeds as high as 1 m/s and at temperatures as low as 90 mK. A 300 micrometer Ge thermometer with response time of less than 1 ms measured the temperature rise resulting from the decay of the turbulence generated. It is believed that homogeneous, isotropic quantum turbulence was formed, since mesh Reynolds numbers in excess of 100,000 were created. The rates and power spectra of the energy increases detected in the helium after grid-pulls are determined. The results are compared to other quantum and classical results, and to the theory of the Kelvin wave cascade in a viscosity-free fluid.      

Continuing Specific Heat Measurements of 3He in 3He - 4He Mixture Films

Additional data from our on-going experiments for the heat capacity of ³ He in ³ He- ⁴ He mixture films on a Nuclepore substrate are reported over the temperature range 90T165 mK, for ³ He coverages between 0.05 and 1.4 bulk-density atomic lagers, on a ⁴ He film of thickness 4.33 bulk-density atomic lagers. This is a two-dimensional Fermi liquid system, in which we can change the ³ He coverage and thus tune the Fermi temperature.     

Specific Heat Measurements of 3He in 3He-4He Mixture Films

Preliminary data for the heat capacity of ³ He in ³ He- ⁴ He mixture films on a Nuclepore substrate are reported over the temperature range 90T165 mK, for ³ He coverages between 0.05 and 1.7 bulk-density atomic layers, and a ⁴ He film thickness of 4.33 bulk-density atomic layers. In this two-dimensional Fermi liquid system, a step structure appears in the specific heat as a function of ³ He coverage, similar to the step previously observed in the magnetization.     

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.     

Fluctuation Effects in the Heat Capacity of 3He–4He Solid Solutions

The impurity heat capacity of solid ³ He–⁴ He mixtures is theoretically investigated both below and above phase separation temperature T _s . It is shown that at T > T _s the temperature behavior of the heat capacity is completely defined by correlation effects in the impuriton subsystem. The proposed theory is in good agreement with experimental data.     

Surface Magnetism of Liquid 3He on 4He Pre-plated Graphite

No Heading The nuclear susceptibility of liquid ³He in Grafoil pre-plated by 2.5 and 3.5 layers of ⁴He has been studied with a cw NMR method at temperatures between 0.7 and 100 mK under various liquid pressures. The 3.5 layers of ⁴He pre-plating suppresses a formation of the first and second solid ³He layer, eliminating most of surface magnetization at saturated vapor pressure. However, with increasing liquid pressure, a magnetization obeying a Curie Weiss law gradually grows in the same way as in the previous experiment for pure liquid ³He. This magnetization, induced by pressurization, is attributable to the formation of solid ³He layer above the pre-plated ⁴He.PACS numbers: 67.80.Jd, 75.70 Cn, 67.70.+n.     

A Preliminary Heat Capacity Measurement of 3He in Aerogel near its Superfluid Transition

We report simultaneous heat capacity and torsional oscillator measurement of ³ He in aerogel near the superfluid transition. The heat capacity has a peak at the temperature T_c where the torsional oscillator shows the onset of superfluid decoupling. The coincidence of these signatures suggests that ³ He in aerogel does undergo a true thermodynamic transition.     

A Novel Method for Heat Capacity Measurements near 2K

We have constructed a calorimeter of novel design for high resolution heat capacity measurements near 2K. This device resembles high resolution paramagnetic salt thermometers used in previous heat capacity measurements on the ⁴ He-Vycor system. ² However, we have modified this thermometer so that our signal is no longer directly proportional to the temperature of the sample, but rather to the change in temperature with respect to time. We demonstrate the technique by measuring the heat capacity of paramagnetic gadolinium trichloride (GdCl ₃ ) salt near its critical temperature of 2.21K.     

Lattice Dynamics of 2D Monoatomic Crystals: Application to 3He on Graphite

The dynamics of a two-dimensional rare gas crystal with triangular lattice is investigated theoretically. Phonon spectra of the 2D crystal are calculated within the model which takes into account the interaction between nearest neighbours in the layer and interaction of layer atoms with the substrate. On the basis of obtained results the temperature dependence of lattice heat capacity has been calculated and a comparison is made with experimental data.     

Low-Temperature Heat Capacity of $$^{}$$He Films on Graphite

Heat capacities of \(^{4}\)He films have been measured at rather low temperatures between 2 and 80 mK and at areal densities between 2 and 24 \(\hbox {nm}^{-2}\). These areal densities correspond to a monolayer fluid and third-layer fluid. For monolayer films, the results do not contradict previous measurements carried out at high temperatures. On the other hand, at some areal densities, small and broad but definite bumps, whose origin has not yet been understood, have been observed around 15 mK. Between 13 and 24 \(\hbox {nm}^{-2}\), the measured heat capacities above 40 mK are proportional to \(T^{2}\) and hardly change with areal density. These behaviors suggest that the second atomic layer does not solidify before the third-layer promotion, at least not into a commensurate solid, such as the so-called 4/7 phase.     

Quasi-2D Flows of 3He – 4He Mixtures

Transport in degenerate ³ He- ⁴ He mixtures in quasi-2D flow channels is discussed. The quasiparticle mean free path combines particle-wall and particle-particle collisions including the interference between them. The temperature, concentration, and polarization dependences of the transport coefficients allow easy extraction of the correlation parameters of random surface roughness from transport data.