Recent Abstracts of our Lab

• Liquid-Vapor Coexistence in Two-dimensional 3He-4He Mixtures(Journal Low Temp Phys. Nov/Dec 1998)
  
  
• Isotopic Ordering in Adsorbed Hydrogen Monolayers (October 16,1999 Physical Review B Vol 60 (16) p.11773)
  
  
• The Heat Capacity of Monolayer Films of Mixtures of 3He and 4He Adsorbed on H2-Plated Graphite(Proceedings, APS March Meeting 1998, Los Angeles, California)
  
  
• Calorimetric Study of the Desorption and Phases of 3He and 4He adsorbed on Two Layers of H2-plated Graphite Above 0.2K (Journal Low Temp Phys., January 1998 Vol 110 (1-2) p.615)
  
  
• Melting and Evaporation of Monolayer H2-D2 Mixtures Adsorbed on D2-plated Graphite (Journal Low Temp Phys., January 1998 Vol 110 (1-2) p.621)
  
  
• Structure of Monolayer Films of Hydrogen Isotopic Mixtures (Physica B, July 1996)
  
  
• Heat Capacity, Adsorption Isotherms, and Quasi-elastic Neutron Scattering (QENS) Measurements of Bilayer Deuterium Hydride Adsorbed on Graphite (Phys. Rev. B, 1996 in press)
  
  
• Heat Capacity Study of 4He Desorbing from Hydrogen-Plated Graphite (Journal Low Temp Phys., July 1995 Vol 100 (1-20) p. 131)
  
  
• Determination of the Thermodynamic Phases of 4He Adsorbed on Hydrogen-Plated Graphite (Journal Low Temp Phys. Vol. 101 (3-4) p.469)
  
  
• Monolayer Helium Adsorbed on Hydrogen-Plated Graphite (Physica B 194-196, p. 635)

Liquid-Vapor Coexistence in Two-dimensional 3He-4He Mixtures

4He single layer films adsorbed on H2- or HD-plated graphite yield liquid-vapor (L-V) phase diagrams which appear to be in agreement with expectations from two-dimensional (2d) corresponding states arguments and quantum Monte Carlo calculations. It is clear that the L-V critical point is in the vicinity of 0.9K. 3He films adsorbed on similar substrates have not shown a L-V critical point. On the contrary, at densities and temperatures comparable to those which yield 4He L-V coexistence, 3He is a variable density 2d Fermi fluid. There are expectations that if a 2d L-V critical point exists for 3He it will be at rather low temperatures. In this paper we present a heat capacity study of 3He-4He mixture films adsorbed on two-plus layers of H2, for 3He concentrations, x, between 0 and 50%, for temperatures between 0.2 and 2K, and for coverages up to 0.06\A^{-2}. We have mapped the L-V coexistence region boundaries for x=0.1, 0.35, and 0.50, and have obtained portions of the L-V region boundaries for other concentrations. Results to be reported include the L-V critical temperatures of the mixtures, and the low temperature density of the liquid mixtures. Implications on the existence of a L-V critical point for 2d 3He, and possible superfluidity in the 2d liquid mixtures will be discussed.

Isotopic Ordering in Adsorbed Hydrogen Monolayers

Quantum ordering of isotopic hydrogen mixtures H2-D2 adsorbed in the monolayer range on graphite (001) is studied at low temperature in the solid phase by small angle neutron scattering and neutron diffraction measurements. A clustering process is observed from 8K downwards, at and above monolayer completion. This trend towards phase separation depends strongly on coverage and isotopic concentration. For instance, for densities corresponding to the root3 by root3 R30degrees commensurate structure, i.e. 1/3 smaller than monolayer completion, the adsorbed layer remains a random mixture down to 3K. At larger temperatures of about 20K, in the fluid state, the diffusion coefficient of HD molecules in a HD-D2 mixture is measured by quasi-elastic neutron scattering. Surprisingly enough, the translational mobility of these molecules is enhanced with respect to the pure HD fluid phase at the same coverage and temperature. This result is consistent with the previous observations of the bond softening of the heavier molecules D2 due to quantum motion at the corresponding densities.

The Heat Capacity of Monolayer Films of Mixtures of 3He and 4He Adsorbed on H2-plated Graphite

We will report on measurements of the heat capacity of single layer 3He-4He mixtures adsorbed on 2.15 layers of H2-plated graphite for temperatures between 0.2K and 8K, surface densities n,below 0.065atoms/Angstroms^2 and values of the 3He concentration, x, of approximately 5%, 10%, 14% and 50%, depending on n. In this "high temperature" range, we have studied the effect of mixing on the desorption of the monolayer and on its two-dimensional liquid-vapor condensation. The mixture measurements will be compared to those on the pure isotopes (x=0% and 100%) physisorbed on a similar substrate(R. C. Ramos Jr., P. S. Ebey, and O. E. Vilches, Journal Low Temp Phys., Jan 1998).

Calorimetric Study of the Desorption and Phases of 3He and 4He adsorbed on Two Layers of H2-plated Graphite Above 0.2K

We have conducted new heat capacity studies of films of 3He and 4He adsorbed on 2.15 layers of hydrogen plating exfoliated graphite for temperatures T > 0.2 K and coverages (n) over the entire first layer and beginning of the second layer. Low-density 3He films show two-dimensional Fermi fluid behavior with no liquid-vapor condensation, in contrast to similar density films of 4He that exhibit 2-d condensation below 0.85K. We have mapped the n-T melting line and estimated monolayer completion density using heat capacity isotherms. By fitting 3He films' desorption specific heats to a model developed by Ebey et. al., we have calculated the 3He binding energy (21.3K to 21.7K) and compare it to a previous measurement for 4He films on the same substrate (25.1K to 25.5K)

Melting and Evaporation of Monolayer H2-D2 Mixtures Adsorbed on D2-plated Graphite

The two-dimensional (2-d) phases of the second layer in bilayer films of H2 or D2 adsorbed onto graphite anre similar to their bulk three-dimensional (3-d) counterparts. Differences between 2-d and 3-d lie in the temperatures of the critical and triple points, the magnitude of the heat of fusion, and the change in density upon melting - all significantly lower in 2-d. Using AC calorimetry, we have studied the 2-d melting and evaporation of partial monolayers if H2-D2 mixtures adsorbed onto D2-plated graphite as a function of the D2 concentration in the second layer. In addition, we have observed a reduction in triple point temperatures of the second layer if H2 adsorbed onto partial monolayers of D2. Using ideal solution theory, we have estimated the degree of fractionation due to 2-d evaporation. Other interesting properties at the three-phase melting point will be discussed.

Structure of Monolayer Films of Hydrogen Isotope Mixtures (Physica B, July 1996)

Hydrogen-Deuterium mixtures adsorbed on graphite have been studied by neutron diffraction for 2K < T < 18 K and various H2-D2 ratios. One commensurate (sqrt(3) x sqrt(3) 30 deg) and one incommensurate solid (full monolayer) coverages were measured. A single diffraction line at a position intermediate between the 10 lines of pure H2 and D2 was observed, showing that the solidus a well-ordered mixed phase. the strong increase of the peak intensity as T decreases for a 50% mixture at monolayer completion is interpreted as a trend towards phase separation.

Heat Capacity, Adsorption Isotherms, and Quasi-elastic Neutron Scattering (QENS) Measurements of Bilayer Deuterium Hydride Adsorbed on Graphite (Physical Review B, 1996 in press)

Bilayer films of deuterium hydride (HD) adsorbed on exfoliated graphite have been studied using heat capacity, adsorption isotherms and QENS measurements, The second layer if HD shows solid (S), liquid (L) and gaseous (G) (vapor (V)) phases, with two-dimensional (2-d) characteristics, We determined the S-L-V triple line at T(2-d) = 8.44 K, and the K-V critical temperature at T (2-d) = 11.45 K. QENS identified the L-phase and measured its translational diffusion coefficient D (approximately 10^-5 cm^2/sec at 9.25K). Isosteric heats, latent heat and entropy change at melting, and solid Debye temperatures are calculated, The results are used to construct a general graph of critical and triple points of adsorbed gases as a function of their quantum parameter.

Heat Capacity Study of 4He Desorbing from Hydrogen-Plated Graphite (Journal Low Temp Phys. July 1995 Vol.100(1-2) p. 131)

We report measurements of the surface binding energy of 4He adsorbed on 1.45, 2.15 or 3.15 layers of hydrogen plating graphite. The heat capacity of partial monolayer coverages of helium was measured using adiabatic heat pulse calorimetry from 0.2 K to 9.0 K. Desorption of low surface density 4He films was indicated above approximately 2.0 K by a broad peak in the heat capacity. A model was developed to calculate the specific heat of an ideal gas in two dimensions. This model was used to obtain the binding energies from our experimental data. The binding energies per atoms are -39, -25.8 and -22 +/- 0.5 K, respectively, for the three hydrogen platings. The 2-d compressibility of the partial solid second layer of H2 in the 1.45 layer H2 coverage was calculated from an increase in its melting temperature with increase of 4He 2-d pressure.

Determination of the Thermodynamic Phases of 4He Adsorbed on Hydrogen-Plated Graphite (Journal Low Temp Phys. Vol. 101 (3-4) p. 469)

Thermodynamic measurements have been used to investigate the surface phases of monolayer 4He adsorbed on graphite plated with 1.45, 2.15 and 3.15 layers of Hydrogen. In each case, the 4He preferentially adsorbed on the lower completed layer. In this paper, we report the measurements of 4He on the 1.45 layer H2 graphite plating. For this plating the 4He goes into a condensed phase below 0.87 K. The high temperature-low density film entropy has been calculated.

Monolayer Helium Adsorbed on Hydrogen-Plated Graphite (Physica B 194-196, p. 635)

A preliminary study has been made of monolayer 4He adsorbed on Hydrogen adsorbed on graphite. Adiabatic heat capacity measurements in the range of 1.9 K to 8.0 K show that at low densities, the He film has close to an ideal 2-D specific heat of C/Nk = 1.0 at 2K and that the film is almost completely desorbing from the surface by 8.0 K.