B. E. Clements

4He on weakly attractive substrates: Structure, stability, and wetting behavior

Using a microscopic variational approach we examine the structure and the excitation spectrum of layered4He liquids absorbed to alkali metal and graphite substrates. We find that the alkali metal substrates produce a less pronounced layering structure than the shorter-range graphite/solid helium potential. For the excitations, three features are in common to the substrates: First, for coverages of a monolayer or more, a surface mode is present. Second, a bulk mode which gains strength as the coverage is increased, is identifiable for films with sufficiently high coverage. Finally, a two-dimensional mode that propagates within the first layer is observed for the more attractive substrates. We also present results that we obtain by using the nonlocal density functional theory. We document the reliability and shortcomings of this approach by making a detailed comparison of experimental, Monte Carlo, and variational theory results for the structure, energetics, and excitations. We also give a brief discussion on the wetting properties of helium on alkali metal and graphite/solid helium substrates.

Theoretical Analysis of Neutron Scattering Spectra of 4He Films on Graphite

We compare microscopic calculations of the dynamic structure function of helium films adsorbed to a graphite substrate with neutron scattering data. Starting from a generalized Feynman theory of collective excitations, we include successively, three-phonon interactions, self-energy corrections, and experimental broadening and thereby improve the agreement between theoretical predictions and experiments. The inclusion of three-phonon vertices allows high-lying excited states to decay into lesser energetic ones and thus leads to a natural linewidth. It is shown that the theoretical linewidth of the excitations is typically smaller than the experimental resolution, and that much structure of the spectrum in the neutron scattering data is obscured under a broad plateau between the ripplon and the phonon excitations. This analysis leaves little doubt of the existence of layer-phonons and that the observed shoulder in the dynamic structure function, near the roton minimum, is actually a 2D roton propagating in the inner-most liquid layer.

Phase transitions in the growth of4He films

Using a microscopic, variational approach we examine the growth of4He absorbed to graphite and alkali substrates. We find that superfluid layers are formed and their behavior as a function of coverage is closely related to the one of a purely two-dimensional superfluid. The growth of a new layer undergoes a phase transition from a cluster formation into the connected superfluid when the coverage is increased. Based on the important connection to the two-dimensional fluid we propose a microscopic theory of quantum vortices in4He films at zero temperature, in which single vortices are treated as quasiparticles. We calculate the energy needed to create the single vortex, vortex inertial mass, microscopic interaction between vortices and binding energy of the vortex-antivortex pair as a function of density. We predict that at the4He superfluid density less than about 0.037 Å−2 the binding energy of the pair becomes negative, indicating a phase transition into a new state where vortex-antivortex pairs are spontaneously created.

Dynamics of3He impurities in4He films

Using a microscopic theory we calculate the binding energy of3 He impurities in films of4 He absorbed to a graphite substrate. Without adjustable parameters, we obtain excellent agreement with the experimental binding energies for the ground state of the He impurity. By introducing a timedependent variational wave function, the impurity atom acquires a hydrodynamic effective mass for its motion parallel to the surface due to hydrodynamic backflow, and the excited states have a finite lifetime. When these effects are included, both the energy of the first excited state of the impurity, and the effective mass of the ground state, agree well with experimental data.A comparison with recent density functional results is carried out. It is shown that the substrate bound states on strong potentials are spurious and due to the inappropriate treatment of the effective mass within that theory.

Dynamics of {sup 3}He impurities in {sup 4}He films

Using a microscopic variational theory we calculate the binding energy of3He impurities in films of4He absorbed to a graphite substrate. Without adjustable parameters, we obtain excellent agreement with the experimental binding energies for the ground state of the3He impurity.To calculate excited states, we then introduce a time-dependent variational wave function. In that way, the impurity acquires a hydrodynamic effective mass for its motion parallel to the surface due to hydrodynamic backflow. Excited states have a finite lifetime. When these effects are included, both the energy of the first excited state of the impurity, and the effective mass of the ground state, also agree well with experimental data.

Dynamics of helium films

We present quantitative calculations for the static structure and the dynamics of quantum liquid films on a translationally invariant substrate. The excitation spectrum is calculated by solving the equations of motion for time-dependent one- and two-body densities. We find significant corrections to the Feynman spectrum for the phonon-like collective excitations.

Dynamics of quantum films

From no assumptions other than a microscopic many-body Hamiltonian, we calculate the elementary excitation spectrum of mono- and double-layer helium films. Essential to quantitative predictions is the inclusion of time-dependent short-range correlations of the liquid.

Temperature dependence of third sound in helium monolayers

Abstract The temperature dependence of third sound is determined from a variational-density-matrix approach. At zero temperature a liquid4He monolayer, adsorbed to a weakly attractive substrate, supports third sound only within a restricted range of coverages. Elevating the temperature appreciably decreases the value of the minimum stable coverage, increases the maximum sound velocity, and slightly decreases the maximum stable coverage.

Structure and growth of quantum films

Abstract The growth of liquid 4 He films on weakly attractive substrates occurs in a number of well-defined discrete steps. Above a minimum stable coverage, a liquid monolayer uniformly covers the surface. Upon increasing the coverage a second and a third liquid layer grow through the formation of two-dimensional “clusters” on top of liquid layers.