Superfluid State of 4He on Graphane and Graphene–Fluoride: Anisotropic Roton States

We explore the phase behavior of Helium films on two variants of graphene: graphane (graphene coated with H, denoted GH) and graphene–fluoride (GF). A semiempirical interaction with these substrates is used in T=0 K Path Integral Ground State and finite temperature Path Integral Monte Carlo simulations. We predict that ⁴He forms anisotropic fluid states at low coverage. This behavior differs qualitatively from that on graphite because of the different surface composition, symmetry and spacing of the adsorption sites. The ⁴He ground state on both substrates is thus a self-bound anisotropic superfluid with a superfluid fraction ρ _s /ρ lower than 1 due to the corrugation of the adsorption potential. In the case of GF such corrugation is so large that ρ _s /ρ=0.6 at T=0 K and the superfluid is essentially restricted to move in a multiconnected space, along the bonds of a honeycomb lattice. We predict a superfluid transition temperature T? 0.25 (1.1) K for ⁴He on GF (GH). We have studied the elementary excitation spectrum of ⁴He on GF at equilibrium density finding a phonon–maxon–roton dispersion relation that is strongly anisotropic in the roton region. We conclude that these new platforms for adsorption studies offer the possibility of studying novel superfluid phases of quantum condensed matter.     

Bose fluid in restricted geometry: High-energy neutron scattering

The existence is discussed of a low temperature phase with infinite susceptibility in a Bose fluid in two dimensions and in thin films. In particular the high temperature approach to the relative phase transition is considered by a second-order theory based on the use of finite temperature Green functions. It is shown that the small momenta singularity of the momentum distribution present in the low temperature phase gives an anomalous contribution to the high-energy neutron scattering and its temperature dependence is very similar to the one expected in the case of scattering by a system with Bose-Einstein condensation and to the experimental data on bulk liquid ⁴ He. It is suggested that ⁴ He in a porous medium with an appropriate statistical distribution of pore size has an intermediate phase, characterized by an infinite susceptibility, between the high temperature normal phase and the low temperature phase with Bose-Einstein condensation.     

A study of the liquid phase of4He using an improved shadow wave function

The shadow wavefunction is generalized by introducing a pseudopotential with an attractive part for the auxiliary variables. We report variational Monte Carlo results for the ground state of a system of ⁴ He atoms in the liquid phase. The variational energy is close to the best energy obtained with the standard Jastrow plus triplet terms at the equilibrium density, and our results improve over those at higher densities. There is a marked improvement in the radial distribution function g(r) compared with that obtained using a purely repulsive auxiliary particle pseudopotential. In addition, the attractions between the shadow particles can be made strong enough to describe a self-binding system.     

Disorder Phenomena in Quantum Solids with Vacancies

Journal of Low Temperature Physics - We have investigated disorder phenomena in solid 4He near megting in presence of vacancies by means of Quantum Monte Carlo (MC) simulations using the Shadow...     

Roton wave–packets in 3–Dimensional and 2–Dimensional 4He

We have obtained new information on the elementary excitations in superfluid ⁴ He by studying local observables relative to roton wave–packets built from accurate variational representations of the excited states. The same technique has been applied also to study a pure 2–Dimensional system, where the spectrum, computed through a variational Monte Carlo simulation, still shows a modified phonon–maxon–roton dispersion relation. In the roton region, collective quantum effects are present, but when the average momentum of the wave–packet matches with the wave–vector of the roton minimum the interference effects between neighboring atoms disappear and the local density and momentum essentially coincide with those of a single particle excitation. A roton, as well as a single particle excitation, has a velocity vector field which is very similar to that of a smoke–ring scaled down to atomic size.     

Variational theory of rotons in superfluid4He

We have extended the shadow wave function for excited states by including an explicit backflow term in the shadow variables. The variational computation gives a roton energy 9.18 K, only 6% above experiment. Similar agreement is found at freezing density. Also the strength Z_q of the single excitation peak greatly improves. The uniform agreement between theory and experiment at all wave vectors suggests that there is no qualitative difference in the wave function of phonon, maxon and roton excitations. Only the amount of backflow and short range correlations greatly varies with k.     

Theory of the universal and nonuniversal quantities of fluids at the critical point

We show that the hierarchical reference theory is an accurate global theory of fluids at least above the critical temperatureT _c. The hierarchy is truncated at the first equation, the one connecting the free energy to the pair correlation function, with an Ornstein-Zernike ansatz. In this approximation the theory can be considered as a sophisticated generalization of the optimized random phase approximation which has genuine nonclassical critical exponents and for which scaling is satisfied. We study the system of hard spheres plus the Lennard-Jones attractive well and find a good agreement with measuredPVT, specific heat, correlation length, and structure factor in rare gases. The accuracy of the theory remains very good up to freezing density.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.