Metastable solid 4He and the possible role of point defects.

The metastable phase of solid 4He and the possible role of point defects in its destabilization are investigated by the introduction of a trial function of the shadow class with an explicit symmetrical kernel. This is a trial function that ensures the possible exchange of atoms and the delocalization of atoms and defects in a very effective manner. We show that the formation energy for vacancies is equal to zero at a pressure Pc = 20 ± 2 atm, which is in excellent agreement with the experimental observation. The pressure at which a self-interstitial also has a formation energy equal to zero, is in agreement with the density where vacancies have the same property. Formation energies of a 3He interstitial or a substitutional impurity were estimated. Other properties of interest for systems made from 4He atoms are estimated and compared with results from the literature whenever available.

Ground-State Properties of Unitary Bosons: From Clusters to Matter.

The properties of cold Bose gases at unitarity have been extensively investigated in the last few years both theoretically and experimentally. In this Letter we use a family of interactions tuned to two-body unitarity and very weak three-body binding to demonstrate the universal properties of both clusters and matter. We determine the universal properties of finite clusters up to 60 particles and, for the first time, explicitly demonstrate the saturation of energy and density with particle number and compare with bulk properties. At saturation in the bulk we determine the energy, density, two- and three-body contacts, and the condensate fraction. We find that uniform matter is more bound than three-body clusters by nearly 2 orders of magnitude, the two-body contact is very large in absolute terms, and yet the condensate fraction is also very large, greater than 90%. Equilibrium properties of these systems may be experimentally accessible through rapid quenching of weakly interacting boson superfluids.

Solid

Properties of helium atoms in the solid phase are investigated using the multiweight diffusion Monte Carlo method. Two different importance function transformations are used in two series of independent calculations. The kinetic energy is estimated for both the solid and liquid phases of ^{4}He. We estimate the melting and freezing densities, among other properties of interest. Our estimates are compared with experimental values. We discuss why walkers biased by two distinctly different guiding functions do not lead to noticeable changes in the reported results. Criticisms concerning the bias introduced into our estimates by population control and system size effects are considered.

Efficient implementation of the Hellmann-Feynman theorem in a diffusion Monte Carlo calculation.

Kinetic and potential energies of systems of (4)He atoms in the solid phase are computed at T = 0. Results at two densities of the liquid phase are presented as well. Calculations are performed by the multiweight extension to the diffusion Monte Carlo method that allows the application of the Hellmann-Feynman theorem in a robust and efficient way. This is a general method that can be applied in other situations of interest as well.

Dislocation mobility in a quantum crystal: the case of solid 4He.

We investigate the structure and mobility of dislocations in hcp 4He crystals. In addition to fully characterizing the five elastic constants of this system, we obtain direct insight into dislocation core structures on the basal plane, which demonstrates a tendency toward dissociation into partial dislocations. Moreover, our results suggest that intrinsic lattice resistance is an essential factor in the mobility of these dislocations. This insight sheds new light on the possible correlation between dislocation mobility and the observed macroscopic behavior of crystalline 4He.