Ground-state energy and stability limit of small 3He drops

Small and stable drops of 3He atoms can only exist above a minimum number of particles, due to the combination of the 3He atom Fermi statistics and its light mass. An accurate estimation of this minimum number using microscopic theory has been difficult due to the inhomogeneous and fermionic nature of these systems. We present a diffusion Monte Carlo calculation of 3He drops with sizes near the minimum in order to determine the stability threshold. The results show that the minimum self-bound drop is formed by N=30 atoms with preferred orbitals for open shells corresponding to maximum value of the spin.Comment: 5 pages, 4 figure

Dynamic Structure Function in 3he-4he Mixtures

Relevant features of the dynamic structure function $S(q,\omega)$ in $^3$He-$^4$He mixtures at zero temperature are investigated starting from known properties of the ground state. Sum rules are used to fix rigorous constraints to the different contributions to $S(q,\omega)$, coming from $^3$He and $^4$He elementary excitations, as well as to explore the role of the cross term $S^{(3,4)}(q,\omega)$. Both the low-$q$ (phonon-roton $^4$He excitations and 1p-1h $^3$He excitations) and high-$q$ (deep inelastic scattering) ranges are discussed.Comment: 29 pages, Plain TeX, 11 figures available by request from [email protected]

4He adsorbed outside a single carbon nanotube

The phase diagrams of $^4$He adsorbed on the external surfaces of single armchair carbon nanotubes with radii in the range 3.42 -- 10.85 \AA \ are calculated using the diffusion Monte Carlo method. For nanotubes narrower than a (10,10) one, the ground state is an incommensurate solid similar to the one found for H$_2$ on the same substrates. For wider nanotubes, the phase with the minimum energy per particle is a liquid layer. Curved $\sqrt 3 \times \sqrt 3$ registered solids similar to the ones found on graphene and graphite were unstable for all the tubes considered.Comment: 6 pages, accepted for publication in Phys. Rev.

Zero-temperature phase diagram of the second layer of 4^{\bf 4}He adsorbed on graphene

The phase diagram at zero temperature of $^4$He adsorbed on an helium incommensurate triangular solid on top of a single graphene sheet has been obtained using the diffusion Monte Carlo method. We have found that, in accordance with previous experimental and simulation results for graphite, the ground state of $^4$He on this setup is a liquid that, upon compression, transforms into a triangular solid. To define the stability limits of both liquid and solid phases, we considered not only the adsorption energies of the atoms located on the second layer but the average energy of the atoms in both layers. Our results show that the lower density limit for a stable liquid in the second layer is 0.163 $\pm$ 0.005 \AA$^{-2}$ and that the lower limit for the existence of an incommensurate solid on the second layer is 0.186 $\pm$ 0.003 \AA$^{-2}$. Both values are in overall agreement with the results of torsional oscillator experiments and heat capacity measurements on graphite. The 4/7 and 7/12 registered solids are found to be metastable with respect to triangular incommensurate arrangements of the same density.Comment: 7 pages, accepted for publication in Phys. Rev.

Finite H2 concentrations in superfluid 4He

We have studied the solubility of molecular hydrogen in bulk liquid $^4$He at zero temperature using the diffusion Monte Carlo method and realistic interatomic potentials between the different species of the mixture. Around the $^4$He equilibrium density, the H_2 molecules clusterize in liquid-like drops blocking the existence of a uniform dilution. On the contrary, at higher densities the cluster formation is less feasible and metastable dilute solutions may exist.Comment: 2 pages, 2 eps figures, contribution to the LT22 Conferenc