993 research outputs found
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 in
He-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 , coming from He and He
elementary excitations, as well as to explore the role of the cross term
. Both the low- (phonon-roton He excitations and
1p-1h He excitations) and high- (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 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 on the same substrates. For wider nanotubes, the phase with the
minimum energy per particle is a liquid layer. Curved
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 He adsorbed on graphene
The phase diagram at zero temperature of 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 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 0.005 \AA and that the lower limit for
the existence of an incommensurate solid on the second layer is 0.186
0.003 \AA. 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 He at
zero temperature using the diffusion Monte Carlo method and realistic
interatomic potentials between the different species of the mixture. Around the
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
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