3,581 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]
First-principles modeling of three-body interactions in highly compressed solid helium
We present a new set of three-body interaction models based on the
Bruch-McGee (BM) potential that are suitable for the study of the energy,
structural and elastic properties of solid 4He at high pressure. Our ab initio
three-body potentials are obtained from the fit to total energies and atomic
forces computed with the van der Waals density functional theory method due to
Grimme, and represent an improvement with respect to previously reported
three-body interaction models. In particular, we show that some of the
introduced BM parametrizations reproduce closely the experimental equation of
state and bulk modulus of solid helium up to a pressure of ~ 60 GPa, when used
in combination with standard pairwise interaction models in diffusion Monte
Carlo simulations. Importantly, we find that recent predictions reporting a
surprisingly small variation of the kinetic energy and Lindeman ratio on
quantum crystals under increasing pressure are likely to be artifacts produced
by the use of incomplete interaction models. Also, we show that the
experimental variation of the shear modulus, C44, at P < 25 GPa can be
quantitatively described with the new set of three-body BM potentials. At
higher pressures, however, the agreement between our C44 results and
experiments deteriorates and thus we argue that higher order many-body terms in
the expansion of the atomic interactions probably are necessary in order to
better describe elasticity in very dense solid 4He.Comment: 11 pages, 7 figure
Possible superfluidity of molecular hydrogen in a two-dimensional crystal phase of sodium
We theoretically investigate the ground-state properties of a molecular
para-hydrogen (p-H2) film in which crystallization is energetically frustrated
by embedding sodium (Na) atoms periodically distributed in a triangular
lattice. In order to fully deal with the quantum nature of p-H2 molecules, we
employ the diffusion Monte Carlo method and realistic semi-empirical pairwise
potentials describing the interactions between H2-H2 and Na-H2 species. In
particular, we calculate the energetic, structural and superfluid properties of
two-dimensional Na-H2 systems within a narrow density interval around
equilibrium at zero temperature. In contrast to previous computational studies
considering other alkali metal species such as rubidium and potassium, we find
that the p-H2 ground-state is a liquid with a significantly large superfluid
fraction of ~30%. The appearance of p-H2 superfluid response is due to the fact
that the interactions between Na atoms and H2 molecules are less attractive
than between H2 molecules. This induces a considerable reduction of the
hydrogen density which favours the stabilization of the liquid phase.Comment: 7 pages, 6 figures, submitte
The Limit of Mechanical Stability in Quantum Crystals: A Diffusion Monte Carlo Study of Solid 4He
We present a first-principles study of the energy and elastic properties of
solid helium at pressures below the range in which is energetically stable. We
find that the limit of mechanical stability in hcp 4He is = -33.82 bar,
which lies significantly below the spinodal pressure found in the liquid phase
(i.e., -9.6 bar). Furthermore, we show that the pressure variation of the
transverse and longitudinal sound velocities close to do not follow a
power law of the form , in contrast
to what is observed on the fluid.Comment: 4 pages, 4 figure
Temperature Dependence of the Vacancy Formation Energy in Solid He
We studied the thermal effects on the behavior of incommensurate solid He
at low temperatures using the path integral Monte Carlo method. Below a certain
temperature, depending on the density and the structure of the crystal, the
vacancies delocalize and a finite condensate fraction appears. We calculated
the vacancy formation energy as a function of the temperature and observed a
behavior compatible with a two-step structure, with a gap of few K appearing at
the onset temperature of off-diagonal long-range order. Estimation of the
energy cost of creating two vacancies seems to indicate an effective attractive
interaction among the vacancies but the large error inherent to its numerical
estimation precludes a definitive statement.Comment: Contribution to the Special Issue on "Quantum Crystals": 9 pages, 3
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