38 research outputs found
The Debye-Waller Factor in solid 3He and 4He
The Debye-Waller factor and the mean-squared displacement from lattice sites
for solid 3He and 4He were calculated with Path Integral Monte Carlo at
temperatures between 5 K and 35 K, and densities between 38 nm^(-3) and 67
nm^(-3). It was found that the mean-squared displacement exhibits finite-size
scaling consistent with a crossover between the quantum and classical limits of
N^(-2/3) and N^(-1/3), respectively. The temperature dependence appears to be
T^3, different than expected from harmonic theory. An anisotropic k^4 term was
also observed in the Debye-Waller factor, indicating the presence of
non-Gaussian corrections to the density distribution around lattice sites. Our
results, extrapolated to the thermodynamic limit, agree well with recent values
from scattering experiments.Comment: 5 figure
The helium trimer with soft-core potentials
The helium trimer is studied using two- and three-body soft-core potentials.
Realistic helium-helium potentials present an extremely strong short-range
repulsion and support a single, very shallow, bound state. The description of
systems with more than two helium atoms is difficult due to the very large
cancellation between kinetic and potential energy. We analyze the possibility
of describing the three helium system in the ultracold regime using a gaussian
representation of a widely used realistic potential, the LM2M2 interaction.
However, in order to describe correctly the trimer ground state a three-body
force has to be added to the gaussian interaction. With this potential model
the two bound states of the trimer and the low energy scattering helium-dimer
phase shifts obtained with the LM2M2 potential are well reproduced.Comment: 15 pages, 3 figures, submitted to Few-Body System
Microscopic study of the He2-SF6 trimers
The He2-SF6 trimers, in their different He isotopic combinations, are studied
both in the framework of the correlated Jastrow approach and of the Correlated
Hyperspherical Harmonics expansion method. The energetics and structure of the
He-SF6 dimers are analyzed, and the existence of a characteristic rotational
band in the excitation spectrum is discussed, as well as the isotopic
differences. The binding energies and the spatial properties of the trimers, in
their ground and lowest lying excited states, obtained by the Jastrow ansatz
are in excellent agreement with the results of the converged CHH expansion. The
introduction of the He-He correlation makes all trimers bound by largely
suppressing the short range He-He repulsion.
The structural properties of the trimers are qualitatively explained in terms
of the shape of the interactions, Pauli principle and masses of the
constituents.Comment: 17 pages, 5 figures. Submitted to PR
The ^4He trimer as an Efimov system
We review the results obtained in the last four decades which demonstrate the
Efimov nature of the He three-atomic system.Comment: Review article for a special issue of the Few-Body Systems journal
devoted to Efimov physic
Universality in the Three-Body Problem for 4He Atoms
The two-body scattering length a for 4He atoms is much larger than their
effective range r_s. As a consequence, low-energy few-body observables have
universal characteristics that are independent of the interaction potential.
Universality implies that, up to corrections suppressed by r_s/a, all
low-energy three-body observables are determined by a and a three-body
parameter \Lambda_*. We give simple expressions in terms of a and \Lambda_* for
the trimer binding energy equation, the atom-dimer scattering phase shifts, and
the rate for three-body recombination at threshold. We determine \Lambda_* for
several 4He potentials from the calculated binding energy of the excited state
of the trimer and use it to obtain the universality predictions for the other
low-energy observables. We also use the calculated values for one potential to
estimate the effective range corrections for the other potentials.Comment: 23 pages, revtex4, 6 ps figures, references added, universal
expressions update
Quantum Rotation of HCN and DCN in Helium-4
We present calculations of rotational absorption spectra of the molecules HCN
and DCN in superfluid helium-4, using a combination of the Diffusion Monte
Carlo method for ground state properties and an analytic many-body method
(Correlated Basis Function theory) for the excited states. Our results agree
with the experimentally determined effective moment of inertia which has been
obtained from the spectral transition. The correlated basis function
analysis shows that, unlike heavy rotors such as OCS, the J=2 and higher
rotational excitations of HCN and DCN have high enough energy to strongly
couple to rotons, leading to large shifts of the lines and accordingly to
anomalous large spectroscopic distortion constants, to the emergence of
roton-maxon bands, and to secondary peaks in the absorption spectra for J=2 and
J=3.Comment: accepted by Phys. Rev. B; changes: included referee suggestions,
removed typos, added 10 ref
Espécies crípticas em Pagamea coriacea sensu lato (Rubiaceae): evidências morfológicas, ecológicas e de comportamento reprodutivo em um contexto simpátrico
In this study we explore morphological and ecological variation in sympatric populations of Pagamea coriacea s.l. - a species complex from white-sand vegetation in the Amazon. A total of 147 trees were sampled and monitored at three nearby sites in Central Amazon, Brazil. Multivariate analyses of morphology indicated two distinct groups (A and B), which also differed in bark type, each containing subgroups associated with sexual dimorphism. However, a single hermaphroditic individual was observed within group B. As expected, all pistillate plants produced fruits, but 23% of the staminate plants of group B, and 5% of group A also produced fruits. This variation suggests that the sexual systems of both groups are between dioecy and gynodioecy. There was an overlap in flowering phases between the two groups, but the pattern of floral maturation differed. Ecologically, plants of group B were found in more shaded habitats and over sandstone bedrocks, while group A was prevalent in deeper sandy soils as canopy plants. The significances of morphological and environmental differences were tested by a multivariate analysis of variance, and a canonical discriminant analysis assessed the importance of variables. The coexistence in sympatry of two discrete morphological groups in the P. coriacea s.l., with different habitat preferences and reproductive behaviors, indicates they represent distinct species.O objetivo deste estudo foi testar se o complexo de espécies Pagamea coriacea inclui múltiplas espécies, usando evidências morfológicas, ecológicas e de comportamento reprodutivo. Um total de 147 árvores foram amostradas e monitoradas em três locais próximos na Amazônia Central, Brasil. Análises multivariadas de dados morfológicos indicaram dois grupos discretos (A e B), que coincidem com diferenças no tipo de casca, cada um por sua vez com subgrupos que representam dimorfismo sexual. Contudo, um indivíduo distintamente hermafrodita foi encontrado no grupo B. Todas as plantas pistiladas produziram frutos, mas também foram observados frutos para 23% das plantas estaminadas do grupo B, e para 5% das estaminadas do grupo A. Essa variação sugere que ambos grupos possuem um sistema sexual entre dioicia e ginodioicia. Houve sobreposição das fases de floração e frutificação entre os dois grupos, mas o padrão de maturação de flores foi diferente. Quanto ao hábitat, o grupo B predominou em situações de sombra e em solos arenosos rasos sobre lajes de pedra, enquanto o grupo A ocorreu em solos arenosos mais profundos e no dossel da vegetação. A coexistência de dois grupos morfológicos discretos no complexo P. coriacea s.l., com diferenças ecológicas e de comportamento reprodutivo, indica que correspondem a duas espécies distintas