Srećko Kilić

Helium molecules within carbon nanotubes

Abstract The ground-state properties of 3 He and 4 He dimers and trimers in infinite carbon nanotubes are studied. The ground state eigen-functions of a helium atom in tubes of different radii are obtained numerically and then fitted by analytic expressions. Total wave function is assumed to be a product of Jastrow–Feenberg pair correlation and one particle functions. After extensive Monte Carlo (MC) calculation in two successive stages, simple variational MC and diffusion MC it is found that binding depends on the tube radius and it has the maximum for a certain tube width. This radius is between 5 and 7 A , and the average distance between atoms in such tubes is ranges from 5 to 10 A . The difference in binding between bosons and fermions disappears for R→0.

Binding of Two Helium Atoms in Confined Geometries. II. Dimerization on Flat Attractive Substrates

We study the binding of dimers of helium isotopes in a geometry that corresponds to the adsorption on a flat substrate. By varying the width of the holding potential, we go continuously from a rigorously three-dimensional to a rigorously two-dimensional geometry. It is shown that the binding energy is significantly enhanced when the width of the holding potential is approximately equal to the range of the pair interaction.

He Atoms Adsorbed in Carbon Nanotubes and Nanotube Bundles

We calculate the energetics of 4He and 3He atoms in the presence of one, two, and three carbon single-walled nanotubes (SWNTs). The spectrum is obtained for three characteristic sites: inside a tube in the presence of other tubes, within an interstitial channel, and in a groove. Calculations have been performed using both an axially symmetric averaged helium-carbon potential and for a potential constructed as a sum of individual carbon-helium interactions. Thermodynamic properties of helium atoms adsorbed in carbon nanonotube bundles are then studied using the single-particle spectrum. A range of one-dimensional (1D) to two-dimensional (2D) behavior is observed.

Few helium atoms in quasi two-dimensional space

Abstract Two, three and four 3 He and 4 He atoms in quasi two-dimensional space above graphite and cesium surfaces and in “harmonic” potential perpendicular to the surface have been studied. Using some previously examined variational wave functions and the Diffusion Monte Carlo procedure, it has been shown that all molecules: dimers, trimers and tetramers, are bound more strongly than in pure two- and three-dimensional space. The enhancement of binding with respect to unrestricted space is more pronounced on cesium than on graphite. Furthermore, for 3 He 3 ( 3 He 4 ) on all studied surfaces, there is an indication that the configuration of a dimer and a “free” particle (two dimers) may be equivalently established.

3He Dimer and Trimer in Two Dimensions

Using variational procedure in infinite two dimensional space, the existence of one dimer and one trimer (spin-1/2) bound state, with binding energies below −0.014 mK and −0.0057 mK (respectively), is definitively found. In a holding harmonic potential the strongest binding is achieved for the width of about 3 Å. For dimer at 3.2 Å it is −8.39 mK and for trimer at 3Å it is −5.5 mK.

Helium 4 Dimer in Two Coaxial Adjacent Nanotubes

The ground state properties of the helium 4 dimer, within the geometry defined by two adjacent nanotubes with different radii and hard core walls, are considered using the Monte Carlo technique. In this geometry, that may be realized as a generalization of a cylindrical one, the dependence of the binding energy on the radius leads to an effective force which moves the molecule towards the region of minimal energy. Thus, in tubes with non-homogeneous cross sections, the motion of particles is realized in dimer form.

A theory of elementary excitation interactions in bose fluids

Using a generalized Brillouin-Wigner-Feenberg perturbation method, an expression for the interaction energy between quasiparticles, which is described by “ladder state diagrams”, is obtained. In the roton region it depends on density (pressure) and changes its sign when density increases if the bare two-body potential qualitatively has short range repulsion and longer range attraction. In the low momentum limit the interaction energy contains two constants what we interpret as a sign of the anharmonicity.

Binding of small number of He4 atoms

Abstract The wave function used in the analysis includes the correlations of short-range exp (-(α/r)β) and of long-range exp(-sr). Using Monte Carlo calculation for the binding energy of the 4 atoms of He4 we obtained the value E=-0,8·10-23 J. A comparison of the results obtained for the trimer (He4)3 and for the molecule (He4)4 shows an increase in binding energy per atom.

Application of the Ursell-Mayer method in the theory of spin-polarized atomic hydrogen

Abstract Employing the Ursell-Mayer method and Ljolje semi-free gas model analytic relations describing ground state properties (energy, pressure, compressibility, sound velocity, radial distribution function and one-particle density matrix) of spin-polarized atomic hydrogen were derived. The expressions are valid up to density 2 10 26 atoms/m 3 . It was found out that at density of 2 10 26 atoms/m 3 the condensation of particle in momentum space is 88% (at absolute zero).