Structural Properties Of Small Clusters Research Articles

Path integral simulations of confined parahydrogen molecules within clathrate hydrates: Merging low temperature dynamics with the zero-temperature limit.

Clathrate hydrates, or cages comprised solely of water molecules, have long been investigated as a clean storage facility for hydrogen molecules. A breakthrough occurred when hydrogen molecules were experimentally placed within a structure-II clathrate hydrate, which sparked much interest to determine their feasibility for energy storage [Mao et al., Science 297, 2247-2249 (2002)]. We use Path Integral Molecular Dynamics (PIMD) and Langevin equation Path Integral Ground State (LePIGS) for finite temperature and zero-temperature studies, respectively, to determine parahydrogen occupancy properties in the small dodecahedral (512) and large hexakaidecahedral (51264) sized cages that comprise the structure-II unit cell. We look at energetic and structural properties of small clusters of hydrogen, treated as point-like particles, confined within each of the different sized clathrates, and treated as rigid, to determine energetic and structural properties in the zero-temperature limit. Our predicted hydrogen occupancy within these two cage sizes is consistent with previous literature values. We then calculate the energies as a function of temperature and merge the low temperature results calculated using finite temperature PIMD with the zero-temperature results using LePIGS, demonstrating that the two methods are compatible.

Self-organized nanoscale multilayer growth in hyperthermal ion deposition

In the course of thin film growth by co-deposition of low energy mass selected carbon and metal (Au or Fe) ions, an effect of self-organization was found. Although carbon and metal ions were deposited quasi-simultaneously, a multilayer film structure of alternately metal-rich and metal-deficient layers was grown. The period of these layers is of the order of a few nanometers $(\ensuremath{\sim}6\text{\ensuremath{-}}20\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$, and the metal-rich layers consist of metallic nanocrystals. The multilayer formation process is discussed in comparison with earlier studies on $\mathrm{C}\ensuremath{-}\mathrm{Cu}$ and $\mathrm{C}\ensuremath{-}\mathrm{Ag}$ films with respect to the structural properties of small clusters of the different metals, the influence of sputtering yields, and the deposition parameters. For a variety of compound thin film materials we expect a multilayer structure to develop during simultaneous sputter deposition or ion beam deposition of the components. The suppositions for this scenario are: (a) the deposited elements are immiscible or there are immiscible phases of a compound material, (b) the sputtering yields of the film components imposed by the impinging species are in an appropriate range, and (c) one compound segregates at the surface.

Magnetism and structure of small clusters: An exact treatment of electron correlations.

The electronic, magnetic, and structural properties of small clusters are studied in the framework of the single-band Hubbard Hamiltonian. Results for various ground-state and excited-state many-body properties are presented, which were calculated exactly by means of Lanczos's numerical diagonalization method. A full geometry optimization is performed for N\ensuremath{\le}8 atoms by considering all possible nonequivalent cluster structures with fixed nearest-neighbor bond lengths. The most stable structure and the corresponding total spin S are obtained rigorously as a function of the Coulomb interaction strength U/t and number of electrons \ensuremath{\nu}. The resulting interplay between electron correlations, magnetism, and cluster structure is analyzed and the main trends as a function of N, U/t, and \ensuremath{\nu} are derived. The stability of cluster ferromagnetism is studied from two complementary points of view. First, for N\ensuremath{\le}8 and \ensuremath{\nu}=N+1, we determine exactly the stability of the ferromagnetic ground state with respect to electronic excitations and structural changes. It is shown that in small clusters the structural changes can be as important to the temperature dependence of the magnetization as the purely electronic excitations. Second, we determine the stability of the saturated ferromagnetic state with respect to single spin flips as a function of the band filling \ensuremath{\nu}/N. In this case a few selected larger clusters (7\ensuremath{\le}N\ensuremath{\le}43) in the strongly correlated limit (U/t\ensuremath{\rightarrow}+\ensuremath{\infty}) are considered. It is shown that the \ensuremath{\nu}/N dependence of the spin-flip energy \ensuremath{\Delta}${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{sf}}$ shows interesting electronic-shell-like oscillations, which reflect the characteristics of the single-particle energy-level structure and its dependence on the symmetry and size of the cluster. Finally, we conclude by discussing some of the limitations of the model together with relevant extensions. \textcopyright{} 1996 The American Physical Society.

EXACT MANY-BODY APPROACH TO MAGNETISM AND CLUSTER STRUCTURE

The magnetic and structural properties of small clusters having N≤8 atoms were determined rigorously in the framework of the Hubbard model by performing the exact many-body electronic calculations together with a complete geometry optimization. We discuss the resulting interplay between electron correlations, magnetism, and cluster structure at T=0, as well as the stability of cluster ferromagnetism against temperature-induced electronic excitation and structural changes.