Ab-Initio Simulation Laboratory

Motivated by the synthesis of the layered structure CoO2via Li atom deintercalation from LixCoO2, herein, we investigated the electronic structure, lattice dynamics, electron–phonon interaction, and superconductivity of monolayer CoO2 using first-principles calculations. This 2D material was predicted to have a ferromagnetic ground state with a metallic band structure and the total magnetization of 0.83μB. Remarkably, the non-spin polarized calculations show that the monolayer CoO2 possesses phonon-mediated superconductivity at 25–28 K owing to its intermediate to strong electron–phonon coupling (EPC). The rather strong EPC in this compound is mainly driven by the acoustic phonons, making this compound one of the highest-temperature superconductors among the existing 2D materials. Moreover, the CoO2 sheets could be synthesized via exfoliation from bulk CoO2 owing to the relatively small interlayer binding energy while maintaining its stability under normal experimental conditions. Compared to its bulk and bilayer counterparts, monolayer CoO2 was found to have highest EPC.

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Single differential and total cross sections of the electron-impact ionization of the hydrogen atom are calculated numerically in the two-potential distorted-wave approximation for excess energies from 0 to 1 eV. Partial-wave contributions to the cross sections are also investigated. The near-threshold cross section is parametrized by the power gamma and the proportionality constant a(0) for models with various asymptotic charges, and the dependence of a(0) on the asymptotic charge is studied. The validity range of the threshold power law is also discussed.

We have carried out first-principles calculations of Pb (111) films up to 25 monolayers to study the oscillatory quantum size effects exhibited in the surface energy and work function. These oscillations are correlated with the thickness dependence of the energies of confined electrons, which can be properly modeled by an energy-dependent phase shift of the electronic wave function upon reflection at the interface. It is found that a quantitative description of these quantum size effects requires a full consideration of the crystal band structure.

Atomically uniform Pb films are successfully prepared on Si(111), despite a large lattice mismatch. Angle-resolved photoemission measurements of the electronic structure show layer-resolved quantum well states which can be correlated with dramatic variations in thermal stability. The odd film thicknesses N=5, 7, and 9 monolayers show sharp quantum well states. The even film thicknesses N=6 and 8 do not, but are much more stable than the odd film thicknesses. This correlation is discussed in terms of a total energy calculation and Friedel-like oscillations in properties.

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We have used ab initio methods to study the possible transition between icosahedral (ico) and cuboctahedral (fcc) structures in lead nanoclusters of sizes up to 309 atoms. Spontaneous fcc-to-ico transition in Pb-13 was observed in the ab initio molecular dynamics (MD) simulations at various temperatures. The transition path can be described predominantly by an angular variable s, which can, generally be applied to the similar transitions in clusters of larger sizes and was observed to follow the Mackay model. We have calculated the two-dimensional energy surface that describes the transition in Pb-13 and found a barrierless fcc-to-ico transition path, which is consistent with the observed spontaneous transition in the ab initio MD simulations. The atomic displacements in the transition were identified as one of the vibrational eigenmodes of these two Pb-13 clusters. For clusters of larger sizes (Pb-n, where n = 55, 147, and 309), the possible transitions following similar paths were determined not to be barrierless and the sizes of the barriers were determined by the ab initio elastic band method.

The formation process of Al magic clusters on the Si(111)-7 x 7 surface was investigated by means of a variable-temperature scanning tunneling microscope (STM) in situ and was interpreted using density-functional theory (DFT) calculations. At a growth temperature of 450 degrees C, Al atoms hopped among the corner, center, and T4 sites and also across the dimer rows on the Si(111)-7 x 7 surface. At low coverage below 0.08 ML, a single Al atom was adsorbed on the corner or center site. When the coverage was increased to 0.08 ML, Al dimers and trimers appeared, and Al magic clusters were also observed. However, no Al tetramers or pentamers were experimentally confirmed. Careful analysis of STM images suggests that Al trimers could be key precursors for the formation of Al magic clusters, and DFT calculations verified this interpretation. Total-energy calculation results using DFT reveal that this is due to the small energy gain from Al trimer to Al tetramer. These results are important for understanding the atomic structure and the formation mechanism of the magic clusters on the Si(111)-7 x 7 surface.