Petri Salo

Energetics and Vibrational States for Hydrogen on Pt(111)

We present a combination of theoretical calculations and experiments for the low-lying vibrational excitations of H and D atoms adsorbed on the Pt(111) surface. The vibrational band states are calculated based on the full three-dimensional adiabatic potential energy surface obtained from first principles calculations. For coverages less than three quarters of a monolayer, the observed experimental high-resolution electron peaks at 31 and 68meV are in excellent agreement with the theoretical transitions between selected bands. Our results convincingly demonstrate the need to go beyond the local harmonic oscillator picture to understand the dynamics of this system.

Energetics and atomic mechanisms of dislocation nucleation in strained epitaxial layers

We numerically study the energetics and atomic mechanisms of misfit dislocation nucleation and stress relaxation in a two-dimensional atomistic model of strained epitaxial layers on a substrate with lattice misfit. Relaxation processes from coherent to incoherent states for different transition paths are studied using interatomic potentials of Lennard-Jones type and a systematic saddle-point and transition-path search method. The method is based on a combination of a repulsive potential minimization and the nudged elastic band method. For a final state with a single misfit dislocation, the minimum-energy path and the corresponding activation barrier are obtained for different misfits and interatomic potentials. We find that the energy barrier decreases strongly with misfit. In contrast to continuous elastic theory, a strong tensile-compressive asymmetry is observed. This asymmetry can be understood as a manifestation of the asymmetry between repulsive and attractive branches of the pair potential, and it is found to depend sensitively on the form of the potential.

Minimum energy paths for dislocation nucleation in strained epitaxial layers

We study numerically the minimum energy path and energy barriers for dislocation nucleation in a two-dimensional atomistic model of strained epitaxial layers on a substrate with lattice misfit. Stress relaxation processesfrom coherent to incoherent states for different transition paths are determined using saddle point search based on a combination of repulsive potential minimization and the Nudged Elastic Band method. The minimum energy barrier leading to a final state with a single misfit dislocation nucleation is determined. A strong tensile-compressive asymmetry is observed. This asymmetry can be understood in terms of the qualitatively different transition paths for the tensile and compressive strains.

Electronic properties of H on vicinal Pt surfaces: First-principles study

In this work, we use the first-principles density-functional approach to study the electronic structure of a H atom adsorbed on the ideal Pt(111) and vicinal Pt(211) and Pt(331) surfaces. Full three-dimensional potential-energy surfaces (3D PESs) as well as local electronic density of states on various adsorption sites are obtained. The results show that the steps modify the PES considerably. The effect is nonlocal and extends into the region of the (111) terraces. We also find that different type of steps have different kind of influence on the PES when compared to the one of the ideal Pt(111) surface. The full 3D PESs calculated in this work provide a starting point for the theoretical study of vibrational and diffusive dynamics of H adatoms adsorbed on these vicinal surfaces.

Relaxation of twisted vortices in the Faddeev-Skyrme model

Abstract We study vortex knotting in the Faddeev–Skyrme model. Starting with a straight vortex line twisted around its axis we follow its evolution under dissipative energy minimization dynamics. With low twist per unit length the vortex forms a helical coil, but with higher twist numbers the vortex becomes knotted or a ring is formed around the vortex.

Topologically nontrivial configurations associated with Hopf charges investigated in the two-component Ginzburg-Landau model

We study the stability of Hopfions embedded in the Ginzburg-Landau (GL) model of two oppositely charged components. It has been shown by Babaev et al. [Phys. Rev. B 65, 100512 (2002)] that this model contains the Faddeev-Skyrme (FS) model, which is known to have topologically stable configurations with a given Hopf charge, the so-called Hopfions. Hopfions are typically formed from a unit-vector field that points to a fixed direction at spatial infinity and locally forms a knot with a soft core. The GL model, however, contains extra fields beyond the unit-vector field of the FS model and this can in principle change the fate of topologically non-trivial configurations. We investigate the stability of Hopfions in the two-component GL model both analytically (scaling) and numerically (first order dissipative dynamics). A number of initial states with different Hopf charges are studied; we also consider various different scalar potentials, including a singular one. In all the cases studied, we find that the Hopfions tend to shrink into a thin loop that is too close to a singular configuration for our numerical methods to investigate.

First-principles study for the adsorption of segments of BPA-PC on a-Al2O3(0001)

We have studied the adsorption of bisphenol-A-polycarbonate (BPA-PC) on the alpha-Al2O3(0001) surface using density-functional theory (DFT) with van der Waals (vdW) corrections. The BPA-PC polymer can be divided into its chemical fragments which are phenylene, carbonate and isopropylidene groups. We have calculated the adsorption energy and geometry of the BPA-PC segments that consist of two to three adjacent groups of the polymer. Our DFT results show that the adsorption is dominated by the vdW interaction. It is also important to include the interaction of nearest-neighbor groups in order to provide a realistic environment for the adsorption of the polymer onto the surface. Our results also show that the BPA-PC molecule attaches to the alumina surface via the carbonate group located in the middle of the molecule chain.

Characteristics of S adsorption on Pd vicinal surfaces

Results of first principles calculations for two types of vicinals of Pd(1 1 1) and Pd(1 1 0), with 3-atom wide terraces, are reported. We discuss the structure of the stepped surfaces, and the preferred adsorption sites and adsorption energies for S. We show that the adsorbate can have considerable effect on the relaxations and registry of substrate atoms and that the effect is most pronounced for Pd(3 2 0) which also displays the strongest adsorption energies for S. 2003 Elsevier Science B.V. All rights reserved.

Mechanisms of dislocation nucleation in strained epitaxial layers

Using molecular static simulations we have studied possible mechanisms of stress relaxation with misfit dislocation nucleation in strained heteroepitaxial layers. Two-dimensional models of atomic systems with Lennard-Jones type potential were considered. Combination of total energy minimization with spherical repulsion and nudged elastic band calculation allowed us to study possible transitions between an initial coherent interface state and a final state with a single dislocation. Different transition paths involving either successive relaxation of layers or concerted sliding along atomic rows have been identified as possible mechanisms with low activation energies. The role of these mechanisms in heteroepitaxial thin film growth and possible implementations in three dimensions are discussed.

Surface instability and dislocation nucleation in strained epitaxial layers

We have studied numerically the stability and defect nucleation in epitaxial layers on a substrate with lattice mismatch. Stress relaxation and energy barriers for misfit dislocation nucleation are estimated using modern methods for saddle point search based on a combination of activation with local repulsive potential and the Nudged Elastic Band method. Stress relaxation processes correspond to different transition paths from coherent to incoherent states of the epitaxial layer.Using a two-dimensional atomistic model with Lennard-Jones interacting potential, we and different equilibrium critical thickness and activation energy behavior for dislocation nucleation of epitaxial films under tensile and compressive strain. For tensile strain, the energy barrier decreases with thickness while it reaches a constant value for compressive strain.