Modelling of defect states in covalent amorphous solids

Includes abstract.Includes bibliographical references (leaves 181-202).This study consists of the tight binding molecular dynamics simulations, of the bulk and defect structures, in hydrogenated amorphous silicon. Emphasis is given to the systematic analysis of the changes in the network structures, which occur due to the incorporation of hydrogen, at different levels of concentration. The atomic-level stress in the simulated structures is defined, in a manner that is consistent with the local structure. The concept of configurational landscapes is introduced, as a new analysis framework for the correlations between the stress fluctuations and the modifications of the local structure

Gr\"uneisen parameter as an entanglement compass

The Gr\"uneisen ratio $\Gamma$, i.e., the singular part of the ratio of thermal expansion to the specific heat, has been broadly employed to explore both finite-$T$ and quantum critical points (QCPs). For a genuine quantum phase transition (QPT), thermal fluctuations are absent and thus the thermodynamic $\Gamma$ cannot be employed. We propose a quantum analogue to $\Gamma$ that computes entanglement as a function of a tuning parameter and show that QPTs take place only for quadratic non-diagonal Hamiltonians. We showcase our approach using the quantum 1D Ising model with transverse field and Kane's quantum computer. The slowing down of the dynamics and thus the ``creation of mass'' close to any QCP/QPT is also discussed.Comment: 5 pages, 3 figures, comments are wellcome

Inhibiting the Laydown of Polymeric Carbon and Simultaneously Promoting Its Facile Burn-Off during the Industrial-Scale Production of Hydrogen with Nickel-Based Catalysts: Insights from Ab Initio Calculations

This paper presents a computational study of the mechanistic models for the laydown of carbon species on nickel surface facets and the burn-off models for their gasification mechanism in methane steam reforming based on density functional theory. Insights into catalyst design strategies for achieving the simultaneous inhibition of the laydown of polymeric carbon and the promotion of its burn-off are obtained by investigating the influence of single atom dopants on nickel surfaces. The effects of single atom dopants on adsorption energies are determined at both low and high carbon coverages on nickel and used to introduce appropriate thermodynamic descriptors of the associated surface reactions. It is found that the critical size of the nucleating polymeric carbon adatom contains three atoms, i.e., C3. The results show that the burn-off reaction of a polymeric carbon species is thermodynamically limited and hard to promote when the deposited carbon cluster grows beyond a critical size, C4. The introduction of single atom dopants into nickel surfaces is found to modify the structural stability and adsorption energies of carbon adatom species, as well as the free energy profiles of surface reactions for the burn-off reactions when CH4, H2O, H2, and CO species react to form hydrogen. The results reveal that materials development strategies that modify the sub-surface of the catalyst with potassium, strontium, or barium will inhibit carbon nucleation and promote burn-off, while surface doping with niobium, tungsten, or molybdenum will promote the laydown of polymeric carbon. This study provides underpinning insights into the reaction mechanisms for the coking of a nickel catalyst and the gasification routes that are possible for the recovery of a nickel catalyst during the steam reforming of methane for large-scale production of hydrogen

Higher-Order First Integrals of Autonomous Non-Riemannian Dynamical Systems

We consider autonomous holonomic dynamical systems defined by equations of the form q¨a=−Γbca(q)q˙bq˙c−Qa(q), where Γbca(q) are the coefficients of a symmetric (possibly non-metrical) connection and −Qa(q) are the generalized forces. We prove a theorem which for these systems determines autonomous and time-dependent first integrals (FIs) of any order in a systematic way, using the ’symmetries’ of the geometry defined by the dynamical equations. We demonstrate the application of the theorem to compute linear, quadratic, and cubic FIs of various Riemannian and non-Riemannian dynamical systems

Electrodynamics of Topologically Ordered Quantum Phases in Dirac Materials

First-principles calculations of the electronic ground state in tantalum arsenide are combined with tight-binding calculations of the field dependence of its transport model equivalent on the graphene monolayer to study the emergence of topologically ordered quantum states, and to obtain topological phase diagrams. Our calculations include the degrees of freedom for nuclear, electronic, and photonic interactions explicitly within the quasistatic approximation to the time-propagation-dependent density functional theory. This field-theoretic approach allows us to determine the non-linear response of the ground state density matrix to the applied electromagnetic field at distinct quantum phase transition points. Our results suggest the existence of a facile electronic switch between trivial and topologically ordered quantum states that may be realizable through the application of a perpendicular electric or magnetic field alongside a staggered-sublattice potential in the underlying lattice. Signatures of the near field electrodynamics in nanoclusters show the formation of a quantum fluid phase at the topological quantum phase transition points. The emergent carrier density wave transport phase is discussed to show that transmission through the collective excitation mode in multilayer heterostructures is a unique possibility in plasmonic, optoelectronic, and photonic applications when atomic clusters of Dirac materials are integrated within nanostructures, as patterned or continuous surfaces