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Search Results: 1 - 10 of 32734 matches for " Daniel Dundas "
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Multielectron effects in high harmonic generation in N2 and benzene: simulation using a non-adiabatic quantum molecular dynamics approach for laser-molecule interactions
Daniel Dundas
Physics , 2012, DOI: 10.1063/1.4718590
Abstract: A mixed quantum-classical approach is introduced which allows the dynamical response of molecules driven far from equilibrium to be modeled. This method is applied to the interaction of molecules with intense, short-duration laser pulses. The electronic response of the molecule is described using time-dependent density functional theory (TDDFT) and the resulting Kohn-Sham equations are solved numerically using finite difference techniques in conjunction with local and global adaptations of an underlying grid in curvilinear coordinates. Using this approach, simulations can be carried out for a wide range of molecules and both all-electron and pseudopotential calculations are possible. The approach is applied to the study of high harmonic generation in N2 and benzene using linearly-polarized laser pulses and, to the best of our knowledge, the results for benzene represent the first TDDFT calculations of high harmonic generation in benzene using linearly polarized laser pulses. For N2 an enhancement of the cut-off harmonics is observed whenever the laser polarization is aligned perpendicular to the molecular axis. This enhancement is attributed to the symmetry properties of the Kohn-Sham orbital that responds predominantly to the pulse. In benzene we predict that a suppression in the cut-off harmonics occurs whenever the laser polarization is aligned parallel to the molecular plane. We attribute this suppression to the symmetry-induced response of the highest-occupied molecular orbital.
High-harmonic generation in benzene with linearly- and circularly-polarised laser pulses
Abigail Wardlow,Daniel Dundas
Physics , 2015,
Abstract: High harmonic generation in benzene is studied using a mixed quantum-classical approach in which the electrons are described using time-dependent density functional theory while the ions move classically. The interaction with both circularly- and linearly-polarised infra-red ($\lambda = 800$ nm) laser pulses of duration 10 cycles (26.7 fs) is considered. The effect of allowing the ions to move is investigated as is the effect of including self-interaction corrections to the exchange-correlation functional. Our results for circularly-polarised pulses are compared with previous calculations in which the ions were kept fixed and self-interaction corrections were not included while our results for linearly-polarised pulses are compared with both previous calculations and experiment. We find that even for the short duration pulses considered here, the ionic motion greatly influences the harmonic spectra. While ionization and ionic displacements are greatest when linearly-polarised pulses are used, the response to circularly-polarised pulses is almost comparable, in agreement with previous experimental results.
Molecular effects in the ionization of N$_2$, O$_2$ and F$_2$ by intense laser fields
Daniel Dundas,Jan M. Rost
Physics , 2004, DOI: 10.1103/PhysRevA.71.013421
Abstract: In this paper we study the response in time of N$_2$, O$_2$ and F$_2$ to laser pulses having a wavelength of 390nm. We find single ionization suppression in O$_2$ and its absence in F$_2$, in accordance with experimental results at $\lambda = 800$nm. Within our framework of time-dependent density functional theory we are able to explain deviations from the predictions of Intense-Field Many-Body $S$-Matrix Theory (IMST). We confirm the connection of ionization suppression with destructive interference of outgoing electron waves from the ionized electron orbital. However, the prediction of ionization suppression, justified within the IMST approach through the symmetry of the highest occupied molecular orbital (HOMO), is not reliable since it turns out that, e.g. in the case of F$_2$, the electronic response to the laser pulse is rather complicated and does not lead to dominant depletion of the HOMO. Therefore, the symmetry of the HOMO is not sufficient to predict ionization suppression. However, at least for F$_2$, the symmetry of the dominantly ionized orbital is consistent with the non-suppression of ionization.
Length matters: keeping atomic wires in check
Brian Cunningham,Tchavdar N. Todorov,Daniel Dundas
Physics , 2015, DOI: 10.1557/opl.2015.197
Abstract: Dynamical effects of non-conservative forces in long, defect free atomic wires are investigated. Current flow through these wires is simulated and we find that during the initial transient, the kinetic energies of the ions are contained in a small number of phonon modes, closely clustered in frequency. These phonon modes correspond to the waterwheel modes determined from preliminary static calculations. The static calculations allow one to predict the appearance of non-conservative effects in advance of the more expensive real-time simulations. The ion kinetic energy redistributes across the band as non-conservative forces reach a steady state with electronic frictional forces. The typical ion kinetic energy is found to decrease with system length, increase with atomic mass, and its dependence on bias, mass and length is supported with a pen and paper model. This paper highlights the importance of non-conservative forces in current carrying devices and provides criteria for the design of stable atomic wires.
Nonconservative dynamics in long atomic wires
Brian Cunningham,Tchavdar N. Todorov,Daniel Dundas
Physics , 2014, DOI: 10.1103/PhysRevB.90.115430
Abstract: The effect of nonconservative current-induced forces on the ions in a defect-free metallic nanowire is investigated using both steady-state calculations and dynamical simulations. Non-conservative forces were found to have a major influence on the ion dynamics in these systems, but their role in increasing the kinetic energy of the ions decreases with increasing system length. The results illustrate the importance of nonconservative effects in short nanowires and the scaling of these effects with system size. The dependence on bias and ion mass can be understood with the help of a simple pen and paper model. This material highlights the benefit of simple preliminary steady-state calculations in anticipating aspects of brute-force dynamical simulations, and provides rule of thumb criteria for the design of stable quantum wires.
Non-conservative current-driven dynamics: beyond the nanoscale
Brian Cunningham,Tchavdar N. Todorov,Daniel Dundas
Physics , 2015,
Abstract: Long metallic nanowires combine crucial factors for non-conservative current-driven atomic mo- tion. These systems have degenerate vibrational frequencies, clustered about a Kohn anomaly in the dispersion relation, that can couple under current to form non-equilibrium modes of motion growing exponentially in time. Such motion is made possible by non-conservative current-induced forces on atoms, and we refer to it generically as the waterwheel effect. Here the connection be- tween the waterwheel effect and the stimulated directional emission of phonons propagating along the electron flow is discussed in an intuitive manner. Non-adiabatic molecular dynamics show that waterwheel modes self-regulate by reducing the current and by populating modes nearby in fre- quency, leading to a dynamical steady state in which non-conservative forces are counter-balanced by the electronic friction. The waterwheel effect can be described by an appropriate effective non- equilibrium dynamical response matrix. We show that the current-induced parts of this matrix in metallic systems are long-ranged, especially at low bias. This non-locality is essential for the characterisation of non-conservative atomic dynamics under current beyond the nanoscale.
Robert Griffiths Hodgins (1920–2010)
Neil Dundas
South African Journal of Science , 2010, DOI: 10.4102/sajs.v106i7/8.347
Understanding Code Patterns - Analysis, Interpretation & Measurement
Jitesh Dundas
Computer Science , 2011,
Abstract: This research paper aims to find, analyze and understand code patterns in any software system and measure its quality by defining standards and proposing a formula for the same. Every code that is written can be divided into different code segments, each having its own impact on the overall system. We can analyze these code segments to get the code quality. The measures used in this paper include Lines of Code, Number of calls made by a module, Execution time, the system knowledge of user and developers, the use of generalization, inheritance, reusability and other object-oriented concepts. The entire software code is divided into code snippets, based on the logic that they implement. Each of these code snippets has an impact. This measure is called Impact Factor and is valued by the software developer and/or other system stakeholders. Efficiency = (Code Area / Execution Time) * Qr
Automaton based detection of affected cells in three dimensional biological system
Jitesh Dundas
Computer Science , 2011,
Abstract: The aim of this research review is to propose the logic and search mechanism for the development of an artificially intelligent automaton (AIA) that can find affected cells in a 3-dimensional biological system. Research on the possible application of such automatons to detect and control cancer cells in the human body are greatly focused MRI and PET scans finds the affected regions at the tissue level even as we can find the affected regions at the cellular level using the framework. The AIA may be designed to ensure optimum utilization as they record and might control the presence of affected cells in a human body. The proposed models and techniques can be generalized and used in any application where cells are injured or affected by some disease or accident. The best method to import AIA into the body without surgery or injection is to insert small pill like automata, carrying material viz drugs or leukocytes that is needed to correct the infection. In this process, the AIA can be compared to nano pills to deliver or support therapy. NanoHive simulation software was used to validate the framework of this paper. The existing nanomedicine models such as obstacle avoidance algorithm based models (Hla K H S et al 2008) and the framework in this model were tested in different simulation based experiments. The existing models such as obstacle avoidance based models failed in complex environmental conditions (such as changing environmental conditions, presence of semi-solid particles, etc) while the model in this paper executed its framework successfully.Come systems biology, this field of automatons deserves a bigger leap of understanding especially when pharmacogenomics is at its peak. The results also indicate the importance of artificial intelligence and other computational capabilities in the proposed model for the successful detection of affected cells.
Law of Connectivity in Machine Learning
Jitesh Dundas
Computer Science , 2011,
Abstract: We present in this paper our law that there is always a connection present between two entities, with a selfconnection being present at least in each node. An entity is an object, physical or imaginary, that is connected by a path (or connection) and which is important for achieving the desired result of the scenario. In machine learning, we state that for any scenario, a subject entity is always, directly or indirectly, connected and affected by single or multiple independent / dependent entities, and their impact on the subject entity is dependent on various factors falling into the categories such as the existenc
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