Length matters: keeping atomic wires in check

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.Comment: 6 pages, 5 figures, conference proceedings from 2014 MRS fall meetin

Nonconservative dynamics in long atomic wires

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.Comment: 20 pages, 8 figure

Molecular effects in the ionization of N2_2, O2_2 and F2_2 by intense laser fields

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.Comment: 19 pages, 5 figure

Inelastic quantum transport: the self-consistent Born approximation and correlated electron-ion dynamics

A dynamical method for inelastic transport simulations in nanostructures is compared with a steady-state method based on non-equilibrium Green's functions. A simplified form of the dynamical method produces, in the steady state in the weak-coupling limit, effective self-energies analogous to those in the Born Approximation due to electron-phonon coupling. The two methods are then compared numerically on a resonant system consisting of a linear trimer weakly embedded between metal electrodes. This system exhibits enhanced heating at high biases and long phonon equilibration times. Despite the differences in their formulation, the static and dynamical methods capture local current-induced heating and inelastic corrections to the current with good agreement over a wide range of conditions, except in the limit of very high vibrational excitations, where differences begin to emerge.Comment: 12 pages, 7 figure

Probing the role of excited states in ionization of acetylene

Ionization of acetylene by linearly-polarized, vacuum ultraviolet (VUV) laser pulses is modelled using time-dependent density functional theory. Several laser wavelengths are considered including one that produces direct ionization to the first excited cationic state while another excites the molecules to a Rydberg series incorporating an autoionizing state. We show that for the wavelengths and intensities considered, ionization is greatest whenever the molecule is aligned along the laser polarization direction. By considering high harmonic generation we show that populating excited states can lead to a large enhancement in the harmonic yield. Lastly, angularly-resolved photoelectron spectra are calculated which show how the energy profile of the emitted electrons significantly changes in the presence of these excited states

Iron and Steel Statistical Bulletin quarterly 6-1975

High-order harmonic generation (HHG) from aligned acetylene molecules interacting with mid infra-red (IR), linearly polarized laser pulses is studied theoretically using a mixed quantum-classical approach in which the electrons are described using time-dependent density functional theory while the ions are treated classically. We find that for molecules aligned perpendicular to the laser polarization axis, HHG arises from the highest-occupied molecular orbital (HOMO) while for molecules aligned along the laser polarization axis, HHG is dominated by the HOMO-1. In the parallel orientation we observe a double plateau with an inner plateau that is produced by ionization from and recombination back to an autoionizing state. Two pieces of evidence support this idea. Firstly, by choosing a suitably tuned vacuum ultraviolet pump pulse that directly excites the autoionizing state we observe a dramatic enhancement of all harmonics in the inner plateau. Secondly, in certain circumstances, the position of the inner plateau cut-off does not agree with the classical three-step model. We show that this discrepancy can be understood in terms of a minimum in the dipole recombination matrix element from the continuum to the autoionizing state. As far as we are aware, this represents the first observation of harmonic enhancement over a wide range of frequencies arising from autoionizing states in molecules.Comment: 8 pages, 8 figures; discussion extended on referees' recommendations, extra figures added, and explanation of cutoff position revise

A discrete time-dependent method for metastable atoms in intense fields

The full-dimensional time-dependent Schrodinger equation for the electronic dynamics of single-electron systems in intense external fields is solved directly using a discrete method. Our approach combines the finite-difference and Lagrange mesh methods. The method is applied to calculate the quasienergies and ionization probabilities of atomic and molecular systems in intense static and dynamic electric fields. The gauge invariance and accuracy of the method is established. Applications to multiphoton ionization of positronium and hydrogen atoms and molecules are presented. At very high intensity above saturation threshold, we extend the method using a scaling technique to estimate the quasienergies of metastable states of the hydrogen molecular ion. The results are in good agreement with recent experiments.Comment: 10 pages, 9 figure, 4 table

Prediction of broad-spectrum pathogen attachment to coating materials for biomedical devices

Bacterial infections in healthcare settings are a frequent accompaniment to both routine procedures such as catheterization and surgical site interventions. Their impact is becoming even more marked as the numbers of medical devices that are used to manage chronic health conditions and improve quality of life increases. The resistance of pathogens to multiple antibiotics is also increasing, adding an additional layer of complexity to the problems of employing safe and effective medical procedures. One approach to reducing the rate of infections associated with implanted and indwelling medical devices is the use of polymers that resist the formation of bacterial biofilms. To significantly accelerate the discovery of such materials, we show how state of the art machine learning methods can generate quantitative predictions for the attachment of multiple pathogens to a large library of polymers in a single model for the first time. Such models facilitate design of polymers with very low pathogen attachment across different bacterial species that will be candidate materials for implantable or indwelling medical devices such as urinary catheters, cochlear implants and pacemakers

Evaluation of the Healthy Start Voucher Scheme in UK: a natural experiment using the Growing Up in Scotland record linkage study and the Infant Feeding Survey

Background: Having a good start in life during pregnancy and infancy has been shown to be important for living both a healthy life and a longer life. Despite the introduction of many policies for the early-years age group, including voucher schemes, with the aim of improving nutrition, there is limited evidence of their impact on health. Objectives: To assess the effectiveness of the Healthy Start voucher scheme on infant, child and maternal outcomes, and to capture the lived experiences of the Healthy Start voucher scheme for low-income women. Design: This was a natural experiment study using existing data sets, linked to routinely collected health data sets, with a nested qualitative study of low-income women and an assessment of the health economics. Setting: Representative sample of Scottish children and UK children. Participants: Growing Up in Scotland cohort 2 (n = 2240), respondents to the 2015 Infant Feeding Study (n = 8067) and a sample of 40 participants in the qualitative study. Interventions: The Health Start voucher, a means-tested scheme that provides vouchers worth £3.10 per week to spend on liquid milk, formula milk, fruit and vegetables. Main outcome measures: Infant and child outcomes – breastfeeding initiation and duration; maternal outcomes – vitamin use pre and during pregnancy. Results: The exposed group were women receiving the Healthy Start voucher (R), with two control groups: eligible and not claiming the Healthy Start voucher (E) and nearly eligible. There was no difference in vitamin use during pregnancy for either comparison (receiving the Healthy Start voucher, 82%; eligible and not claiming the Healthy Start voucher, 86%; p = 0.10 vs. receiving the Healthy Start voucher, 87%; nearly eligible, 88%; p = 0.43) in the Growing Up in Scotland cohort. Proportions were similar for the Infant Feeding Study cohort (receiving the Healthy Start voucher, 89%; eligible and not claiming the Healthy Start voucher, 86%; p = 0.01 vs. receiving the Healthy Start voucher, 89%; nearly eligible, 87%; p = 0.01); although results were statistically significantly different, these were small effect sizes. There was no difference for either comparison in breastfeeding initiation or breastfeeding duration in months in Growing Up in Scotland, but there was a negative effect of the Healthy Start voucher in the Infant Feeding Survey. This contrast between data sets indicates that results are inconclusive for breastfeeding. The qualitative study found that despite the low monetary value the women valued the Healthy Start voucher scheme. However, the broader lives of low-income women are crucial to understand the constraints to offer a healthy diet. Limitations: Owing to the policy being in place, it was difficult to identify appropriate control groups using existing data sources, especially in the Infant Feeding Study. Conclusions: As the Healthy Start voucher scheme attempts to influence health behaviour, this evaluation can inform other policies aiming to change behaviour and use voucher incentives. The null effect of Healthy Start vouchers on the primary outcomes may be due to the value of the vouchers being insufficient to change the broader lives of low-income women to offer a healthy diet. Future work: The methods developed to undertake an economic evaluation alongside a natural experiment using existing data can be used to explore the cost-effectiveness of the Healthy Start voucher scheme. Funding: This project was funded by the National Institute for Health and Care Research (NIHR) Public Health Research programme and will be published in full in Public Health Research; Vol. 11, No. 11. See the NIHR Journals Library website for further project information