Nicholas Kioussis

Generalized-stacking-fault energy surface and dislocation properties of aluminum

We have employed the semidiscrete variational generalized Peierls-Nabarro model to study the dislocation properties of aluminum. The generalized-stacking-fault (GSF) energy surface entering the model is calculated by using first-principles density functional theory (DFT) and the embedded-atom method (EAM). Various core properties, including the core width, dissociation behavior, energetics, and Peierls stress for different dislocations have been investigated. The correlation between the core energetics and the Peierls stress with the dislocation character has been explored. Our results reveal a simple relationship between the Peierls stress and the ratio between the core width and the atomic spacing. The dependence of the core properties on the two methods for calculating the GSF energy (DFT vs EAM) has been examined. Although the EAM gives the general trend for various dislocation properties, it fails to predict the correct finer core structure, which in turn can affect the Peierls stress significantly (about one order of magnitude). (c) 2000 The American Physical Society.

Anomalous bias dependence of spin torque in magnetic tunnel junctions.

We predict an anomalous bias dependence of the spin transfer torque parallel to the interface, Tparallel, in magnetic tunnel junctions, which can be selectively tuned by the exchange splitting. It may exhibit a sign reversal without a corresponding sign reversal of the bias or even a quadratic bias dependence. We demonstrate that the underlying mechanism is the interplay of spin currents for the ferromagnetic (antiferromagnetic) configurations, which vary linearly (quadratically) with bias, respectively, due to the symmetric (asymmetric) nature of the barrier. The spin transfer torque perpendicular to interface exhibits a quadratic bias dependence.

Multiscale modelling of nanomechanics and micromechanics: an overview

Recent advances in analytical and computational modelling frameworks to describe the mechanics of materials on scales ranging from the atomistic, through the microstructure or transitional, and up to the continuum are reviewed. It is shown that multiscale modelling of materials approaches relies on a systematic reduction in the degrees of freedom on the natural length scales that can be identified in the material. Connections between such scales are currently achieved either by a parametrization or by a ‘zoom-out’ or ‘coarse-graining’ procedure. Issues related to the links between the atomistic scale, nanoscale, microscale and macroscale are discussed, and the parameters required for the information to be transferred between one scale and an upper scale are identified. It is also shown that seamless coupling between length scales has not yet been achieved as a result of two main challenges: firstly, the computational complexity of seamlessly coupled simulations via the coarse-graining approach and, secondly, the inherent difficulty in dealing with system evolution stemming from time scaling, which does not permit coarse graining over temporal events. Starting from the Born–Oppenheimer adiabatic approximation, the problem of solving quantum mechanics equations of motion is first reviewed, with successful applications in the mechanics of nanosystems. Atomic simulation methods (e.g. molecular dynamics, Langevin dynamics and the kinetic Monte Carlo method) and their applications at the nanoscale are then discussed. The role played by dislocation dynamics and statistical mechanics methods in understanding microstructure self-organization, heterogeneous plastic deformation, material instabilities and failure phenomena is also discussed. Finally, we review the main continuum-mechanics-based framework used today to describe the nonlinear deformation behaviour of materials at the local (e.g. single phase or grain level) and macroscopic (e.g. polycrystal) scales. Emphasis is placed on recent progress made in crystal plasticity, strain gradient plasticity and homogenization techniques to link deformation phenomena simultaneously occurring at different scales in the material microstructure with its macroscopic behaviour. In view of this wide range of descriptions of material phenomena involved, the main theoretical and computational difficulties and challenges are critically assessed.

Hydrogen-enhanced local plasticity in aluminum: an ab initio study.

Dislocation core properties of Al with and without H impurities are studied using the Peierls-Nabarro model with parameters determined by ab initio calculations. We find that H not only facilitates dislocation emission from the crack tip but also enhances dislocation mobility dramatically, leading to macroscopically softening and thinning of the material ahead of the crack tip. We observe strong binding between H and dislocation cores, with the binding energy depending on dislocation character. This dependence can directly affect the mechanical properties of Al by inhibiting dislocation cross-slip and developing slip planarity.

Room-temperature high on/off ratio in suspended graphene nanoribbon field-effect transistors

We have fabricated suspended few-layer (1-3 layers) graphene nanoribbon field-effect transistors from unzipped multi-wall carbon nanotubes. Electrical transport measurements show that current annealing effectively removes the impurities on the suspended graphene nanoribbons, uncovering the intrinsic ambipolar transfer characteristic of graphene. Further increasing the annealing current creates a narrow constriction in the ribbon, leading to the formation of a large bandgap and subsequent high on/off ratio (which can exceed 10(4)). Such fabricated devices are thermally and mechanically stable: repeated thermal cycling has little effect on their electrical properties. This work shows for the first time that ambipolar field-effect characteristics and high on/off ratios at room temperature can be achieved in relatively wide graphene nanoribbons (15-50 nm) by controlled current annealing.

The Peierls-Nabarro model revisited

We re-examine two important issues within the Peierls-Nabarro model, which are critical in obtaining accurate values for the Peierls stress. The first is related to the sampling scheme (double versus single counting) of the misfit energy across the glide plane and the second is the effect of atomic relaxation on the Peierls stress. We argue that the double-counting scheme is physically more appropriate. An analytical formula is derived for the Peierls stress of dislocations in alternating lattices. The atomic relaxation is shown to play an important role on the Peierls stress for narrow dislocations.

Interaction of vacancies with a grain boundary in aluminum: A first-principles study

We present a theoretical study of the interaction of vacancies with a tilt grain boundary in aluminum based on the density functional theory. The grain boundary volume expansion and vacancy induced contraction are calculated and compared for the nearest-neighbor atoms, and the former is found to be smaller than the latter. The formation energy of a vacancy placed at various layers in the grain boundary has been calculated and we find that it costs more energy to form a vacancy at the boundary plane than in bulk, although the rest of the grain boundary region does attract vacancies. The microscopic mechanisms of grain boundary sliding and migration are investigated thoroughly with and without a vacancy. We find that although the vacancy can hinder the grain boundary motion by tripling the energy barrier of sliding and migration, it cannot inhibit or even delay the migration process. The vacancy placed at the first layer from the interface is found to be trapped at the layer and not able to follow the migrating interface.

First principles determinations of the bonding mechanism and adsorption energy for CO/MgO(001)

Abstract Using the highly precise local density functional full potential linearized augmented plane wave (FLAPW) method, we have studied the bonding mechanism and other electronic properties of CO physisorption on the MgO(001) substrate. The calculated adsorption energy and the blue shift of the CO stretch frequency are 0.28 eV/molecule and 33 cm −1 , in good agreement with the corresponding experimental data of 0.3–0.43 eV/molecule and 35 cm −1 , respectively. The charge density contours indicate that a significant charge redistribution (bonding) is involved in the CO/MgO(001) interaction, in contrast with previous cluster calculations which suggested that the CO–MgO interaction is of the Van der Waals type.

An improved QM/MM approach for metals

We present an improved quantum mechanical (QM) and molecular mechanical (MM) coupling method for the study of metallic systems. The improved method is based on the earlier work of Choly et al (2005 Phys. Rev. B 71 094101). In this approach, quantum mechanical treatment is spatially confined to a small region, surrounded by a larger molecular mechanical region. This approach is particularly useful for systems where quantum mechanical interactions in a small region, such as lattice defects or chemical impurities, can affect the macroscopic properties of a material. We discuss how the coupling across the different scales can be accomplished efficiently and accurately for metals. The method is tested by performing a multiscale simulation of bulk aluminium (Al) where the coupling errors can be easily analysed. We then apply the method to study the core structure and Peierls stress of an edge dislocation in Al.

Thermally stable voltage-controlled perpendicular magnetic anisotropy in Mo|CoFeB|MgO structures

We study voltage-controlled magnetic anisotropy (VCMA) and other magnetic properties in annealed Mo|CoFeB|MgO layered structures. The interfacial perpendicular magnetic anisotropy (PMA) is observed to increase with annealing over the studied temperature range, and a VCMA coefficient of about 40 fJ/V-m is sustained after annealing at temperatures as high as 430 °C. Ab initio electronic structure calculations of interfacial PMA as a function of strain further show that strain relaxation may lead to the increase of interfacial PMA at higher annealing temperatures. Measurements also show that there is no significant VCMA and interfacial PMA dependence on the CoFeB thickness over the studied range, which illustrates the interfacial origin of the anisotropy and its voltage dependence, i.e., the VCMA effect. The high thermal annealing stability of Mo|CoFeB|MgO structures makes them compatible with advanced CMOS back-end-of-line processes, and will be important for integration of magnetoelectric random access memory into on-chip embedded applications.