Atomistic Theory Research Articles

Fine structure of light-hole excitons in nanowire quantum dots

Quantum dots with light-hole ground states could find numerous applications including faster quantum bit operations or coherent conversion of photons into electron spins. Typically, however, holes confined in epitaxial quantum dots are of heavy-hole character. I show, by use of atomistic tight-binding theory, that the hole ground state undergoes a transition from heavy holelike to light holelike with increasing height of a nanowire InAs/InP quantum dot. The fine structure of the light-hole exciton consists of a dark ground state and three bright states. Two of the bright states are quasidegenerate and are in-plane polarized, whereas, the third energetically higher bright state is polarized in the perpendicular out-of-plane direction. The light-hole exciton fine structure is robust against alloying.

On the induction period for crystallization in solute−solvent systems by polythermal method

The data of induction period tin derived from the data of maximum undercooling ΔTmax at different cooling rates R for some solute−solvent systems at different saturation temperature T0 are discussed using classical and atomistic theories of three‐dimensional nucleation. In the case of potassium nitrate aqueous solutions the tin data are analyzed in terms of the maximum supersaturation ratio Smax calculated from temperature dependence of solubility and the maximum undercooling ΔTmax and the values of solid−solution interfacial energy γ and number i* of molecules in the stable critically‐sized nuclei predited from tin(lnSmax) and tin(ΔTmax) data are compared. These results revealed that: (1) the values of supersaturation calculated as lnS from solubility data are lower than the real values expected from the chemical potential difference Δμ and that the description of Δμ in terms of undercooling ΔTmax is realistic, and (2) analysis of induction period data for solute−solvent systems should be carried out using applied undercooling ΔT or maximum undercooling ΔTmax instead of the usually used lnS or lnSmax. Examination of tin(ΔTmax) data for different systems showed that: (1) the value of γ lies between 1.7 and 13.2 mJ/m2 while that of i* between 2 and 50, and (2) in several cases atomistic nucleation theory describes the tin(ΔTmax) data adequately.

Genetic design of enhanced valley splitting towards a spin qubit in silicon

The long spin coherence time and microelectronics compatibility of Si makes it an attractive material for realizing solid-state qubits. Unfortunately, the orbital (valley) degeneracy of the conduction band of bulk Si makes it difficult to isolate individual two-level spin-1/2 states, limiting their development. This degeneracy is lifted within Si quantum wells clad between Ge-Si alloy barrier layers, but the magnitude of the valley splittings achieved so far is small—of the order of 1 meV or less—degrading the fidelity of information stored within such a qubit. Here we combine an atomistic pseudopotential theory with a genetic search algorithm to optimize the structure of layered-Ge/Si-clad Si quantum wells to improve this splitting. We identify an optimal sequence of multiple Ge/Si barrier layers that more effectively isolates the electron ground state of a Si quantum well and increases the valley splitting by an order of magnitude, to ∼9 meV.

Extended Interplanar Linking in Graphite Formed from Vacancy Aggregates

The mechanical and electrical properties of graphite and related materials such as multilayer graphene depend strongly on the presence of defects in the lattice structure, particularly those which create links between adjacent planes. We present findings which suggest the existence of a new type of defect in the graphite or graphene structure which connects adjacent planes through continuous hexagonal sp2 bonding alone and can form through the aggregation of individual vacancy defects. The energetics and kinetics of the formation of this type of defect are investigated with atomistic density functional theory calculations. The resultant structures are then employed to simulate high resolution transmission electron microscopy images, which are compared to recent experimental images of electron irradiation damaged graphite.

Semiclassical Monte-Carlo approach for modelling non-adiabatic dynamics in extended molecules

Modelling of non-adiabatic dynamics in extended molecular systems and solids is a next frontier of atomistic electronic structure theory. The underlying numerical algorithms should operate only with a few quantities (that can be efficiently obtained from quantum chemistry), provide a controlled approximation (which can be systematically improved) and capture important phenomena such as branching (multiple products), detailed balance and evolution of electronic coherences. Here we propose a new algorithm based on Monte-Carlo sampling of classical trajectories, which satisfies the above requirements and provides a general framework for existing surface hopping methods for non-adiabatic dynamics simulations. In particular, our algorithm can be viewed as a post-processing technique for analysing numerical results obtained from the conventional surface hopping approaches. Presented numerical tests for several model problems demonstrate efficiency and accuracy of the new method.

The Untenability of Atomistic Theory of Meaning

Atomistic theory of meaning or 'meaning atomism' expresses that each representation either in a linguistic or mental system is completely definable by itself. It explains that meaning of a sentence is determined independently of other sentences, which implies understanding a sentence does not require any further sentences for its support because, each sentence conveys a specific sense, and it is about the state of affairs of the world. Thus, 'sense' assists to identify the referent of a sentence. A sentence is meaningful if the state of affairs is found in the phenomenal world, and if it does not find in the phenomenal world, then it is judged as meaningless sentence. Meaning of a sentence is thus determined by its truth-value that stands as referent of the sentence. In this context, the paper argues how meaning atomism is inadequate to capture the whole complexity of meaning in the language system. Further, it highlights the logical problems in atomistic theory of meaning for not being qualified as a comprehensive theory of meaning. In this regard, we have considered some of the well-known traditional theories of meaning, such as; picture theory of meaning, verifiability theory of meaning, Tarski's T-Schema, Frege's notion of sense determination of propositions among others, and offer a few crucial arguments on the failure of atomistic theory of meaning in a language system.

Bottom-up modeling of Al/Ni multilayer combustion: Effect of intermixing and role of vacancy defects on the ignition process

Vapor deposited multilayered aluminum/oxide and bimetallics are promising materials for Micro Electro Mechanical System technologies as energy carriers, for instance, microinitiators or heat microsources in biological or chemical applications. Among these materials, the Al/Ni couple has received much attention both experimentally and theoretically. However, the detailed relation between the chemical composition of the intermixed interfacial regions and its impact on the ignition capabilities remains elusive. In this contribution, we propose a two-fold strategy combining atomistic density functional theory (DFT) calculations and a macroscopic 1D model of chemical kinetics. The DFT calculations allow the description of the elementary chemical processes (involving Al, Ni atoms and vacancies basic ingredients) and to parameterize the macroscopic model, in which the system is described as a stack of infinite layers. This gives the temporal evolution of the system composition and temperature. We demonstrate that the amount of vacancies, originating from the deposition process and the Al and Ni lattice mismatch, plays a critical role on both the ignition time and the temperature. The presence of vacancies enhances the migration of atoms between layers and so dramatically speeds up the atomic mixing at low temperatures far below ignition temperature, also pointing to the relation between experimental deposition procedures and ageing of the nanolaminates.

Dissolution Kinetics of Nanocrystals

A simple series of test-tube experiments is all it takes to quantify a largely neglected nano-effect responsible for a dramatic increase-by orders of magnitude-in the surface-area-normalized rate of dissolution of nanocrystals. Though the observed variation in this specific rate as a function of size is unprecedented, the effect may be rationalized in terms of the classic atomistic theory of crystal growth and dissolution.

Adaptive multiscale method for two-dimensional nanoscale adhesive contacts

There are two separate traditional approaches to model contact problems: continuum and atomistic theory. Continuum theory is successfully used in many domains, but when the scale of the model comes to nanometer, continuum approximation meets challenges. Atomistic theory can catch the detailed behaviors of an individual atom by using molecular dynamics (MD) or quantum mechanics, although accurately, it is usually time-consuming. A multiscale method coupled MD and finite element (FE) is presented. To mesh the FE region automatically, an adaptive method based on the strain energy gradient is introduced to the multiscale method to constitute an adaptive multiscale method. Utilizing the proposed method, adhesive contacts between a rigid cylinder and an elastic substrate are studied, and the results are compared with full MD simulations. The process of FE meshes refinement shows that adaptive multiscale method can make FE mesh generation more flexible. Comparison of the displacements of boundary atoms in the overlap region with the results from full MD simulations indicates that adaptive multiscale method can transfer displacements effectively. Displacements of atoms and FE nodes on the center line of the multiscale model agree well with that of atoms in full MD simulations, which shows the continuity in the overlap region. Furthermore, the Von Mises stress contours and contact force distributions in the contact region are almost same as full MD simulations. The method presented combines multiscale method and adaptive technique, and can provide a more effective way to multiscale method and to the investigation on nanoscale contact problems.

Atomistic theory of emission from dark excitons in self-assembled quantum dots

We present atomistic theory of emission from dark excitons in self-assembled quantum dots. Using the atomistic tight-binding model coupled with the configuration-interaction approach, we calculate the emission spectrum of a dark exciton confined in a single, self-assembled quantum dot. We demonstrate that the dark exciton state is optically active and that it emits photons in the plane of the dot, polarized along its vertical axis. This finite radiative lifetime is due to the mixing of hole subbands and can be tuned over several orders of magnitude by engineering the size and shape of the quantum dot.

Rate-limiting processes in the fast SET operation of a gapless-type Cu-Ta2O5 atomic switch

The speed of the SET operation of a Cu/Ta2O5/Pt atomic switch from a high-resistance state to a low-resistance state was measured by transient current measurements under the application of a short voltage pulse. The SET time decreased exponentially with increasing pulse amplitude, reaching as low as 1 ns using moderate pulse voltages. This observation shows that oxide-based atomic switches hold potential for fast-switching memory applications. From a comparison with atomistic nucleation theory, Cu nucleation on the Pt electrode was found to be the likely rate-limiting process determining the SET time.

EARLY IBĀḌĪ THEOLOGICAL ARGUMENTS ON ATOMS AND ACCIDENTS

AbstractThe bulk of Orientalist research regarding Islamic theological literature has neglected Ibāḍī theological opinions related to cosmology, which has led to an incomplete understanding of Islamic theology in the West and to a significant gap in Western scholarship. The omission of this important movement of Islam is understandable, considering the unavailability, lack of publication, circulation and translation of Ibāḍī texts. Therefore, this study seeks to address some of these gaps in the scholarship on early Islamic theology. The goals of this article are: 1) to characterize the classical Ibāḍī theological literature dealing with the atomistic theory of substance and accident; 2) to review the texts of Ibāḍī scholars as they argued and engaged with other Islamic theological schools: pre-Bahshamiyya Muʿtazilites (Abū Hāshim al-Jubbāʾī, d. 321/933) and Ashʿarites (Abū al-Ḥasan al-Ashʿarī, d. 324/936), in relation to the atomistic theory of substance and accident during the 3rd/9th century; 3) to survey the wide range of opinions among the early Ibāḍī theologians, and to examine the specific sources and themes that may have influenced this multifarious school of thought. Likewise, it is the aim of this article to demonstrate the common features in the Ibāḍī approach to producing theological literature during the formation of Islamic theology, and to explore how these early theologians may have gained access to cosmological themes that predate Islam.

Atomistic Theory of Ostwald Ripening and Disintegration of Supported Metal Particles under Reaction Conditions

Understanding Ostwald ripening and disintegration of supported metal particles under operating conditions has been of central importance in the study of sintering and dispersion of heterogeneous catalysts for long-term industrial implementation. To achieve a quantitative description of these complicated processes, an atomistic and generic theory taking into account the reaction environment, particle size and morphology, and metal-support interaction is developed. It includes (1) energetics of supported metal particles, (2) formation of monomers (both the metal adatoms and metal-reactant complexes) on supports, and (3) corresponding sintering rate equations and total activation energies, in the presence of reactants at arbitrary temperature and pressure. The thermodynamic criteria for the reactant assisted Ostwald ripening and induced disintegration are formulated, and the influence of reactants on sintering kinetics and redispersion are mapped out. Most energetics and kinetics barriers in the theory can be obtained conveniently by first-principles theory calculations. This allows for the rapid exploration of sintering and disintegration of supported metal particles in huge phase space of structures and compositions under various reaction environments. General strategies of suppressing the sintering of the supported metal particles and facilitating the redispersions of the low surface area catalysts are proposed. The theory is applied to TiO(2)(110) supported Rh particles in the presence of carbon monoxide, and reproduces well the broad temperature, pressure, and particle size range over which the sintering and redispersion occurred in such experiments. The result also highlights the importance of the metal-carbonyl complexes as monomers for Ostwald ripening and disintegration of supported metal catalysts in the presence of CO.

Breakdown of nucleation theory for crystals with strongly anisotropic interactions between molecules

We study the nucleation of model two-dimensional crystals in order to gain insight into the effect of anisotropic interactions between molecules on the stationary nucleation rate J. With the aid of kinetic Monte Carlo simulations, we determine J as a function of the supersaturation s. It turns out that with increasing degree of interaction anisotropy the dependence of ln J on s becomes step-like, with jumps at certain s values. We show that this J(s) dependence cannot be described by the classical and atomistic nucleation theories. A formula that predicts the identified J(s) behavior is yet to be derived and verified, and the present study provides the necessary data and understanding for doing that.

Perfluoropolyether Lubricant Interactions With Novel Overcoat via Coarse-Grained Molecular Dynamics

In this paper, we investigated physiochemical properties of new lubricant candidates for head-disk interface through various perfluoropolyether lubricant films on diamond, diamond-like carbon, and graphene overcoat surfaces via large scale coarse-grained bead-spring molecular dynamics stemming from the atomistic theory. Lubricant film conformations were characterized by investigating perpendicular component of molecular conformation, which determines the thickness of monolayer lubricant film. The distribution of functional endgroups and the mobility were analyzed via self-diffusion process. Here, we illustrate the effects of endgroup structure and carbon-surface structure on the film conformation and the mobility by expanding the multiscale simulation methodology and select candidates for future HDI design.

Nucleation and growth phenomena in nanosized electrochemical systems for resistive switching memories

The impact of the electrochemical nucleation on the switching kinetics in many nanoscaled redox-based resistive switching memories is critically discussed. In the case of the atomic switch, the system is site invariant and the nucleation process is strictly localized below the STM tip. Using RbAg4I5 solid electrolyte, nucleation was found to be rate limiting. The electrochemical metallization memory cells (gapless type atomic switch) operate at conditions closer to the conventional nucleation. They introduce additional difficulties for interpretation of the experimental results due to formation of hillocks, of surface oxide barrier films, induction of strain, and the influence of the high electric field. In valence change memories, the nucleation seems to be less important because of the higher applied voltages. The results are discussed in the context of the atomistic theory of electrochemical nucleation. We believe that re-analysis of the experimental data for many systems will reveal that the nucleation is limiting the switching time.

Thermodynamic Consistency between Analytic Integral Equation Theory and Coarse-Grained Molecular Dynamics Simulations of Homopolymer Melts

We present the equation of state for a coarse-grained model of polymer melts where each chain is represented as a soft colloidal particle centered on its center-of-mass. The formalism is based on the solution of the Ornstein–Zernike equation and is analytical, allowing for the formal investigation of the elements that ensure thermodynamic consistency in coarse-grained models of polymer melts. By comparing predictions from our expressions with those from computer simulations of the coarse-grained system and with atomistic polymer integral equation theory, we demonstrate that both structural and thermodynamic consistency with the atomistic level description is maintained during our coarse-graining procedures.

Multiscale approach for simulations of Kelvin probe force microscopy with atomic resolution

The distance dependence and atomic-scale contrast recently observed in nominal contact potential difference (CPD) signals simultaneously recorded by Kelvin probe force microscopy (KPFM) using noncontact atomic force microscopy (NCAFM) on defect-free surfaces of insulating as well as semiconducting samples have stimulated theoretical attempts to explain such effects. Especially in the case of insulators, it is not quite clear how the applied bias voltage affects electrostatic forces acting on the atomic scale. We attack this problem in two steps. First, the electrostatics of the macroscopic tip-cantilever-sample system is treated by a finite-difference method on an adjustable nonuniform mesh. Then the resulting electric field under the tip apex is inserted into a series of atomistic wavelet-based density functional theory (DFT) calculations. Results are shown for a realistic neutral but reactive silicon nanoscale tip interacting with a NaCl(001) sample. Bias-dependent forces and resulting atomic displacements are computed to within an unprecedented accuracy. Theoretical expressions for amplitude modulation (AM) and frequency modulation (FM) KPFM signals and for the corresponding local contact potential differences (LCPD) are obtained by combining the macroscopic and atomistic contributions to the electrostatic force component generated at the voltage modulation frequency, and evaluated for several tip oscillation amplitudes $A$ up to 10 nm. For $A$ $=$ 0.1 \AA{}, the computed LCPD contrast is proportional to the slope of the atomistic force versus bias in the AM mode and to its derivative with respect to the tip-sample separation in the FM mode. Being essentially constant over a few volts, this slope is the basic quantity that determines variations of the atomic-scale LCPD contrast. Already above $A$ $=$ 1 \AA{}, the LCPD contrasts in both modes exhibit almost the same spatial dependence as the slope. In the AM mode, this contrast is approximately proportional to ${A}^{\ensuremath{-}1/2}$, but remains much weaker than the contrast in the FM mode, which drops somewhat faster as $A$ is increased. These trends are a consequence of the macroscopic contributions to the KPFM signal, which are stronger in the AM-mode and especially important if the sample is an insulator even at subnanometer separations where atomic-scale contrast appears.

The Role of Adaptive Martensite in Magnetic Shape Memory Alloys

AbstractMagnetic shape memory materials require a high twin boundary mobility and low hysteresis for applications mainly as actuators, sensors, and magnetocaloric cooling elements. Usually, outstanding properties are found only in samples with a modulated martensitic structure. Here, we analyze the question why a modulated structure is beneficial and show evidence that the modulated martensite is not an equilibrium phase but a nanoscale microstructure of non‐modulated (NM) martensite. In this review, we combine results from continuum and atomistic theory, as well as local and integral measurements on the model system Ni–Mn–Ga. Following the concept of adaptive martensite the modulated phase forms to minimize elastic energy near the phase boundary by introducing low‐energy twin boundaries between lamellae of the NM martensite that have widths of a few unit cells.

Coarse-grained atomistic simulations of dislocations in Al, Ni and Cu crystals

This paper presents the application of a recently developed concurrent atomistic continuum (CAC) methodology in coarse-grained (CG) atomistic simulations of dislocation nucleation and migration in face-centered cubic (fcc) Al, Ni and Cu crystals, using an EAM force field and an adaptive mesh refinement strategy. The CAC method is based on recently developed atomistic field theory (AFT) that frames the problem in terms of a continuum field representation of lattice and sublattice atomic arrangements. Four sets of CG models with different finite element mesh refinement are constructed to test the accuracy and efficiency of the CG method relative to full molecular dynamics (MD). Simulation results show that the CG method is able to reproduce key phenomena of dislocation dynamics in initially defect free fcc crystals, including strain bursts caused by dislocation nucleation and migration, the structure of leading and trailing partial dislocations separated by equilibrium stacking faults in Al, formation of intrinsic stacking fault ribbons in Ni and Cu, and 3D migration of curved dislocations, all comparable with results of MD simulations. CG simulations have also revealed that the yield strength of Al depends on the thickness of the specimens as well as the operative deformation mechanisms. The effects of mesh size and adaptive mesh refinement on CG simulation results are analyzed and discussed.